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Slip in Electrical Induction Motors

By Susi | Published on Nov 02,2015
112

The 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 stators' rotating magnetic field 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 useful torque.

The difference between the synchronous speed of the magnetic field, and the shaft rotating speed is slip - and would be some number of RPM or frequency.

The slip increases with an increasing load, thus providing a greater torque.

It is common to express the slip as a ratio between shaft rotation speed and synchronous magnetic field speed. The Slip is often expressed as

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.

Motor Size (hp)

0.5

5

15

50

250

Typical Slip (%)

5

3

2.5

1.7

0.8

Synchronous speed at different
number of poles and frequency

No. of poles

50 Hz

60 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 the motor starts rotating the slip is 100 % and the motor current is at maximum. The slip and motor current are reduced when the rotor starts to turn.

Slip Frequency

Frequency decrease 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.



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