# الموضوع: معلومات تهمك بخصوص المحركات الحثية 3 phase Induction Motor Questions & answers

1. ## معلومات تهمك بخصوص المحركات الحثية 3 phase Induction Motor Questions & answers

3 phase Induction Motor Questions & answers
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1) Explain the operation of induction motor in simple words ?

When 3 phase ac supply is given to stator of the induction motor rotating magnetic field will be generated. This magnetic field (flux) cuts the stationary conductors of the rotor of induction motor. Therefore an emf is induced by Faraday's law of electro-magnetism. As the rotor conductors are short circuited, current starts flowing in the windings of the rotor this produces a magnetic field. Due to interaction of both the magnetic fields a torque will be developed. If the torque generated is sufficient to overcome the load torque rotor starts rotating.

2) What is Synchronous speed?

Speed at which the rotating magnetic flux rotates is called synchronous speed (Ns). its value depends upon the number of poles and the supply frequency.
NS=(120F)/P

3) Define Slip in Induction Motor?

Slip is the difference between the speed of the stator rotating magnetic field and the speed of the rotor of the induction motor expressed as a fraction of synchronous speed is known as slip of the induction motor.

4) Why Induction Motor always runs below synchronous speed ?

Induction motor works on the principle of induction. When both rotating magnetic field of stator and rotor of induction motor rotates with same speed no emf is induced in the rotor conductors resulting no torque is produced in the rotor of induction machine and motor will stop. Therefore in order to have torque for rotating the rotor induced emf is required. Hence Induction motor rotates with speeds less than synchronous speed.
5) Why the exciting current of induction motor is so high as compared to the power transformer?
Unlike that of a power transformer, the magnetic circuit of the induction motor has an air-gap which has a high Reluctance (magnetic resistance) . Therefore, the exciting current of induction motor (3O to 4O% of full-load current) is much higher than that of the power transformer.

6) How direction of 3 phase Induction Motor can be changed ?

Direction of 3 phase induction motor can be changed by changing any two supply phases of the induction motor. Changing any two phases will result in change in rotating magnetic field to opposite direction. Therefore rotor direction is also changed.

7) Why the starting torque of Squirrel cage induction motor is LOW?

It suffers from the disadvantage of a low starting torque. It is because the rotor bars are permanently short-circuited and it is not possible to add any external resistance to the rotor circuit to have a large starting torque.

8- Why motor take heavy current at starting?

When 3 phase supply is given to the stator of an induction motor, magnetic field rotating in space at synchronous speed is produced. This magnetic field is cut by the rotor conductors, which are short circuited. This gives to induced current in them.
Since rotor of an induction motor behaves as a short circuited secondary of a transformer whose primary is stator winding, heavy rotor current will require corresponding heavy stator balancing currents. Thus motor draws heavy current at starting

9) What is the effect of staring of induction motor on connected line?

Large starting current will produce large line-voltage drop. This will adversely affect the operation of other electrical equipment connected to the same lines. Therefore, it is desirable and necessary to reduce the magnitude of stator current at starting and several methods are available for this purpose.

10) What are the different starting methods of 3 phase induction motor ?

The common methods used to start induction motors are:

(i)Direct-on-line starting

(ii) Stator resistance starting

(iii)Autotransformer starting

(iv) Star-delta starting

(v)Rotor resistance starting

11) When we can get Maximum Torque in Induction Motor while starting ?

Starting torque for 3 phase induction motor is low. In order to increase the starting torque an external resistance should be inserted while starting of the 3 phase induction motor. It is to be remembered that starting torque of slip ring induction motor is high compared to squirrel cage rotor machine due to the provision of adding external resistance while starting the machine.

12) What are the different losses in 3 phase induction motor ?

Different losses in the 3 phase induction motor are stator copper loss, stator iron loss, rotor copper loss, friction and windage loss and stray load loss.
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