Electric motor

An electric motor converts electricity into mechanical motion. The reverse task, that of converting mechanical motion into electricity, is accomplished by a generator. The two devices are identical except for their application.

Most electric motors work by electromagnetism, but motors based on other electromechanical phenomena, such as electrostatic forces and the piezoelectric effect, exist. The overarching concept is that a force is generated when a current-carrying element is subjected to a magnetic field. In a cylindrical motor, the rotor rotates because a torque is developed when this force is applied at a given distance from the axis of the rotor.

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. Though this frame is often called the armature, that term is often erroneously applied. Correctly, the armature is that part of the motor across which the input voltage is suppled or that part of the generator across which the output voltage is generated. Depending upon the design of the machine, either the rotor or the stator can serve as the armature.

Kits for making very simple motors are used in many schools. See Westminster motor kits.

DC motors

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 of the motor is proprtional to the voltage, and the torque is proprtional to the current. 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.

However there are a number of limitations in the classic design, many due to the need for brushes to rub against the commutator. The rubbing creates friction and the higher the rpm the harder the brushes have to press to maintain good contact. Not only does this friction make the motor noisy it creates an upper limit on the rpm and means the brushes eventually wear out and need to replaced. The imperfect electric contact also causes electrical noise in the attached circuit. These problems vanish when you turn the motor inside out, putting the permanent magnets on the inside and the coils on the outside thus designing out the need for brushes in a brushless design.

Wound field DC motor

The permanent magnets on the outside (stator) of a DC motor may be replaced by electromagnets. By varying the field current it is possible to alter the speed/torque ratio of the motor. Typically the field winding will be placed in series with the armature widing to get a high torque low speed motor, and in parallel with the armature to get a high speed low torque motor. Further reductions in field current are possible to gain even higher speed but with corresponding less torque. This technique is ideal for electric traction and many similar applications where its use can eliminate the requirement for a mechanically variable transmission.

Universal motors

A variant of the Wound field DC motor is the universal motor. The name derives from the fact that it may use AC or DC supply current, allthough in practice they are nearly always used with AC supplies. The principle is that in a wound field DC motor the current in both the field and the armature (and hence the resultant magnetic fields) will alternate at the same time, and hence the mechanical force generated is always the same. In practice the motor must be specially designed to cope with the AC current (impedence/reluctance must be taken into account), and the resultant motor is generally less efficient than an equivalent pure DC motor. The advantage of the universal motor is that AC supplies may be used on motors which have the typical characteristics of DC motors, specifically high starting torque and very compact design if high running speeds are used. The negative aspect is the maintenance and reliability problems caused by the commutator, and as a result such motors will rarely be found in industry but are the most common type of AC supplied motor in devices such as food mixers and power tools which are only used intermittently.

AC motors

AC motors generally come in two flavors: single phase and three phase.

Single-phase AC motors

The most common single-phase motor is the shaded-pole synchronous motor, which is most commonly used in devices requiring lower torque such as electric fans, microwave ovens and other small household appliances.

Another common single-phase AC motor is the induction motor, commonly used in major appliances such as washing machines and clothes dryers. These motors can generally provide greater starting torque by using a special startup winding in conjunction with a starting capacitor and a centrifugal switch.

When starting, the capacitor and startup windings are connected to the power source via a set of spring loaded contacts on the rotating centrifugal switch. The capactor helps increase the motor's starting torque. Once the motor reaches design operating speed, the centrifugal switch activates, opening a set of contacts which disconnect the series connected capacitor and startup winding from the power source. The motor then operates soley on the run winding.

Three-phase AC motors

For higher-power applications the three phase (or polyphase) AC induction motor is used. This uses the phase differences between the three phases of the polyphase 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. This type of motor is becoming more common in traction applcations such as locomotives where it is known as the asychronous traction motor. 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 an alternator.

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 off 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.

Stepper motors

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.

Linear motors

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. You can find a linear motor in a maglev (Transrapid) train, where the train "flies" over the ground.