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Induction Motors

 

Induction Motor Rotor

Because the induction motor works by magnetic repulsion rather than attraction like the synchronous motor, it has been called a "repulsion induction" motor.

If there were no friction in the system, the rotor would turn at synchronous speed, but the motor would produce no useful torque. Under this condition, there would be no relative motion between the rotor bars and the rotating stator field, and no current would be induced in them. As soon as any load is applied to the motor the speed is reduced, causing the rotor bars to cut the magnetic lines of force of the stator field, and creating the repulsion force in the rotor. The induced magnetic field in the rotor migrates around in the direction of the rotation, and the speed of this migration is dependent on the applied load. This means the RPM will always be less than synchronous speed. The difference between the actual speed and synchronous speed is called the "slip". The greater the slip, the greater the induced current in the rotor bars, and the greater the output torque. The current in the stator windings also increases in order to create the larger currents in the bars.

For these reasons, the actual speed of an induction motor is always dependent on the load.

 

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