Slip in Electrical Induction Motors

Difference between the synchronous and asynchronous speed - also named rated speed

An AC (Alternating Current) induction motor consists of two assemblies - a stator and a rotor. The interaction of currents flowing in the rotor bars and the rotating magnetic field in the stator generates a torque. In an actual operation 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 magnetic field, and the shaft rotating speed is slip - measured in RPM or frequency.

Slip increase with increasing load - providing a greater torque.

electric motor current torque curves

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)


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.

Motor Size
Typical Slip
5 3 2.5 1.7 0.8

Number of poles, frequencies and synchronous induction motor speed

No. of polesFrequency (Hz)
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.

Related Topics

  • Electrical - Electrical units, amps and electrical wiring, wire gauge and AWG, electrical formulas and motors

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