Questions & Answers about THREE PHASE INDUCTION MACHINES

1..  What are the effects of increasing rotor resistance on starting current and         starting  torque?

The additional external resistance reduces the rotor current and hence the current drawn from the supply.
It improves the starting torque developed by improving the power factor in high proportion to the decrease in rotor current.

2.What is slip of an induction motor?

The slip speed expressed as the ratio of synchronous speed is defined as slip.
Percentage slip     S=Ns-N/Ns*i00

3. How the frequency of rotor emf is related to the slip in an I M?
Frequency of rotor emf/current fr=Sfs

4.What is the normal value of slip of an I M operating at full load?

3 -   5%

5.Why is not possible for the rotor speed of an I M to be equal to the speed of its rotating magnetic field?

The machine will not be able to develop any mechanical torque to run as a motor.


6.  State the condition at which the  torque developed in a 3 phase induction motor is maximum.

When R2=SX2


7. What are the advantages of slip-ring I M over cage I M?


(i) Rotor circuit is accessible for external connection.

(ii) By adding external resistance to the rotor circuit the starting current is reduced with the added advantage of improving starting torque.

(iii) Additional speed control methods can be employed with the accessibility in the rotor circuit.


8.What are the losses occurring in an I M and on what factors do they depend?

Magnetic losses Wi Electrical losses Wcu Mechanical losses Wm
For I M operating in normal condition (with constant voltage and frequency) magnetic  and mechanical losses remain constant whereas electrical losses vary in square proportion to the current.


9. What care should be taken at the time of construction to reduce eddy current losses in I M?


Make the resistance of the core body as large as possible.This is achieved by  laminating the stator core,stacked and revetted at right angles to the path of eddy current.The laminations are insulated from each other by thin coat of varnish.


10. Why is there not appreciable magnetic losses in the rotor core of Induction motors?


Although the rotor core is also subjected to magnetic flux reversals and since the frequency of flux reversals in the rotor, fr = Sfs, is very small, the iron loss incurred in the rotor core is negligibly small.
                                          

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