Dingfeng Capacitor -- What Is an AC Motor Capacitor?

Aug 9,2019

In the 1880s, Nikola Tesla developed a series of alternating current (AC) electric motors. They relied on polyphase power--that is, two or three AC electric feeds in sync with each other, with one feed designed to reach its maximum before the others. Polyphase power produces a rotating magnetic field which drives the motor. Today, our homes have single-phase AC power. To make the AC motors in your appliances work, engineers added capacitors to create an extra phase.


AC Motor

An AC electric motor has an inner rotor surrounded by a set of coils. A three-phase AC motor runs different sets of coils. One phase may be approaching a maximum in its cycle, the next is at the maximum, the next is decreasing from the maximum. Only one set of coils at a time makes a maximum-strength magnetic field. As each phase goes through its cycles, the maximum magnetic point rotates around the motor’s circumference, driving the rotor.


Starter Capacitor

With single-phase power, all of a motor’s coils begin their cycle at the same time. The magnetic field doesn’t rotate, so the rotor can’t move. Engineers worked around this by using a separate starter coil in series with a capacitor. A capacitor is a small cylinder-shaped electronic device that stores and releases electric charge. Its capacity is measured in units called farads, with starter capacitors typically having around 10 microfarads (millionths of a farad). Combined with the coil, the capacitor creates a second phase that leads the first by 90 degrees. This is enough to create a rotating magnetic field and start the motor. Once the motor comes up to speed, a centrifugal switch disconnects the starter coil and capacitor, otherwise, they would interfere with the motor’s efficiency.


Start-Run Capacitors

A variation of the starter capacitor scheme uses two capacitors: a large one to start the motor, and a smaller one to keep it running. This improves the performance of larger electric motors.


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