This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Principle of Operation”.
1. The rotor of a three phase induction motor can never attain synchronous speed.
a) True
b) False
View Answer
Explanation: Because then there will be no torque developed and flux cutting.
2. The direction of rmf when a single phase supply is given to stator of three phase induction motor is?
a) Zero
b) Ns
c) 2Ns
d) -Ns
View Answer
Explanation: There will be no rmf generated as a single phase is only fed to the three phase of induction motor.
3. The direction of rmf when a three phase supply is given to stator of three phase induction motor is?
a) Zero
b) Ns
c) 2Ns
d) -Ns
View Answer
Explanation: There will be rotating magnetic field will be produced causing the emf to be induced and the current will flow in the rotor at a speed of synchronous speed.
4. Two three phase induction motors A and B are identical in all respects except that motor A has a larger air-gap than motor B. Which motor will have more no load current?
a) A
b) B
c) Both A and B
d) Neither A nor B
View Answer
Explanation: As the air gap of motor A is more, it will require more flux to excitation and so the no load current.
5. Two three phase induction motors A and B are identical in all respects except that motor A has a larger air-gap than motor B. Which motor will have poorer power factor?
a) A
b) B
c) Both A and B
d) Neither A nor B
View Answer
Explanation: Due to larger air gap of motor A, it will need more of exciting current. So the no load angle will increase and power factor will deteriorate.
6. Two three phase induction motors A and B are identical in all respects except that motor A has a larger air-gap than motor B. Which motor will have better full-load power factor?
a) A
b) B
c) Both A and B
d) Neither A nor B
View Answer
Explanation: At full load conditions, the excitation current is not very high and so the power factor improves.
7. A three phase induction motor is sometimes calles a generalized transformer in so far as voltage and frequency transformation has been concerned. How a three phase induction motor operates when rotor frequency is equal to stator frequency?
a) It will not operate
b) It will operate as induction motor only
c) It will operate as induction generator
d) It will operate in braking mode
View Answer
Explanation: As the rotor and stator frequencies are equal, there will be no slip and so the zero emf will be produced.
8. A three phase induction motor is sometimes called a generalized transformer in so far as voltage and frequency transformation has been concerned. How a three phase induction motor operates when rotor frequency is greater than stator frequency?
a) Rotor will be driven against rmf
b) Rotor will be driven in direction of rmf
c) No emf will be induced
d) The losses will be maximum
View Answer
Explanation: As the rotor rotates at larger speed than the stator, the rmf speed is less than the rotor speed and it tries to rotate in the opposite direction to rmf.
9. A three phase induction motor is sometimes called a generalized transformer in so far as voltage and frequency transformation has been concerned. How a three phase induction motor operates when rotor frequency is less than stator frequency?
a) It operates as induction motor
b) Emf is induced in rotor
c) Rmf will rotate at synchronous speed
d) All of the mentioned
View Answer
Explanation: It will operate as induction motor.
10. A three phase induction motor is sometimes called a generalized transformer in so far as voltage and frequency transformation has been concerned. Then the rotor emf E2 and the rotor current I2 are zero at _________
a) Ns
b) 2Ns
c) Zero
d) Slip of 50%
View Answer
Explanation: The rotor emf and current are zero at the speed of synchronous speed of the machine.
11. During no load test the wattmeter reading will be _________
I. stator copper loss II. stator core loss III. rotor core loss IV. friction and windage losses V. Rotor copper loss
a) I, II, IV
b) II, III, IV
c) I, III, II, IV
d) II, IV
View Answer
Explanation: Blocked rotor test will give stator losses and the friction and windage losses.
12. If use auto transformer method starting to start an induction motor to replace star-delta method, then the required tapping on the transformer will be _________
a) 57.73%
b) 86.7%
c) 57%
d) 66.66%
View Answer
Explanation: k = 1/1.73 = 57.73%.
13. During the blocked rotor test the wattmeter reading will be _________
I. stator copper loss II. stator core loss III. rotor core loss IV. friction and windage losses V. Rotor copper loss
a) I, V
b) II, III, IV
c) I, III, II, IV
d) II, IV
View Answer
Explanation: Blocked rotor test will account for copper losses.
14. The starting method for a 3-phase squirrel-cage induction motor which is inferior in view of poor starting torque per ampere of line current drawn is?
a) series-inductor method of starting
b) direct-on-line starting
c) auto-transformer method
d) star-delta method
View Answer
Explanation: Due to inductor method, reactance increases and the starting torque decreases.
15. Reactor method of starting is preferred over resistance method because _________
I. power factor improves II. increase starting torque III. lower losses IV. more effective in reducing voltage
a) III, IV
b) I, II, III
c) II, III, IV
d) I, II
View Answer
Explanation: By reactor method of starting, losses will be lesser and voltage reduction is effective than the resistance method of starting.
Sanfoundry Global Education & Learning Series – Electrical Machines.
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