MCQs on Electrical Machines


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01․ To control the speed of an induction motor, the supply frequency is reduced by 10%. For the same magnetizing current to remain constant, the supply voltage must be
reduced by 10%
reduced by 20%
increased by 10%
increased by 20%

Flux ∝ V/f Flux ∝ Im To maintain magnetizing current constant, if frequency is reduced by 10 %, then voltage also to be reduced by 10 %.

02․ In V/f speed control of 3-phase induction motor, with increase in frequency, the slip at which maximum torque occurs
increase.
decrease.
either of these.
none of the above.

By increasing the frequency, speed will increase. As speed increases, slip at which max torque occurs will decrease.

03․ In star-delta starting of squirrel cage induction motor compared to DOL starting the starting current and torque are
1/√3, 1/3
1/3, 1/3
1/√3, 1/√3
1/3, 1/√3

Line current with star-delta starting = I/√3 Line current with DOL starting = √3×I The ratio of Line current with star-delta starting and Line current with star-delta starting is 1/3. Starting torque with star-delta starter = (VL/√3)Β². Starting torque with DOL starter = (VL)Β². So the torque ratio is 1/3. Hence with star-delta starting, the starting torque is also reduced to one third of starting torque obtained with direct switching.

04․ As compared to DOL starting, a cage induction motor with star-delta starting shall have
more starting torque.
more starting time.
reduced starting current.
smoother acceleration.

DOL starting is to start only small rating motors upto 5 HP. There is a large starting current which lasts for few seconds also known as inrush current which does not damage the motor as the motor is predesigned to withstand this inrush current. In DOL starting the starting torque is not affected since there is no reduction in the supply voltage. In star-delta starting the stator winding is connected in star. The voltage impressed in each phase is only 57.7% of line voltage. The starting current also correspondingly reduces to 1/3 and starting torque is also reduced to 1/3.

05․ An induction motor has synchronous speed of 1500 rpm. What will be the slip when it is running at a speed of 1450 rpm?
+3.33%
-3.33%
+3%
-3%


06․ For the purpose of starting an induction motor, star/delta switch is an equivalent to auto transformer of ratio
33.3%
66.3%
50%
57.7%

Star - delta starting is the most simple way and cheapest to start induction motor. It offers one step control only. It requires a tri pole double throw switch, which make the stator winding connected in to star or delta. During starting the stator winding is connected in star. Therefore the voltage impressed in each phase is only 57.7% of line voltage. The starting current is also correspondingly reduced to 57.7% only. As the motor starts, accelerates and running at rated slips, the stator winding is thrown on to delta using tri pole double throw switch, where the voltage across each phase is rated and produce rated torque.

07․ A 4 pole, 3-phase induction motor is running at 4% slip at full load. If the speed of the motor is 720 rpm, the supply frequency is
50 Hz
60 Hz
25 Hz
40 HZ


08․ A voltmeter gives 120 oscillations per minute when connected to the rotor of an induction motor. The frequency is 50Hz. The slip of the motor is
2%
5%
4%
25%


09․ An induction motor is, in general analogous to
two winding transformer with secondary short circuited.
two winding transformer with secondary open circuited.
auto transformer.
none of the above.

The rotor consists of thick laminations, made up of silicon steel with skewed slots which has less depth. Solid copper conductors are placed which are short circuited at both ends using end rings made up of copper. This gives a completely closed rotor circuit. Induction motor stator winding is equivalent to primary of a transformer and its rotor winding is equivalent to secondary of a transformer but it is short circuited with a rotating mechanical equivalent of r2 / s.

10․ 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 50 Hz is applied, it will run at
1500 rpm
1000 rpm
zero speed
750rpm

To avoid cogging effect in slip ring induction motor the conditions are
  • The number of poles on the rotor should not be equal to stator poles.
  • Rotor poles are always less than the stator poles.
  • Rotor poles also should not be in integral multiples of stator slots.
For all the above conditions the motor will not start at all hence speed will be zero.

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