Slip in Induction Motor or in Induction Machine

Induction Motor Slip

The slip in an induction motor or in an induction machine is an important parameter that describes the relative speed between the synchronous speed and the actual speed of the induction motor expressed as the percentage of synchronous speed shown in equation (1).

As we know, induction motors are asynchronous motors, they never run at synchronous speed. In induction motors, the rotor rotates at speed slightly less than the synchronous speed. The difference between these two speeds is called slip speed. In other words, we can say that the speed of the stator RMF with respect to the rotor is known as slip speed.

Formula for Slip

By Definition

s.N_s  =  N_s  -  N

N  =  N_s - s.N_s

………………….     (1) 

Equation (1) represents the formula for slip

N_s = 120F / P

 

Where, 

• N_s =  Synchronous Speed
• N  =  Actual Speed of Motor
• S  =  slip 
• sN_s =  Slip speed
• F = Supply Frequency
• P = Number of Poles on Stator

Effect of slip on the Induction Motor Parameters

 

Effect on the Rotor Frequency

Let us say, the frequency of the voltage applied to the stator winding is F and the Frequency of voltage and current induced in the rotor F’.

F’ = s.F

 

Effect on the Rotor Reactance

Let X_r be the rotor reactance

X_r  =   2πFL  at starting

But under running condition

F’ = s.F

X_r’  =  2π s.F.L

So, 

X_r’  = s.X_r

 

Effect on the Induced EMF in Rotor

At the starting of induction motor, the induced EMF in the rotor is E_r.

E_r  = 4.44 k_w F’.N.ɸ

At starting slip is equal to 1

F’ =  F

So,

E_r  = 4.44 kw F.N.ɸ

But under the slip condition F’ = sF

So,

E_r’  = 4.44 kw s.F.N.ɸ

E_r’  =  s.E_r

Similarly 

I_r’ = E_r’ / Z_r’

Importance of Slip in Induction Motor

The slip in electrical machines (specifically in induction machines) is an important parameter to be considered while studying the induction machine. It greatly affects the performance of the induction motor. Efficiency and Torque in induction motor significantly depends on the slip. If there is no slip or slip is equal to zero then the actual motor speed is equal to the synchronous speed and there is no relative motion between the rotor and the stator rotating magnetic field. If there is no relative motion between rotor and stator rotating magnetic field then there is no induced EMF and current in the rotor. Hence no torque is developed in the induction motor. 

Torque in an induction motor 

  ……………  (2)

Under the running condition s is very low so (sX₂)² is negligible and can be neglected

So, for low slip value 

So, from the above equation it is clearly seen that for low slip value Torque is directly proportional to slip.

 

Different Values of Slip and their Significance

The slip of the induction motor lies between 0 to 1. Zero means the 0% of the synchronous speed 1 means the 100% of synchronous speed. This range of slip between 0 to 1 is theoretical. Practically slip of an induction motor is between 3 to 5% and in some cases, it will be 8%. Higher value of slip ranging between 0% to 100% of the synchronous speed is not preferable.

 

Why is the higher value of slip not preferable?

In induction motor torque slip characteristics, we see that torque is directly proportional to slip until the value of slip at which maximum torque occurs. Beyond that value of slip at which maximum torque occurs, torque is inversely proportional to slip. If we increase the slip further then torque will decrease. So, for that reason the higher value of slip is not preferable.

From equation (2), we see that if slip is high then sX₂ is high and it cannot be neglected. So, for the higher value of slip sX₂ is much greater than R₂ and it can be neglected for high slip region. Then the torque equation for high slip region 

 

It can be clearly seen that torque is Inversely proportional to slip for high value of slip from above equation.

 

When slip is nearly equals to zero

An induction motor at no load runs at a speed near equals to the synchronous speed (discussed in induction motor). So, at no load slip is nearly equal to zero. As load increases speed of the motor decreases then slip increases.

When slip is 1

At the starting of the induction motor the actual speed of the motor is zero so the slip is equal to 1.

 

When Slip is zero

If the Actual speed of the motor is equals to the synchronous speed of the motor then the slip is zero. It basically means the motor runs at synchronous speed or the motor is a synchronous motor. 

 

When slip is greater than 1

If the direction of rotation of the induction motor is reversed then the relative speed of the between motor actual speed and the synchronous speed is high. Simultaneously the slip is greater than 1. It is the case of plugging i.e. electrical braking where the changing two phases of the induction motor will instantly make the motor to rotate in the opposite direction. 

s   =   (N_s - (-N) ) / 100

s   =   (N_s + N) / 100

 

When slip is negative

If the Induction motor is rotated at speed greater than the synchronous speed then slip is negative (N > N_s). This situation is only possible when the rotor of the induction motor is rotated with the help of external means or prime mover. It is the case when an induction motor behaves as the induction generator. In that case induction machines supply active power. 

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