Electrical MCQ Induction motor 1-3

1). An induction motor has a rotor resistance of 0.002Ω/phase. If the resistance increased to 0.004Ω/phase then the maximum torque will

a)    Reduced to half

b)    increased by 100%

c)    Increased by 200%

d)    Remain unaltered

Answer: d) Maximum torque ∝ V². When the supply voltage is low, the starting torque is low and when the poles on the stator and rotor are equal,due to locking tendency motor caused by the slot harmonics reduces to start

the maximum torque is independent of rotor resistance and is inversely proportional to rotor reactance. So ,If we incease the rotor resistance the maximum torque is unaltered

2). The power factor of squirrel cage induction motor is

a)    low at light loads only

b)    low at heavy load only

c)    Both a and b   

d)    Low at rated load only  

Answer: a) The operating power factor on no load or light loads is very low, because of magnetizing current. The losses on no load are only core losses which are negligible. Therefore magnetizing current is greater than core loss component current and the no load power factor is very low around 0.2 lagging. as the load increases the core loss component current will increase and the power factor will improve.

3). Cogging of induction motor occurs at

a)    High voltage and when stator and rotor poles are equal    

b)    Low voltage and when stator and rotor poles are equal

c)    High voltage and when stator and rotor poles are not equal

d)    Low voltage and when stator and rotor poles are not equal

Answer: b) starting Torque ∝ V². When the supply voltage is low, the starting torque is low and when the poles on stator and rotor are equal, due to locking tendency motor caused by the slot harmonics reduced to start.

4). Cogging and crawling are phenomena associated with

a)    cage induction machines and they are essentially the same   

b)    the former during starting and the latter at a fraction of its rated speed

c)    the former at a fraction of its rated speed and the latter during starting

d)    any of the above

Answer: b)

5).Cogging in the induction motor occurs when

a)    number of stator teeth - number of rotor teeth = odd number  

b)    number of stator teeth - number of rotor teeth = even number

c)    number of stator teeth - number of rotor teeth = 0

d)    number of stator teeth - number of rotor teeth = negative number

Answer: c)

6). A 4 pole 50Hz induction motor is running at 1470 rmp. Find the speed of stator magnetic field with respect to rotor?

a)    1470 rpm

b)    1500 rpm

c)    30 rpm

d)    zero

Answer: c) The relative speed of stator magnetic field with respect to rotor id Ns-Nr

7).No load test of 3-phase induction motor gives which of the following ?

a)    constant loss  

b)    variable loss

c)    both a and b

d)    non of the above

Answer: a)

8). An induction motor is , in general analogous to

a)    Two winding transformer with secondary slot circuited

b)    Two winding transformer with secondary open circuited

c)    Auto transformer

d)    non of the above

Answer: a)

9). A 3-phase slip ring induction motor is wound for 4 poles on stator and 6 poles on the rotor. when 3-phase balanced voltage source at 50Hz is applied, it will run at

a)    1500 rpm

b)    1000 rpm

c)    zero speed    

d)    750 rpm

Answer: c) To avoid cogging effect in slip ring induction motor the condition are i) The number of poles on the rotor should not be equal to stator poles ii) Rotor poles are always less than the stator poles iii) Rotor poles also should not be in integral multiple of stator slots. For all the above conditions the motor will not start at all hence speed will be zero

10). A voltmeter gives 120 oscillations per minute when connected to rotor of an induction motor. The frequency is 50Hz. The slip of the motor is

a)    2%

b)    5%

c)    4%

d)    25%

Answer: c)

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