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"Slip" is a term used to describe the difference between synchronous speed and actual speed in induction motors. Slip is always present in induction motors, because the rotor speed must always be lower than synchronous speed to induce the rotor field.
The amount of slip for a given load is not equal for all motor designs, and if more than one motor is connected to the same load, motors with differing slip will not share the load equally. An engineer may want to specify the percentage of load from each motor when running two different motors sharing the same load. The controller(s) would be set up to measure the currents, infer the torques, and reduce the frequency of one of the motors to share the load as desired. This "artificial slip" would be set so that the other motor would take up the desired portion of the load. In motor controller terminology, this addition of slip is called "droop" control.
The converse is also true. In the generator world, "droop" refers to the reduction in speed (and frequency) that a generator would experience when loaded if the input did not change. Once again, it is important because when multiple generators are on the same grid, generators have differing droop as they are loaded, so the generator with the lowest droop would otherwise tend to pick up most of any additional load. Droop control is necessary so that the load is shared; each generator must increase its governor setting to "catch up" and stay closely in phase with others on the grid so that load sharing will occur.
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.
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 SizeWhen 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.
Frequency decreases when slip decrease.
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.
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.
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.