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Electrical Induction Motors - Slip

Slip is the difference between an electrical induction motor's synchronous and asynchronous speed.

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An AC (Alternating Current) induction motor consists of a stator and a rotor and the interaction of the currents flowing in the rotor bars and the rotating magnetic field in the stator generates the torque that turns the motor. In normal operation with a load the rotor speed always lags the magnetic field's speed allowing the rotor bars to cut magnetic lines of force and produce a useful torque.

The difference between the synchronous speed of the electric motor magnetic field, and the shaft rotating speed is slip - measured in RPM or frequency.

Slip increases with increasing load - providing a greater torque.

electric motor current torque curves slip

It is common to express the slip as the ratio between the shaft rotation speed and the synchronous magnetic field speed. 

s = (ns - na) 100% / ns         (1)

where

s = slip

ns = synchronous speed of magnetic field (rev/min, rpm)

na = shaft rotating speed (rev/min, rpm)

When the rotor is not turning the slip is 100 %.

Full-load slip varies from less than 1 % in high hp motors to more than 5-6 % in minor hp motors.

Electrical Induction Motors -  Slip
Motor Size
(hp)
0.551550250
Typical Slip
(%)
5 3 2.5 1.7 0.8

Number of poles, frequencies and synchronous induction motor speed

Electrical Induction Motors - Speed vs. no. of Poles and Frequency
No. of Magnetic PolesSpeed (rpm)
Frequency (Hz)
5060
2 3000 3600
4 1500 1800
6 1000 1200
8 750 900
10 600 720
12 500 600
16 375 450
20 300 360

Slip and Voltage

When a motor starts to rotate the slip is 100 % and the motor current is at maximum. Slip and motor current are reduced when the rotor begin turning.

Slip Frequency

Frequency decreases when slip decrease.

Slip and Inductive Reactance

Inductive reactance depends on the frequency and the slip. When the rotor is not turning the slip frequency is at maximum and so is the inductive reactance.

A motor has a resistance and inductance and when the rotor is turning the inductive reactance is low and the power factor  approaches to one.

Slip and Rotor Impedance

The inductive reactance will change with the slip since the rotor impedance is the phase sum of the constant resistance and the variable inductive reactance.

When the motor starts rotating the inductive reactance is high and impedance is mostly inductive. The rotor has a low lagging power factor. When the speed increases the inductive reactance goes down equaling the resistance.

Classification of Induction Motors

Electrical induction motors are designed for different applications regarding characteristics like breakaway torque, pull-up torque, slip and more - check NEMA A, B, C and D classification of electrical inductions motors.

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