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.
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.
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.
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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