# Objective Questions on Induction Motor | 2

1. Slip exists in

Because in induction motor relative speed between stator magnetic field and rotor is exists i.e., slip will exists but not in synchronous motor.

2. What happens when d.c supply is given to rotor in induction motor ?

The synchronous motor is double excited machine i.e., giving AC at stator side
and DC at rotor side.

3. The principal of starting of 3 - Φ IM is similar to

At standstill condition when supply is given to the stator winding of 3 phase IM, then 3 phase rotating flux of the stator induces emf in the rotor winding as in the case of a transformer with short circuited secondary.

4. 3rd harmonics are eliminated in induction motor by using

Because in star connection phases or winding are 120° apart so that are they eliminated but it is not possible in delta connection.

5. If the air gap of IM is increased, ____________ will increase.

When a constant voltage is applied to the stator of the IM, the mutual flux remains constant. Now reluctance, offered to the mutual flux, increases with air gap and therefore the same mutual flux [ = (mmf / reluctance)] will require more magnetizing mmf and hence an increased amount of magnetizing electric current will be required.

6. An IM cannot run at synchronous speed because

If rotor runs at synchronous speed then there will be no flux cutting between stator and rotor, and hence no EMF will be produced and as a result there will be no electric current flowing through the rotor, and consequently rotor torque will be zero.

7. How can the synchronous speed of a 3 - Φ IM be varied?

Synchronous speed Ns = ( 120f ) / P , So Ns can be varied by varying both frequency and number of poles.

8. In a 3 - Φ induction motor running at full load which of these parameters is stationary with respect to the stator mmf wave?

Stator mmf wave rotates at synchronous speed where as stator is fixed. Rotor winding rotates at a speed less than synchronous speed and depends on slip. Both rotor and stator mmf waves rotate in the air gap in the same direction at synchronous speed when balanced 3 - Φ supply is given.

9. In a 3 - Φ I.M. with line frequency f1 and winding factor kw1 and kw2 for stator and rotor respectively the ratio of per phase stator winding emf E1 to per phase rotor winding emf E2 at standstill is given by

At standstill per phase value of stator EMF
E1 = √2 × πf1kw1N1φ
The Emf induced by E1 to rotor at standstill by transformer action is
E2 = √2 × πf1kw2N2φ
Where φ is flux induced.
∴ E1 / E2 = (√2πf1kw1N1φ)/(√2πf1kw2N2φ)
= N1kw1 / N2kw2

10. A 3 - Φ, 6 pole, 50 Hz IM has a rotor speed of 987 rpm find rotor frequency?

Ns= 120f / P = (120 × 50) / 6 = 1000 rpm.
Nr = 987 rpm
s = ( Ns - Nr ) / Ns = 0.013
fr = sf = 0.013 × 50 = 0.65 Hz

11. What does happen if tripled harmonics given to induction motor?

Sinusoidal wave form is an ideal form and is nearly impossible to realize such a wave form in practice. All the alternating wave forms deviate to a lesser or higher degree from this ideal sinusoidal shape. When a sinusoidal input is applied to electronic circuit the output is distorted wave form. This distorted wave is because of nonlinear characteristics of electronic circuit. Those distorted wave is called harmonics. The component having triple times of the fundamental are known as third harmonics. Generally all third harmonics are equal in all phases. These harmonics is applied to induction motor it will fail to start because rotating magnetic field never develops. Hence it results in fail to start.

12. Considering a galvanometer is connected to the rotor of a 3 - Φ, 4 pole, 50 Hz IM and is making 130 oscillations in a minute, then calculate the rotor speed?

Since the galvanometer is connected to the rotor, one complete of galvanometer will correspond to 1 cycle of rotor frequency.
∴ Rotor frequency f2 = sf1 = 130 / 60 = 2.17 Hz
∴ s = f2/f1 = 2.17 / 50 = 0.043
Rotor speed = Ns(1-s) = ( 120 × 50 ) / 4 × ( 1 - 0.043 ) = 1435 rpm .

13. A 3 - Φ, 6 pole, 50 Hz IM is running at a full load speed of 980 rpm. If it delivers an air gap power of 98000 Watt, calculate the rotor ohmic loss?

Pg = 98,000 Watt
Ns = (120f) / P = 1000 rpm
Nr= 980 rpm
∴ s = ( Ns - Nr ) / Ns = 0.02
∴ Rotor ohmic loss = sPg = 0.02 × 98,000 = 1960 Watt

14. Which of these types of motor is self starting?

In a 3 - Φ IM, 3 phase rotating flux is produced when energized from a 3 phase source. The 3 phase rotating flux cuts the stationary rotor windings and as a result Emf is produced. Due to this rotor induced EMF electric current flows through the short circuited rotor bars. This electric current interacts with the 3φ rotating flux and produces rotating torque, as a result the rotor starts rotating at a speed below synchronous speed. Hence it is called self starting as it starts without any auxiliary means.

15. A 3 - Φ, 4 pole IM is supplied from a 50 Hz source. Calculate rotor frequency when rotor runs 600 rpm?

Ns = ( 120 × 50 ) / 4 = 1500 rpm
Nr = 600 rpm
s = ( 1500 - 600 ) / 1500 = 0.6
fr= sf = 0.6 × 50 = 30 Hz

16. What happens if fifth multiples of harmonics is given to induction motor?

The sinusoidal components having frequency five times of fundamental are known as fifth harmonics. All the fifth harmonics have a phase displacement of 120° and having a negative phase sequence of R, B, Y (opposite rotation to the fundamental). The fifth multiples of harmonics are 5th, 11th, 17th, 23rd etc. Hence induction motor will rotates in reverse direction because of reverse phase sequence of fifth harmonics.

17. Rotating magnetic field is produced in an induction motor because of

Rotating magnetic field is developed in induction motor. Why? In order to develop rotating magnetic field the induction motor, it needs 120° phase shift of supply and winding in stator winding.

18. Air gap flux density is of the order of

The air gap is present between stator and rotor. This air gap should be as low as possible. A reduced air gap will increase power factor. Hence air gap is should be low i.e., 0.35 - 0.5 gap flux density.

19. The reduced air gap results in an increase

As the air gap increased between stator and rotor it will increase magnetizing electric current and hence it results in low power factor. Hence a reduced air gap results in an increase power factor. Increase in magnetizing electric current is not desirable in any a.c machine.

20. Under operating conditions the rotor conductor should be _________________ in induction motor.

Current will flow only when after the formation of closed path. Under operating conditions rotor circuit of an induction motor should always be closed otherwise electric current can not flow hence no torque is produced.