comprehensive viva

07E303 / DC Machines & Transformers / 2 Marks Q &A

UNIT I

ELECTROMECHANICAL ENERGY CONVERSION

1. How electromechanical energy conversion occurs? It occurs through the medium of the magnetic stored energy.

2. What is prime mover?The basic source of mechanical power which drives the armature of

the generator is called prime mover.

3. Give the materials used in machine manufacturing Three materials are used in machine manufacturing.

(i) Steel – to conduct magnetic flux(ii) Copper – to conduct electric current

(iii) Insulation

4. What is MMF? Give its expression.MMF is the work done in moving a unit magnetic pole once around

the magnetic circuit.MMF = N I (Ampere turns)

Where N = Number of turns I = Current in Ampere

5. Define magnetic field intensity.It is the MMF per unit length. Ampere Turn/metre

H = N I/L where N = Number of Turns; I = Current in Ampere; L = Length in metre.

6. What is reluctance?It is the property of the magnetic material to oppose the magnetic

lines of flux.S=F/Φ where F = MMF; Φ = Flux; S = Reluctance in Ampere Turns per Weber.

7. What is quasi static field?It is the field pattern which is fixed in space but field intensity at

every point varies as a replica of time variation of current.8. What is leakage flux?

The flux which takes a path which is not intended for it is called leakage flux.

9. What is fringing effect?While passing through the non magnetic medium the magnetic

lines of force try to bulge out because the lines of force repel each other. This is fringing effect.

Prepared by: P.S. Mayurappriyan & J. Karpagam, Department of EEE, BIT, Sathy


10.Compare electric and magnetic circuitsMagnetic circuit Electric circuit

1. Closed path of magnetic flux Closed path for electric current2. Flux = mmf / reluctance Current= Emf / Resistance3. Reluctance=1 / aμ0 μr Resistance R = ρl/a4. Magnetic flux density B = φ/a Current density J = I/a5. MMF drop = φ S Voltage drop = I R6. Magnetic field intensity H = N I/l Electric field intensity E = V/d

11.How is the direction of induced emf determined? Direction of induced emf is determined by using

Right hand gripping rule and Right hand cork screw rule

12.State Lenz’s law.Any induced emf will circulate a current in such a direction as to

oppose the cause producing it. e= -N dΦ/dt

13.What is self inductance?The emf induced in a coil due to change of flux in the same coil is

known as self inductance.

14.Give the expression for magnetizing force F = B I l sin θ (Newtons)

Where I is the current carried by the conductor in Ampere

l is the effective length of the conductor in metres B is the flux density in Wb/m2

15.What are the factors on which hysteresis loss depends?The hysteresis loss depends on the magnetic flux density,

frequency f and the volume of the material V.

16.What is core loss? What is its significance in electric machines?When a magnetic material undergoes cyclic magnetization, two

kinds of power losses occur on it – hysteresis and eddy current loss which together are known as core loss. It is important in determining heating, temperature rise, rating and efficiency of transformers, machines and other ac run magnetic devices.

17.Derive an equation for energy density for a magnetic circuit. Energy stored in the magnetic circuit = ½ LI2

L = N2 aμ0 μr / l Energy Density = ½ I2 N2 aμ0 μr / l = ½ B2 H2 aμ0 μr since H =NI/l

= ½ B2 la / μ0 μr

= ½ B2a2 /S



= ½ Φ2 /R Joules

18.What is eddy current loss?When a magnetic core carries a time varying flux voltages are

induced in all possible paths enclosing flux. Result is the production of circulating current in core. These induced currents do no useful work are known as eddy current and have power loss known as eddy current loss.

19.How are hysteresis and eddy current losses minimized?Hysteresis loss can be minimized by selecting materials for core

such as silicon steel & steel alloys with low hysteresis coefficient and electrical resistivity.

Eddy current losses are minimized by laminating the core.

20.Write the equation for energy transfer.Energy input from = mechanical energy output + increase in

electric sources energy stored in magnetic field + energy converted into heat. For motors : electric and mechanical energy terms have positive values

For generators: they have negative values.

21.What is the basic feature of an electro magnetic energy conversion device?They contain air gaps in their magnetic circuits in their moving parts.

22.What is the energy conversion medium in a singly-excited magnetic field system?

The magnetic circuits have air gap between stationary and moving members in which considerable energy is stored in the magnetic field. This field acts as the energy conversion medium and its energy is the reservoir between electric and mechanical system.

23.Why does the energy storage in a magnetic material occur mainly in the air gap?

The reluctance of the air gap is much larger than the magnetic material. Hence the predominant energy is stored occurs the air gap and the properties of the air gap are determined by the dimension of the air gap.



24.Draw a schematic diagram indicating flow of energy in the conversion of mechanical energy to electrical form.

25.Why do all practical energy conversion devices make use of the magnetic field as a coupling medium rather than an electric field?

It can be shown that the force density on the bounding surface of a magnetic field near saturation is of the order of 1.02 x 106 Nm2.

In electric field for the field intensity near breakdown the force density is only about 39.8 Nm2. The vast difference in the force density in magnetic and electric fields is because of the fact that in magnetic field = 4 x 10-7 H/m while in electric field 0 = 8.85 x 10-12 C/Nm2.

26.What is single excited magnetic field system? Give an example.If the electromechanical devices have only one set of exciting system

it is called single excited magnetic field system. Example: Reluctance Motor.

27.What is multiply excited magnetic field system? Give an example.If the electromechanical devices have more than one set of exciting

system it is called multiply excited magnetic field system. Example: DC Motor.

28.Enumerate the chief advantages of electric energy over other forms of energy.

1. It can be generated in centrally in bulk and transmitted economically over long distances.

2. Its voltage levels can be raised or lowered with ease and economically.

3. Electric lighting has no competitor.4. Electric motors can be produced in all sizes right from few Watts to

Mega Watts.


Mechanical Source

Coupling Field

Electrical Sink

Electrical Losses

Mechanical LossesGross

Electrical Input

Net Mechanical

Output

Ideal Conversion Process

Net Electrical

Output

Gross Mechanical

Input


5. Domestic applications apart from lighting and fans are multiplying everyday.

6. All air-conditioner loads are now electric driven.7. Attempts are being made to replace automobile engines with electric

motors.

29.Give the relation between energy and co-energy for linear system.Energy and Co-energy are numerically equal in linear system.

30.Define leakage flux. Does it take part in electromechanical energy conversion process via the magnetic field?

Flux that links one coil (say on the stationery member) only partially passes through the core, does not link the coil on the moving or rotating member or vice versa, is called leakage flux.

This flux though small, links each of the coils only. As a result, the leakage flux of one member does not interact with the current of the other member and so it does not produce mechanical torque / force. Thus it does not take part in energy conversion process.

31.How is voltage generated in rotating machines?In rotating machines voltage is generated in windings or group of

coils by rotating them through a magnetic field or by mechanically rotating a magnetic field past the winding or by designing the magnetic circuit so that the reluctance varies with rotation of the rotor.

32.What is the expression for torque in terms of the resultant mmf wave Fsr?T = -P/2 . Π/2. μ0 Dl / g . Fr Fsr sinδ

33.What are distributed windings?Windings which are spread over a number of slots around the air gap

periphery.

34.Why salient pole construction is a characteristic of hydroelectric generators?Because hydraulic turbines operate at relatively low speeds and a

large number of poles are required to produce the required frequency.

35.Torque is proportional to ------------------------------.Torque is proportional to the interacting fields and to the sine of the

electrical space angle between their magnetic axes.

36.How will you find the direction of emf using Fleming’s Right Hand Rule?The thumb, the forefinger and the middle finger of the right hand are

held so that these fingers are mutually perpendicular, then forefinger gives the direction of field, thumb represents the direction of motion of conductor and middle finger gives the direction of induced current.



37.How will you find the direction of force produced using Fleming’s Left Hand Rule?

The thumb, forefinger and middle finger of the left hand are held so that those fingers are mutually perpendicular then forefinger gives the direction of field, middle finger gives the direction of current and thumb represents the direction of motion of conductor.



UNIT II

DC GENERATORS

1. What is the function of carbon brush used in DC generator?The function of carbon brush is to collect current from the

commutator and supply to the external load circuit and to the field circuit.

2. Distinguish between lap winding and wave winding used in dc machine. Lap winding is designed for high current and low voltage machines Wave winding is designed for low current and high voltage

machines

3. Write the number of parallel paths in a lap and wave connected windings In a lap wound machine, the number of parallel paths is equal to

the number of poles. But in wave wound machine, the number of parallel paths is always two irrespective of number of poles.

4. Name the three things required for the generation of emf.o Presence of armature conductors o Presence of magnetic field o Relative motion between conductor and magnetic field

5. What is meant by self excited and separately excited dc generator? Self-excited generator are those whose field magnets are

energized by the current produced by the generator themselves Separately excited generator are those whose field magnets are

energized from an independent external source of dc current

6. What is the basic difference between dc generator and dc motor?Generator converts mechanical energy into electrical energy. Motor

converts electrical energy into mechanical energy. But there is no constructional difference between the two.

7. Write down the emf equation of dc generator. Give the meaning of each symbol

Emf induced E = ФZNP/60A (Volts)whereФ = Flux per pole in WeberZ = Total number of armature conductors N = Speed of armature in rpmP = Number of polesA = Number of parallel paths

8. What is pole pitch?The periphery of the armature is divided for a number of poles of the

generator. The center to center distance between two adjacent poles is called



pole pitch. It is also equal to the number of armature slots or armature conductors per pole.

9. How can the voltage in a DC generator be increased?Increasing the main field flux and the speed of the armature can

increase the voltage in a DC generator.

10.What is critical resistance of a DC shunt generator?The value of resistance of shunt field winding beyond which the

shunt generator fails to build up its voltage is known as “critical resistance“.

11.What are the conditions to be fulfilled for a shunt generator to build up voltage?

a) There must be some residual magnetism in the field poles.b) The shunt field resistance should be less than critical resistance.c) The field coils should be connected with the armature in such a way

that current flowing through them should increase the emf induced by the residual magnetism.

12.What do you mean by residual flux in DC generator?The magnetic flux retained in the poles of the machine even without

field supply is called the residual flux.

13.A DC generator fails to self excite. List the cause for the failure for the failure.a) Residual magnetism may not there in the polesb) Direction of rotation may be wrongc) The field resistance may be more than critical resistanced) There may be disconnection in the field windinge) Brush contact may be poorf) The field coils may be connected with the armature to oppose the

emf due to residual magnetism

14.What are open circuit characteristics of DC shunt generator?It is the relation between the field current and the emf induced in

the armature.

15.How can one differentiate between long shunt compound generator and short shunt compound generator?

In a short shunt compound generator the shunt field circuit is shorter i.e. across the armature terminals. In a long shunt compound generator the shunt field circuit is connected across the load terminals.

16.Why is the emf not zero when the field current is reduced to zero in a dc generator?

Even after the field current/magnetizing force is reduced to zero the machine is left out with some flux as residue. Emf due to this residual flux is available when field current is zero.



17.Define the term ‘critical speed’ in dc shunt generator.Critical speed is defined as the speed at which the generator is to

be driven to cause self-excited generator to build up its emf for the given field circuit resistance.

18.On what occasions dc generators may not have residual flux? The generator may be put for its first operation after its construction In previous operation the generator would have been fully

demagnetized

19.How the critical field resistance of a dc shunt generator is estimated from its OCC?

Critical field resistance can be obtained from OCC by drawing a straight line passing through the origin and tangent to the initial straight line portion of OCC. The slope of this line gives the value of critical field resistance for the given speed at which OCC is obtained.

20.Define the term armature reaction in dc machines.The interaction between the flux set up by the current carrying

armature conductors with the main field flux is defined as armature reaction.

21.What are the two unwanted effects of armature reaction?Cross magnetizing effect / Distorting effectDemagnetising effect

22.Differentiate between geometric neutral axis (GNA) and magnetic neutral axis (MNA).

GNA is the axis which is situated geometrically or physically in the mid way between adjacent main poles. MNA is the axis which passes through the zero crossing of the resultant magnetic field waveform in the air gap.

23. In which part of the dc machine is the compensating winding situated?In the slots provided in the main pole phases.

24.What are the various types of commutation? Linear commutation Sinusoidal commutation

25.Name the two methods of improving commutation.Emf commutationResistance commutation

26.What is reactance emf in dc machine?The self induced emf in the coil undergoing commutation which

opposes the reversal of current is known as reactance emf.



27.Define the term commutation in dc machines.The changes that take place in winding elements during the period of

short circuit by a brush is called commutation.

28.How and why the compensating winding in dc machine excited?As the compensation required is proportional to the armature current

the compensating winding is excited by the armature current.

29.How is the interpole winding in dc machine excited?Interpole winding is connected in series with the armature circuit and is

excited by the armature current.

30.To what polarity are the interpoles excited in dc generators?The polarity of the interpoles must be that of the next main pole along

the direction of rotation in the case of generator.

31.Why are carbon brushes preferred for dc machines?The high contact resistance carbon brushes help the current in the coil

undergoing commutation to attain its full value in the reverse direction at the end of commutation. The carbon brushes also lubricate and give less wear and tear on commutator surface.



UNIT III

DC MOTORS

1. What is the basic difference between DC generator and DC motor?Generator converts mechanical energy into electrical energy. Motor

converts electrical energy into mechanical energy. But there is no constructional difference between the two.

2. What is the basic principle of operation of DC motor?When a current carrying conductor is placed in a magnetic field, a

mechanical force is exerted on the conductor which develops the torque.

3. What is torque proportional to?Torque is proportional to the interacting fields and to the sine of the

electrical space angle between their magnetic axes.

4. Distinguish between shunt and series field coil constructions.Shunt field coils are wound with wires of small cross section and

have more number of turns. Series field coils are wound with wires of larger cross section and have less number of turns.

5. How does a DC motor differ from DC generator in construction?Generators are normally placed in closed room, accessible only to

skilled operators. Therefore on ventilation point view they may be constructed with large opening in the frame. Motors on the other hand, have to be installed right in the place of use which may have dust, dampness, inflammable gases, chemical fumes etc. To protect the motors against these elements, the motor frames are made either partly closed or totally closed or flame proof etc.

6. How will you change the direction of rotation of a DC motor?Either the direction of the main field or the direction of current

through the armature conductors is to be reversed.

7. What is back emf in DC motors?As the motor armature rotates, the system of conductor come

across alternate North and South pole magnetic fields causing an emf induced in the conductors. The direction of the emf induced in the conductors. The direction of the emf induced is in the direction opposite to the current .As this emf always opposes the flow of current in motor operation it is called back emf.

8. Write down the equation for back emf of DC motor.Eb = P Z N P / 60 A, [Volts]



9. Write down the equation for torque developed in DC motor.Td = Ia Z P / 2 A , [N-m]

Td Ia

10.Why DC motors are not operated to develop maximum power in practice? The current obtained will be much higher that the rated

current The efficiency of operation will be below 50 %

11.Under what condition the mechanical power developed in a DC motor will be maximum?

Condition for mechanical power developed to be maximum is Eb = Va /2 or I a= Va / 2Ra

12.Why shaft torque is always less than that developed inside the armature in a DC motor?

Mechanical power developed inside the armature is Pd = EbIa = 2NTd / 60

Mechanical power output available on the shaft is Po = 2NT / 60

Pd – Po = Wi + Wm Therefore shaft torque T is less than torque developed in the armature Td to meet the iron loss Wi and mechanical loss Wm.

13.Why is the starting current high in a DC motor?The absence of back emf at the time of starting causes the

armature current to shoot up to about 20 times the normal current, if no limiting resistance is included.

14.What is the need for starter in a DC motor?Starters are used in DC motors to limit the high starting current

within about 2 to 3 times the rated current by adding resistance in series with the armature circuit. Apart from starting resistances, starters are invariably fitted with protective devices such as no voltage protection and over-load protection.

15.What is the function of over-load release coil provided in a DC motor starter?Due to any overload in the motor, if the line current increases

above a preset value, the excess magnetic force causes the lifting of an iron piece. As the iron piece makes an upward movement, a contactor fitted along with it causes the two terminals of NVR coil to get short circuited. Hence the electromagnet fitted with NVR coil loses its magnetic force and releases the starter handle from the ON position towards OFF position, thus protecting the motor against over-load.

16.What is the function of a no-voltage release coil provided in a DC motor starter?



As long as the supply voltage is on healthy condition the current through the NVR coil produce enough magnetic force of attraction and retain the starter handle in the ON position against spring force. When the supply voltage fails or becomes lower than a prescribed value the electromagnet may not have enough force and the handle will come back to OFF position due to spring force automatically. Thus a no-voltage or under voltage protections are given to the motor.

17.How does 4 point starter differ from 3 point starter?In 3 point starter, NVR coil is connected in series with the shunt

field coil. The exciting current through the NVR coil in 3 point starter is same as the shunt field current of the motor. In 4 point starter, NVR coil along with a high resistance connected across the supply voltage. Thus the exciting current through NVR coil of a 4 point starter is purely proportional to the supply voltage and independent of shunt field current.

18.Enumerate the factors on which the speed of a DC motor depends.N = Eb /

= (Va-IaRa)/ The speed of dc motor depends on three factors. Flux in the air gap Resistance of the armature circuit Voltage applied to the armature

19.List the different methods of speed control employed for DC series motor. Field diverter method Regrouping of field coils Tapped field control Armature resistance control Armature voltage control for single motor Series parallel control for multiple identical motors

20.Draw the N Vs Eb characteristics of a dc motor for two different field currents.

If<Ifn

Nn

Ifn

Ehn

21.What is the relation between electrical degree and mechanical degree?Electrical degree θe and mechanical degree are related to one

another by the number of poles P, the electrical machine has, as given by the following equation. θe = (P/2) θm

22.What is the meaning of electrical degree?Electrical degree is used to account the angle between two points

in rotating electrical machines. Since all electrical machines operate with the help of magnetic fields, the electrical degree is accounted with reference to



the polarity of magnetic fields. 180 electrical degrees is accounted as the angle between adjacent North and South poles.

23.List out some examples of prime movers.I.C. Engines, Steam engine, Turbine or Electric Motors.

24.Give some applications of DC motor.Shunt : driving constant speed, lathes, centrifugal pumps, machine tools, blowers and fans, reciprocating pumpsSeries : electric locomotives, rapid transit systems, trolley cars, cranes and hoists, conveyorsCompound : elevators, air compressors, rolling mills, heavy planners.

25.Explain how the back emf of a motor causes the development of mechanical power?

Net electrical power into motor armature is Pmot = EbIa

= m Z P Ia / 2 A= m { Z P Ia / 2 A) = m T = Pmech

So it is power absorbed by Eb that gets converted to mechanical form.

26.Why a differentially compound motor is not used in practice?As the motor is loaded, the series winding reduces the flux

produced by the shunt winding and so its N-T characteristic curves are upwards. Under accidental overload, flux / pole tend to zero and motor speed can increase to dangerous values. Hence they are not used in practice.

27.What do you mean by constant torque operation and constant HP operation of speed control in DC shunt motor?

Constant torque operation of speed control is Armature control method and constant HP method is Field control method.

Speed and torque equations of DC motor are N = KN (Va – IaRa)/ = KN Va / as armature drop is neglected

T = KT Ia At load torque TL = Tmotor at base speed Nb, apply full voltage to field; so If

= If(max). Adjust armature voltage Va to rated value. With field remaining fixed for constant torque, Ia should remain fixed at rated value. For speeds above rated value, reduce Va, the speed reduces as per the first equation, the motor draws rated Ia at constant load torque. The speed cannot be raised above Nb as rated Ia cannot be exceeded unless the torque is allowed to be reduced.

Keeping Va at rated value if If is reduced, the speed would increase but torque would reduce. This indeed is constant Hp drive.

28.State one advantage and disadvantage in the application of each of the three basic types of DC motors.

a. Shunt Motor:



Advantage: Substantially constant speed i-e low speed regulationDisadvantage: Cannot be used for constant speed application

b. Series Motor:Advantage: High torque low speed (at start) and low torque at high speed. This is typical requirement for traction type of loadDisadvantage: Accidental no load can cause the motor to run at dangerously high speed.

c. Compound Motor:Advantage: Negligible speed regulation for cumulatively compound motor.Disadvantage: Higher cost.

29.List all the important information on name plate of a DC motor.kW, nominal speed, armature current, insulation class.

30.Why field control is considered superior than armature control method of DC shunt motor?

a. The regulating resistance which has to carry only a small current is easily available.

b. Power wasted in regulating the resistance is very small and hence this method is more economical.



UNIT IV

TRANSFORMERS

1. State the principle of operation of a transformer.Transformer operates on the principle of mutual induction between

inductively coupled coils. When AC source is connected to one coil, flux is produced in the core, which links both the coils. As per Faraday’s Laws of Electromagnetic Induction, EMF is induced in the secondary coil also. If the external circuit is closed the power is supplied.

2. What are the main parts of a transformer? What type of material is used for the core?

Laminated core and primary and secondary windings are the main parts. The core is built up of thin soft iron or high-grade silicon steel laminations to provide a path of low reluctance to the magnetic flux.

3. What is an ideal transformer?An ideal transformer is one which does not involve any power losses and

also does not have any leakage of magnetic field.

4. Mention two types of constructions used in transformer.Core type, shell type and Spirakore type. Spirakore type is a latest

transformer and is used in big transformers.

5. Mention the difference between core and shell type transformers. In core type, the windings surround the core considerably and in shell

type the core surround the winding.

6. What is the purpose of laminating the core in transformers? To reduce eddy current loss in the core.

7. Why do we use iron core in a transformer?Transformer action demands only the existence of alternating flux linking

the two windings. No doubt such action will be obtained if an air core is used. But it will be obtained much more effectively if an iron core is used. It is because the flux is then substantially confined to a definite path (i-e iron path) having a much more permeability than air.

8. Where is core type and shell type construction suitable for a transformer? In core type transformer, one limb of the core one half of primary and

secondary windings wound on it, low voltage winding being the innermost for mechanical strength. This type affords better cooling surface than shell type and is therefore more suitable for transformers which remain fully loaded.

In shell type, both the windings are wound on central limb. This type is applicable for poor power factor circuits.



9. What is meant by turns ratio in transformer?It is the ratio of number of turns in the secondary winding N2 to number

of turns in the primary winding N1.

10.Why is the rating of a transformer given in kVA? Copper loss of a transformer depends on current and iron loss on

voltage. Hence total transformer loss depends on volt ampere (VA) and not on the phase angle between voltage and current, i.e., it is independent of load power factor. That is why the rating of a transformer is in kVA.

11.List four applications of a transformer. It can raise or lower the voltage or current in an AC circuit. It can act as an impedance transferring device by increasing or

decreasing the value of a capacitor, inductor or resistance in an AC circuit.

It can isolate two circuits electrically. It can be used to prevent DC from passing from one circuit to

another.

12.Give the emf equation of a transformer and define each term. Emf induced in primary coil E1 = 4.44 fΦ mN1 (Volts) Emf induced in secondary coil E2 = 4.44fΦ mN2 (Volts) Where f is the frequency of AC input in Hertz Φ m is the maximum value of flux in the core in Webers N1, N2 are the number of primary and secondary turns.

13. Name two important electrical performances of transformers?i) Voltage regulation ii) Efficiency

14.What will happen if the primary of a transformer is connected to a DC supply?

The primary will draw a steady current and hence produce constant flux. Consequently no back emf will be produced. The primary winding will draw excessive current due to low resistance of the primary. This result in over heating of primary windings and the fuses will blow.

15. How does flux leakage occur in transformer?The entire flux set up by one winding during load condition may not be

able to reach the other winding to carry on the useful purpose of energy transformation. Some of the fluxes will be present in and around the respective windings only, causing self induced emf in them. These fluxes are called leakage fluxes.

16.How do you reduce hysteresis loss in a transformer? Hysteresis loss can be reduced by selecting suitable core material.

Silicon steel is having less Stein Metz hysteresis coefficient.



17.The efficiency of a transformer is always higher than that of rotating electrical machines. Why?

In rotating machines, there are mechanical losses (frictional and windage losses) due to the rotating parts. As there is no rotating part in a transformer, efficiency of transformer is always higher than rotating electric machines.

18. Explain on the material used for core construction. The core is constructed of transformer sheet steel laminations

assembled to provide a continuous magnetic path with a minimum of air gap included. The steel used is of high silicon content sometimes heat treated to produce a high permeability and a low hysteresis loss at the usual operating flux densities. The eddy current loss is minimized by laminating the core, the laminations being insulated from each other by light coat of core-plate vanish or by an oxide layer on the surface .the thickness of laminations varies from 0.35 mm for a frequency of 59 Hz and 0.5 mm for a frequency of 25 Hz.

19. List the advantages of stepped core arrangement in a transformer. To reduce the space effectively To obtain reduced length of mean turn of the windings To reduce I2R loss

20. Why are breathers used in transformers? Breathers are used to entrap the atmospheric moisture and thereby

not allowing it to pass on to the transformer oil. It is also used to permit the oil inside the tank to expand and contract as its temperature increases and decreases. Also to avoid sledging of oil i.e. decomposition of oil, breather is used. Addition of 8 parts of water in 1000000 reduces the insulation quantity of oil. Normally silica gel in pink colour is filled in the breather. This colour will be changed to white due to continuous usage, which is an indication of degradation in absorption property. Then it is normally heated and reused.

21. When will a Bucholz relay operate in a transformer? Bucholz rely is a protective device in a transformer. If the temperature of the coil exceeds its limit, Bucholz relay operates and gives an alarm.

22. What is the function of transformer oil in a transformer? Nowadays instead of natural mineral oil, synthetic oils known as

ASKRELS (trade name) are used. They are noninflammable; under an electric arc do not decompose to produce inflammable gases. PYROCOLOR oil possesses high dielectric strength. Hence it can be said that transformer oil provides, (i) good insulation and (ii) cooling.



23. A 1100/400 V, 50 Hz single phase transformer has 100 turns on the secondary winding. Calculate the number of turns on its primary.

We know V1 / V2 = K = N2 / N1

Substituting 400/1100 = 100/N1 N1 = 100/400 x 1100 = 275 turns

24. What are the applications of step-up and step-down transformers? Step-up transformers are used in generating stations. Normally the

generated voltage will be either 11 kV or 22 kV. This voltage is stepped up to 110 kV or 220 kV or 400 kV and transmitted through transmission lines. (In short it may be called as sending end).

Step-down transformers are used in receiving stations. The voltage are again stepped down to 11 kV or 22 kV and transmitted through feeders. (In short it may be called as receiving end). Further these 11 kV or 22kV are stepped down to 3 phase 400 V by means of a distribution transformer and made available at consumer premises. The transformers used at generating stations and receiving stations are called power transformers.



UNIT V

TESTING OF DC MACHINES AND TRANSFORMERS

1. State the condition for maximum efficiency.Constant Losses = Variable Losses

2. State the advantages of Swinburne’s test. No load Test Less power consumed Efficiency can be predetermined for both motor and generator

3. Is it possible to conduct Swinburne’s test on DC series motor? Justify.No. Because Swinburne’s Test is No-load Test. No-load Test cannot be performed on DC series motor.

4. Does the transformer draw any current when secondary is open? Why? Yes, it (primary) will draw the current from the main supply in order to

magnetise the core and to supply iron and copper losses on no load. There will not be any current in the secondary since secondary is open.

5. What do you mean by no-load current of a transformer? It is the input drawn by a transformer from the supply mains when its

secondary windings are kept opened (or) no-load is connected to secondary.

6. What are the functions of no-load current in a transformer? No-load current produces flux and supplies iron loss and copper loss on no-load.

7. Why the iron losses in a transformer are independent of the load current? The hysteresis and eddy current losses depend upon the maximum flux

density in the core and the frequency. Since from no-load to full-load the flux linking with the core and the supply frequency remains constant, these losses remain constant, i.e., iron loss is independent of load current.

8. How does change in frequency affect the operation of a given transformer? With a change in frequency, iron loss, copper loss, regulation, efficiency

and heating varies and thereby the operation of the transformer is affected.

9. How will you transfer the quantities from one circuit to another circuit in a transformer?1. Secondary to primary 2. Primary to secondary Symbol Value Symbol Value



V2 \ V2/k VL \ kV1 I 2\ kI2 IL \ I1/k R2 \ R2/k2 RL \ k2R1 X2 \ X2/k2 XL’ k2X1 ZL \ ZL/k2

10.Define voltage regulation of a transformer When a transformer is loaded with a constant primary voltage, the

secondary voltage decreases for lagging power factor load, and increases for leading pf load because of its internal resistance and leakage reactance. The change in secondary terminal voltage from no load to full load expressed as a percentage of no load or full load voltage is termed as regulation.

% regulation down = (0V2-V2) x 100/0V2

% regulation up = (0V2-V2) x 100/V2

11.Can the voltage regulation of a transformer go to negative? If so under what condition?

Yes. If the load has leading power factor.

12.Full load copper loss in a transformer is 1600 watts. What will be the loss at half load?

If x is the ratio of actual load to full load then copper loss = x2 (full load copper loss). Here Wc = (0.5)2 x 1600 = 400 Watts.

13.What is the angle by which no-load current will lag the ideal applied voltage? In an ideal transformer, there are no copper loss and no core loss,

(i.e. loss free core). The no load current is only magnetizing current. Therefore the no-load current lags behind by an angle of 90˚. However the windings possess resistance and leakage reactance and therefore the no-load current lags the applied voltage slightly less than 90˚.

14.Distinguish between power transformer and distribution transformer. Power transformers have very high power ratings in the order of MVA.

They are used in generating and receiving stations. Sophisticated controls are required. Voltage ranges will be very high.

Distribution transformers are used in consumer side. Voltage levels will be medium. Power ranging will be small in order of kVA. Complicated controls are not needed.

15.What is the purpose of providing ‘taps’ in transformer and where these are provided?

In order to attain the required voltage, ‘taps’ are provided. Normally it will be provided at low voltage side.



16. What are the advantages of 3-phase transformers over 3 numbers of single phase transformers?oA single phase transformer occupies less space than installing 3

numbers of 1-phase transformers of equal capacity.oCost of single 3-phase transformer is less than 3 numbers of 1 phase

transformers.o It is enough to install only one transformer.

17.State the conditions under which OC and SC tests are conducted in a transformer.

Since no load current, I0 is very small, pressure coils of the wattmeter and the voltmeter should be connected such that the current taken by them should not flow through the current coil of the wattmeter. The transformer should be energized with rated voltage when conducting OC test.

For conducting SC test, the secondary winding should be short-circuited and a very low voltage should be given for the circulation of full-load current.

18. What is the purpose of conducting OC test on a transformer?oTo find shunt branch elements of the equivalent circuit (Ro & Xo)oTo find iron loss at rated voltage.

19.What is the purpose of conducting SC test on a transformer?oTo find total resistance, impedance and hence reactance referred to a

particular side.oTo find full load copper loss.

20. Why OC test is generally performed on LV side of a transformer?The test voltage required, the rated voltage is less on LV side. This will

also permit to use voltmeter and wattmeter of lower voltage range.

21. Why SC test is generally performed on HV side of a transformer?The rated current is less on HV side. This will also permit to use

ammeter and wattmeter of lower current range.

22.What are the advantages of OC and SC tests of a transformer over the load test?

The performance of a transformer can be calculated on the basis of its equivalent circuit, which contains R0, X0, R01 and X01 (or R02 and X02 referred to secondary) These constants or parameters can be easily determined by the OC and SC tests. These tests are very economical and convenient because they furnish the required information without actually loading the transformer.

23. What is the condition for obtaining maximum efficiency of a transformer?



The efficiency will be maximum when the variable losses are equal to the constant (iron) losses.

24. What do you understand by all-day efficiency?The ordinary or commercial efficiency of a transformer is given by the

ratio between output (in Watts) and the input (in Watts) but the performance of a distribution transformer is determined by the all-day efficiency. It is defined as the total energy output (kWH) in 24 hrs a day to the total energy input in kWH for the same 24 hours of the day.

25. List the merits of an autotransformer.o Continuously varying voltage can be obtained.o Requires less copper and is more efficient.o Voltage regulation is superior.

26. What are the components of magnetic losses in transformer and on what factors they depend?

i) Hysteresis loss Wh = Ch Bm1.6 f (Watts)

ii) Eddy current loss We = Ce Bm2 f2 (Watts)

Iron losses Wi = Wh + We Hence magnetic losses depend on flux density in the core and the supply frequency.

27. Why are iron losses considered as constant losses in transformer?Iron losses depend on supply frequency and flux density in the core. For

all normal operations, the frequency of flux reversals which is same as supply frequency is constant and the value of flux density more or less remains constant. Hence iron losses remain constant under all load conditions. i-e from no-load to full-load.

28. What are the various losses in a transformer?i) Magnetic losses / Iron losses – Constant lossesii) Eelctric losses / Copper losses – Variable losses

29.Why the range of efficiency is is transformers higher than those of other electrical machines?

Transformers operate at higher efficiency when compared to other electrical machines. This is due to the absence of mechanical losses which is due to the absence of moving parts.


comprehensive viva

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