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Slip in Electrical Induction Motors
The difference between the synchronous and asynchronous speed - also named rated speed - is "the slip"
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An AC (Amplitude 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 generate 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.
This speed difference is called the slip. The slip increase with load and is necessary for torque production. Slip speed is equal to the difference between rotor speed and synchronous speed. Percent slip is slip multiplied by 100. When the rotor is not turning the slip is 100 %.
The Slip can be 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)
Full-load slip varies from less than 1 % in high hp motors to more than 5-6 % 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 voltage is at maximum. The slip and voltage are reduced when rotor starts to turn.
Slip and 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 frequency and slip are 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 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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