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NAME OF LABORATORY: ELECTRIC DRIVES LAB SUBJECT CODE: EX-702 NAME OF DEPARTMENT ELECTRICAL ENGINEERING

Experiment 01

OBJECT: - Speed control of universal motor using “TRIAC”.APPARATUS REQUIRED:-

1. TRIAC control kit2. Connecting cords 3. CRO 4. CRO probes 5. Multimeter6. Single Phase Universal Motor

THEORY-:The universal motor is a rotating electrical machine similar to a dc motor but designed to operate either from direct current or single phase alternating current. The stator and rotor windings of the motor are connected in series through the rotor commutator. Therefore the universal motor is also known as an AC series motor or an AC commutator motor.The universal motor can be controlled either as a phase-angle drive or as a chopper drive.In the phase-angle application, the phase angle control technique is used to adjust the voltage applied to the motor. A phase shift of the gate’s pulses allows the effective voltage, seen by the motor, to be varied. The phase-angle drive requires just a triac.The advantage of universal motor is that AC supplies may be used on motors which have the typical characteristics of DC motors, specifically high starting torque and very compact design if high running speeds are used. The negative aspect is the maintenance and short life problems caused by the commutator. As a result such motors are usually used in AC devices such as food mixers and power tools which are used only intermittently. Continuous speed control of a universal motor running on AC easily obtained by using thyristor circuit, while stepped speed control can be accomplished using multiple taps on the field coil. Household blenders that advertise many speeds frequently combine a field coil with several taps and a diode that can be inserted in series with motor (causing the motor to run on half wave rectified AC). Universal motors generally run at high speeds, making them useful for appliances such as blenders, vaccum cleaners and hair dryers where high RPM operation is desirable. They are also commonly used in portable power tools, such as drills, circular and jigsaws, where the motor’s characteristics work well,. Many vaccum cleaner and weed trimmer motors exceed


10,000 RPM, while dremel and other similar miniature grinders will often exceed 30,000 RPM.

TRIGGER CIRCUIT:- Phase control IC TCA 785 is used to control thyristors, triac and transistors.The trigger pulses can be shifted with in a phase angle between 00 and 1800.

CIRCUIT DISCRIPTION:-R sync resistor provides synchronization signal from main supplyIC has in built three functional blocks

1. Zero consisting detector 2. Sync ramp generator3. Control comparator

Sync ramp signal is connected internally to control comparator.Depending on the control voltage set by potmeter, comparator gives zero pulses Q1 and Q2 exactly 1800 apart. Q1 and Q2 is Ored using diodes D1 and D2 and is applied to gate of triac BT136.

Fig (B) Pulse Wave form

EXPERIMETAL SETUP:-Triac is main power controlling device.


AC input to the circuit is from 0-100 volt/2Amp isolation transformer.2 pin cord of the motor is to be connected to the motor terminals AS shown in diagram

PROCEDURE:-1. Connect universal ,motor to the unit2. Make power on to the unit3. Observe that the speed of motor is controlled by varying the ( speed adjust)

phase angle adjust potmeter.4. Observe the wave form across triac and motor on CRO

Do not connect / disconnect motor to the unit when power is on.

Fig (A) Block diagram of speed control of Universal Motor

OBSERVATION TABLE-


CONCLUSION-

APPLICATION -TRIAC control for up to 50 mA gate trigger currentA phase control with a directly controlled triac is as shown in figure. The triggering angle of the triac can be adjusted continuously between 00 and 1800 with the aid of an external potentiometer. During the positive half-wave of the line voltage, triac receives a positive gate pulse from the IC output pin 15. During the negative half-wave, it also receives a positive trigger pulse from pin 14 .The trigger pulse width is approx. 100 µs.

S.No

Speed (R.P.M) Control voltage(volts)

Output Voltage(volts)

12345


Experiment No.2OBJECT: - To study of close loop system for a light intensity control system.APPARATUS REQUIRED:-

7. Light Intensity control kit.8. Connecting cords 9. CRO 10. CRO probes 11. Multimeter12. Resistive load ( Internally connected )

THEORY:-IN a process control system, automatic controller is an intelligent element, which controls and maintains physical parameters of the process such as temp, pressure, flow, liquid level,PH etc at predetermined level.

An automatic controller compares the actual value of the process variable (PV) with the desired value (set point SP) and depending on the deviation (e= SP – PV) produces a control signal, which will reduce the deviation to a zero or small value, there by maintaining the desired process variable at desired (set point )level.Block diagram of close loop control system is shown in figure

.

Fig.(A)Industrial automatic controllers may be classified according to their control action as,

1. Two position or ON/OFF controller,2. Proportional controller3. Integral controller4. Proportional plus integral controller (PI)5. Proportional plus derivative controller(PD)6. Proportional plus integral plus derivative controller (PID)


TWO POSITIONS /ON – OFF CONTROLLER:-The actual element has only fixed position, (ON & OFF).This control is relatively simple and less expensive and for this reason , is very widely used in both industrial and domestic control system.

PROPORTIONAL CONTROLLER:-For a controller with proportional control action, the relationship between the output of the controller m (t) and the actuating error signal e(t) is

m (t) = Kp.e(t)In Laplace – transformed quantities,

M (S)E(S)

=Kp

Where, Kp = proportional sensitivity or the gainAs shown in block diagram, proportional controller is essentially an amplifier with an adjustable gain.

Fig.(B)


Fig.(C)

Proportional band is the term used to define the gain or sensitivity of the proportional controller. Proportional band is the percentage change in the input to the controller (error signal) required to cause 100% change in the output of the controller. Thus small proportional band corresponds to high gain or high proportional sensitivity.

OFFSET:-Suppose for a given conditions of SP & PV the controller is giving some output X, expressed as X = Kp . e.Now if there is load disturbance (say load is increased) the output power must be increased to bring the error back to ‘e’ . But for the output power to remain at a new higher level (X1 = Kp e1), there must be some finite error e1.

e = error signal for normal load.e1 = error signal for increased load

OFFSET is a characteristic of the proportional control. The only way to reduce the offset and hold the process parameter at the setpoint is to increase the proportional gain. But too high gain introduces instability of the system.Without making the system instable, offset can only be reduced by adding integral control action to the controller.INTEGRAL CONTROL ACTION: Integral action is a control action in which the value of the manipulated variable m(t) is changed at a rate proportional to the deviation e(t). thus if the deviation is double over a previous value, the final control will increase twice as fast

M(t) = Ki ∫o

t

e (t )dt

The transfer function of the integral control is,M(S)E(S)

= KiS

The integral control action is sometimes called Reset control.In practice, integral control action is used along with proportional control action.


Fig.(D)

Fig.(E)

This is termed as proportional + integral control is defined as,

M(t) = Kp e(t) + KpTi ∫

o

t

e (t )dt

Or the other transfer function of the controller is, M(S)E(S)

= Kp (1 + 1Ti s)

Kp = Proportional sensitivity or gain Ti = Integral time.The combined effect of proportional plus integral action is the reduction of offset to appreciable value.However process takes longer time to stabilize.DERIVATIVE ACTION:Derivative control action may be defined as a control action in which the magnitude of the manipulated variable is proportional to the rate of change of deviation. The net effect of the derivative action id to shift the manipulated variable ahead by derivative time Td.Because the derivative control operates on the rate of change of the actuating error and not the actuating error itself, this mode is always used in combination with proportional action or proportional + integral action.P+D CONTROL ACTION:This action is represented as,


M (t) = Kp e(t) + Kp Td d [e (t )]dt

Where Kp= proportional sensitivity. Td = Derivative time And transfer function is M(S)

E(S) = Kp [ 1 + Td S]

Fig.(F)

Fig.(G)As seen in response curve, the derivation control action has an anticipatory character and initiates an early corrective action as, it amplifies the noise signals and may cause a saturation effect in the actuator.P I D CONTROL ACTION:The combination of proportional control action, derivative control action and integral control action is termed proportional plus derivative plus integral control action. This combined action has the advantages of each of the three individual control actions. This action can be represented as,

m (t) = Kp e(t) + Kp Td d e(t )dt

+ KpTi ∫

o

t

e ( t )dt

Or the transfer function is,M(S)E(S)

= Kp (1+Td S + 1Ti S )

Kp = proportional sensitivity.Td = Derivative time.


Ti = integral time.

Fig.(G)Block diagram of PID control

Fig.(H) Response of PID for unit step input

LIGHT INTENSITY CONTROLSystem consist of,

1) Power control unitOutput voltage controlled by changing the firing angle of SCR Half controlled bridge. Trigger circuit:9 volt step down transformer is used to provide synchronization signal. IC TCA 785 has built three functional blocks,a) Zero crossing detector.b) Sync ramp generator.c) Control comparator.


Sync ramp signal is connected internally to control comparator.Depending on the control voltage ( either from controller, count in AUTO mode or set by potmeter in manual mode ) , comparator gives two pulses Q1 and Q2 exactly 1800 apart.Pulse transformer PTX1, PTX2, is used to isolate the firing pulses from power circuit.The output of bridge acts as variable DC supply to the 24 volt LAMP.

2) Process 24 volt LAMP is mounted in close box.Light intensity is controlled by varying DC input to the lampLDR is used as light sensor, and is mounted infront of lamp in box.Feedback circuit is configured using LDR in circuit.The output voltage across LDR (feedback voltage ) increases as the light intensity increases.At minimum intensity the, feedback voltage is 2.00 volt (aprox.)At maximum intensity the , feedback voltage is 4.5 volt (aprox.)

Fig.(I)HALF CONTROLLED RECTIFIER


Fig.(J) Firing Circuit3)Control

P+I controller is provided for the system.Set point SP is adjustable from 2.00 volt to 4.50 volt.The output voltage across LDR (feed back voltage) acts PV to error Amplifier. The output of error amplifier E, acts as an input to P and I controller.The output of controller count, act as input to power control circuit.AUTO/MAN switch changes the controller from Auto to manual mode.In AUTO mode, cout is input to comparator of trigger circuit which ultimately determines the firing angle of SCR controlled bridge there by controlling the voltage to the lamp.In manual mode, the manual output adjust potmeter provides variable output voltage to comparator of trigger circuit ultimately determines the firing angle of SCR controlled bridge there by controlling the voltage to the lamp.


Fig.(K)

ERROR AMPLIFIERProcedure:

1) Keep light box closed.Make power on to the unit.Put Auto / Manual switch in manual mode.Connect CRO across output of the SCR controlled bridge.Also connect voltmeter across output of the bridge.Vary output voltage by adjusting manual adjust output potmeter, note the firing angle and output voltage of the bridge , Vout . Also note the feedback voltage at PV socket.Tabulate the result

2) Keep light box closed.Make power on to the unit.Put Auto / Manual switch in AUTO mode.

Set SP for say 3.0 volt.Observe the PV voltage, error voltage and controller output Cout.Provide step input by changing SP from 3.0 volt to 4.0 volt and note PV voltage, error voltage and controller output Cout.


Also note output voltage of the bridge Vout.Create the disturbance by slightly opening the light box and note the change in PV and Vout.

Obsevation Table-

S.No. Firing angle Output voltage volt

Feedback voltage

1

2

3

Result-


Experiment 03 and 04OBJECT:-

(1) Speed control of three phase induction motor using V / F control.

(2)To set the Acceleration and Deceleration time for start and stop of three

Phase induction motor.

APPARATUS REQUIRED:-

13. Three phase induction motor control kit14. Three phase induction motor15. One computer P-4

BASIC PRINCIPLE:-

Adjustable (controlled variable) speed of motors is the requirement in industries for –

1) Machine operations or process parameter control needs.2) Energy savings (to avoid unwanted wastage of energy).3) Automation of plants.4) Increase in productivity

Variable voltage inverter (VVI) & current source inverters (CSI) can be used for speed control to some extend. Variable voltage inverter (VVI) provides variable RMS voltage output. This variable RMS voltage controls speed of AC motor, in fact it is the driving current of the AC motor that result in speed & so CSI provides improved performance.

But both of these methods have many disadvantages like

1) No energy savings2) No smooth speed variation,3) No power factor improvement etc.

For smooth speed control, like DC motors, AC motors can not be controlled simply by controlling the voltage applied to it & so controlling the speed of AC induction motor is complicated & involves complex techniques. Simplest method for


controlling speed of AC induction motor is to use PWM technique. Following block diagram explains simple PWM method of power conversion for AC drive.

Fig.(A) BLOCK DAIGRAM

Basic principle of PWM control is to provide uniform triangular wave as a carrier single to comparator circuit to which input is sinosidal signal that act as a modulating signal. Output of the comparator is pulse with modulated (PWM) signal. (i.e. square wave pulses of variable width proportional to corresponding value of modulating signal amplitude.) we can call it as sine wave weighed pulses.

These PWM pulsed are used to control firing of power devices in half or full bridge configuration. Incase of full bridge configuration instead of single comparator two comparators are used in complimentary configuration. ( i.e. one comparator controls positive half cycle of power input & second controls negative half cycle ) This complete process controls RMS current output of the inverter circuit.


Fig.(B) GRAPH DAIGRAM

Fig.(C) CIRCUIT DAIGRAME

IGBT Fired in sequence to deliver output power


PWM INVERTERS:. There are four different types of control algorithms giving different performance level as under.

a) V / Hz invertersb) High performance vector drivec) Flux vector controld) Field oriented control

V / HZ i.e. volts to hertz ratio control is basic control method in PWM inverter that provides variable frequency drive it provides good speed & torque control for non precision application like fan & pump control, but has poor response of torque / speed at low speeds.

Senseless vector control provides better speed regulation even at low speed . it has ability to produce high starting torque.

Flux vector control provides high precision speed & torque control with dynamic response.

Field oriented control provides best speed and torque control in AC motor. It gives DC motor performance with AC motor & mostly suits for DC applications.

Fig.(D)

SINEWAVE WEIGHTED PULSED OUTPUT & CURRESPONDING CURRENT


Fig.(E)

Fig.(F): 3 Phase AC Motor Speed control (VFD)

PROCEDURE :-

EXPERIMENT :No 1

1) Connect motor to the main unit at terminals UVW provided on left side of the unit.Connect speed sensor cable to the 5-pin speed sensor socket.


Connect 230 volt AC mains at terminal L N E provided on right side of unitMake power on to the unit.Set parameter Pr. 00 = 00 (speed reference as digital keypad) and

Pr. 01 = 01 (operation control as panel switches) .2) Set the speed of the motor by changing SET FREQUENCY on drive.

START the motor by START / STOP switch on panel.Vary the potmeter slowly in proper steps from minimum to maximum and every time note, speed of the motor in RPM, output frequency (Hz) voltage and current.Use START / STOP switch on panel to STOP the motor.Tabulate the result.Plot the graph in between Speed & Frequency and Voltage & Frequency on the graph paper.

S. No.

Frequency(Hz)

Frequency of The Motor(RPM)

Voltage(Volt)

Current(Ampere)

1

2

3

4

5

3) Plot the graph of speed against frequency & voltage against frequency


Fig.(G)

A) Setting the required display parameter on display of drive.

1) Set frequency on digital keypad Press mode button till display F 60.0 (frequency) Use UP / DN key to change the value of frequency. Modified value will automatically get updated.

2) Display actual output frequency. Press mode button till display H 60.0 (frequency)

3) Display output current A.Press mode button till display A 1.7 (current)4) Direction status

Status of FW /REV direction and control of motor.Press mode button till display Frd or Rev displays.Direction can be changed on digital keypad if Pr. 01 is 00.Change the direction by UP / DN key

5) Direction can be changed by FW / REV switch on panel if parameter Pr. 01 is 01.

B) Setting SPEED References:Basic program parameter Pr. 00 sets the source of the speed references for the drive.The drive speed command can be obtained from a number of different sources.


OPTIONS:

*0 Master Frequency Determined by digital keypad (Change by UP/DN key)

1 Master Frequency Determined by 0 – 10 volt.

2 Master Frequency Determined by 4 – 20 mA input

3 Master Frequency Determined by RS 485 COMM port

*4 Master Frequency Determined by potentiometer on digital keypad.

Note : only two “ * ” mark references are used in our study.

Press mode button till display parameter “ P “ on 7 – segment display.

Select the parameter Pr. 00 for basic program group and set the required value ‘0’ or ‘4’ and press ENTER. Verify the operation.

EXPERIMENT:NO 2

-Setting Acceleration and Deceleration time start and stop of the motor .

Acceleration and Deceleration time can be set in program parameter.

1) Acceleration time 1 – Pr. 10Sets the rate of Acceleration for all speed increases.Acceleration Rate = Maximum Frequency

AccelerationTime

Values : Minimum = 0.1 Secs.Maximum = 600.0Secs.

Press mode button till display parameter “P” appears on 7 – segment display Select the Pr. 10 parameter and press ENTER. Set the required value between minimum and maximum and press ENTER.


2) Deceleration time 1 – Pr. 11Sets the rate of Deceleration for all speed increases.Deceleration Rate = Maximum Frequency

DecelerationTime

Values : Minimum = 0.1 Secs.Maximum = 600.0Secs.

Press mode button till display parameter “P” appears on 7 – segment display Select the Pr. 11 parameter and press ENTER. Set the required value between minimum and maximum and press ENTER.

3) Setting Operation Control :Basic program parameter Pr. 01 sets the source of the operation command for the drive. Parameter Pr. 01 decides the source of operation command.

Options : *0 Operation Determined by keypad button RUN / STOP.

1 Operation determined by external control switches provided on panel START /

STOP and FW / REV.

(Parameter Pr. 38 is set for 01 for panel control operation switches.)

Fig.(H) GRAPH DAIGRAM


Make system on. Set the acceleration and deceleration parameters as described above. Say Ta = 20 secs and Td = 20 secs. Set some input frequency and start the motor. Measure the time required to reach the maximum frequency. (Time Ta). Stop the motor. Measure the time required to reach the frequency to zero (Time Td).

RESULT-


EXPERIMENT NO.5

OBJECT: - To observe the performance of three phase fully controlled rectifier with microprocessorAPPARATUS REQUIRED:-

(1)3 – phase Rectifier control kit(2)Microprocessor 8085 kit(3)3 – phase supply with Auto Transformer (4)Connecting leads(5)Lamp load

INTRODUCTION :-Single phase a.c. to d.c. converters are generally limited to a few kilovolts, and for higher levels of d.c. power output, three phase line commutated converters are used owing to restrictions on unbalanced loading, line harmonics, current surge and voltage dips. Increase I ripple frequency also reduces the filter size. Converters which can be operated both in rectifying and inverter modes are called fully controlled converters. When power flow can only occur from ac – to – dc, the converter is known as semi-converter or half controlled converter. Fully controlled three – phase converters find application in high – voltage dc power (HVDC) transmission, dc and ac motor drives with regenerative breaking capabilities.

THEORY:-CIRCUIT DESCRIPTION AND PRINCIPLES :-

Figure 1 A shows the power circuit configurations of a three phase fully controlled converter in which all the rectifying elements are thyristors . Figure 1 B shows the wave forms of supply voltages, converter output voltage under continuous load current condition, firing instants for controlling the output voltage and sequence of firing. Thyristor are gated on at an interval of 600 in the sequence in which they are numbered. Triggering angle , also called firing delay angle, is defined with respect to cross over points of the phase voltage at which an equivalent diode would start to conduct. In the positive group of thyristors, Viz, Th1, Th3 and Th5 turning on of one thyristor turns off a conducting thyristor in the group. So is the case with negative group of thyristors, Viz, Th2, Th4 and Th6. As a result with highly inductive load, carrying continuous current, each thyristor would conduct for a period of 1200 in a cycle with commutation occurring every 600.


Fig(1)a

Fig(1)b

Since at any instant two thyristors should be in the conducting state no current would flow if at start a single thyristor is given a pulse. This means that


each thyristor should always be supplied with gate pulses 600 apart so that at start two thyristors can be triggered simultaneously.

Reference to Fig. 1 B shows the ideal dc average output voltage ( average height of the full line wave) of the converter under continuous load current is

Eo = 3√2π

EL – L CosαWhere EL _ L is the line – to – line rms voltage, and α is the delay angle.

If thyristor drops and supply side inductances are taken into account, average load voltage is quite closely given by

Eavg = Eo – 2 – 3ωg Lgπ IL

Where , Lg = Supply side inductance per phaseωg = 2π times the supply frequencyIL = Average load current.

For a delay angle α greater than 600, the instantaneous output voltage will have a negative part in its periodicity for continuous load current (fig. 2A). with a resistive load, current is always in phase with voltage. As current through a thyristor can not be negative the output voltage cannot take any negative value. The range of control for delay angle α, with resistive load, is from 00 to 1200. For α< 600, the ideal output voltage, with resistive load, is the same as in fig. 1B for continuous load current. Where as , for α> 600. The output voltage wave form will be as shown in fig. 2B. the ideal average of the converter output voltage with resistive load is given by –

Eo = 3√2π

EL – L [ 1 + Cosα (α + π3 ) ] for π/3 < α < 2π/3

Eo = 3√2π

EL – L Cosα for 0 < α < π/3A fully controlled converter can be made a semi converter by placing a freewheel diode across the load as shown in fig. 3. This circuit has the same output voltage characteristics as that of the full converter with resistive load since output voltage can never go negative because of the freewheel diode. Another configuration of a three phase semi conductor is a half controlled converter bridge, shown in fig. 4A, where half the devices are thyristors the reminder being diodes. Thyristors get turned off either on the firing of another thyristor or by the action of the freewheeling diode. However in order to avoid half waving effect in the case of


trigger failure of the thyristors a freewheel diode is a necessity. Figs. 4B and 4C shows the output voltage waveforms. For delay angle α < π/3, output voltage wave is continuous and for α > π/3 output voltage wave is discontinuous. The average output voltage is given by –

Eo = 3√2π

EL – L [ 1 + Cosα ] for 0 < α < π/3ROLE OF MICROPROCESSOR :-

(1)8085 microprocessor kit has 8255 PPI configure in mode 0 with port A & B as input & port C as output.

(2)8353 is used to provide the required firing angle delay. It is configured in mode 00 with counter 0 as operating counter & two data byte loading. Output of 8253 counter gives interrupt RST 7.5 after the set delay angle.

(3)There 220 V / 12 V step down transformers are used in typical pattern as shown in circuit diagram to read six modes of operation at A2, A1 and A0 points. These read by 8255 input port at A0, A1 , A2 and decides the pair of SCR to be made ON for the current mode.

(4)RST 6.5 interrupt reads the BCD input from TWS (Thumb Wheel Switch) & scale it for proper value depending on the required firing angle.

FLOW CHART :-START :- Set stack pointer

Initialize 8255 portPort A, B …………………… InPort C .…………………. OutCall ISR 6.5 for getting angle for first time.Read A0, A1, A2 and note the mode.

MAIN :- In port A & note the change in mode.If NO change JUMP to MAIN.

LOOP :- In port A & On change in mode load 8253 with firing angle.JUMP BACK TO LOOP.

ISR 7.5:- Mask all interrupt.Get L value from location 1090 H.Set HL pointer for sequence table.OUT required pattern for making the pair of SCR ON.Enable Interrupt.RETURN.


FIRST TIME OPERATION :-(1)Connect R, Y , B phase in proper sequence at R, Y, B Input terminals of

isolation transformer.Keep external 3 – phase main switch OFF.Keep main power switch on unit OFF.Keep G1 G2 G3 trigger switch OFF.Keep G4 G5 G6 trigger switch OFF.Keep gate supply on switch OFF.

(2)Connect 8085 microprocessor kit to the main unit by cord provided with the unit. Make 8085 microprocessor kit ON.benICs message id displayed on kit.

(3)Make external 3 – phase main switch ON so that 3 – phase supply is connected to sync. transformer.Do not make ON main – power of 3 – phase SCR converter on panel.

(4)Set required firing angle on Thumb wheel switch ( TWS ).Execute the program as follows.

Go4000Exec.

E is display on kit.Provide 6.5 interrupt from kit by pressing VI key.Reset the kit by pressing ‘ RES ‘ key.Check the value at memory location 1088.It should be firing angle in HEX as follows.

If 0-59 ……………………as it is in Hex.60 ……………………0261-119 ……………………as 1-59 (converted in Hex)120-176 ……………………as 1-59 (converted in Hex)

(5)Check the wave forms at A0 A1 A2 status points on main unit.Confirm that it should be exactly same as shown in diagram.

(6)If 4 & is as stated above then only proceed further.Observe the waveform at G1K1 to G6K6 as per sequence table given in figure and verify the table. If any discrepancy found do not proceed further.


(7) a) Configure the bridge for fully controlled rectifier by connecting A1, A2, A3 points of thyristors T1, T2, T3, to K4, K5, K6 points of thyristors T4, T5, T6 by patch cords respectively.Connect lamp load of 60 W / 230 V bulbs in series across output.

b) keep G1 G2 G3 and G4 G5 G6 trigger switch ON. Keep gate supply ON switch OFF. c) Make main 3 – phase power switch ON. Set firing angle on TWS between 0-120 only. For firs time set it to say 40.

Execute the program stored at memory location 4000 H.NOW MAKE GATE SUPPLY ON SWITCH ‘ ON ‘ and observe the output wave form across load on CRO.NOW DO NOT TOUCH G1 K1 to G6 K6 POINTS ON PANEL.Trace the observed waveform on graph paper.

d) Change the firing angle in steps of 10 between 0-120 only on TWS and set the given firing angle by pressing VI key on microprocessor kit.Observe the change in output voltage & waveforms.For firing angle between 90-120 connect freewheeling diode across Converter to avoid negative surge.Tabulate the results for firing angle against average DC output voltage Measured by DC voltmeter to be connected externally at output.


Fig2(a)

Fig2(b)


e) Change the firing angle in steps of 10 between 0-60 on TWS and set the Given firing angle by pressing Vi key on microprocessor kit.Observe the change in output voltage & waveforms.Tabulate the results for firing angle against average DC output voltage measured by DC voltmeter to be connected externally at output.

8) Make main power switch on unit OFF.Make external power switch OFFMake G4 G5 G6 trigger switch OFF.(and should be kept off for half controlled operation ).Make gate supply on switch OFF.

OBSERVATION TABLE-

S.No. Firing angle Average DC Output voltage

1234

CONCLUSION-

-Kit diagram



OBJECT: - To observe the performance of three phase Semi controlled rectifier with microprocessorApparatus required:-

(1)3 – phase Rectifier control kit(2)Microprocessor 8085 kit(3)3 – phase supply with Auto Transformer (4)Connecting leads(5)Lamp load

Introduction :-Single phase a.c. to d.c. converters are generally limited to a few kilovolts, and for higher levels of d.c. power output, three phase line commutated converters are used owing to restrictions on unbalanced loading, line harmonics, current surge and voltage dips. Increase I ripple frequency also reduces the filter size. Converters which can be operated both in rectifying and inverter modes are called fully controlled converters. When power flow can only occur from ac – to – dc, the converter is known as semi-converter or half controlled converter. Fully controlled three – phase converters find application in high – voltage dc power (HVDC) transmission, dc and ac motor drives with regenerative breaking capabilities.THEORY:-CIRCUIT DESCRIPTION AND PRINCIPLES :- A half controlled converter, when compared to a fully controlled converter has no starting problem. But has higher harmonic content in the load voltage and the supply current waveforms.


Semi control convertor--ROLE OF MICROPROCESSOR :-

(1)8085 microprocessor kit has 8255 PPI configure in mode 0 with port A & B as input & port C as output.

(2)8353 is used to provide the required firing angle delay. It is configured in mode 00 with counter 0 as operating counter & two data byte loading. Output of 8253 counter gives interrupt RST 7.5 after the set delay angle.

(3)There 220 V / 12 V step down transformers are used in typical pattern as shown in circuit diagram to read six modes of operation at A2, A1 and A0 points. These read by 8255 input port at A0, A1 , A2 and decides the pair of SCR to be made ON for the current mode.

(4)RST 6.5 interrupt reads the BCD input from TWS (Thumb Wheel Switch) & scale it for proper value depending on the required firing angle.

FLOW CHART :-


START :- Set stack pointerInitialize 8255 portPort A, B …………………… InPort C .…………………. OutCall ISR 6.5 for getting angle for first time.Read A0, A1, A2 and note the mode.

MAIN :- In port A & note the change in mode.If NO change JUMP to MAIN.

LOOP :- In port A & On change in mode load 8253 with firing angle.JUMP BACK TO LOOP.

ISR 7.5:- Mask all interrupt.Get L value from location 1090 H.Set HL pointer for sequence table.OUT required pattern for making the pair of SCR ON.Enable Interrupt.RETURN.

FIRST TIME OPERATION :-(8)Connect R, Y , B phase in proper sequence at R, Y, B Input terminals of

isolation transformer.Keep external 3 – phase main switch OFF.Keep main power switch on unit OFF.Keep G1 G2 G3 trigger switch OFF.Keep G4 G5 G6 trigger switch OFF.Keep gate supply on switch OFF.

(9)Connect 8085 microprocessor kit to the main unit by cord provided with the unit. Make 8085 microprocessor kit ON.benICs message id displayed on kit.

(10) Make external 3 – phase main switch ON so that 3 – phase supply is connected to sync. transformer.Do not make ON main – power of 3 – phase SCR converter on panel.

(11) Set required firing angle on Thumb wheel switch ( TWS ).Execute the program as follows.

Go4000


Exec.E is display on kit.Provide 6.5 interrupt from kit by pressing VI key.Reset the kit by pressing ‘ RES ‘ key.Check the value at memory location 1088.It should be firing angle in HEX as follows.

If 0-59 ……………………as it is in Hex.60 ……………………0261-119 ……………………as 1-59 (converted in Hex)120-176 ……………………as 1-59 (converted in Hex)

a) Configure the bridge for half controlled rectifier by connecting A1 A2 A3 points of thyristor T1 T2 T3 to cathode of diodes D4 D5 D5 by patch coard, respectively.Repeat the procedure of part 7.Vary the firing angle from 0 – 175 for half controlled operation .Trace the observed waveform on graph paper.Note the average DC output voltage across load for given firing angle and tabulate the result.

b) Change the firing angle in steps of 10 between 0 – 60 on TWS and set the given firing angle by pressing VI key on microprocessor kit.Observe the change in output voltage & waveforms.Tabulate the results for firing angle against average DC output voltage measured by DC voltmeter to be connected externally at output.

S.No. Firing angle Average DC Output voltage


CONCLUSION-

OPCODE- For 8085Main :- Microproccesor code-Address Opcode Mnemonic CommentSTART 4000 31, FF, 17 LXI SP 17FF

03 3E, 92 MVI 9205 D3, 03 OUT 0307 CD, 00 42 CALL ISR6.5

MAIN 0A DB, 00 IN PORT AC E6, 07 ANI 07E 32, 89, 10 STA 1089

L1 11 DB, 00 IN PORT A13 E6, 07 ANI, 0715 21, 89, 10 LXIH 108918 BE CMP M19 CA, 11, 40 JZ L11C 77 MOV MA1D 3E, 19 MVI 191F 30, FB SIM, EI21 3E, 30 MVI A 30


23 D3, 13 OUT 1325 3E, 00 MV1 A 0027 D3, 13 OUT 1329 3A, 87, 10 LDA 10872C D3, 10 OUT 102E C3, 11, 40 JMP L1

ISR 7.5 :-Address Opcode Mnemonic Comment4300 3E, IF MVI A IF MASK 7.5 & 6.5 INTERUPT02 30 SIM03 3A, 90, 10 LDA 1090 LOAD ACC WITH ‘L’06 6F MOV L A VALUE & ADJUST HL07 26, 40 MVI H 40 POINTER AS PER REQUIRED

MODE SEQUENCE 09 3A, 89, 10 LDA 10890C 85 ADDL0D 6F MOV L A0E 7E MOV A M GET PROPER FIRING STATUS0F D3, 02 OUT C AND OUT ON C PORT11 FB EI ENABLE INTERRUPT12 C9 RET RETURN


Experiment No. 7

OBJECT: - Closed loop Speed control of DC motor using Thyristor controlled techniques.

Apparatus required:-

16. Thyristor control kit17. Connecting cords 18. D.C shunt motor19. Multimeters

Theory-:Among all existing electric motors, the separately excited DC machine has the best ability to

fulfill the demands of adjustable drive systems, as its speed can be varied over a wide range through armature voltages and field flux control. The possibility of speed control through these parameters gives increasing matching ability to drive requirements.

If the motor field is excited from a source separate from that supplying the armature, as it is usually in case with speed controlled drives, two distinct ranges of operation are obtained as shown in figure.

Fig (1) Power – speed curve for a D.C. motor.


Fig (2) Torque – speed curve for a D.C. motor.

In the armature voltage control range, the field excitation is held at its rated value and the armature voltage is varied to control the speed. Above the base speed, the armature voltage is constant at its rated value and the field is weakened. In this region, the torque decreases proportional to increase in speed and, therefore, the power output above the rated speed remains constant.

In modern DC derives, the classical motor – generator set has been replaced by a thyristorised power converter which provides faster response at a lower total cost.

Thyristor converters have distinct advantages over the rotary converters, and in view of their excellent control qualities and high efficiency, have become economically realizable power controllers for small as well as medium and high power drives systems.

These thyristor converters are very suitable for closed loop control system, required for good speed accuracy. Close loop feedback systems generally have the advantages of greater accuracy, improved dynamic response and reduced effect of disturbances such as loading.

TRANSFER FUNCTION OF DC MOTOR

A dc motor is often employed in a control system where an appreciable amount of shaft power is required. The DC motors are much more efficient than two-phase AC servo motors.


The DC motors have separately excited fields. They are either armature – controlled with fixed field or field – controlled with fixed armature current. For example, DC motors used in instruments employ a fixed permanent magnetic field, and the control signal is applied to the armature terminals.

The performance characteristics of the armature controlled DC motor resemble the idealized characteristics of the two – phase AC servomotor.

Consider the armature – controlled DC motor shown in figure. In this system.

Ra = armature – winding resistance, Ohms

La = armature – winding inductance, henrys

ia = armature-winding-current amperes

if = field current, amperes

ea = applied armature voltage, volts

eb = back emf, volts

Ɵ = angular displacement of the motor shaft, radians

T = torque delivered by the motor, N-M

J = equivalent moment of inertia of the motor and load referred to the motor

N-M

F = equivalent viscous – friction coefficient of the motor and load referred to the motor shaft, N-M/rad/sec.

The torque T delivered by the motor is proportional to the product of the armature current ia and the air gap flux Ψ, which in turn is proportional to the field current or

Ψ = K f if

Where K f is a constant. The torque t can, therefore, be written

T = K f if K 1ia

Where k1is constant.

In the armature-controlled DC motor, the field current is held constant.


For a constant field current, the flux becomes constant, and the torque becomes directly proportional to the armature current so that,

T = Kia ---------------------(1)

Where K is a motor-torque constant. When the armature is rotating, a voltage proportional to the product of the flux and angular velocity is induced in the armature. For a constant flux, the induced voltage Eb is directly proportional to the angular velocity dƟ/dt.

Thus,

eb= KbdƟ/dt --------------------------------(2)

Where K bis a back emf constant.

The speed of an armature – controlled DC motor is controlled by the armature voltageea. The armature voltage eais supplied by an amplifier (or by a generator, which is supplied by an amplifier). The differential equation for the armature circuit is

La d ia

dt + ra ia +eb = ea-----------------------(3)

The armature current produces the torque which is applied to the inertia and friction: hence

jd2Ɵ

dt 2 + f dƟdt = T = K ia----------------------------(4)

Assuming that all initial conditions are zero and taking the Laplace transforms of Eqs. (2),(3) and (4), we obtain the following equations;

K bS Ɵ (S) = Eb(S) -------------------------(5)

(LaS + Ra) I a(S) + EbS = Ea (S) ------------(6)

(JS2 + fS)Ɵ(S) = T(S) = K I a(S) --------------(7)

Considering Ea (S) as the input and Ɵ(S) as the output, we can construct, the block diagram from Eqns.(5) , (6) and (7) as shown in figure. The effect of the back emf is seen to be the feedback signal proportional to the speed of the motor. This back emf thus increases the effective damping of the system. The transfer function of this system is,


Ɵ(S)Ea(S) =

KS [La J S2+(La f +Ra J ) S+Ra f+ K K b]

-----------(8)

The inductance La in the armature circuit is usually small and may be neglected. If La is neglected, then the transfer function given by Eqn. (8) reduces to

Ɵ(S)Ea (S ) =

Km

S (T m S+1) -------------------------------(9)

Where,

Km= K/(Ra f + K K b) = motor gain constant

T m = RaJ/ (Ra f + K K b) = motor time constant

From Eq. (9), it can be seen that the transfer function involve the term 1/S. thus, this system possesses an integrating property. In Eq. (9) notice that the time constant of the motor is smaller for smaller Raand smaller J. with small J , as the resistance Rais reduced, the motor – time constant approaches zero, and the motor acts as an ideal integrator.

THYRISTOR CONTROLS FOR THE MOTOR CONTROL;

Thyristor controlled converter is used to control the speed and torque of DC motors.


Depending on the rating and application of the DC motor, different types of Thyrister is used in power circuit.

In all these configurations, the DC output voltage applied to the motor is controlled by varying the firing angle.

CLOSED LOOP CONTROL SYSTEM:

A controller has basically the following two functions:

1) To match the controlled variable to the set reference value2) To achieve an optimum time response.

A comparision between the reference value and the controlled variable is made at the input of the controller. The controlled variable can be the speed, or current or voltage of A DC motor. The difference between the reference value and controlled value (or actual value) results in the control error.This error is suitably processed in the controller and the output of the controller controls the motor, such as to make the actual value and the reference value equal. This is illustrated in figure.

The speed of the DC motor is controlled using a IGBT converter. The actual speed is compared with the desired speed and if there is any error between the two, it is processed in the controller and its output in turn suitably, changes the pulse width of the PWM control in the converter to make the actual speed equal to the desired speed. The time response of the control system is usually divided into two parts: the transient response, and the steady state response. If C(t) is the time response, then, in general,

C(t) – Ct (t) + Css (t)


Where Ct (t) denotes the transient portion and Css is the steady state portion of the response. In control systems, the steady state response is simply the response when time reaches infinity.

Transient is defined as the part of the response, which goes to zero as time becomes large. Therefore, Ct(t) has the property,

limt → ∞

Ct (t )=0

In a motor control system, the speed cannot change instantaneously to a step input, transient is usually observed. The steady state response, when compared with the input, gives an indication of the accuracy of the system.

If the output steady state response does not agree with the input exactly, the system is said to have a steady state error. An optimized response is said to be achieved when, upon a disturbance the controller returns the controlled variable as fast as possible and with little overshoot, to its original value. The same applies when the reference value is altered. Figure shows the transient response of the controlled variable in a controlled loop when the reference value is suddenly altered (unit step input) at the controller input.

SPEED CONTROL OF DC MOTOR:

The speed is set using potmeter. The set value is compared with the voltage proportional to the actual value of speed, obtained from the tachogenerator. The output of the tachogenerator is filtered to reduce the ripple content. The error is fed to the controller controls the firing angle of the pulses applied to turn on the thyristor in the converter. As the firing angle changes, the output voltage of the converter changes which in the turn changes the (armature voltage) speed of the DC motor.

EXPERIMENTAL SETUP FOR DC MOTOR SPEED CONTROL

Setup consists of the following components with their specifications mentioned below.


1) CONTROL UNIT ( Thyristor controlled CONVERTER):Power input : 230 VOLT MainsPower output : 20 – 220 Volt DC outputControl input : 0-10 VoltThe motor current and motor voltage is indicated on respective ammeter and Voltmeter provided on panel.

2) DC MOTOR WITH TACHOGENERATORA) MOTOR : DC SHUNT MOTOR

Armature : 220 Volt DCField : 200 Volt DCF L Speed : 1500 RPMF L current : 5 AmpCapacity : 1 HP

B) Tachogenerator : Coupled to the shaft of the motorInput : 0 – 2000 RPMOutput : 10 Volt/ 1000 RPM

3) Speed measurement : Digital tachometer


A) Sensor : inductive proximity switchSensing distance : 2 to 5 mmSignal Type : Digital pulse input 3-wire Pulse Rate : 1 PPR

B) Display unit: 5 Digit, 7-sigment LED display Controller : Micro controller based Frequency counter Range : 60 to 6000 RPM (Auto Range selection)Sampling rate : 1 sample per second Time base : Quartz crystal controlledResolution : 0.1 RPM upto 999.9 RPM and 1.0 RPM above 1000 RPMPower : 230 Volt AC Mains

PROCDURE:

1) Connect DC shunt motor to the unit at DIN rail connector connect 230 Volt AC Mains to the unit.Make mains power on to the unit

2) Speed of the motor is controlled by adjusting speed adjust potmeter.Vary the speed in proper steps, and every time note armature voltage, armature current, and speed of the motor. Also measure the output of tachogenerator using external voltmeter.Tabulate the result.

OBSERVATION TABLE-

S.No. Armature Voltage Va ( VOLT)

Armature CurrentIa (Amp)

SpeedN (RPM)

Tacho Output Volt

1

2

3

4

CONCLUSION-

· web view- speed control o f universal motor using “triac” . apparatus...

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