How Do AC Voltage Electric Motors Work?
Electric motors run on forces created by changing magnetic fields. Since alternating current (AC) changes direction, it offers a head start for motor design. AC motors use a set of wire coils surrounding a rotating part made of a permanent magnet or wire grid. The changing currents produce changing magnetic fields in the outer coils. This drives the rotating part, producing rotary motion. Most AC motors run on only one of the power company's three current phases, but some use all of them.
Brushes
- A simple AC motor design uses two permanent magnets placed above and below a rotating wire coil. They're aligned so that the coil faces the north pole of one magnet and the south pole of the other. Two conducting brushes touch two rings, which feed the inner coil with electric current. Magnetic forces make the coil rotate so that the south pole turns to the north pole of the permanent magnet, and vice versa. When the current alternates (changes direction), the magnetic south and north of the coil swap places, and the motor continues to turn.
Synchronous
- The synchronous AC motor is ideal if you want a very accurate motor speed, as for an electric clock. This design has a set of coils surrounding a rotor, but in this case, the rotor is a permanent magnet. Coils arranged in north-south pairs attract the north-south poles of the permanent magnet on the rotor, turning it. The design can accommodate many pairs of coils. The motor's speed depends on the number of coils. A 60-cycle AC current will run a two-coil synchronous motor at 3,600 revolutions per minute (RPM), or 60 revolutions per second. A four-coil motor turns at 1800 rpm, and a 12-coil turns at 600 rpm.
Induction
- A more advanced AC motor doesn't use brushes or conducting rings. Instead, it uses an effect called induction where a changing magnetic field produces an electric current. The electric current in a conductor, in turn, has its own magnetic field. Coils surrounding the rotor induce a current in the rotor, and, once again, magnetic forces turn the rotor around in a rotary motion. Because the inner conductor is shaped as a round, cage-like grid, engineers refer to this design as a "squirrel-cage" motor. Not having the rings and brushes make the motor more durable and efficient. Brushless motors don't have the sparking and wear that brushes entail.
Shaded-Pole
- For an AC induction motor to work, its coils must produce magnetic fields that rotate around the motor's axis. Household AC current comes in a rippling wave with a single phase, though two phases are needed--one 90 degrees behind the other--to make a properly rotating field. Engineers developed many ways around this problem, one being the shaded-pole motor. It uses a set of copper rings that surround part of the motor's outer coils. The rings make the coils produce magnetic fields 90 degrees out of phase with the other coils. This effect is not very strong, so shaded-pole motors are used for fans and other light-duty appliances.
Three-Phase
- Modern power plants produce AC current in three separate phases. Each phase is 120 degrees apart from the next, forming a complete 360 degree cycle. Powerful industrial electric motors are generally wired for this three-phase electricity. These motors have coils spaced 120 degrees apart, each coil being driven by one phase of the electricity. This arrangement produces a rotating magnetic field simply and efficiently.
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