#
**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).

The induction motor is an asynchronous motor and it never runs on
synchronous speed so there is a slight difference between synchronous
speed and actual motor speed. And this speed difference between
synchronous speed and the actual motor speed is known as slip speed. Or in
other words we can say that the speed of the stator RMF with respect to
the rotor is known as **slip speed**.

By Definition

s.Ns = Ns - N

N = Ns - s.NS

…………………. (1)

Equation (1) represents the formula for slip

Ns = 120F/P

Where,

Ns = Synchronous Speed

N = Actual Speed of Motor

S = slip

sNs = Slip speed

F = Supply Frequency

P = Number of Poles on Stator

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

####
**Effect on the Frequency**

Let 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 Xr
be the rotor reactance

Xr = 2Ï€FL at starting

But under running condition

F’ = s.F

Xr’ = 2Ï€ s.F.L

So,

Xr’ = s.Xr

####
**Effect on the Voltage**

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

Er = 4.44 kw F’.N.É¸

At starting slip is equal to 1

F’ = F

So,

Er = 4.44 kw F.N.É¸

But under the slip condition F’ = sF

So,

Er’ = 4.44 kw s.F.N.É¸

Er’ = s.Er

Similarly

Ir’ = Er’ / Zr’

###
**Importance of Slip in Induction Motor**

The slip 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 in induction motor. 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 (sX_{2})^{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 equal to1**

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 equal to 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 = (Ns - (-N) ) / 100

S = (Ns + N) / 100

####
**When slip is negative**

If the Induction motor is rotated at speed greater than the synchronous
speed then **slip is negative** (N > Ns). 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.

__related posts__

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

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

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