# Polyphase Induction Motor MCQ

### 1. In induction motor, percentage slip depends on

(a) supply frequency
(b) supply voltage
(c) copper losses in motor
(d) none of the above

Answer: (c) copper losses in motor

### 2.The efficiency of an induction motor ……….is than that of a transformer.

(a) lower
(b) higher
(c) either of the above
(d) none of the above

### 3.If the rotor circuit of a squirrel cage induction motor is open, the rotor will

(a) run at very high speed
(b) run at very low speed
(c) make noise
(d) not run

### 4.The advantage of a slip-ring induction motor over a squirrel cage induction motor is that

(a) it has higher efficiency
(b) it has higher power factor
(c) it can be started with the help of rotor resistance starter
(d) none of the above

Answer: (c) it can be started with the help of rotor resistance starter

(b) lagging
(c) unity
(d) zero

### 6.A 3-phase slip-ring induction motor is always started with

(a) a starting winding
(b) squirrel cage winding
(c) no external resistance in rotor circuit
(d) full external resistance in rotor circuit

Answer: (d) full external resistance in rotor circuit

### 7.Out of the following methods of starting a 3-phase induction motor, which one requires six stator terminals ?

(a) Direct-on-line
(b) Star-delta
(c) Auto-transformer
(d) Rotor rheostat

### 8.Each of the following statements regarding stator flux of an induction motor is true except

(a) It induces E.M.F. in the rotor bars
(b) Its magnitude depends on the motor load
(c) it is constant in magnitude
(d) it revolves around the stator at synchronous speed

### 9.The rotor of an induction motor cannot run with synchronous speed because

(a) Lenz’s law would be violated
(b) induction motor would then become a synchronous motor
(c) rotor torque would then become zero
(d) air friction prevents it to do so

Answer: (c) rotor torque would then become zero

### 10.When a load on a squirrel-cage induction motor is increased from no-load to full-load, which of the following statement is incorrect ?

(a) Slip increases.
(b) The magnetizing component of the stator exciting current is increased.
(c) Stator power factor is improved.
(d) Torque developed by the motor increases till it equals the applied torque.

Answer: (b) The magnetizing component of the stator exciting current is increased.

(a) 0.2 lag
(b) 0.4 lag
(c) unity

### 12.The principle of operation of a 3-phase induction motor is similar to that of

(a) transformer with a shorted secondary
(b) synchronous motor
(c) capacitor-start induction run motor
(d) repulsion-start induction motor

Answer: (a) transformer with a shorted secondary

### 13.The speed of a 3-phase wound motor can be increased by

short-circuiting slip-rings
(a) increasing applied voltage
(b) decreasing applied voltage
(c) decreasing external resistance in the rotor circuit

Answer: (c) decreasing external resistance in the rotor circuit

### 14.The A.C. motor which would be best suited to drive a centrifugal pump for discharging a variable quantity of water against a fixed had is the

(a) repulsion motor
(b) synchronous motor
(c) squirrel-cage
(d) slip-ring Induction motor

### 15.For starting a Schrage motor, 3-phase supply is connected to

(a) rotor via slip rings
(b) regulating windings
(c) secondary winding via brushes
(d) stator as in an ordinary induction motor

Answer: (a) rotor via slip rings

### 16.The primary winding of a Schrage motor is located

in stator
(a) in lower part of rotor
(b) in upper part of rotor
(c) partly in stator and pertly in rotor

Answer: (a) in lower part of rotor

### 17.When a stationary 3-phase induction motor is switched on with one phase disconnected

(a) It will start but run very slowly.
(c) Remaining intact fuses will be blown out due to heavy rush of current.
(d) It is likely to burn out quickly unless immediately disconnected.

Answer: (d) It is likely to burn out quickly unless immediately disconnected.

### 18.The pull-out torque for a normal squirrel cage induction motor usually occurs at a percentage slip of about

(a) 10 to 15 per cent
(b) 20 to 25 per cent
(c) 36 to 45 per cent
(d) 50 to 60 per cent

Answer: (b) 20 to 25 per cent

### 19.Speed variations or a squirrel cage induction motor are essentially similar to those of

(a) D.C. shunt motor
(b) D.C. series motor
(c) synchronous motor
(d) differential compound motor

### 20.If air gap of the induction motor is increased, its

(a) power factor will decrease
(b) magnetizing current will decrease
(d) magnetizingcurrent will increase
(d) power factor will increase

### 21 . Two of the power supply terminals to a 3-phase induction motor get interchanged during reconnection after maintenance of the motor. When put back into service, the motor will

(a) get heated up and damaged
(b) rotate in the same direction as it was prior to maintenance
(c) fail to rotate
(d) rotate in the reverse direction to that prior to maintenance
(e) none of the above

Answer: (d) rotate in the reverse direction to that prior to maintenance

### 22. In a three-phase induction motor, the relative speed of stator flux with respect to is ………… is zero.

(a) space
(b) rotor
(c) stator winding
(d) rotor flux

### 23. In a 3-phase induction motor reactance under running conditions is less than its standstill value because of decrease in

(a) stator magnetic flux
(b) mutual flux linking the stator and rotor conductor
(c) rotor inductance
(d) frequency of rotor E.M.F.

Answer: (d) frequency of rotor E.M.F.

### 24. In case of a 3-phase induction motor, an increase in rotor resistance affects the motor performance in the following way

(a) the motor efficiency decreases
(b) the motor efficiency increases
(c) starting current decreases
(d) starting current increases
(e) the maximum torque is reduced

### 25. A squirrel cage induction motor running on no-load is loaded, which of the following statements is incorrect ?

(a) Current in the rotor bars decreases
(b) Motor speed decreases
(c) Torque developed by the rotor increases
(d) Stator flux cuts the rotor bars more rapidly
(e) Stator flux keeps rotating synchronously

Answer: (a) Current in the rotor bars decreases

### 26. Which of the following statements about 3-phase induction motor is incorrect ?

(a) It is capable of operation under a wide range of power factors both legging and leading.
(b) It starts up from rest and has not be synchronized.
(c) Its no-load line current may be as large as 40% of the full load current.
(d) It has no commutator and no slip rings.
(e) It has no moving contact between stator and rotor.

Answer: (a) It is capable of operation under a wide range of power factors both legging and leading.

### 27.Which of the following statement about a double squirrel cage induction motor is incorrect ?

(a) It has better speed regulation than squirrel cage motor.
(b) Its upper cage is of low resistance and lower cage of high resistance.
(c) It is particularly useful where frequent starting under heavy loads is required.
(d) It maintains high efficiency during normal operating conditions.

Answer: (b) Its upper cage is of low resistance and lower cage of high resistance.

### 28. When a double squirrel cage motor is started, the current induced in the rotor

(a) flows mostly through the upper winding.
(b) flows mostly through the lower winding.
(c) is directly to the proportional impedance offered by each cage.
(d) is equally divided between the two windings.

Answer: (a) flows mostly through the upper winding.

### 29. Regarding single-phasing of a 3-phase induction motor under running conditions which of the following statement is incorrect ?

(a) It will stop and blow the remaining fuses.
(b) It will continue to run without damage if it is carrying half load or less.
(c) It will stop and promptly burnt out if it is heavily overloaded.
(d) It will try to keep running though over-heating of part of its winding if it is carrying full-load or slight over- load.

Answer: (a) It will stop and blow the remaining fuses.

(a) P/s
(b) sP
(c) s2P
(d) P/s2