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Induction Motors

Yellow Submarine Original Artwork copyright 2012

 

Induction motors are the simplest motors to manufacture. The rotor is made of a shaft with a sandwich of steel layers (laminations). The laminations have a ring of punchings. When aluminum or copper is cast into the punchings, a conductive cage called a squirrel cage results. That is all that is required for the electrical/magnetic circuit in the rotor.

The stator needs 3 phase poles with copper windings. If there are 2 poles, the motor will turn near 3600 RPM (3000 RPM) at 60H (50 Hz). A 4 pole motor will rotate near 1800 RPM. A 6 pole motor will rotate near 1200 RPM.

I use the word near since induction motors always slip. The rotor has no magnetizing current induced unless it is slipping with respect to the magnetic field in the stator. Typically the rotor slips by 50 RPM, depending on the load on the motor.

Be sure to specify the motor for variable speed drive duty. Insulation is upgraded for Pulse Width Modulated (PWM) inverter duty. In addition, separate ventilation is required for variable speed motors to ensure adequate air flow.

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