An electric motor is more properly described as an electric machine, for the same piece of apparatus can be used either as a motor or as a generator of electricity. There is no type of machine that can convert electrical energy into mechanical energy that cannot be used equally well to convert in the opposite direction if so required.
All types of electric motor use the principles of electromagnetism in which either electric currents flowing in wires situated in a magnetic field experience mechanical force or in which electromagnets apply force to a ferromagnetic (intrinsically highly magnetic) material. The electric current used by motors can be either direct current (DC), as obtained from a battery, or alter-nating current (AC), which is generally more convenient to use, mainly because the supply voltage can be raised or lowered effectively by static transformers.
Controls
The speed of some types of AC motors can be easily and efficiently controlled by using a form of thyristor called a triac. Triacs are electronic switches that remain off until triggered by a short pulse of current, after which time they turn on and remain on until the voltage across them falls to zero (which occurs twice in each current cycle). There are two main control techniques for using triacs. Phase control (also called conduction angle control) alters the power fed to the motor by adjusting the point in each half cycle when the trigger pulse is applied—early for high power, late for low power. It is simple to implement but can generate a lot of electromagnetic interference at mid power, when there is a high voltage across the triac immediately before it switches. Burst firing control triggers the triac only near the beginning of half cycles and so is less prone to producing interference.
In many applications, the speed or position of a motor shaft must be measured in order to allow accurate adjustments to be made. The transducer used for making these measurements is called a shaft encoder, which, in its most basic form, consists of a light shining through radial slots in a disk attached to the motor shaft. When the motor is turning, flashes of light are produced that can be detected by a photocell and used to calculate the motor speed. For increased accuracy, the shaft encoder may be connected to the motor through gearing so that it produces pulses at a higher rate. If one of the slots is made significantly wider than all of the others, the resultant long pulse of light can be distinguished and the motor shaft position calculated by counting subsequent pulses.
For reversible motors, a second photocell slightly offset from the original is used to determine direction of rotation by detecting which photocell is illuminated first by each slot. Alternatively, a bank of photocells can be used in conjunction with a complex pattern of slots to give a unique light pattern for every position of the shaft. If a normal binary code is used for the pattern sequence, some shaft movements would result in changes at more than one photocell. If the photocells were read during the transition, an incorrect result might be obtained. To overcome this problem, a sequence called Gray code is used that has the property that only one bit changes between consecutive counts. Special integrated circuits are available to translate between Gray and binary if required.
