## Why induction motor can not run at synchronous speed ?

#### When the supply is given to the induction motor the field winding set-ups a magnetic field rotating at synchronous speed N_{s} ( i.e., N_{s} = 120f / number of poles ), and the field is known as a rotating magnetic field. As the rotor is rotating in this magnetic field, the magnitude of rotor induced currents will be proportional to the relative speed between the stator rotating field and rotor speed and this current always lags with the main field current. Thus it is difficult to maintain synchronism between main field flux speed and rotor rotating speed.

Let us assume that the rotor rotates at synchronous speed, so the emf induced in the rotor conductors would become zero and consequently, there is no torque on the rotor causing the rotor to fails back in speed. Once the rotor falls back in speed, the relative speed increases, and again rotor picks up the speed. But due to the inertia of the rotor, this does not happen in practice and the rotor continues to rotate with a speed slightly less than the synchronous speed of the rotating field.

Therefore, an induction motor will always rotate at a speed N less than the speed of synchronous speed N_{s} ( speed of rotating magnetic field ).

*i.e., N < N*

_{s}## What is Slip in Induction Motor ?

#### We have seen that there will be a difference in the speed of the rotating magnetic field and rotor rotating speed. This difference in speed expressed by the percentage of synchronous speed N_{s} is known as slip of the motor. Therefore, The slip 's' is defined as the difference between the speed of the main magnetic field ( N_{s} ) and the rotor rotating speed ( N ).

Thus slip,

#### The actual speed of the motor can be expressed, from the above equation as

#### From the equation of slip s, it may be noticed that the relative speed or slip speed ( N_{s} - N ) of the rotor and rotating magnetic field is directly proportional to the slip.- At starting or standstill position of the rotor, N = 0. At this condition the slip s = 1, and this is the maximum value of slip.
- The value of slip cannot be zero because when s = 0, N = N
_{s}, and the torque produced will be zero. This can be understood by the torque-slip characteristics of the induction motor.
- At no-load slip is very low ( near to 0 ) about 1% and at full load, it is about 3-5%. The variation in slip and therefore speed, from no-load to full load is very small. Thus we can say a 3-phase induction motor is also a constant speed motor.

_{s}, and the torque produced will be zero. This can be understood by the torque-slip characteristics of the induction motor.## Effect of Slip on Induction Motor Parameters :

### Rotor Frequency :

#### In an induction motor, the relation between synchronous speed, supply frequency, and the number of stator poles is given by,

#### When the rotor rotates at a speed 'N' then the rotor conductor cut the rotating magnetic field at the relative speed i.e., N_{s} - N. The frequency of the rotor induced emf or current 'f_{r}' can be expressed as

#### Dividing equation ( 2 ) by ( 1 ), we get

#### Therefore, f_{r} = s f at the running condition. So rotor frequency in running condition is slip times the supply frequency.- Suppose, at starting, N = 0 and slip = 1, hence f
_{r} = f. - Under the normal running condition, the rotor frequency is very small as the slip is very small.

_{r}= f.