eep 203 electromechanics laboratory

LABORATORY MANUAL

for

EEP 203 ELECTROMECHANICS

DEPARTMENT OF ELECTRICAL ENGINEERING

INDIAN INSTITUTE OF TECHNOLOGY DELHI- 110016

DEPARTMENT OF ELECTRICAL ENGINEERING

EEP 203 ELECTROMECHANICS LABORATORY (0-0-3) 1.5 Credits

Sem. Ist, Session 2011-2012

List of Experiments

1 Study of the steady state performance of a single‐phase transformer.

2 Study of the steady state performance of a separately excited DC generator.

3 Study of the steady state performance of a three‐phase alternator.

4 Study of the steady state performance of a three‐phase induction motor.

5 Study of the steady state performance of three‐phase transformers.

6 Study of the speed control of a DC motor.

7 Study of the steady state performance of a single‐phase capacitor‐type cage induction motor for a single winding and two winding configurations.

8 Study of steady state performance of a variable frequency control fed Three‐Phase Induction Motor Drive.

9 Study of the steady state performance of a three‐phase synchronous motor.

10 Study of the steady state performance of a grid connected three‐phase squirrel cage induction generator.

Course Coordinator

1


Experiment-I

Study of the steady state performance of a single-phase transformer

1.1 Motivation Insulation considerations limit the voltage of generation to a few kilovolts (10kV). But in order to reduce the transmission losses, the electric power is transmitted over long distances at the highest possible voltage (220 kV, 400kV…). Again due to considerations of safety, the power has to be distributed to the consumers at much lower voltages. In fact electric power is transformed several times from one voltage to another, with the help of power transformers, before it is made available at the consumer terminals. To find the performance of large power transformers by direct load test, a huge amount of energy has to be wasted. Moreover, it is difficult to obtain a suitable load large enough for direct loading. Thus for large transformers, the performance characteristics( efficiency, regulation etc) are computed from the knowledge of losses and equivalent circuit parameters, which in turn are determined by conducting simple tests like OC and SC test. 1.2 Objectives Study of the steady state performance of a single-phase transformer (a) Conduct (i) open circuit test at varying voltages, (ii) short circuit test, and (iii) windings

resistances measurement and compute equivalent circuit parameters and study the voltage/current on these parameters.

(b) Conduct load test and draw: (i) voltage regulation with output VA, and (ii) efficiency with output VA at UPF load.

(c) Compute following characteristics: (i) voltage regulation with output VA, (ii) efficiency with output VA at unity, 0.8 lagging and 0.8 leading power factor loads.

(d) Compute loads for conditions of (i) maximum efficiency and (ii) zero voltage regulation for the test transformer.

1.3 Theory It is well known that the equivalent circuit of a single phase transformer can be approximately represented as shown in Fig.1.1. The parameters R0 and X0, which take into account the two components of the no-load current, can be determined by conducting an OC test. The parameters R1 and X1 are determined from the SC test. These parameters depend to a certain extent on the actual load conditions of the transformer. The winding resistance can be measured using precision multimeter at primary and secondary side of the transformer. By loading the transformer with different nature and amount of loads (lagging, leading and unity pf) the conditions of positive, negative and zero voltage regulation can be obtained. Further the loading at which maximum efficiency occurs can be obtained. 1.4 Equipment and Components (a) Single Phase Transformer (b) Single Phase Auto-transformer (16A, 0-230V) (c) Two low pf Wattmeter’s (d) Two ac Voltmeters (e) Two ac Ammeters

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1.5 Procedure, Connection Diagrams, Experimentation and Precautions Note down the name plate details of the transformer and identify the terminals. Observe the windings and constructional features. 1.5.1 Open Circuit Test The open circuit test is usually done on the low voltage side, keeping the high voltage side open. Make the connections as shown in Fig.1.2. Apply rated voltage (V0) using auto-transformer and note down the corresponding power input (W0) and current drawn (I0). Repeat the above for different input voltages and tabulate the readings as in Table 1.1

Table- 1.1 Voltage applied V0 (volts) Current drawn I0 (amps) Power Input W0 (watts)

1.5.2 Short Circuit Test

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The short circuit test is usually done on the high voltage side, keeping the low voltage side short circuited. Make connections as shown in Fig. 1.3. Apply the required voltage (Vsc) so that the current drawn (Isc) is equal to the rated current. Note the corresponding power input (Wsc). Repeat the above for different values of short circuit currents and tabulate the readings as Table 1.2. Precautions:

• Make sure that autotransformer output voltage is zero before switching on the input supply

A very small input voltage is required to allow the rated short circuit current through secondary winding

Table- 1.2 Voltage applied Vsc (volts) Current drawn Isc (amps) Power Input Wsc (watts)

1.5.3 Load Test The load test is performed by adjusting the primary input voltage to its rated value. Then load the transformer up to its full load value in steps. Make connections as shown in Fig. 4. Vary the connected load until load VA equals the rated transformer VA. Record the input VA, input power, output VA and output power. Repeat the above for different nature of loads (lagging, leading and unity pf) and tabulate the reading as Table 1.3

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Precautions:

• Check the rating of the transformer before applying the load and restrict the load according to current rating of the transformer

Table- 1.3

Primary voltage

applied V1 (volts)

Primary Current I1

(amps)

Primary side VA

Secondary Terminal voltage

V2 (volts)

Secondary Current I2

(amps)

Secondary side VA

Fig1.4 Connection diagram of load test on transformer.

1.6 Data Sheet 1-Φ transformer: kVA rating: Primary voltage: Secondary voltage: Frequency: 1.7 Data Processing and Analysis

5

Losses Iron loss (for rated voltage) =…………….watts Full load copper loss =…………….watts Load at maximum efficiency=…………….VA Load at zero voltage regulation=………….VA Plots the following graphs (a) I0 vs V0 (b) W0 vs V0 (c) Isc vs Vsc (d) Wsc vs Vsc (e) VR vs load VA at upf (f) Efficiency vs output VA at upf (g) VR vs load VA at 0.8 lagging pf (h) Efficiency vs output VA at 0.8 lagging pf (i) VR vs load VA at 0.8 leading pf (j) Efficiency vs output VA at 0.8 leading pf Comment on the shape of the above graphs. Equivalent Circuit Parameters The four parameters of the equivalent circuit are R0, X0, R1 and X1. R0 and X0 are obtained from the OC test and R1 and X1 are obtained from the SC test as follows From OC test No load pf (cosΦ0)=W0/V0I0=………… sin Φ0=…………. Ic=I0cos Φ0=……………amps Im= I0sin Φ0=……… …amps Rc=V0/Ic=……………ohms X0= V0/Im=………….ohms From SC test, Total impedance referred to the high voltage side, Z2=Vsc/Isc=…………ohms Total resistance referred to the high voltage side, R2=Wsc/Isc

2 =…….....ohms X2=√(Z2

2-R22)=…….ohms

Total resistance referred to the low voltage side, R1= R2(N1/N2)2=…….ohms Similarly X1=X2(N1/N2)2=…….ohms Efficiency at any load (‘x’ times full load) at a given pf Let given pf be cosΦ Output at ‘x’ times full load = x(rated kVA.1000).cosΦ=……..watts Iron loss (Wi=constant)=W0=……..watts Copper loss at ‘x’ times full load, Wcx=x2(full load cu loss)=…..watts Therefore percentage efficiency= (output)/(output+losses) The load at which maximum efficiency occurs is cu loss= iron loss Regulation at full load at a given pf The percentage regulation can be approximately put down for the general case as Percentage regulation =(No load terminal voltage-full load terminal voltage)/(no load voltage) Percentage regulation = rcosΦ±xsinΦ

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(the + sign for lagging pf and –sign for leading pf) Where r= percentage resistance=(I1R1/E1).100=(I2R2/E2).100=…….. x= percentage reactance=(I1X1/E1).100=(I2X2/E2).100=…….. 1.8 Post-Experimental Quiz

a) Why is a transformer more efficient than rotating electrical machines? b) Are the equivalent circuit parameters of a transformer constant under all operating

conditions? If not, what are the possible reasons? c) How will you justify in taking the open circuit input as iron loss only? d) Can the regulation of transformer be negative? if so, when? e) Form the view point of ‘short circuit” risk should a transformer have a good or bad

regulation? f) How is magnetic leakage reduced to a minimum in commercial transformer. g) What effect are produced by change in voltage. h) Does the transformer draw any current when its secondary is open? i) How does change in frequency affect the operation of a given transformer?

References 1. Stephen J. Chapman, “Electric machinery fundamentals” The McGraw-Hill Companies,

New York 2005. 2. Arthur Eugene Fitzgerald, Charles Kingsley, Stephen D. Umans “Electrical machinery”

Tata McGraw-Hill publishing company Limited, New Delhi, India,2009. 3. Harold W Gingrich “Electrical machinery, transformers, and control” Englewood Cliffs,

N.J. Prentice-Hall, ©1979. 4. Harris Joseph Ryan “A text-book of electrical machinery” vol. 1. Electric, magnetic, and

electrostatic circuits.” New York, J. Wiley & Sons; London, Chapman & Hall, 1903. 5. Samarjit Gosh, “Electrical Machines,” First Indian Print, 2005, Pearson Education

(Singapore) Pte. Ltd.

1


Experiment-II

Study of the steady state performance of a separately excited DC generator 2.1 Motivation Inspite of the advantages of ac systems, the dc machines continue to find use in a wide range of industrial applications because of their flexibility and versatility. The special features which determine the choice of a dc machine for a particular application are the torque speed characteristics of motors and the voltage-load characteristics of generators. The knowledge of the limits within which these characteristics can be varied, and of the way, in which such variations could be obtained are also important. Study of these features for a dc machine is accordingly the motivation for this experiment. 2.2 Objectives Study of the steady state performance of a separately excited DC generator. (a) Conduct OCC test and draw (i) armature voltage with field current, and (ii) armature

voltage with speed. (b) Conduct load test at constant speed and draw (i) armature voltage with load current. (c) Compute critical resistances for the self excitation at different speeds and critical speed

using MATLAB Simulink and the data of same machine. (d) Compute following characteristics: (i) armature voltage with load current, and (ii) Emf with

armature current using MATLAB Simulink and the data of same machine. 2.3 Theory 2.3.1 Open Circuit Characteristics An important relation essential in the determination of dc generator performance is the relation between field current or field ampere turns and armature emf. The resulting curve at the desired speed is the magnetization characteristic or the open-circuit characteristics (OCC). The magnetization characteristics at several different speeds can be obtained from any one characteristic by recognizing that the voltage is directly proportional to speed for a fixed flux or field current. 2.3.2 Load Characteristics The load characteristic of a dc generator at a particular speed is the relationship between armature voltage of the generator and its load current at that speed. It is called the external characteristic if the plot is between the terminal voltage vs load current and the internal characteristic if the plot is between the generated emf vs load current. In a separately excited dc generator, the field current is independent of armature conditions. At constant field current and constant speed, the terminal voltage in this case drops off somewhat as load current increases because of the increased armature resistance drop and reduction in flux due to armature reaction. 2.3.3 Process of Self Excitation and Critical Resistances As long as some residual flux remains in the field poles and the field winding mmf produces the flux that aids the residual flux with field winding resistance is less than the critical resitance, the shunt generator is capable of building up the terminal voltage. When the generator is rotating at its rated speed, the residual flux in the field poles, however small it may be, induces an emf in the armature winding. Because the field winding is connected across the armature, the induced emf sends a small current through the field winding. This small current sets up a flux that aids the residual flux. The total flux per pole increases the induced emf which in turn increases the field current. The action is therefore cumulative till the no-load voltage.

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The value of no-load voltage at the armature terminals depends upon the field-circuit resistance. A decrease in the field-circuit resistance causes the shunt generator to build faster to a higher voltage as shown in Fig.2.1. The value of the field circuit resistance that makes the field resistance line tangent to the magnetization curve is called the critical resistance. The speed at which field resistance becomes the critical resistance is called the critical speed.

Fig.2.1 Voltage build up for various values of field circuit resistances.

2.4 Equipment and Components (a) Separately excited dc motor-generator set (b) Two Rheostat of suitable range for field control (c) Two dc Ammeters (d) Two dc Voltmeters 2.5 Procedure, Connection Diagrams, Experimentation and Precautions Note down the name plate details of the dc shunt motors and separately excited dc generator and identify the terminals. Observe the constructional features. 2.5.1 Magnetization Characteristics of a Separately Excited DC Generator Connect as in Fig. 2.2 with the generator separately excited. Start the dc shunt motor using the starter and bring it to rated speed by adjusting its external field circuit resistance. Set the field current of the generator, If to its rated value. Reduce the field current in steps to the minimum and open the field switch to make If=0. Note the terminal voltage Vt of the generator. Close the field switch and increase the field currents in steps up to the rated value and note the corresponding terminal voltage. Take care not to let the field current fall back in value during the increasing process. Decrease If back to zero in similar steps and record the same readings taking care not to let If rise in value during this process. Tabulate the data in Table 2.1 Now set If at the rated value, vary the speed and note the terminal voltage of the generator keeping If constant throughout this test. Tabulate the data in Table 2.2 Precautions:

• Make sure that field connections of DC shunt motor are proper and three-point starter return to its zero position before every fresh start.

• Use proper range of DC instruments only 2.5.2 Load Characteristics of Separately Excited DC Generator

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Connect the generator for separate excitation. With the speed at the rated value, adjust the field current of the generator to obtain the rated voltage on open circuit. Keeping the field current and speed constant, take readings of terminal voltage, Vt covering the entire range of load current. Note the corresponding input voltage and current to the driving motor and tabulate in Table 2.3. Measure the armature and field winding resistances of the generator by using precision multimeter. Precautions:

• Check the rating of generator and driving motor and apply the electrical load accordingly

Fig.2.2 OCC & Load test on a separately excited DC generator

Table- 2.1

Rated Speed=

S. No. Field current(If) Terminal voltage (Vt)

4

Table- 2.2 Rated If=

S. No. Speed in rpm (N) Terminal voltage (Vt)

Table- 2.3 Load test

S. No. Load Current IL (in amp.) Vt

Input Voltage to Motor (in volts)

Input Current to Motor (in amp.)

1.6 Data Sheet Name plate details of the shunt motor and separately excited DC generator set: Name of Manufacturer: Machine No. Class of Insulation: kW: RPM: Voltage: Amperes: Rating: Connections: Frequency: 1.7 Data Processing and Analysis Plots the following graphs

(a) Magnetizing curve and the speed voltage curve

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(b) Terminal voltage vs load current starting both scales at origin (c) From external characteristics draw internal characteristics

Comment on the shape of the above graphs. Critical Resistance Speed1 =……………RPM Rc=…………….ohm Speed2 =……………RPM Rc=…………….ohm Critical speed=……...RPM Rc=……………ohm 1.8 Post-Experimental Quiz a) Why does the open circuit characteristic differ for increasing and decreasing values of field

current b) Determine the critical resistance of field circuit for normal speed and the critical speed

corresponding to the normal field resistance of the machine c) How will you determine the load characteristic of a given machine using its OCC d) Why does the total flux in a dc machine decrease with load even through the field current is

constant e) Is the armature reaction mmf in dc machine stationary in space. f) How are brushes connected in dc generator? g) What is the standard direction of the rotation of the dc generator? h) What are the indications & causes of an overloaded generator? i) What is the procedure for shutting down a generator? References

1. Arthur Eugene Fitzgerald, Charles Kingsley, Stephen D. Umans “Electrical machinery” Tata

McGraw-Hill publishing company Limited, New Delhi, India,2009. 2. Samarjit Gosh, “Electrical Machines,” First Indian Print, 2005, Pearson Education

(Singapore) Pte. Ltd. 3. D.V. Richardson, “Rotating Electric Machinery and Transformer Technology,” Prentice Hall

Company, Reston, Virginia, 1978. 4. Harold W Gingrich “Electrical machinery, transformers, and control” Englewood Cliffs,

N.J. : Prentice-Hall, ©1979. 5. I. J. Nagrath and D.P. Kothari, “Electric Machines”, TMH, New Delhi, 2004. 6. M.G. Say and E.O. Taylor, "Direct Current Machines,” ELBS Pitman, IInd Edition, London,

1985. 7. A.E. Clayton and N.N. Hancock, "The Performance and Design of Direct Current

Machines,” CBS Publishers and Distributors, Third Edition, Delhi, 2001.

1


Experiment-III

Study of the steady state performance of a three-phase alternator 3.1 Motivation The most commonly used machine for generation of electrical power for commercial purpose is the synchronous generator or alternator. An alternator works as a generator when its rotor carrying the field system is rotated by a prime-mover. The terminal voltage of an alternator changes with load. The consumer’s voltage, however, must be maintained within pre-specified limits. This demands that the machine be designed with low voltage regulation. But a machine with low voltage regulation is uneconomical and is subjected to much mechanical and electrical stresses in case of accidental short circuits. However, in most cases low voltage regulation is not necessary since automatic voltage control equipment is normally used to avoid voltage fluctuations with load. The voltage regulation is important characteristic of an alternator and its predetermination is essential for its normal operation as well as for designing suitable excitation control schemes. 3.2 Objectives Study of the steady state performance of a three-phase alternator (a) Conduct the (i) OCC test, (ii) SCC test, and (iii) stator resistance measurement the

machine. (b) Conduct load test and draw (i) voltage regulation with output VA, and (ii) voltage with

output VA at UPF load. (c) Compute following characteristics: (i) voltage regulation with output VA, (ii) voltage with

output VA at unity, 0.8 lagging and 0.8 leading power factor loads. Use the parameters calculated in part (a) for simulation.

(d) Compute following characteristics: (i) power factor vs. power, (iii) armature current vs. power, (iv) V curves and (v) inverted V curves in gird connected mode. Use the parameters calculated in part (a) for simulation.

3.3 Equipment and Components (a) DC motor driven three phase Alternator set (b) Two Rheostat of suitable range for field control (c) One dc Ammeter (0-1A) (d) Three AC Ammeters (0-10A) (e) Three AC Voltmeters (0-500V) (f) Two UPF wattmeter (600V, 10A) (g) Suitable three phase resistance loads

3.4 Procedure, Connection Diagrams, Experimentation and Precautions Note down the name plate details of the dc shunt motors and alternator and identify the terminals. Observe the constructional features. 3.4.1 Open Circuit Test The open-circuit test, or the no-load test, is performed by driving the generator at its rated speed while the armature winding is left open as shown in Fig. 3.1. The field current is varied in

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suitable steps and the corresponding values of the open-circuit voltage varied in suitable steps and corresponding values of the open-circuit voltage between any two pair of terminals of the armature windings are recorded. The OCC follows a straight-line relation as long as the magnetic circuit of the synchronous generator does not saturate. In the linear region, most of the applied mmf is consumed by the air-gap; the straight line is appropriately called the air-gap line. As the saturation sets in, the OCC starts deviating from the air-gap line. Record the results as shown in Table 3.1 3.4.2 Short Circuit Test The short-circuit test provides information about the current capabilities of a synchronous generator. It is performed by driving the generator at its rated speed when the terminals of the armature winding are shorted as shown in Fig. 3.1. An ammeter is placed in series with one of the three shorted lines. The field current is gradually increased and the corresponding value of the armature current is recorded. The maximum armature current under short circuit should not exceed 1.5 times the rated armature current of the generator. When the per phase short-circuit current is plotted as a function of the field current, the graph is called the short circuit characteristic of a generator. Tabulate the results as shown in Table 3.2. Precautions:

• Make sure starting the dc motor that the external exciter field winding resistance of alternator is at maximum value and no voltage is applied to the exciter

3.4.3 Stator Resistance Measurement The resistance is measured between armature terminals of alternator either by using voltmeter-ammeter method or using high precision multimeter. With the help of recorded values per phase resistance is calculated depends on machine is star or delta connected. If it is star connected, per phase resistance is given as (Rmeas/2) and if it is delta connected it is given as (3/2) Rmeas. 3.4.4 Load Test The voltage regulation of an alternator is the per unit voltage rise at its terminals when a given load at a given power factor is thrown-off, the excitation and speed remaining same. Regulation is governed by the armature resistance, leakage reactance and to a large extent by the armature reaction and can be pre-determined by using load test or using any one method like synchronous impedance method, magneto-motive force method or zero power factor method. In load test alternator is loaded with different power factor loads and change in terminal voltage is observed from its rated terminal voltage. The connections are as shown in Fig. 3.2. Tabulate the results as shown in Table 3.3. Precautions:

• Check the rating of alternator and apply electrical load accordingly 3.4.5 Determination Of Synchronous Reactance For a particular field current I

f, the internal voltage E

f could be found from the OCC and the

short-circuit current flow Isc,A

could be found from the SCC. Then the synchronous reactance Xs

could be obtained using

2 2, ,

fs unsat a s unsat

scA

EZ R X

I= + =

2 2, ,s unsat s unsat aX Z R= −

3

Synchronous Reactance under saturated condition

( )2 2, ,

rated fs sat a s sat

scB

V EZ R X

I

== + =

,2 2

, ,s sat s sat aX Z R= −

3.4.6 Short Circuit Ratio Another parameter used to describe synchronous generators is the short-circuit ratio (SCR). The SCR of a generator defined as the ratio of the field current required for the rated voltage at open circuit to the field current required for the rated armature current at short circuit. SCR is just the reciprocal of the per unit value of the saturated synchronous reactance calculated by

Fig(3.1) Open circuit & Short circuit test connection diagram

[ ]

_

_

_

1. .

f Vrated

f Iscrated

s sat

ISCR

I

X in p u

=

=

4

Fig (3.2) Load test connection diagram

Table- 3.1 No load test

Rated Speed=

Table- 3.2

Short Circuit Test Rated Speed=

S.

No. Alternator Field

Current If Armature phase

Current Ia Armature phase

Current Ib Armature phase

Current Ic

S. No.

Alternator Field Current If

Armature phase voltage Va

Armature phase voltage Vb

Armature phase voltage Vc

5

Table- 3.3 Load Test

S. No.

Armature Voltage Wattmeter reading

PL

Load Current Power Factor Va Vb Vc iLa iLb iLc

3.5 Data Sheet Name plate details of the dc shunt motor alternator set: Name of Manufacturer: Machine No. Class of Insulation: kW: RPM: Voltage: Amperes: Rating: Connections: Frequency: Excitation: 3.6 Data Processing and Analysis Stator Resistance Phase a =……………ohm Phase b =……………ohm Phase c =……... ohm Plot the following graphs from measured data

(a) Voltage regulation with output VA (b) Output voltage with output VA

Use MATLAB simulink and using sim power system block set simulate the dc motor-alternator set using obtained parameters and plot the following graphs

(a) voltage regulation with output VA, (b) Voltage vs load VA at upf (c) Voltage vs load VA at 0.8 lagging pf (d) Voltage vs load VA at 0.8 leading pf

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Comment on the shape of the above graphs. 3.7 Post-Experimental Quiz a) What do you understand by saturated and unsaturated synchronous reactances? Which of the

two values is higher? b) What are the typical values of synchronous reactance in pu? c) Why is the zero power factor test often carried out at somewhat reduced voltage? d) Is it possible to plot the complete ZPF characteristic from one experimental observation? e) Which of the method of finding regulation gives the most acceptable results and why? f) Explain why armature reaction is always compensated in dc machine but not in alternators. g) State the purpose of damper winding? h) If the synchronous machine has no damper windings on the rotor, what reactance would you

get if you perform the same test which was done fo X’d and X’q i) what do you observe when the synchronous motor in the maximum lagging current test is

about to fall out of step? j) Can a dc generator be converted in to an alternator? how k) What is the direct connected alternator?

References

1. Dale R Patrick; Stephen W Fardo “Rotating electrical machines and power systems” Lilburn,

Ga. : Fairmont Press, 1997. 2. J D Edwards “Electrical machines: an introduction to principles and characteristics” New

York : Macmillan, 1986. 3. Brian Moore, John Donaghy “Electrical machines Basic principles series” Pitman, 1988 4. A.S. Langsdorf, “Theory of alternating current machinery,” TMH, new Delhi, 2001. 5. John Hindmarsh “Electrical machines and their applications” Oxford [u.a.] Pergamon Pr.,

1991. 6. Arthur Eugene Fitzgerald, Charles Kingsley, Stephen D. Umans “Electrical machinery” Tata

McGraw-Hill publishing company Limited, New Delhi, India,2009 7. J. H. Walker and T. Stuart Walker, “Large Synchronous Machines: Design, Manufacture, and

Operation,” Clarendon Pr, 1996. 8. Samarjit Gosh, “Electrical Machines,” First Indian Print, Pearson Education (Singapore) Pte.

Ltd, 2005.

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Experiment-IV

Study of the steady state performance of a three-phase induction motor

4.1 Motivation A large percentage of the electrical power generated in the world is consumed by induction motors, as these are the main drive motors used in the industries. Practicing engineers should be conversant with the performance characteristics. Equivalent circuit parameters of the machine should be known for predicting the performance. While motor designer calculates the parameters using design details, measured values are preferable for prediction. All parameters would not be constant under all operating conditions as they would be affected by temperature, winding currents, saturation, skin effect etc, and these have to be accounted for as far as practicable. 4.2 Objectives Study of the steady state performance of a three-phase induction motor (a) Conduct (i) no load test at different voltages, (ii) blocked rotor test, and (iii) stator winding

resistance test on a three-phase induction motor and compute its equivalent circuit parameters and F&W losses.

(b) Conduct the load test and draw: (i) Speed vs. Output Power, (ii) Stator Current vs. Output Power, (iii) Power factor vs. Output Power, (iv) Efficiency vs. Output Power at the rated voltage and frequency.

(c) Compute the following characteristics: (i) Speed vs. Output Power, (ii) Current vs. Output Power, (iii) Power factor vs. Output Power, (iv) Efficiency vs. Output Power using same parameters as in part 1(a).

4.3 Theory The steady-state performance of a poly-phase induction motor can be obtained using per phase equivalent circuit and it is shown in Fig. 4.1. The symbols are, V1= input voltage per Phase R1, X1= resistance and leakage reactance of the stator per phase R2, X2= resistance and leakage reactance of the rotor per phase Xm= magnetizing reactance Rc= core loss resistance I1, I2= stator and rotor currents (referred to stator) per phase The developed torque is given as, T=3I2

2R2/(sωs) N.m Where ωs=synchronous speed in radians/sec s= pu slip The parameters could be determined by ‘no-load’ and ‘blocked-rotor’ tests, the former determines Rc and Xm while the latter yields R1, R2, X1, X2. Following equations could be used for calculation:

2

1 1 1

22 2

21

2

11

e mm

e m

m

m

Z R jXR

Z jXsjR X

ZR jX

Z ZZ Z

Z ZV

IZ

= +

= +

=+

= ++

=

12

2

22 2

1

1 1

( )The torque T=3

factor, pf= cos tan ( / )

R=Re(Z) X=Im(Z)Input power P 3 *

P (1 )Effeciency =P /

m

m

in

o s

o in

I ZI

Z Z

I Rs

Power R X

where

V I pfOutput power s T

Pω

−

=+

⎡ ⎤⎣ ⎦

== −

Fig. 4.1 Per Phase Equivalent Circuit of a Poly-phase Induction Motor

4.4 Equipment and Components (a) A three phase squirrel cage induction motor coupled with dc generator (b) Three-phase auto-transformer (16A, 0-415V) (c) One AC Ammeter (0-5/10A) (d) One AC Voltmeter (0-500V) (e) Two Low pf wattmeter (600V,10A) (f) Suitable dc loads (440V, 10A) (g) Tachometer

4.5 Procedure, Connection Diagrams, Experimentation and Precautions Note down the name plate details of the both machines and identify the terminals. Observe the constructional features. Note the type of rotor used and the winding connections.

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4.5.1 Measurement of Stator Winding Resistance Make connections as shown in Fig. 4.2 for a star connected stator. Similar connections can be used for a delta-connected stator. Apply low voltage so that current through the windings is well below the rated value. 4.5.2 Light Running Test Connect the machine as in Fig. 4.3. Start the motor by applying the normal frequency reduced voltage to the stator and gradually increases the voltage to its rated value. In case of slip-ring motor short circuit the slip rings before starting. Note down the readings of voltmeter, ammeter, wattmeters and tachometer at different voltages. Precautions:

• Use low power factor meters for power measurements and careful about the deflections of both instruments, In case of reverse deflection, change the current wires.

• Take care about inrush current of motor during starting • Make sure autotransformer zero setting before every fresh start

Fig. 4.2 Connection Diagram for Stator Winding Resistance Measurement

Fig. 4.3 Connection Diagram for Light Running Test and Block Rotor Test 4.5.3 Block Rotor Test With the above connections, keep the rotor blocked, and record the readings of various instruments for different steps of input current varying from zero to 1.5 pu. If values change

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noticeably for different rotor positions, an average set of readings should be taken. Since very low value of voltage can inject rated current under this condition precaution should be taken not to apply high voltages. Precautions:

• Use upf meters for power measurements for better accuracy. • Make sure autotransformer zero setting before switching on the input supply and small

input voltage is sufficient for allowing the rated current of machine • Hold rotor in block position tightly

4.5.4 Load Test With the connections shown in Fig. 4.4, apply the rated voltage and frequency to the induction motor and then record the supply voltage, supply current and the wattmeter readings corresponding to rated voltage, no-load current and no-load power losses. Switch on the field supply voltage of the DC generator adjust the field voltage to its rated value, then switch on the load connected to armature step by step and record the armature current, Tabulate the reading in Table 4.3. Precautions:

• Use upf meters for power measurements for better accuracy. • Make sure autotransformer zero setting before switching on the input supply

4.6 DATA SHEET (a) Name Plate Details of the Machine Name of the manufacturer: Rated output: Rated voltage: Supply Frequency: No. of phases: Rated speed: No. of poles: Rated current: Type of rotor: Type of starting method: Winding connections for stator/rotor: (b) Average stator winding resistance/phase=……..ohm (c) Average rotor winding resistance/phase=………ohm

5

VROTOR

AUTO

TRANSFORMER

400 v PHASE50 Hz

L

C

M

M L

C

A1

A2

220 V DC

SUPPLYVariable Load

V

V

A

G

Fig. 4.4 Connection Diagram for load Test

Table- 4.1

Light Running Test

Table- 4.2

Block Rotor Test

S. No.

Input Voltage (V) Input Current (I) Input Power= W1+W2

S. No.

Input Voltage (V0)

Input Current (I0) Input Power= W1+W2 Speed

6


S. No.

Input Voltage

(V)

Input Current

(I)

Input Power= W1+W2

Power Factor Speed

Efficiency Armature Current

4.7 Data Processing and Analysis Parameters of the Equivalent Circuit The no-load power input mainly represents core losses, as copper losses could be ignored. Rc=V2

0/Pc Ic=Vc/Rc Im

2=I02-Ic

2

Knowing Im, Xm can be calculated from the relation, Xm=V0/Im In the case of blocked rotor test, the equivalent series impedance referred to stator is given by, Zsc=Rsc+jXsc = (voltage per phase/short circuit current per phase) The equivalent series resistance referred to stator is Rsc=R1+R2=(Input power per phase/(current per phase)2) And Xsc=X1+X2=√(Zsc

2-Rsc2)

The ac resistance R1 is known by multiplying dc resistance by skin effect factor. Thus R2 can be evaluated. The separation of stator and rotor lekage reactance is difficult. For most machines of normal design it is sufficiently accurate to take X1=X2=(Xsc/2) In the block rotor test Xm is neglected. 4.8 Report a) Plot no load power input V applied voltage. Find out iron loss at rated voltage. Calculate

Rc and Xm at different voltages and plot them. b) Plot blocked rotor input power/ input current. Calculate Rsc and Xsc and plot them

/current. c) Plot using measured data Speed vs. Output Power. d) Plot using measured data Stator Current vs. Output Power. e) Plot using measured data Power factor vs. Output Power. f) Plot using measured data Efficiency vs. Output Power. Simulate the induction machine using measured parameters a) Plot using measured data Speed vs. Output Power. b) Plot using measured data Stator Current vs. Output Power. c) Plot using measured data Power factor vs. Output Power. d) Plot using measured data Efficiency vs. Output Power.

4.9 Post-Experimental Quiz

7

a) Why should there be a difference between dc and ac resistance? b) Will the magnetizing branch parameters remain constant at all voltages? Explain. c) Are the core loss and mechanical losses constant for all operating conditions? Comment. d) How does the core loss vary with voltage and why? e) Critically comment on the characteristics you obtained? f) What factors determine the direction of rotation of the machine? g) Why are induction motors called asynchronous? h) How can the direction of rotation of the motor be reversed? i) How does the slip vary with load? j) Enumerate the possible reasons if a 3 phase motor fails to start. k) What is the basis of operation of a three phase induction motor?

References

1. S K Bhattacharya “Electrical machines” Tata McGraw-Hill Pub. Co. Ltd., New Delhi,

©2009. 2. Arthur Eugene Fitzgerald, Charles Kingsley, Stephen D. Umans “Electrical Machinery” Tata

McGraw-Hill publishing company Limited, New Delhi, India,2009 3. Mohamed Abdus Salam “Fundamentals of electrical machines” Oxford Alpha Science 2005 4. Say, M.G. "Alternating Current Machines" Fifth Edition. London: Pitman (1983). 5. I. J. Nagrath and D.P. Kothari, “Electric Machines”, TMH, New Delhi, 2004. 6. Guru Bhag Singh, H. Hiziroglu “Electric Machinery And Transformers” 3nd ed. Oxford

University Press, 2005. 7. Daniels, A.R. "Introduction to Electrical Machines" Macmillan.(1985).

1


Experiment-V

Study of the steady state performance of three-phase transformers

5.1 Motivation The importance of three-phase system in generation, transmission and distribution of power is well known. To transform the three-phase electric power from one voltage to another, three-phase transformers are required both at the generating and the distribution ends of a power system. It is therefore essential to learn about the performance of three phase transformers connected in different possible manners. Initially it was a common practice to use suitably interconnected three single phase transformers instead of a single 3-phase transformer. But, these days the latter is gaining popularity because of improvement in design and manufacture but principally because of better acquaintance of operating men with the three phase type. As compared to a bank of single phase transformers, the main advantages of a 3-phase transformer are that it occupies less floor space for equal rating, weighs less , costs less and further, that only one unit is to be handled and connected. In order to keep down the third harmonic voltages in a Y-Y bank of transformers, tertiary winding connected in Δ is provided. This provides a path for zero sequence current during ground fault condition. Such a winding may also help to stabilize the neutral of the fundamental frequency voltages and prevent third harmonic currents in the lines and ground. Hence it is interesting to study such multi-winding transformers. In some cases the tertiary windings are designed for voltages which may be useful to supply local circuits in a power system. 5.2 Objectives Study of the steady state performance of three-phase transformers (a) Conduct no load test to verify the voltage and current relationships for (i) Y-Y, (ii) Y-

Delta, (iii) Delta-Star, (iv) Delta-Delta connections (b) Conduct load tests and verify voltage and current relations and draw: (i) voltage regulation

with output VA, and (ii) voltage with output VA at UPF load for (i) Y-Y, (ii) Y-Delta, (iii) Delta-Y, and (iv) Delta-Delta connections.

(c) Compute following characteristics: (i) voltage regulation with output VA, and (ii) voltage with output VA at unity, 0.8 lagging and 0.8 leading power factor loads.

(d) Compute loads for conditions of (i) maximum efficiency and (ii) zero voltage regulation for the test transformer

5.3 Theory 4.3.1 Star-Star Connection This permits grounding the neutral points of both primary and secondary three phase circuits. When the primary neutral is not connected to the source terminal, it is necessary to use Δ connected tertiary windings in order to avoid imbalance in the system. The relations are given as, √3 Vphase =Vline ( primary), √3 Vphase =Vline ( secondary) Vphase(primary) = (a)* Vphase(secondary) Iline(primary) = Iphase(primary Iline(secondary) = Iphase(secondary)

2

R

Y

B

R

Y

B

ThreePhase Input

( Primary)

ThreePhase Output

( Secondary)

Fig. 5.1

5.3.2 Star-Delta Connection This connection is normally used at the receiving end of high voltage transmission lines. The relations are given as, √3 Vphase =Vline ( primary), Vphase =Vline( secondary) Vphase(primary) = (a)* Vphase(secondary) Iline(primary) = Iphase(primary) Iline(secondary) = √3 Iphase(secondary)

ThreePhase Input

(Primary)

ThreePhase Output

( Secondary)

R

Y

B

R

Y

B

Fig. 5.2

5.3.3Delta-Star Connection This gives higher secondary voltage for transmission purposes than the connections with Δ secondary’s without increasing the strain on the insulation of the transformers. It is the connection commonly used at the generating end of transmission lines. The Y neutral is generally grounded. The relations are given as, Vphase =Vline ( primary), √3 Vphase =Vline ( secondary) Vphase(primary) = (a)* Vphase(secondary) Iline(primary) = √3 (Iphase(primary)) Iline(secondary) = Iphase(secondary

3

Fig. 5.3

5.3.3Delta-Delta Connection The ratio of primary to secondary line voltages remains equal to the ratio of transformation ‘a’. The main advantage of this connection lies in the fact that the system can still operate on 58% of its rated capacity even in case of failure of one of the transformers. The remaining two transformers work in open Δ or V. The relations are given as, Vphase =Vline ( primary), Vphase =Vline ( secondary) Vphase(primary) = (a)* Vphase(secondary) I line(primary) = √3 (I phase(primary)) I line(secondary) = √3 (I phase(secondary))

Fig. 5.4

5.4 Equipment and Components (a) Three identical single phase transformers of suitable ratings (b) Three phase autotransformer (0-16A) (c) AC Voltmeter (0-500V) (d) Ammeters (0-10/30A) (e) Two LPF wattmeter’s (500V, 20A) (f) Two UPF wattmeter’s (500V, 20A) (g) Three-phase loading rheostat

4

5.5 Procedure, Connection Diagrams, Experimentation and Precautions Fig. 5.5 shows the connection diagram for a Y-Y connected three-phase transformer.

(i) Carry out the polarity test for all three single phase transformers (ii) Connect the primaries and secondaries as shown in Fig. 5.5 for star-star connection

and repeat for other connections. (iii) In each case connect the primaries to the appropriate three-phase supply. Make

measurements of open circuits primary and secondary voltages (both line and phase voltage). Tabulate the readings as per Table 5.1,

Table 5.1

DELTA-STAR

PRIMARY

SECONDARY

Vphase Vline Iline Iphase Vphase Vline Iline Iphase

STAR-DELTA

PRIMARY

SECONDARY


STAR-STAR

PRIMARY

SECONDARY


5

DELTA-DELTA

PRIMARY

SECONDARY


5.5.4 Load Test Fig. 5.3 shows the connections for load test for star-star connections.The load test is performed by adjusting the primary input voltage to its rated value. Then load the transformer up to its full load value in steps. Vary the connected load until load VA equals the rated transformer VA. Record the input VA, input power, output VA and output power. Repeat the above for different nature of loads (lagging, leading and unity pf) and tabulate the reading as Table 5.2, repeat the above for all transformer connections. Precautions:

• Check the rating of transformer and apply the load accordingly

L

CM

L

CM

V

V

A

A

A

R

Y

B

N

V

V

R

Y

B

n

V

V

Fig. 5.6 Connection diagram for No Load Test with Star/star Connection

6

Fig. 5.6 Connection diagram for Load Test with Star/star Connection

Table- 5.2 Type of Connection

Primary voltage applied

V1(volts)

Primary Current I1(amps)

Primary side VA

Power W1(watts)

Secondary Terminal voltage

V2 (volts)

Secondary Current I2 (amps)

Secondary side VA

Power W2

(watts)

5.6 DATA SHEET

(i) Compare the results of observations with the theoretical values obtained from the transformer ratings.

(ii) Analyze the waveforms obtained fro various cases and state the reasons of variations (iii) For the unbalanced loading discuss clearly the magnitude of voltage and current

imbalances in Y-Y, Y-delta, delta-Y and delta-delta connections 5.7 Post Experimental Quiz

a) Discuss the advantages and disadvantages of using single three phase transformers instead of three single phase transformers in a three phase system

b) Mention the constructional feature of three phase transformers

7

c) What short of cooling system is used in power transformers d) How is a three phase, four wire connection better than three phase three wire

connection with Y-Y transformers e) If a single phase load is applied between line and neutral of a bank of Y-Y connected

single phase transformers without neutral connection, explain why a smaller load current can be obtained even if the impedance of the load is reduced to zero.

f) Is it possible to deliver single phase load in three wire supply system using any transformer bank.

g) Which of the transformer connections is best suited for three phase ,4 wire service? References

1. S K Bhattacharya “Electrical machines” New Delhi: Tata McGraw-Hill Pub. Co. Ltd.,


McGraw-Hill publishing company Limited, New Delhi, India,2009 3. Mohamed Abdus Salam “Fundamentals of electrical machines” Oxford Alpha Science 2005 4. Say, M.G. "Alternating Current Machines" Fifth Edition. London: Pitman (1983). 5. I. J. Nagrath and D.P. Kothari, “ Electric Machines”, TMH, New Delhi, 2004. 6. Guru Bhag Singh, H. Hiziroglu “Electric Machinery And Transformers” 3nd ed. Oxford


1


Experiment-VI

Study of the speed control of a DC motor 6.1 Motivation DC motors are in general much more adaptable to adjustable speed drives than ac motors which are associated with a constant speed rotating fields. Indeed this susceptibility of dc motors to adjustment of their operating speed over wide ranges and by a variety of methods is one of the important reasons for strong competitive position of dc motors in modern industrial drives. It is thus necessary to gather an idea about speed control methods along with their associated characteristics. 6.2 Objectives (a) Conduct a suitable test for the speed control by varying the resistance in the field circuit

and draw (i) speed-field current, (ii) armature current-speed, (iii) armature current - field current characteristics.

(b) Conduct a suitable test for the speed control by varying the armature terminal voltage using an autotransformer and a diode rectifier circuit and draw (i) speed-armature voltage, (ii) armature current-speed, (iii) armature current-armature voltage characteristics.

(c) Conduct a load test on the DC motor at two speeds and draw (i) speed-torque, (ii) armature current-torque, and (iii) speed-armature current characteristics.

(d) Compute and draw characteristics in Parts (a),(b),and (c) using specification of test machines.

6.3 Theory The torque, T developed and speed, n of a dc motor are given as,

a

t a a

T K IV I RN

K

φ

φ

=−

= (1)

where k is constant decided by the design of the machine. The above equation explains the concepts for different methods of speed control. 6.3.1 Varying Field Excitation (Φ) In shunt and compound motors speed control can be achieved by varying the shunt field circuit resistance. The lowest speed corresponds to zero resistance in field rheostat. Speed can be increased by increasing the field rheostat resistance. The highest speed is limited by armature reaction under weak field conditions, causing motor instability or poor commutation. The dc motors with the shunt field-rheostat speed control is generally referred to as a constant horse power drive, since back emf remains practically constant. The torque on the other hand varies directly with flux and therefore has its highest allowable value at the lowest speed. Field rheostat control is thus best suited to drives requiring decreased torque at high speed. 6.3.2 Varying Armature Terminal Voltage (Vt) A change of the armature terminal voltage results in a change in speed with constant excitation. Usually the power available is constant voltage ac, so the auxiliary equipment in the form of rectifier is required to provide the controlled armature voltage for the motor. In this mode the torque developed by motor is almost equal to rated torque at any speed from zero to rated speed. In this region motor operates in constant torque mode. 6.4 Equipment and Components

2

(a) Test DC motor coupled with DC generator (b) PMMC Voltmeter (0-300VDC)- 02Nos (c) PMMC ammeter (0-20A)-02 Nos. (d) Two rheostats for field control (e) Single phase rectifier (suitable rating) (f) Single phase Autotransformer (suitable rating) (g) Loading device (h) Tachometer 6.5 Procedure, Connection Diagrams, Experimentation and Precautions 6.5.1 Shunt Field Rheostat Control Decide the values and ranges of rheostats, ammeters and voltmeters from the specifications of the test motor and then make the connections as shown in Fig. 6.1. Switch on the field supply for motor and generator both and adjust both the rheostat in such a way that rated field current flow through the field windings. Apply the terminal voltage slowly keeping armature current of separately excited dc motor below its rated armature current till reach the rated speed of the motor. Now gradually increase the field resistance of dc motor and observe the variation of speed with the field current, for no load condition. Repeat the procedure for various constant electrical loads applied on dc generator. Speed control at different constant loads could be taken as studying the variations of speed with field currents. Test may be 0.25, 0.5 and 1.0 pu of armature current. The dc separately excited generator is used to load the dc motor. With separate excitation its field current can be kept constant at rated value and the armature load current gives a measure of torque.

Fig. 6.1 Connection Diagram for Speed Control of Separately Excited DC Motor Using Field

Current Control 6.5.2 Armature Voltage Control Make the connections as shown in Fig. 6.2. Switch on the field supply of both dc motor and generator and adjust the field rheostat in such a way that rated field current flows in both machines. Now set the output of autotransformer to its minimum value and apply rated input voltage to the input of autotransformer. Now increase the output of autotransformer in small

3

steps and observe the armature voltage, armature current and speed of dc motor. Tabulate the results as given in Table 6.2.

Fig. 6.2 Connection Diagram for Speed Control of Separately Excited DC Motor Using Armature Voltage Control 6.6 DATA SHEET Name Plate Details of the Machine Name of the manufacturer: Rated output: Rated voltage: Rated speed: No. of poles: Rated current: Type of rotor: Field voltage:

Table- 6.1

Field Resistance Control

S. No.

Motor Field Current (amp)

Motor Speed (rpm)

Test motor Terminal Voltage

(volt)

Test Motor current (amp)

4

Table- 6.2

Armature Voltage Control and Load Test

S. No.

Motor Field Current (amp)

Motor Speed (rpm)

Test motor Terminal Voltage

(volt)

Test Motor current (amp)

6.8 Report Plot for field resistance control

(a) speed-field current (b) Armature current-speed (c) Armature current - field current characteristics.

Plot for armature voltage control (a) Speed-armature voltage, (b) Armature current-speed, (c) Armature current-armature voltage characteristics.

Using MATLAB-simulink models the experimental setup using the given name-plate rating of machines and simulate to plot all above characteristics. 6.9 Post-Experimental Quiz (a) Comment on the nature of graph speed vs field current obtained in the field control

method (b) Can the dc starter be used for speed control? (c) What are the limitations of field control? (d) What is the difference between speed control and speed regulation of a motor (e) Explain why the speed changes with load (f) Which of the speed control method provide constant horse power drive? (g) Can the field control method be applied to series motor? (h) What is main advantage of ward-leonard system for dc motor speed control .

5

(i) Write the merits & demerits of rheostatic control method used for speed control in dc motor.

References

1. S K Bhattacharya “Electrical machines” New Delhi, Tata McGraw-Hill Pub. Co. Ltd.,

©2009. 2. M.G. Say and E.O. Taylor, "Direct Current Machines,” ELBS Pitman, IInd Edition,

London, 1985. 3. Arthur Eugene Fitzgerald, Charles Kingsley, Stephen D. Umans “Electrical machinery”

Tata McGraw-Hill publishing company Limited, New Delhi, India,2009 4. A.E. Clayton and N.N. Hancock, "The Performance and Design of Direct Current

Machines,” CBS Publishers and Distributors, Third Edition, Delhi, 2001.

1


Experiment-VII

Study of the steady state performance of a single-phase capacitor-type cage induction motor for a single winding and two winding configurations

7.1 Motivation Most of the fractional horse power motors are single phase induction motors. These account for millions of motors, about 20-30% of the commercial value. The knowledge of the performance of such a machine is thus essential. Single phase motors, which are designed to operate from a single phase supply have manufactured in a large number of types to perform a wide variety of useful services in home, offices, factories, workshops etc. Small motors, particularly in the fractional kilo watt sizes are better than any other. In fact, most of the new products of the manufacturers of space vehicles, aircrafts, business machines and power tools, driving fans, blowers, compressors etc. have been possible due to the advances made in the design of fractional-kilowatt motors. Since the performance requirements differ so widely, the motor manufacturing industry has developed many different types of such motors, each being designed to meet special demands. 7.2 Objectives (a) Conduct the (i) no load test, (ii) blocked rotor test, and (iii) stator winding resistance test on

a single-phase induction motor on main winding and auxiliary winding and compute equivalent circuit parameters and turns ratio for a single winding and two windings configurations.

(b) Conduct load test and draw: (i) Torque vs. speed, (ii) Current vs. Speed, (iii) Current vs. Output Power, (iv) Power factor vs. Output Power and (v) Slip vs. Output Power curves with only main winding excited configuration

(c) Compute (i) the value of a capacitor for the starting and (ii) the value of a capacitor at full load speed.

(d) Compute following characteristics: (i) Torque vs. speed, (ii) Current vs. Speed, (iii) Current vs. Output Power, (iv) Power factor vs. Output Power and (v) Slip vs. Output Power on single-winding and two-winding configurations for a starting capacitor

7.3 Theory Structurally, the most common types of single-phase induction motors resemble polyphase squirrel-cage motors except for the arrangement of the stator windings. An induction motor with a squirrel-cage rotor and a single-phase stator winding is represented schematically in Fig. 7.1. Instead of being a concentrated coil, the actual stator winding is distributed in slots to produce an approximately sinusoidal space distribution of mmf. From three phase induction motor theory, we know that a single-phase winding produces equal forward- and backward-rotating mmf waves. By symmetry, it is clear that such a motor inherently will produce no starting torque since at standstill, it will produce equal torque in both directions. However, if it is started by auxiliary means, the result will be a net torque in the direction in which it is started, and hence the motor will continue to run. The behavior of single phase induction machine can be studied by using double revolving theory. In double revolving field concept, a pulsating mmf produced by stator winding of a pure single

2

phase machine can be resolved into two oppositely rotating mmf Ff and Fb of constant and equal magnitude which can be mathematically expressed by,

[ ]cos( ) cos( )2f b

NIF t t

F F F

ω θ ω θ= − + +

= +

where N= effective number of turns for main winding I=main winding current However Φf and Φb produced by mmf Ff and Fb are of equal magnitude only at standstill. Under running condition Φf > Φb. each component of flux produces electromagnetic torque i.e. forward torque Tf produced by forward flux Φb and backward torque Tb by backward flux Φb. net torque produced by induction machine being (Tf-Tb) which is positive along the direction of rotation.

Fig.7.1 Schematic view of single phase induction motor

Capacitor-type motor: Capacitors can be used to improve motor starting performance, running performance, or both, depending on the size and connection of the capacitor. The capacitor-start motor is also a split-phase motor, but the time-phase displacement between the two currents is obtained by means of a capacitor in series with the auxiliary winding, as shown in Fig.7.2 Again the auxiliary winding is disconnected after the motor has started, and consequently the auxiliary winding and capacitor can be designed at minimum cost for intermittent service. By using a starting capacitor of appropriate value, the auxiliary-winding current iaux at standstill can be made to lead the main-winding current Imain by 90 electrical degrees, as it would in a balanced two-phase motor (see Fig. 7.2b). In practice, the best compromise between starting torque, starting current, and cost typically results with a phase angle somewhat less than 90 °. A typical torque-speed characteristic is shown in Fig. 7.2 (c) , high starting torque being an outstanding feature. These motors are used for compressors, pumps, refrigeration and air-conditioning equipment, and other hard-to-start loads. 7.4 Equivalent Circuit The equivalent circuit of a single phase induction motor is as shown in Fig. 7.2 and drawn using double revolving theory. This equivalent circuit can be drawn for any speed. Here the core loss is neglected. The core loss can be taken in to account by placing core loss resistor in parallel with the magnetizing reactance branch. Block Rotor Test (auxiliary winding open):

3

The equivalent circuit is shown in Fig. 7.4(a). During this test rotor is kept blocked and a low voltage supply is given to the stator to maintain the rated current in the main winding. The voltage (V

bm), current (I

bm) and power (P

bm) are measured. During the test s=1 and X

m can be

neglected. The input impedance is Zbm

=(Vbm

/Ibm

). The total resistance in the circuit is

Rbm

=(Pbm

/I2

bm). The total reactance is X

bm=√(Z

2

bm-R

2

bm). The equivalent series resistance (R

bm)

of the motor is given by Rbm

=R1+(R

’

2/2)+(R

’

2/2)=R

1+R

’

2

so R’

2 =R

1 - R

bm . The effective resistance at the line frequency can be calculated because R

1 can

be measured using DC test. The equivalent series reactance (X0) of the motor is given by

Xbm

=X1+(X

’

2/2)+(X

’

2/2)=X

1+X

’

2 As separation of X

1m and X’

2 is not directly possible. Let us make the following assumption-

X1=X

’

2=(1/2)X

bm=√(Z

2

bm-R

2

bm)/2.

Block Rotor Test (main winding open): Fig. 7.4 (b) shows the equivalent circuit. In this test rotor is kept blocked and a low voltage supply is given to the stator to maintain the rated current in the auxiliary winding .The voltage (V

ba), current (I

ba) and power (P

ba) are measured. During the test s=1 and X

m can be neglected.

The total resistance in the circuit is Rba

=(Pba

/I2

ba).Since the rotor winding resistance is already

known, so the rotor resistance referred to the auxiliary winding as R2a

=Rba

-Ra .The ratio ‘a’

effective turns in the auxiliary winding to the main winding is a=√(R2a

/R2).

No-Load Test: Fig. 7.5 shows the equivalent circuit. During this test motor is run without load at rated voltage and frequency. The voltage (V

nl), current (I

nl) and power (P

nl) are measured. During the test s=0

and hence (R’

2/2s) is large compared to (X

m/2). The equivalent reactance (X

nl) of the motor is

given by Xnl

=X1+(X

m/2)+(X

’

2/2)

X1=X

2=0.5X

bm

X

m can be calculated, because X

1 and X

’

2 have already been calculated from blocked rotor test

Xm

=2Xnl

-1.5Xbm

The no-load power factor is given by cosФ

nl=(P

nl/V

nlI

nl)

The no-load equivalent impedance is given by Z

nl=(V

0l/I

nl)

The no-load equivalent reactance is given by

4

Xnl

=Znl√(1-cos

2Ф

nl)

The rotational loss is

Pr=P

nl- I

2

nl(R

1+0.25R

2)

Forward torque Tf=I2.Rf synchronous watts Backward torque Tb=I2.Rb synchronous watts Net developed torque= Tf- Tb Power output=(1-s)Pg Power input=V*IcosΦ Efficiency= {(1-s)Pg}/( V*IcosΦ)

Fig. 7.2Capacitor-start motor: (a) connections, (b) phasor diagram at starting, and (c) typical torque speed characteristic

7.5 Equipment and Components a. Single phase induction motor b. Single phase autotransformer (0-240V)

Fig. 7.3 Equivalent circuit of two winding single phase induction motor

5

c. Ammeter (0-10A) moving iron type d. Voltmeter (0-300V) moving iron type e. AC Wattmeter LPF f. AC Wattmeter UPF

7.6 Procedure, Connection Diagrams, Experimentation and Precautions 7.6.1 No load test i) With auxiliary winding open:- Make connections as shown in the Fig. 7.5. The motor is run at no load at the rated speed by applying the rated voltage. Note the values of input current drawn, input power, pf, input voltage and tabulate in Table 7.1.

Fig. 7.5 No load test with auxiliary winding open

(a) (b)

Fig. 7.4 (a) Block rotor test with auxiliary winding open (b) Block rotor test with auxiliary windingopen

6

Table 7.1

Vo(V) Io(A) Po(W) PF

ii) With main winding open Make the connections as shown in Fig. 7.6. Note the values of input current drawn, input power, pf, input voltage and tabulate in Table 7.2.

A

Rotor

MA

V

220V DC supply

1- phase ac

supply

A

Aux winding

Main winding

C-r

un

C-st

art

Power analyser

Fig. 7.6 No load test with main winding open

Table 7.2

Vo(V) Io(A) Po(W) PF

7.5.2 Blocked rotor test i) With auxiliary winding open: Make the connections as shown in Fig 7.7. The rotor of the induction motor is blocked manually (so as to make slip s=1) and voltage is applied such that rated current flows through the main winding. The input voltage, pf , input power values are noted and tabulate in Table.7.3.

7

Table 7.3

Irated(A) Vin(V) Pi(W) PF

ii) With main winding open: Make the connections as shown in Fig 7.8. The rotor of the induction motor is blocked manually (so as to make slip s=1) and voltage is applied such that rated current flows through the auxiliary winding. The input voltage, pf , input power values are noted and tabulate in Fig.7.4.

ARotorV

1- phase ac

supply

A

Aux winding

Main winding

Power analyser

Fig. 7.7 block rotor test with auxiliary winding open

ARotorV

1- phase ac

supply

A

Aux winding

Main winding

C-r

un

C-s

tart

Power analyser

Fig. 7.8 block rotor test with auxiliary winding open

8

Table 7.4

Irated(A) Vin(W) Pi(W) PF

7.6.2 Load test Make the connections as shown in Fig. 7.9. The field of dc machine is excited by 220V dc source and the induction motor is started by giving it a single phase supply of 220 V, 50Hz through an autotransformer. As the speed of the motor reaches almost 80% of no load speed, the start capacitor is disconnected using a switch. When the motor reaches its rated speed, the armature current of the dc machine is varied by varying the load on the dc machine. The armature current is increased in steps till the total current drawn by the motor reaches its rated value and readings of speed, main winding current, auxiliary winding current, power factor, and input power are recorded in Table 7.5.

C-r

un

C-s

tart

Fig. 7.9 Connection diagram for load test

9

Table 7.5 Armature current Ia

(A)

Total current It (A)

Main winding current

Im (A)

Auxiliary winding current

Iaux (mA)

PF Field voltage

Vf (V)

Field current

If (A)

Speed in

rpm

Input Power

Pi (W)

7.5.3 DC Resistance test: The test is conducted on individual windings of induction motor to obtain the respective resistances. A bridge rectifier is used to obtain dc voltage which is fed from the autotransformer. The DC voltage applied and the current drawn are noted. Two readings are taken for each winding and the resistance is calculated as the average of these two readings. 7.6 DATA SHEET Name Plate Details of the Machine Name of the manufacturer: Rated output: Rated voltage: Rated speed: No. of poles: Rated current: Type of rotor: 7.7 Report Equivalent circuit paramaters: R1= ……….., R’

2=………., X1=………….., X’2=……………..., Xm=………, Ra=……….,

R’2a=…………, X’

2a=…………, a=………., Plot

(a) Torque vs. speed (b) Current vs. Speed, (c) Current vs. Output Power (d) Power factor vs. Output Power (e) Slip vs. Output Power curves

Computed value of Cstart= Computed value of Crun=

10

Using MATLAB-simulink models the experimental setup using the given name-plate rating and equivalent circuit parameters, obtain

(a) Torque vs. speed (b) Current vs. Speed (c) Current vs. Output Power (d) Power factor vs. Output Power (e) Slip vs. Output Power on single-winding and two-winding configurations for a starting

capacitor 7.8 Post-Experimental Quiz (a) What are the approximations made while evaluating the equivalent circuit parameters?

How are these approximations justified? (b) While evaluating the performance from the equivalent circuit for different values of slip,

the backward impedance zb, need only be found for one value of slip and the same can be assumed constant for other normal operating values of slips. Why?

(c) Comment on the agreement between the performance characteristics obtained theoretically and those obtained from the load test.

(d) How does the efficiency of the test motor compare with that of a three phase motor of same rating? Comment.

(e) What are the effects of backward field on the performance of the test motor? (f) List various applications of single phase motors (g) How can we reverse the rotation of a single phase ,split phase motor? (h) What could be the reasons if a split phase motor runs too slow? References

1. S K Bhattacharya “Electrical machines” New Delhi: Tata McGraw-Hill Pub. Co. Ltd., ©2009.

2. Mohamed Abdus Salam “Fundamentals of electrical machines” Oxford Alpha Science 2005

3. Say, M.G. "Alternating Current Machines" Fifth Edition. London: Pitman (1983).

4. P.C. Sen, “Principles of Electric Machines and Power Electronics,” IInd

edition, John Wiley & Sons, 1997

5. I. J. Nagrath and D.P. Kothari, “Electric Machines”, TMH, New Delhi, 2004. 6. Guru Bhag Singh, H. Hiziroglu “Electric Machinery And Transformers” 3nd ed. Oxford

University Press, 2005.

1

EEP 203 ELECTRO-MECHANICS LABARATORY (0-0-3) 1.5 CREDITS

EXPERIMENT- VIII

Study of steady state performance of a variable frequency control fed Three-Phase Induction Motor Drive

8.1 MOTIVATION In many industrial applications, the speed control is required to be varied either in steps or smoothly. In certain special applications such as textile and mining industry a group of motors is required to be run at different speeds with extremely good accuracy. In such applications d.c. shunt motor is being used because of its characteristics and easy speed control. However, the use of squirrel cage induction motor is desirable due to its well known advantages over all other motors namely, rugged construction, low maintenance and high efficiency. The problem however, is that the speed of induction motor cannot be controlled easily. It requires additional expensive equipments. The knowledge of a system whereby speed is controlled by varying the frequency is important for applied engineers. The system is best suited for applications such as mining, chemical industries and textiles.

8.2 OBJECTIVES (a) Conduct the no load test at variable frequency from 5 Hz to 75 Hz and draw (i) stator

voltage with speed, (ii) stator current with speed, (iii) power factor with speed. (b) Conduct the load test at 25 Hz and 50 Hz and draw: (i) Speed vs. Output Power, (ii)

Stator Current vs. Output Power, (iii) Power factor vs. Output Power, (iv) Efficiency vs. Output Power.

(c) Compute the following characteristics: (i) Speed vs. Output Power, (ii) Current vs. Output Power, (iii) Power factor vs. Output Power, (iv) Efficiency vs. Output Power at 25 Hz and 50 Hz.

(d) Compute the starting torque and the starting stator current for (i) constant V/F from 5 Hz to 50 Hz, (ii) constant flux from 5 Hz to 50 Hz.

(e) Compute the following characteristics: (i) torque-speed, (ii) stator current-speed at 10 Hz, 25 Hz and 50 Hz keeping constant V/f ratio.

8.3 THEORY From Faraday’s law, The air gap component of the armature voltage in an AC machine is proportional to the peak flux density in the machine and electrical frequency. Thus, neglecting the voltage drop across the armature resistance and leakage reactance, the stator voltage can be written as,

(1)

is the amplitude of the armature voltage; is the operating frequency; is the peak flux density; , , are the correspondi

peakea rated

rated rated

a e

peak rated rated rated

BfV V

f Bwhere V fB f B V

⎛ ⎞⎛ ⎞= ⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠

ng rated values. The speed of induction motors can be precisely controlled by frequency control and can be made independent of variation in supply voltage, field current and load. Therefore, keeping V

a=V

rated, Eq.(1) can be rewritten as

2

ratedpeak rated

e

fB B

f⎛ ⎞

= ⎜ ⎟⎝ ⎠

(2)

This demonstrates the constant voltage, variable frequency operation. In this mode, a machine operating in saturation at rated voltage and frequency, any reduction in frequency will lead to further increase in flux density. Higher flux density will result in increased core loss and higher machine currents. Therefore, for frequencies less than or equal to rated frequency, the machine is operated at constant flux density, i.e. B

peak=B

rated. This makes the

Eq. (1) as

e a rateda rated

rated e rated

f V VV V

f f f⎛ ⎞

= ⇒ =⎜ ⎟⎝ ⎠

(3)

This is constant voltage per hertz (V/f) operation. It is typically maintained from rated frequency to the low frequency at which armature resistance drop becomes significant component of the applied voltage. For frequencies higher than the rated with the voltage at its rated value, the air-gap flux density will drop below its rated value {referring Eq (1)}. Thus, to maintain the rated flux density the voltage has to be increased, which may result in insulation failure. Therefore, for frequencies above the rated frequency the terminal voltage is kept at rated value. Assuming that machine cooling is not affected by rotor speed, the maximum permissible terminal current will remain constant at its rated value Irated. Therefore, for the frequencies below rated frequency the machine power will be proportional to feVratedIrated. Fig. 1 shows the typical torque-speed characteristics with variable frequency drive.

8.4 EQUIPMENT AND COMPONENTS (a) Three phase induction motor coupled to a separately excited DC motor. (b) Variable voltage-variable frequency AC Drive (c) Fluke-43B Power Analyser

Fig. 8.1 Torque –speed curve

Fig. 8.2 operating regions with V/F operation

3

8.5 PROCEDURE, CONNECTION DIAGRAM, EXPERIMENTATION AND PRECAUTION: No-Load Test: Fig. 3 shows the connection diagram for speed control of induction motor using variable frequency AC drive. Apply the rated voltage to AC drive using variac. As soon as the rated voltage reach rated AC voltage, the green indicator will start glowing. Push the start button and slowly increase the speed controller knob to observe the rise in induction motor speed. Record the results (speed, frequency, stator voltage, stator current and power factor) using Fluke 43B at different speed and tabulate in Table 8.1 for no load test.

Load Test: Switch on the field DC supply of separately excited DC generator. Start the AC drive and increase the speed till motor frequency reaches 25 Hz. Now switch on the load on DC generator step-wise and record the results in Table 8.1.

Reduce the load on generator to minimum and then increase the speed of motor till frequency reaches to 50 Hz. Now switch on the load at DC generator step wise and record the results in Table 8.2. .

Fig. 8.3 Connection diagram of variable frequency AC drive

8.6 DATA SHEET

Name plate details of the machine

4

Name of manufacturer:

Rated output:

Rated voltage:

Rated current:

Supply frequency:

No. of phases:

Rated speed:

No. of poles:

Table-8.1

S. No.

Speed of IM

Input Line

current IL

Power Input Frequency Power factor

Table-8.2

S. No.

Torqe on IM

Speed of IM

Input current

IL

Power Input Armature current

,Ia

Armature voltage, Va

Field Excitation

If W1 W2

8.7 DATA PROCESSING AND ANALYSIS

(a) Determine the Equivalent circuit parameters of Induction Motor (b) Draw the following characteristics: (i) Speed vs. Output Power, (ii) Current vs. Output

Power, (iii) Power factor vs. Output Power, (iv) Efficiency vs. Output Power at 25 Hz and 50 Hz.

Using the obtained simulation parameters develop a variable speed drive model in MATLAB-simulink and determine the following

5

(c) Compute the starting torque and the starting stator current for (i) constant V/F from 5 Hz to 50 Hz, (ii) constant flux from 5 Hz to 50 Hz.

(d) Draw the following characteristics: (i) torque-speed, (ii) stator current-speed at 10 Hz, 25 Hz and 50 Hz keeping constant V/f ratio.

8.8 POST-EXPERIMENTAL QUIZ

a. What will be the change in torque speed characteristics of the induction motor if there is change in V/f ratio

b. Can higher starting torque be obtained at very low frequencies? Comment on the basis of the observations.

c. Can constant torque speed characteristics be achieved at different frequencies? d. What are the advantages and disadvantages of this method of speed control? e. How are the equivalent circuit parameters affected by frequency? f. What is the effect of variation of frequency on maximum torque, starting torque, slip

at maximum torque, if V/f is kept constant? g. At constant V/f, is the developed power at all frequency same at rated input current?

References

1. S K Bhattacharya “Electrical machines” New Delhi: Tata McGraw-Hill Pub. Co. Ltd.,©2009. 2. Arthur Eugene Fitzgerald, Charles Kingsley, Stephen D. Umans “Electrical Machinery” Tata McGraw-Hill publishing company Limited, New Delhi, India,2009 3. Mohamed Abdus Salam “Fundamentals of electrical machines” Oxford Alpha Science 2005 4. Say, M.G. "Alternating Current Machines" Fifth Edition. London: Pitman (1983). 5. I. J. Nagrath and D.P. Kothari, “ Electric Machines”, TMH, New Delhi, 2004. 6. Guru Bhag Singh, H. Hiziroglu “Electric Machinery And Transformers” 3nd ed. Oxford University Press, 2005. 7. Daniels, A.R. "Introduction to Electrical Machines" Macmillan.(1985).

1


Experiment-IX

Study of the steady state performance of a three-phase synchronous motor 9.1 Motivation Synchronous motors are rarely used below 40 kW output in the medium speed range because of their higher initial cost in comparison to that of induction motors. But because of the separate excitation, synchronous motor becomes cheaper than induction motor for low speed and high kW application. Synchronous motors are very much employed as power factor correction. The change in field excitation of synchronous motor affects the power factor and consequently armature current for constant supply voltage and constant input power. These effects can be understood by inverted V-curves & V-curves respectively. The effect of variation in field current on power factor is important characteristic of synchronous motor and provides its wide application as power factor correction & therefore voltage regulation. 9.2 Objectives Study of the steady state performance of a synchronous motor (a) Conduct the (i) OCC test, (ii) SCC test, and (iii) stator resistance measurement the machine. (b) Conduct load test and draw (i) efficiency vs. power, (ii) power factor vs. power, (iii) input current vs. power, (iv) V curves and (v) inverted V curves. (c) Compute following characteristics: (i) efficiency vs. power, (ii) power factor vs. power, (iii) input current vs. power, (iv) V curves and (v) inverted V curves. Use the parameters calculated in part (a) for simulation. 9.3 Equipment and Components (a) DC motor coupled with three phase synchronous machine (b) Two Rheostat of suitable range for field control (c) One PMMC Ammeter (0-2A) (d) One MI Ammeters (0-10A) (e) One MI Voltmeters (0-600V) (f) Two Wattmeters (600V, 10A) (g) One digital Multimeter 9.4 Procedure, Connection Diagrams, Experimentation and Precautions Note down the name plate details of the dc shunt motors and alternator and identify the terminals. Observe the constructional features. 9.4.1 Open Circuit Test The open-circuit test, or the no-load test, is performed by driving the motor as a generator at its rated speed while the armature winding is left open as shown in Fig. 9.1. The field current is varied in suitable steps and the corresponding values of the open-circuit voltage varied in suitable steps and corresponding values of the open-circuit voltage between any two pair of terminals of the armature windings are recorded. The OCC follows a straight-line relation as long as the magnetic circuit of the synchronous generator does not saturate. In the linear region, most of the applied mmf is consumed by the air-gap; the straight line is appropriately called the air-gap line. As the saturation sets in, the OCC starts deviating from the air-gap line. Record the results as shown in Table 9.1 9.4.2 Short Circuit Test

2

The short-circuit test provides information about the current capabilities of a synchronous machine. It is performed by driving the motor as a generator at its rated speed when the terminals of the armature winding are shorted as shown in Fig. 9.1. An ammeter is placed in series with one of the three shorted lines. The field current is gradually increased and the corresponding value of the armature current is recorded. The maximum armature current under short circuit should not exceed 1.5 times the rated armature current of the generator. When the per phase short-circuit current is plotted as a function of the field current, the graph is called the short circuit characteristic of a generator. Tabulate the results as shown in Table 9.2. Precautions: • Make sure during starting the dc motor that the external exciter field winding resistance of synchronous machine is at maximum value and no voltage is applied to the exciter 9.4.3 Stator Resistance Measurement The resistance is measured between armature terminals of alternator either by using voltmeter-ammeter method or using high precision multimeter. With the help of recorded values per phase resistance is calculated depends on machine is star or delta connected. If it is star connected, per phase resistance is given as (Rmeas/2) and if it is delta connected it is given as (3/2) Rmeas. 9.4.4 Load Test Synchronization:-

1. Give connections as per fig. 9.2. 2. Start the d.c motor and bring it to synchronous speed. 3. Adjust the field current ‘If’ of synchronous machine such that ‘Vac’ is nearly equal to

supply (ac) voltage. 4. Under condition of synchronization, voltage, frequency and phase sequence of

generator and supply bus must be same. This is ensured by the synchronising lamp as shown in fig. 9.2, when the two lamps (2, 3) are bright and third (1) is dark. Close the switch at this instant.

VAR control using synchronous motor on no-load (V-curve): 5. Synchronous machine will now work as synchronous motor. Disconnect the d.c

supply to the d.c motor. Now synchronous motor will work on no-load drawing power from the bus.

6. Vary the field current ‘If’ and measure ‘Ia’ in a range such that ‘Ia’ does not exceed rated value. Tabulate ‘If’ and ‘Ia’ (Iac), and also W1, W2 (or use Power Analyzer).

7. Connect d.c machine as a separately excited d.c generator with constant (rated) field current. Load d.c generator at a constant current.

8. Repeat (6). Note :- Record supply frequency and voltage, when tabulating readings. Load test:

9. Keep ‘If’ to cause minimum ‘Ia’ on no-load. Slowly load the d.c generator with its field current fixed. Tabulate ‘Idc’, ‘Vdc’, ‘Vac’, ‘Iac’, ‘Pac’ and ‘If’. Precautions: • Check the rating of synchronous machine and apply electrical load accordingly. • Check for synchronism while connecting the synchronism machine to the AC supply.

3

Fig. 9.1 Open Circuit and Short Circuit Test Connection Diagram

Fig. 9.2 Load Test Connection Diagram

Table- 9.1

Rated Speed= S. No. Alternator Field

Current (If) Armature phase

voltage (Va) Armature phase

voltage (Vb) Armature phase

voltage (Vc)

4

Table- 9.2

Rated Speed= S. No. Alternator Field

Current (If) Armature phase

Current (Ia) Armature phase

Current (Ib) Armature

phase Current (Ic)

Table- 9.3

S. No.

Input Current Ia

Input Voltage V

Input Power

Field Current If

Output Power

Efficiency Power Factor

Table- 9.4

Load Test

If(A) Vac(V) Iac(A) Idc(A) Vdc(V) W1(W) W2(W) Power factor

9.5 Data Sheet Name plate details of the dc shunt motor alternator set: Name of Manufacturer: Machine No. Class of Insulation: kW: RPM: Voltage: Amperes:

5

Rating: Connections: Frequency: Excitation: 9.6 Data Processing and Analysis 9.6.1 Determination Of Synchronous Reactance For a particular field current If, the internal voltage Ef could be found from the OCC and the short-circuit current flow Isc,A could be found from the SCC. Then the synchronous reactance Xs could be obtained using. ZS,unsat =√(Ra

2 + XS,unsat ) = Ef /IscA XS,unsat =√( ZS,unsat 2 -Ra

2) Synchronous Reactance under saturated condition ZS,sat =√(Ra

2 + XS,sat ) = Vrated(Ef) /IscA XS,sat =√( ZS,sat 2 -Ra

2) Tan Ф = √3*[ (W1-W2) / (W1+W2) ]

Pf = Cos Ф Stator Resistance Phase a =……………ohm Phase b =……………ohm Phase c =……... ohm Plot the following graphs from measured data (a) Efficiency vs. power (b) Power factor vs. power (c) Input current vs. power (d) V curves (e) Inverted V curves Use MATLAB simulink and using sim power system block set simulate the dc machine-synchronous motor set using obtained parameters and plot the following graphs (a) Efficiency vs. power (b) Power factor vs. power (c) Input current vs. power (d) V curves (e) Inverted V curves Comment on the shape of the above graphs. 9.7 Post-Experimental Quiz a) Why synchronous motors are not self starting? b) Why a synchronous motor is constant speed motor? c) Why is a 3-phase synchronous motor will always run at synchronous speed? d) What is hunting? e) How will you minimize hunting? f) Explain the basic principle of operation for synchronous motor. g) Write the main features of synchronous motor. References 1. Dale R Patrick; Stephen W Fardo “Rotating electrical machines and power systems” Lilburn, Ga. : Fairmont Press, 1997. 2. J D Edwards “Electrical machines: an introduction to principles and characteristics” New York : Macmillan, 1986. 3. Brian Moore, John Donaghy “Electrical machines Basic principles series” Pitman, 1988.

6

4. A.S. Langsdorf, “Theory of alternating current machinery,” TMH, new Delhi, 2001. 5. John Hindmarsh “Electrical machines and their applications” Oxford [u.a.] Pergamon Pr., 1991. 6. Arthur Eugene Fitzgerald, Charles Kingsley, Stephen D. Umans “Electrical machinery” Tata McGraw-Hill publishing company Limited, New Delhi, India, 2009. 7. J. H. Walker and T. Stuart Walker, “Large Synchronous Machines: Design, Manufacture, and Operation,” Clarendon Pr, 1996. 8. Samarjit Gosh, “Electrical Machines,” First Indian Print, Pearson Education (Singapore) Pte. Ltd, 2005.

1


Experiment-X

Study of the steady state performance of a grid connected three-phase squirrel cage induction generator

10.1 Motivation Over the past few decades, there has been an increasing use of squirrel cage type induction generators, particularly in wind energy conversion systems and micro-hydro power systems. The grid provides frequency and voltage regulation, as well as the reactive power required by the generator. Their advantages in these applications are that they are rugged, less maintaence, high power/weight ratio, self short circuit protection and cheap. It is not essential to operated precisely at synchronous speed. 10.2 Objectives Study of the steady state performance of a grid connected three-phase squirrel cage induction generator. (a) Conduct the (i) no load test, (ii) blocked rotor test, and (iii) stator resistance measurement

the machine. (b) Conduct power generation test and draw (i) efficiency vs. power, (ii) power factor vs. power,

(iii) stator current vs. power, (iv) the speed vs. power curves at rated voltage and frequency.

(c) Compute following characteristics:(i) efficiency vs. power, (ii) power factor vs. power, (iii) stator current vs. power, (iv) the speed vs. power curves at rated voltage and frequency. Use the parameters calculated in part (a) for simulation.

10.3 Theory The steady-state performance of a poly-phase induction motor can be obtained using per phase equivalent circuit and it is shown in Fig. 10.1. The symbols are, V1= input voltage per Phase R1, X1= resistance and leakage reactance of the stator per phase R2, X2= resistance and leakage reactance of the rotor per phase Xm= magnetizing reactance Rc= core loss resistance I1, I2= stator and rotor currents (referred to stator) per phase The developed torque is given as, T=3I2

2R2/(sωs) N.m Where ωs=synchronous speed in radians/sec S = slip The parameters could be determined by ‘no-load’ and ‘blocked-rotor’ tests, the former determines Rc and Xm while the latter yields R1, R2, X1, X2. Following equations could be used for calculation:

1 1 1

22 2

21

2

11

e mm

e m

m

m

Z R jXR

Z jXsjR X

ZR jX

Z ZZ Z

Z ZV

IZ

= +

= +

=+

= ++

=

2

12

222 2

1

1 1

( )The torque T=3

factor, pf= cos tan ( / )

R=Re(Z) X=Im(Z)Input power P 3 *

P (1 )Effeciency =P /

m

m

in

o s

o in

I ZI

Z Z

I Rs

Power R X

where

V I pfOutput power s T

Pω

−

=+

⎡ ⎤⎣ ⎦

== −

10.4 Equipment and Components (a) A three phase squirrel cage induction motor coupled with separately excited dc motor (b) Three-phase auto-transformer (16A, 0-415V) (c) One AC Ammeter (0-5/10A) (d) One AC Voltmeter (0-500V) (e) Two Low pf wattmeter (600V,10A) (f) Two unity pf wattmeter (600V,10A) (g) Suitable dc loads (440V, 10A) (h) Tachometer 10.5 Procedure, Connection Diagrams, Experimentation and Precautions Note down the name plate details of the both machines and identify the terminals. Observe the constructional features. Note the type of rotor used and the winding connections. 10.5.1 Measurement of Stator Winding Resistance Make connections as shown in Fig. 10.2 for a star connected stator. Similar connections can be used for a delta-connected stator. Apply low voltage so that current through the windings is well below the rated value. 10.5.2 Light Running Test Connect the machine as in Fig. 10.3. Start the motor by applying the normal frequency reduced voltage to the stator and gradually increases the voltage to its rated value. In case of slip-ring motor short circuit the slip rings before starting. Note down the readings of voltmeter, ammeter, wattmeters and tachometer at different voltages. Precautions:

Fig. 10.1 Per Phase Equivalent Circuit of a Poly-phase Induction Motor

3

• Use low power factor meters for power measurements and careful about the deflections of both instruments, In case of reverse deflection, change the current wires.

• Take care about inrush current of motor during starting • Make sure autotransformer zero setting before every fresh start

Fig. 10.2 Connection Diagram for Stator Winding Resistance Measurement

Fig. 10.3 Connection Diagram for Light Running Test and Block Rotor Test 10.5.3 Block Rotor Test With the above connections, keep the rotor blocked, and record the readings of various instruments for different steps of input current varying from zero to 1.5 pu. If values change noticeably for different rotor positions, an average set of readings should be taken. Since very low value of voltage can inject rated current under this condition precaution should be taken not to apply high voltages. Precautions:

• Use upf meters for power measurements for better accuracy. • Make sure autotransformer zero setting before switching on the input supply and small

input voltage is sufficient for allowing the rated current of machine • Hold rotor in block position tightly

10.5.4 Power Generation Test Make the connections as shown in Fig. 10.4. Switch on the field winding DC supply and set field current to its rated value. Start the dc motor using starter and set the field current up to its rated current. Record the speed of motor/generator set. Now adjust the field current and increase the speed up to synchronous speed. Now switch on the induction machine supply using autotransformer gradually. Monitor carefully the ammeter and Wattmeters readings. Use field voltage control for further speed increment of DC motor and tabulate the observations in Table 10.3. Take care about the ratings of both machines. Precautions:

• Check the direction of rotation of both machines before conducting the power generation test. It must be same.

4

4.6 DATA SHEET (a) Name Plate Details of the Machine Name of the manufacturer: Rated output: Rated voltage: Supply Frequency: No. of phases: Rated speed: No. of poles: Rated current: Type of rotor: Type of starting method: Winding connections for stator/rotor: (b) Average stator winding resistance/phase=…ohm (c) Average rotor winding resistance/phase=…ohm

Fig. 10.4 Connection Diagram for load Test

Table- 4.1

Light Running Test

S. No.

Input Voltage (V0)

Input Current (I0) Input Power= W1+W2

Speed

5

Table- 4.2

Block Rotor Test S.

No. Input Voltage (V) Input Current (I) Input Power= W1+W2


S. No.

AC Voltage

(VL)

AC Current

(IL)

Pg = W1+W2

Power Factor Speed

Armature Current

(IA)

Armature Voltage

(VA) Pin Efficiency

4.7 Data Processing and Analysis Parameters of the Equivalent Circuit The no-load power input mainly represents core losses, as copper losses could be ignored. Rc=V2

0/Pc, Ic=Vc/Rc, Im

2=I02-Ic

2 Knowing Im, Xm can be calculated from the relation, Xm=V0/Im, In the case of blocked rotor test, the equivalent series impedance referred to stator is given by, Zsc=Rsc+jXsc = (voltage per phase/short circuit current per phase) The equivalent series resistance referred to stator is Rsc=R1+R2=(Input power per phase/(current per phase)2) And Xsc=X1+X2=√(Zsc

2-Rsc2)

The ac resistance R1 is known by multiplying dc resistance by skin effect factor. Thus R2 can be evaluated. The separation of stator and rotor leakage reactance is difficult. For most machines of normal design it is sufficiently accurate to take

6

X1=X2=(Xsc/2) In the block rotor test Xm is neglected. 4.8 Report a) Plot using measured data efficiency vs. output Power. b) Plot using measured data power factor vs. output Power. c) Plot using measured data stator current vs. output Power. d) Plot using measured data Efficiency vs. output Power. e) Plot using measured data speed vs. output Power. Simulate the induction generator using measured parameters. f) Plot using simulation efficiency vs. output Power. g) Plot using simulation power factor vs. output Power. h) Plot using simulation stator current vs. output Power. i) Plot using simulation Efficiency vs. output Power. j) Plot using simulation speed vs. output Power. 4.9 Post-Experimental Quiz a) What are the advantages and disadvantage of induction machine as a generator? b) Draw and explain torque speed characteristics of induction generator. c) How the voltage and frequency control takes place in grid connected induction generator?

Explain. d) Critically comment on the characteristics you obtained? e) Induction generator draws leading VAR? justify. f) Write the limitations of Induction Generator. g) What is the application of Induction Generators.

References

1. S K Bhattacharya “Electrical machines” Tata McGraw-Hill Pub. Co. Ltd., New Delhi,


McGraw-Hill publishing company Limited, New Delhi, India,2009 3. Mohamed Abdus Salam “Fundamentals of electrical machines” Oxford Alpha Science 2005 4. Say, M.G. "Alternating Current Machines" Fifth Edition. London: Pitman (1983). 5. I. J. Nagrath and D.P. Kothari, “Electric Machines”, TMH, New Delhi, 2004. 6. Guru Bhag Singh, H. Hiziroglu “Electric Machinery And Transformers” 3nd ed. Oxford


eep 203 electromechanics laboratory

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