7. Questions & Answers on Electromechanical Energy Conversions

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Electrical Machines Interview Questions and Answers by Manish 4-5 minutes

This set of Electrical Machines Interview Questions and Answers focuses on “EMF Generated in the Armature”. 1. If the speed of rotation of a separately excited dc machine is halved and the filed strength is also reduced to half of the original. Then the emf induced in the armature a) reduces to 1/4th of original b) reduces to half of original c) remains same d) doubles View Answer Answer: a Explanation: Emf is proportional to speed of rotation and flux. 2. If the speed of rotation for a four pole, 220V dc machine is 1 m/s, flux density as 2T having length of bore as 0.1m and radius of 0.5m. The emf induced under pole faces will be a) 0.2 V

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b) 0.1 V c) 0.2*pi V d) 220 V View Answer Answer: a Explanation: E = B*l*v = 2*0.1*1 = 0.2 Volts. 3. The induced emf in the armature of a 4-pole dc machine is a) directly proportional to speed and field strength applied to it b) directly proportional to speed c) directly proportional to field strength applied to it d) inversely proportional to speed and inversely proportional to field strength applied to it View Answer Answer: a Explanation: Emf is proportional to speed of rotation and flux. 4. If the armature current is by mistake increased to 50% for the dc machine, a) then the induced emf remains unchanged b) then the induced emf doubles c) then the induced emf halves d) none of the mentioned View Answer Answer: a 2 of 6

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Explanation: Emf is proportional to speed of rotation and flux. 5. For a four pole, 16 slot armature with two coil-side/slot having lap connected progressive winding has single turn coils. The same machine is then wave wound, the emf induced in the lap wound dc machine to that of wave wound will be a) 2 b) 0.5 c) 1 d) 4 View Answer Answer: b Explanation: E=1/(P/2) =1/(4/2) = 1/2 V. 6. A duplex lap wound armature is used in six pole dc machine with six brush set. The number of current paths in this machine is a) 12 b) 6 c) 3 d) 2 View Answer Answer: a Explanation: The number of parallel paths for a lap wound machine is number of parallel paths. As it is a duplex winding, it will have 6*2 = 12 parallel paths. 7. For a four pole, 16 slot armature with two coil-side/slot 3 of 6

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having wave connected progressive winding has single turn coils has simplex winding. The number of current paths a) 2 b) 6 c) 3 d) 4 View Answer Answer: a Explanation: The number of parallel path for a wave wound machine is equal to two. 8. For a four pole, 200 V dc shunt generator having armature resistance of 1 ohms. The current flowing in the armature is 10 A. What is the back emf generated if the terminal voltage of 220 V supply is fed? a) 230 Volts b) 210 Volts c) 190 Volts d) 200 Volts View Answer Answer: a Explanation: E=V+I*ra = 220+10*1 =230 Volts. 9. For a 4 pole 200 V, 20 A dc generator running at the rated rpm with no load will be a) zero no load losses b) 4000 W no load losses

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c) negligible no load losses d) none of the mentioned View Answer Answer: a Explanation: As there is no load and assuming no armature resistance, the no load losses should ideally be zero. 10. Which of the following cannot reduce the terminal voltage of a dc shunt generator? a) Commutation b) Armature reaction c) Armature ohmic losses d) Any of the mentioned View Answer Answer: a Explanation: Commutation ideally does not reduce the terminal voltage of a dc machine. If it takes place properly at desired timings, the losses can be avoided. Armature reaction, ohmic losses contribute to the losses in the terminal voltage. 11. The emf vs speed characteristics of a dc generator is

a)

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b)

c)

d) View Answer Answer: a Explanation: Emf is proportional to speed of rotation. Sanfoundry Global Education & Learning Series – Electrical Machines. To practice all areas of Electrical Machines for Interviews, here is complete set of 1000+ Multiple Choice Questions and Answers. Answers.

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Torque in DC Machines - Electrical Machines Questions and Answers by Manish 5-7 minutes

This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Torque in DC Machines”. 1. Choose the correct representation, which depict a nonzero torque in the armature.

a)

b)

c) d) any of the mentioned View Answer Answer: a Explanation: Torque is produced when the net force is i s zero

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and there is a couple generated. The directions must obey Lorentz’s law and vector product. 2. What is the principle of torque production in a dc machine? a) Lorentz’s law b) Lenz’s law c) Faraday’s law d) Self inductance i nductance View Answer Answer: a Explanation: Though all the mentioned laws contribute towards the torque generation, but primarily its the Lorentz’s forces which initiate and define the torque direction. 3. The torque induced beyond the pole shoes in the DC machine is a) 0 b) 2/pi *phi *i c) 4/pi *phi *i d) none of the mentioned View Answer Answer: a Explanation: The magnetic field vanishes at the pole terminals and thus the cross product with the magnetic field yields zero. 4. The simple rotating loop between pole faces connected to a battery and resistor through a switch, the specifications of this machine are radius = 0.5m, length 1m, resistance = 2 of 8

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0.3 ohms and magnitude strength = 0.25T is supplied with 120V. Suddenly the switch is closed at t=0, what is observed in the circuit? a) Current will flow but zero induced EMF b) Current will not flow and zero induced EMF c) Current will not flow but EMF is induced d) Current will flow and EMF will also be induced View Answer Answer: a Explanation: Initially the induced emf in the armature is zero. But the current will flow as there is terminal source voltage. 5. The simple rotating loop between pole faces connected to a battery and resistor through a switch, the specifications of this machine are radius = 0.5m, length 1m, resistance = 0.3 ohms and magnitude strength = 0.25T is supplied with 120V. What will be the magnitude of the following current at t=0+? a) 400A b) 200A c) 0 d) any of the mentioned View Answer Answer: a Explanation: I=(V-E)/r =(120-0)/0.3 =400 A. 6. If the torque induced is zero in the dc machine, it can be

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said that a) current is zero b) flux can be zero c) current or flux=0 d) any of the mentioned View Answer Answer: a Explanation: The torque at steady state of operation is zero as the emf induced is equal to the source voltage. Also the the very much starting the current will be zero which will not let the emf to get induced. 7. The simple rotating loop between pole faces connected to a battery and resistor through a switch, the specifications of this machine are radius = 0.5m, length 1m, resistance = 0.3 ohms and magnitude strength = 0.25T is supplied with 120V. What is the steady state angular velocity at no-load? a) 480 rad/s b) 960 rad/s c) 320 rad/s d) 490 rad/s View Answer Answer: a Explanation: v = V/(2*r*l*B) = 120/(2*0.0005*1*0.25) = 480 rad/s. 8. If the torque is in the direction of rotation, the DC machine acts as a) generator

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b) motor c) amplidyne d) any of the mentioned View Answer Answer: a Explanation: The electromagnetic torque generated in the dc generator is in the direction in which the prime mover is rotated.It is produced due to the action of armature current and stator field. 9. The cogging torque is absent in the permanent magnet dc machine is due to a) non-magnetic nature of rotor b) magnetic nature of rotor c) absence of ac supply d) any of the mentioned View Answer Answer: a Explanation: It is due to the fact that no field poles are induced in PMDC to create magnetic flux leading to harmonics. 10. DC Motor torque depends on a) geometry b) magnetic properties c) any of the mentioned d) both geometry and magnetic properties of the structure View Answer Answer: d Explanation: The torque will indeed depend on the 5 of 8

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geometry as well the magnetic materials used in the construction of the dc machine. Though this remains constant with the machine. 11. For a universal motor, the rated supply current is 10A giving a rated torque of 100 N-m. If a current of 5A is applied, what will be generated torque vary to? a) 250 N-m b) 400 N-m c) 500 N-m d) 1000 N-m View Answer Answer: b Explanation: The torque is proportional to the square of the current. So the new torque will be 100*(2^2)= 400 N-m. 12. Pulsating torque frequency in universal motor a) 2f b) f c) f/2 d) 4f View Answer Answer: a Explanation: The torque reverses its direction in the negative cycle of the ac supply and thus it always unidirectional. 13. The variation of current in the armature can be best expressed by which of the below curves?

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a)

b)

c)

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d) An impulse signal View Answer Answer: a Explanation: The emf induced is initially zero which causes a high current in the begining and slowly as the emf is build up, the current reduces to zero at steady state. 14. The velocity of the dc machine at the final state is a) E/Bl b) Bl/E c) EBl d) None of the mentioned View Answer Answer: a Explanation: The velocity at steady state is E/Bl. Sanfoundry Global Education & Learning Series – Electrical Machines. To practice all areas of Electrical Machines, here is complete set of 1000+ Multiple Choice Questions and Answers.. Answers

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7. Questions & Answers on Electromechanical Energy Conversions

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