1. Conduct a suitable test on the given dc series motor and draw the Electrical characteristics and determine the torque developed at half rated output.  ANS: Conduct actual loading test(Break test) and draw the characteristics and from it determine the torque corresponding to half rated output 2. Conduct a suitable test on the given dc shunt generator and determine the critical resistance. 

Ans: Conduct the OCC test on the dc generator. Draw a tangent to the initial part of the OCC… The slope of that line gives the critical resistance

  3. Conduct a suitable test on the given dc shunt generator, draw the internal characteristics and find out E.M.F induced when the load current is 10A.

4.Plot the mechanical characteristics of the given d.c series motor by conducting a suitable test and determine the efficiency at 3/4th full load. 

ANS: Conduct actual loading test(Break test) and draw the characteristics and from it determine the torque corresponding to half rated output

  5 Draw the equivalent circuit of the given single phase transformer by conducting a suitable test. 

ANS: Perform OCC and SCC and obtain the circuit parameters and draw the circuit 6.Pre determine the efficiency of the given single phase transformer at 3/4th full load. 

find out no load losses n full load copper losses frm OC n SC tests 

eff = output/(output + iron loss + (3/4)^2*Cu loss) 7.Conduct a suitable test on the given dc shunt generator and draw the load curve. Also find out the voltage drop across the load at ¾ th full load 8.Determine the regulation of the given d.c compound Generator at ¾th full load current. 9.Conduct a suitable test on the given dc machine and predetermine the O.C.C at ¾ th rated speed and find voltage induced at 110% of its excitations under open circuit condition. 

Perform OCC at 1500*0.75 rpm 10. Find the critical speed of the given d.c machine by conducting a suitable test. 

A11.Predetermine the efficiency of the given d.c machine by conducting a suitable test at half rated load. 12.Draw the equivalent circuit of the given three phase transformer by conducting a suitable test 13. Pre determine the regulation of the given single phase transformer at 3/4th full load and 0.8pf leading. 

Use formula where cos Φ = 0.8 and current + .75 full load current.

14. Conduct a suitable test and determine the fraction of load at which maximum efficiency occurs in the given single phase transformer.  

Load corresponding to max efficiency =( full load* √(iron loss/ full load copper loss)) 15. Pre determine the maximum efficiency of the given single phase transformer. 

Ans: By conducting SC and OC tests, find out the iron loss and full load Cu loss. Then, find the load at which CU loss = iron loss by:

 (x^2)*Full load cu loss = iron loss..

 Where ‘x’ is the fraction of full load current.

 Find out output = x* rated output. Find efficiency = output/(output +iron loss+ cu loss)

 16. Conduct a suitable test and measure the reactive power using single wattmeter for the given balanced 3 phase load.

17. Conduct a suitable test and determine the power factor of the given 3 phase balanced load.  18. Conduct a suitable test and determine the torque at 2% slip on the given 3 phase induction motor.  19. Conduct load test on the given 3 phase induction motor and determine the efficiency at half the rated input current. (Use star-delta starter for starting) 20. Conduct a suitable test and determine the efficiency at half rated load on the given 3 phase induction motor.  21. Conduct a suitable test and separate the different losses on the given dc machines

22. Conduct a suitable test and separate the different losses on the given single phase transformer. 23. Find regulation of single phase Transformer at zero pf 

ANS : sc test     

DC MOTOR VIVA QNS. 1) How can be the direction of rotation of motor be reversed? 2) If the direction of terminals of a dc series is reversed, what will happen? 3) What will happen if ac supply is given to: a) dc shunt motor b) dc series motor. Explain your answer 4) What is the need for starter for a dc motor? 5) What will happen if a shunt motor is connected to the supply line directly? 6) A dc motor is found to stop running after a short period of time.What do think could be the possible causes? How can you remedy each? 7) Comment on the running of a a)dc series b) shunt motor when supplied with only half the rated voltage 8)What will happen to the losses in a dc motor when its supply voltage is: a) doubled b) halved  9) Which is the most efficient method of increasing the speed of a 3.75kW dc shunt motor? 10) What is the main advantage of Ward-Leonard system used for speed control? 11) What are the various speed control methods used to bring down speed of a motor from rated? 12) Give three main applications of dc shunt and series motors 13) Where are dc compound motors used? 14) Which motor is preferred for traction purposes? Why? 15) What is meant by regenerative braking? What are its advantages? Give one of itsmain applications

  16) What is the nature of the emf wave produced in the armature conductors of a dc motor? 17) Give the type of dc motor used in the following cases: a) lathes b) cranes and hoists c) shears and punches d) machine tools 18) Why is the shaft torque of a dc motor less than its armature torque? 19) Sketch the N-T characteristics of dc series and shunt motors 20) Explain the significance of the back emf in a dc motor 21)What is meant by the shaft torque of a motor? 22) In case of dc shunt motors, can flux control be used to reduce the speed below rated speed? 23) Where are differentially compounded shunt motors used? 24) What may be the causes of sparking in a motor? 25) Can a dc motor be made to work as a generator? Discuss the relative ratings in both cases(ie which will be greater etc) 26) Explain the characteristics of all types of dc motors and explain their applications. 27)What will happen if field of a dc shunt motor, running at no load, gets opened accidentally? 28) What will happen if field of a dc shunt motor, running at loaded condition, gets opened accidentally? 29) Why is the speed of dc shunt motor constant? 30)Why is field rheostat in dc shunt motor circuit kept minimum during starting? 31)What are the speed control methods for obtaining speeds greater than rated speed? and for speeds less than rated speed ?

Electrical Lab Viva Questions DC Machines Viva Questions-DC machines 1.Why Series motors cannot be started without load?2. Advantages of DC motors over AC motors3. What are the applications of DC series motors?4. What is back emf and its significance in DC motor?5. How speed is controlled in DC shunt motor?6. Why starter is required in DC motor for starting?

7. What is critical resistance in DC motor?8. What are the different losses in DC motor?9. What is Armature Reaction in DC motor and its impact?10. What is Commutation failure?11. Advantages of Dummy windings in DC motors?12. On what principle does DC motor operates?13. What are the applications of DC Shunt motors?14. What is the typical brush drop in DC motors?15. Draw different characteristic curves of DC motors?16. By carrying out brake test on DC motors what we can understand?17. Explain Fleming's Left hand and Right hand rule and which rule applies to motor and Generator

 DC Generator   

1. What is meant by armature reaction? What are the components of armature reaction2. Which law does d c generator work on3. What is meant by commutator.4. What are brushes made of .5. What are the different types of DC windings6. When are lap and wave windings used.7. What are advantages of Lap windings over wave8. What are the different types of dc generators9. What are the applications of DC shunt generators?10. What are the applications of DC series generators?11. What are the applications of DC compound generators?(differential and commutatively)12. What are the various losses in a DC machine.13. Give the condition for maximum efficiency14. What are compensating windings.15. What are compoles.16. What is meant by commutation .17. what is the reason for sparking in dc machines .18. How can this be over come ?( methods to obtain proper commutation)19. Why do we require equalizing connections.20. Why parallel operation of generators required? What are the conditions for parallel operation?21. Which type of generators is used for parallel operation.22. Define critical resistance of a DC generator.23. what is meant by critical speed.24. What are the conditions for build up of voltage in a shunt generator.25. Explain the load characteristics(external characteristics) of a shunt generator.26. When are series generators used.27. Why is the field rheostat of dc shunt generator kept at maximum value during start.

  

 

Transformer   

1. Why rating of a transformer is in KVA.2. What all factors decides the iron losses in a transformer?( Ans. Frequency, Flux density,

volume of iron used)3. What factors decides the copper losses in a transformer? (load current. Power factor of

load)4. Could a 50 Hz transformer be used on a 60 Hz supply? If so what would happen?(yes but iron losses becomes 11 % higher)5. What is the condition for maximum efficiency of a transformer? What is the load current

at this condition?(copper loss=iron losses)6. Why do you need to do Sumpners test ?(Copper and iron loss occurring simultaneously

so even temperature rise of a transformer under actual conditions can be estimated, while in SCC and OCC either copper or iron loss occurs while under real operating conditions both occurs)

7. What would be the magnitude of no load current of a transformer?(10% full load current)8. Why does a no load current flow ? (or does current flow when secondary is open)9. Are copper losses affected by the load power factor?( yes , because load current changes)10. How do you determine polarity of a transformer ?11. What is regulation of a transformer ?12. Which is a better transformer, one with a) regulation=5% or b) regulation =10% ?( ans. Transformer (a)) )

 13. Draw a phasor diagram of a transformer(under different conditions, no load, loaded etc.)14. Difference between Core type and Shell type of transformers, advantages of each and

when each are used .15. What is Buccholz relay ? (To detect moisture and formation of gases inside tank due to

insulation break down or due to short circuit between phase windings.)16. Why are cores of transformers laminated ?17. What are Askarels ?(transformer oils)18. What are advantages of stepped core ?19. What are the different methods of cooling a transformer ?20. why does a transformer produce a humming sound ?21. Give the EMF equation of a transformer.22. Why is the OCC test conducted in the low voltage side and high voltage left open? ( as in

text + ratings of the instruments are reduced, if we hav to conduct in HV side we would have required larger rating of voltmeter)

23. Why is SCC performed in the HV side and keep HV side shorted ? 

24. How can you separate the Hysterisis and Eddy current losses from OCC test ?25. Why are OCC and SCC conducted to determine equivalent circuit while direct26. loading is not performed? (Ans. Energy consumption is large and difficulty to arrange

very large loads)27. What is the approximate efficiency of a transformer? (Ans. 98, because no moving parts)28. Is efficiency affected by power factor of the load ?( see quest 9)29. What is all day efficiency? For which type of transformer is it specified ?30. What are auto transformers ? How are they advantageous ?31. What are the conditions for parallel operation of transformers ?32. Why is parallel operation necessary?33. What happens if the conditions are not satisfied ?34. What are advantages and when are the different configurations of three phase

transformers preferred? (delta-delta, star-delta, star-star, delta-star)35. What is open delta configuration?36. Why is transmission done in Star configuration ?37. What are instrument transformers? Why are they used?38. Can a transformer be used with d.c. supply ?39. Why is Scott connection used?40. What is meant by tertiary winding of transformer? Why are they used?(Ans. To supply station auxiliaries with low voltages, to connect static capacitors for power factor correction, to reduce impedance offered to zero sequence currents during earth fault, for interconnection between transmission lines, for measurement purposes, used for stabilization.)

1.What is the need for starters?

If a DC motor is started by simply switching it directly on to the power supply, it will draw a much higher current than the normal running current. Small motors can tolerate a brief surge of high starting current, butlarger motors may be damaged if started without limiting the current. A DC motor starter inserts resistors inseries with the motor to limit the current. As the motor accelerates, the resistance is reduced in several stepsand then removed. The starter circuit includes several resistors connected in series and several large relaysthat are closed one at a time to short circuit the resistors.2.Which is the basic protective device in any circuit?Fuseis the basic protective device in any circuit. It is always connected in series between the source and the load.When the flow of electricity exceeds its designed threshold, the fuse is calibrated to permanently open the seriescircuit thereby disconnecting the source from the load. As a result of which the equipment is prevented from damage.3.Name the material used for Fuse.Generally fuse wires are made fromlow temperature melting point materials like lead or tin, Aluminum or copper.4.Why is fuse rated for amperes?The amount of current that the fuse is supposed to stop (by melting) is called the ampererating of the fuse. Therefore the fuse stops the current not the power. Hence it is rated interms of Amperes not watts. 5.What are the protective devices incorporated in DC starters?6.Why the field rheostat of a dc motor kept at minimum resistance position and that of Generator is inmaximum resistance position?7.Why is the Armature rheostat of dc motor kept at maximum resistance position?8.Write the EMF equation of DC and AC machine.EMF equation of DC generator:EMF equation of the Alternator:9.Write the torque equation of DC motor 10.Draw the torque-slip characteristics of 3phase induction motor and explain. 

10.How are the ammeters and voltmeters connected in any circuit?Ammeter- SeriesVoltmeter- Parallel.11.What is the difference between MI and MC type instruments?MI ; Moving coil , used for Ac supply.MC ; Moving galvanometer, used for Dc supply

12.What is the difference between self-excited and separately excited machines?Self-excited generator works on its own feedback whereas separately excited generator need aexternal source to work13.Give the applications of each type of dc motor.  Series Motor:High starting Torque, variable speed for fraction work e.g. electrical locomotives, cranes and hoist trolley, cars,conveyors etc.shunt Motor:Approximately constant speed, medium starting torque during constant speed like boiler and fans, lathe,reciprocating pumps, centrifugal pumps, machine tools.Compound motors:Variable speed, high starting torquea.for punches , elevators, conveyorsb.Heavy planer, ice making machine, air compressor.c.Rolling Hills. Printing press for intermittent high torque load12.What are the losses in dc machine?Cu losses Armature cu lossesShunt Field Cu losses(constant)Series Field Cu lossesIron or Magnetic or Core losses Hysteresis Losses(constant)Eddy current Losses(constant)Machanical Losses Friction losses (constant)Windage losses (constant)13.How is the eddy current loss minimized in dc machine?Eddy currents are minimized in these devices by selecting magnetic core materials that have low electricalconductivity (e.g., ferrites) or by using thin sheets of magnetic material, known as laminations. Electronscannot cross the insulating gap between the laminations and so are unable to circulate on wide arcs 22.Name the parts of dc machine23.What is the use of Commutator and brushes?Commutator: Converts bidirectional current (AC) to unidirectional current (DC)Brushes: It collects the current from the commutator and delivers it to the externalLoad.25.Why is armature resistance less than field resistance of dc shunt machine?26.Why is armature resistance more than field resistance of dc series machine?27.Why is series motor best suited for traction applications?High starting torque and variable speed

31.What are the various types of transformer?a.Core typeb. Shell typec. Spiral core transformerd. Oil filled self cooled.e.f.Air blast typeg.Oil filled water cooled32.What are the methods of speed control of dc motors?Flux control Method and armature controlled method34.What is the difference between two winding transformer and auto transformer?Two winding Transformer Auto transformerHas primary winding and secondary winding Has only one winding

Required more cu for windingsRequired less cu for windings35.Why transformers and alternators are rated with kVA?36.What is meant by power factor? ExplainPower factor = (RMS value) /(Average power)37.What will happen if dc supply is connected to transformer?The transformer works on variable flux (A C flux, RMS value is constant) If we give DC inputthe flux generated is stationary and cannot develop the induced EMF in primary(E1)&secondary (E2). Sothere is no opposition of current flowing primary & Secondary(since E1, E2=0).Therefore high currentpasses through windings and hence burns the windings.38.What are the parts of transformer?Iron core, windings (Both primary and secondary), insulating material and bushings.39.Why single phase induction motors not self-starting? Classify them according to theStarting methods.40.Why is regulation so important for transformers and alternators?41.What are the parts of synchronous machine?Name the material used in various parts42Why is synchronous motor referred as doubly excited machine?49.What is slip of an induction motor?The difference between the synchronous speed (Ns) and the actual speed of the rotor N is called as slip in theinduction motor.59.What is the principle of motor, generator and transformer?Generator: Electromagnetic induction.Motor : Faradays law of inductionTransformer: Mutual Induction60.How will you find the direction of induced emf in generator and the direction of motionof the conductor in motor?Flemings Left hand rule.62.What is the difference between rheostat and potentiometer?The fundamental difference between a potentiometer and a rheostat is that the formeris a three terminal device, while the latter is a two terminal one.The potentiometer has terminals at each end of the resistive element, and therheostat has a terminal at only one end only. Lastly, a pot can be used in place of a rheostat but vice-versa not possible.63.What is L, F, A,N in dc starters?64.What is M, L, C, and V in wattmeter?65.What is LPF, UPF and ZPF?66.What is E and C in autotransformer?68.What are the advantages and disadvantages of autotransformer?The main advantage is thatthe designsaves on copper by using part of the HV side winding for the LV side. This allows autotransformers to be built in somewhat greater unit sizes for the samephysical size.The disadvantage is

thatthe advantageruns out if the voltage ratio is high. I.e. the design is

interesting if the turns ratio is say 2 to 1 but if it is 10 or more to 1 the savings go lost in the detaildesign problems. 71.Why are electrical loads used for generators and motors whereas mechanical loads formotors?76.What is meant by magnetizing current and working component of current with respect totransformer?77.Give the relation between line and phase values of i) star connected network ii) deltaconnected network78.How can the hysteresis loss of any machine be minimized?

Why Induction motors are widely used in Industries?Answer: In Industries 80% of the motor drives used are induction motors. Induction Motors are widely used in Industries because of some features it possess. They are:

Low Cost Simple and Rugged Construction Absence of Commutator  Good power factor Higher efficiency Good Speed Regulation

What is the relation between between  Induction motor and Transformer?Answer: Induction motors are considered as transformers with secondary winding rotating. This is because both transformers and induction motor operates in such a manner that power is transferred to secondary windings (rotor) from primary windings (stator) by mutual induction principle.

Why the efficiency of the Induction motor is less compared to transformer?Answer: Induction motor and Transformer operates on the same principle that power is transferred to secondary by mutual induction. But in transformer, flux will be traveled from primary to secondary winding through the core of the transformer. In induction motor flux travels from stator to rotor through the air gap. Therefore power factor will be less for induction motor compared to transformer which also results in less efficiency for induction motor compared to transformer.

Why Speed Control of Induction Motor is difficult compared to DC shunt motor?Answer: Speed Control of DC shunt motor can be possible by providing resistance in the armature and field circuits and varying the resistors speed changes can be carried out over a wide range. On the other hand speed control of induction motor is costlier and bit tedious. Speed of Induction motors = (120x f x (1-s))/p. Therefore for speed control either supply frequency should change or number of poles should change and other methods can be employed. Care should be taken that in the process efficiency and power factor should be maintained. 

Explain the operation of induction motor in simple words?Answer: When 3 phase ac supply is given to stator of the induction motor rotating magnetic field will be generated. This magnetic field (flux) cuts the stationary conductors of the rotor of induction motor.

Therefore an emf is induced by Faraday's law of electro-magentism. As the rotor conductors are short circuited, current starts flowing in the windings of the rotor this produces a magnetic field. Due to interaction of both the magnetic fields a torque will be developed. If the torque generated is sufficient to overcome the load torque rotor starts rotating. 

12 December 2011

DC Machines Objective test Questions 1.In DC Generator the critical resistance refers to:

a) Armature

b) Field

c) Load

d) Commutator

Answer

2.In DC Generator the residual magnetism will be of the order:

a) 2.5%

b) 10%

c) 15%

d) 20%

Answer

3.Sparkless commutation can be achieved by employing

a) Interpoles

b) Compensating winding

c) High resistance corbon brushes

d) All the above

Answer

4.Comutator in DC machine acts as:

a) Full wave rectifier

b) Half wave rectifier

c) Controlled full wave rectifier

d) Controlled half wave rectifier

Answer

5. In DC machine, commuataor converts

a) dc to ac

b) ac to dc

c) both ac to dc and dc to ac

d) None of the above

Answer

6. Which type of emf is induced in the dc generator

a) Dynamically

b) Static

c) Electrostatic

d) Magentic

Answer

7. If a current carrying conductor is placed in the uniform magnetic field with its plane perpendicular to the direction of magnetic induction then,

a) The net force and torque on the coil are zero

b) net force is zero and torque is finite

c) net force is finite and torque is zero

d) the net force and torque are finite

Answer

8. In DC machine

a) The current and electromagentic force in armature conductors are alternating while those at the terminals are uni-directional

b) The current and emf in armature conductors are uni directional while those at the terminals are alternating

c) The current and the emf in armature conductors and at terminals are unidirectional

d) The emf in armature conductors and at the terminals is alternating while current is uni directional

Answer

9. The induced voltage in a single loop reverses:

a) Once in each revolution

b) Once in each half revolution

c) Once in each one quarter revolution

d) Once in each two revolutions

Answer

10. The yoke of the dc machine is made of:____ magnetic material

a) soft

b) hard

c) non

d) very good

Answer

DC machines Lab Viva Questions

When DC Generator fails to build up the voltage, what are the reasons?

Answer: No residual magnetism, reversal of field connections and the resistance in the load circuit may be greater than critical resistance

What is field flashing?

Answer: In case when the dc machine fails to build up the voltage due to lack of residual magnetism, filed coils are connected to the dc source (battery) for short while for magnetizing the filed poles. Application of external source of direct current to the field of the dc machine is called field flashing

Why do we use starter for dc machine?

Answer: At the time of starting, no back emf is offered by the dc machine, hence huge starting currents flow in to the machine which damage the armature, in order to avoid the damage starter is employed. By using starter armature current is limited during starting the machine by providing external resistance and while starting this external resistance is removed in steps 

Factor that has to be considered while choosing the resistor?

Answer: While choosing the resistor for connecting in filed or armature circuit care should be taken that the resistor current rating is more than the full load current rating of the circuit. For example, if rated field current is about 2Amps. Then resistor connecting in the field circuit should be chosen in such a manner that the current rating of the resistor is greater than 2Amps.

What are the different losses in dc machine?

Answer: 

Armature and field copper loss Iron and magnetic losses (hysteresis and eddy current loss) Mechanical losses Stray-load losses

Drawbacks of Brake test?

Answer: Some of the drawbacks of the brake test are:

This test is performed with small motors only Internal losses cannot be determined

Induction Machine Viva Questions Define Slip in Induction Machine?Answer: Slip is the difference between the speed of the stator rotating magnetic field and the speed of the rotor of the induction motor expressed as a fraction of synchronous speed is known as slip of the induction motor.Why Induction Machine always runs below synchronous speedAnswer: Induction motor works on the principle of induction. When both rotating magnetic field of stator and rotor of induction motor rotates with same speed no emf is induced in the rotor conductors resulting no torque is produced in the rotor of induction machine and motor will stop. Therefore in order to have torque for rotating the rotor induced emf is required. Hence Induction motor rotates with speeds less than synchronous speed.How direction of 3 phase Induction Motor can be changedAnswer: Direction of 3 phase induction motor can be changed by changing any two supply phases of the induction motor. Changing any two phases will result in change in rotating magentic field to opposite direction. Therefore rotor direction is also changed.What are the different starting methods of 3 phase induction motor?Answer: Different starting methods employed while starting 3 phase induction motors are:

Direct on line starting method Primary resistance starting method Auto transformer starting Star-Delta Starting method

When we can get Maximum Torque in Induction Motor while starting?Answer: Starting torque for 3 phase induction motor is low. In order to increase the starting torque an external resistance should be inserted while starting of the 3 phase induction motor. It is to be remembered that starting torque of slip ring induction motor is high compared to squirrel cage rotor machine due to the provision of adding external resistance while starting the machine.What are the different losses in 3 phase induction motor?

Answer: Different losses in the 3 phase induction motor are stator copper loss, stator iron loss, rotor copper loss, friction and windage loss and stray load loss.What is the relation between torque and supply voltage in 3 phase induction motor?Answer:  Torque T = sV2

Where Star (wye) Delta transformer is used in Power system Star-Delta Transformer (Y-Δ ):Primary Y (High Voltage side) and Secondary ( Low Voltage side) Δ is generally used at the generating side for stepping up the voltage levels. Power is generated at 11kV to 33kV having current ratings in thousands of kilo amps. This generation voltage has to be stepped up to higher voltage for transmission as stepping up the voltage results in the reduction of losses while transmissionGeneration side of the transformer (LV side) voltage applied is low and current flowing through the windings of the transformer is high. At secondary side (LV side), delta connection (Δ) is employed because in delta connection Iphase=ILine/√ 3 and Vphase=VLine.Current carrying capacity of windings depends on the thickness of the conductor. Large current flowing through the conductor means more copper utilised for the windings. Therefore in order to reduce the flow of huge currents by above equation delta winding is employed in the LV side of the transformer. On the other hand star type (Y) of connection is employed on the HV side of the transformer. In star connection Vphase=VLine/√ 3 and Iphase=ILine.Voltage withstand ability of conductor depends on the insulation strength of the conductor. Hence to reduce the phase voltage Star (Y) connection is employed in the high voltage (HV side) of the transformer.