AC motors are electromagnetic devices that have an alternating current as a power source. Their function is to convert mechanical energy into electrical energy that can perform work. They operate on the principle of Faradays' law of induction, which states that a rapidly changing magnetic field produces an induced emf or voltage.
Nikola Tesla invented the first AC motor. He originally worked for Thomas Edison, whose company was the main builder of direct current systems. They had a falling out over Tesla's advocacy of alternating currents as a source of power, and he left the company. He sold his patent of an AC system to Edison's rival, George Westinghouse. Tesla's vision eventually won out over Edison's. Edison's company later on became known as General Electric.
They are based on Faradays' law of induction, which states that a rapidly changing magnetic field produces an induced electromotive force or emf. Electromotive forces are voltages.
Wires that carry current produce magnetic fields. Michael Faraday discovered that when a wire is in a changing magnetic field, a current is induced just as if there were a source of emf. The more rapid the changing field, the greater the generated emf.
The two most common types are induction and synchronous.
Induction motors are brushless. The magnetic source is an electromagnet that uses the same induced voltage that the coils use.
Synchronous motors turn at the same speed as the magnetic field. This is done by using a DC part in either the stator or the rotor. The stator may use permanent magnets or the current in the coils of the rotor is manipulated to run in the same direction, instead of switching.
Induction motors are more commonly used. They are simpler to make, less expensive to produce and are fairly rugged.
Because motors are generators that operate in reverse, they produce an emf just like generators do. It is called a back emf and does not immediately appear when a motor is initially turned on. The back emf is a consequence of Lenz's law, which states that an induced emf produces a magnetic flux that resists the change. The back emf works to reduce the current in the loop, thus slowing the motor down, and it gets larger as the speed of the motor increases. When a load isn't present, it becomes nearly equal to the size of the input voltage. The load also works to slow the down, and so the power requirements of the motor increases. This contributes to energy waste and inefficiency in their operation.
Most motors are AC. They are found in refrigerators, trains, computers, hard drives, industrial machinery, household appliances and many other electronic devices. They are used in certain engines in electric and hybrid vehicles.