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Why Does an AC Motor Need a Capacitor to Start?

by John Papiewski; Updated March 16, 2018

Electric motors fall under a few basic types: Direct current (DC), single phase alternating current (AC), and multiphase AC. Each one of these types has many designs. The AC motors used in your dishwasher, vacuum cleaner and washing machine run on single-phase AC. Though single-phase AC motors run efficiently, they can’t get started without some assistance. A capacitor adds a temporary extra phase to start the motor.

Magnetic Repulsion

Most electric motor designs, whether AC or DC, use the forces of opposed magnetic fields to spin a rotor. To accomplish this, the motor has a set of magnetic fields on the rotor and a set surrounding it. As the rotor turns, the magnetic fields switch, keeping like magnetic poles (north with north, south with south) facing each other. Since like poles repel each other, this forces the rotor to keep turning. The magnetic repulsion forces continue through the entire 360-degree rotation of the rotor.

AC Motors

The simplest kind of AC motor needs three-phase electricity to work. The polyphase motor takes advantage of three overlapping current cycles, called phases, to drive the magnetic forces in the motor. Each of the three separate phases connects to a set of magnet coils spaced 120 degrees apart. While this is fine for commercial and industrial settings, the electric current coming into your home has only one or two phases. A single-phase motor needs additional parts for it to run correctly.

Single-Phase Problem

A motor’s coils driven by a single AC phase all alternate at the same time, reversing north and south poles in unison. This creates a problem called zero starting torque. While it can run a motor already spinning, it has no “kick” to get a motor to turn from a dead stop. You can start it by spinning it by hand, but who wants to hand-start a vacuum cleaner?

Starting Capacitor and Switch

A capacitor, connected to a separate coil on the motor, creates an alternating electric current ahead of the main phase by 90 degrees. This happens because the current through a capacitor leads the voltage by 90 degrees. During a motor’s start-up, a switch connects a capacitor and a special starting coil to the motor. After the motor reaches its operating speed, the switch disconnects the capacitor. If the capacitor remains connected to the motor, it puts a drag on the motor’s efficiency.

Run-Start Capacitors

Another, slightly more expensive design uses two capacitors, a larger-value one to start the motor and a smaller one to keep it running. This design also uses a switch to manage the motor’s start-up. For larger single-phase motors, this helps improve power.

About the Author

Chicago native John Papiewski has a physics degree and has been writing since 1991. He has contributed to "Foresight Update," a nanotechnology newsletter from the Foresight Institute. He also contributed to the book, "Nanotechnology: Molecular Speculations on Global Abundance."

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