An electric motor converts electricity into mechanical motion.
Most electromagnetic motors are rotary, but linear types also exist. In a rotary motor, the rotating part (usually on the inside) is called the rotor, and the stationary part is called the stator. The motor contains electromagnets that are wound on a frame called the armature. Kits for making very simple motors are used in many schools. See Westminster motor kits.
One of the first electromagnetic rotary motors, if not the first, was invented by Michael Faraday in 1821, and consisted of a free-hanging wire dipping into a pool of mercury. A permanent magnet was placed in the middle of the pool. When a current was passed through the wire, the wire rotated around the magnet, showing that the current gave rise to a circular magnetic field around the wire. This motor is often demonstrated in school physics classes, but brine is sometimes used in place of the toxic mercury.
The classic DC motor has an armature of electromagnets. A rotary switch called a commutator reverses the direction of the electric current twice every cycle, to flow through the armature so that the electromagnets push and pull on permanent magnets on the outside of the motor.
DC motor speed generally depends on a combination of the voltage and current flowing in the motor coils and the motor load or braking torque. The speed is typically controlled by altering the voltage or current flow by using taps in the motor windings or by having a variable voltage supply. As this type of motor can develop quite high torque at low speed it is often used in traction applications such as locomotives.
The classic single phase AC motor is a shaded-pole synchronous motor[?]. It strongly resembles a DC motor, but the outside magnets are made from electromagnets. One outside, stationary magnet (the "shaded pole") is intentionally weakened so that the motor has a preferred direction of starting.
For high power applications the three phase AC motor is used. This motor uses the phase differences between the three phases of the electrical supply to create a rotating electromagnetic field in the motor. Often, the rotor consists of a number of copper conductors embedded in steel. Through electromagnetic induction the rotating magnetic field induces current to flow in these conductors, which in turn sets up a counterbalancing magnetic field and this causes the motor to turn in the direction the field is rotating. This type of motor is known as an induction motor. In order for it to operate it must always run slower than the frequency of the power supply feeding it causes the magnetic field in the motor to rotate, otherwise no counterbalancing field is produced in the rotor. If the rotor coils are fed a separate field current to create a continuous magnetic field, one has a synchronous motor, because the motor will rotate in synchronism with the rotating magnetic field produced by the 3 phase AC power. Synchronous motors can also be used as generators.
AC motor speed primarily depends on the frequency of the AC supply and the amount of slip, or difference in rotation between the rotor and stator fields, determines the torque that the motor produces. The speed in this type of motor has traditionally been altered by having additional sets of coils or poles in the motor that can be switched on and of to change the speed of magnetic field rotation. However, developments in power electronics mean that the frequency of the power supply can also now be varied to provide a smoother control of the motor speed.
Another kind of electric motor is the stepper motor, where an internal rotor containing permanent magnets is controlled by a set of external magnets that are switched electronically. A stepper motor is a cross between a DC electric motor and a solenoid. Simple stepper motors "cog" to a limited number of positions, but proportionally controlled stepper motors can rotate extremely smoothly. Computer controlled stepper motors are one of the most versatile forms of positioning systems, particularly when part of a digital servo-controlled system.
A linear motor is essentially an electric motor that has been "unrolled" so that instead of producing a torque (rotation), it produces a linear force along its length by setting up a travelling electromagnetic field. Linear motors are most commonly induction motors or stepper motors.
The construction of an electric motor is very similar to that of a dynamo. In fact, a machine can be built that acts as either a motor or a dynamo.