**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}Â =Â N_{s}Â -Â N

NÂ =Â N_{s} - s.N_{s}

Equation (1) represents the formula for slip

N_{s} = 120F / P

Where,Â

- N
_{s }=Â Synchronous Speed - NÂ =Â Actual Speed of Motor
- SÂ =Â slipÂ
- sN
_{s}=Â Slip speed - F = Supply Frequency
- P = Number of Poles on Stator

**Effect of slip on the Induction Motor Parameters**

Â

**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 Fâ€™.

Fâ€™ = s.F

**Effect on the Rotor Reactance**

Let X_{r} be the rotor reactance

X_{r}Â = Â 2Ï€FLÂ at starting

But under running condition

Fâ€™ = s.F

X_{r}â€™Â =Â 2Ï€ s.F.L

So,Â

X_{r}â€™Â = 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}Â = 4.44 k_{w} Fâ€™.N.É¸

At starting slip is equal to 1

Fâ€™ =Â F

So,

E_{r}Â = 4.44 kw F.N.É¸

But under the slip condition Fâ€™ = sF

So,

E_{r}â€™Â = 4.44 kw s.F.N.É¸

E_{r}â€™Â =Â s.E_{r}

SimilarlyÂ

I_{r}â€™ = E_{r}â€™ / Z_{r}â€™

**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.Â

Torque in an induction motorÂ

Â â€¦â€¦â€¦â€¦â€¦Â (2)

Under the running condition s is very low so (sX_{2})^{2 }is
negligible and can be neglected

So, for low slip valueÂ

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

Â

**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.

Â

**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 sX_{2} is
high and it cannot be neglected. So, for the higher value of slip sX_{2}
is much greater than R_{2} and it can be neglected for high slip
region. Then the torque equation for high slip regionÂ

Â

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

Â

**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**.

Â

**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.Â

Â

**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.Â

sÂ Â = Â (N_{s} - (-N) ) / 100

sÂ Â = Â (N_{s} + N) / 100

Â

**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.Â

__related posts__

**#_****speed control methods of three phase induction motor**

**#_torque slip characteristics of three phase induction motor**

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