Aircraft DC motors
DC electric motors are the inverse of the DC generator. They comprise armature field windings and commutator/brushgear and are similarly self excited. The main elements of importance in relation to motors are the speed and torque characteristics, i.e. the variations of speed and torque with load respectively. Motors are categorised by their field winding configuration (as for generators) and typical examples are series-wound electric motors, shunt-wound electric motors and compound-wound (a combination of series- and shunt-wound). Each of these types of motor offers differing performance characteristics that may be matched to the application for which they are intended.
A specialized form of series motor is the split-field motor where two sets of series windings of opposite polarity are each used in series with the armature but parallel with each other.
Either one set of field windings or the other may receive power at any one time and therefore the motor may run bi-directionally depending upon which winding is energized. When used in conjunction with suitable switches or relays this type of motor is particularly useful for powering loads such as fuel system valves where there may be a requirement to change the position of various valves several times during flight. Limit switches at the end of the actuator travel prevent the motor/actuator from overrunning once the desired position has been reached. Split-field motors are commonly used for linear and rotary position actuators when used in conjunction with the necessary position feedback control. DC motors are most likely to be used for linear and rotary aerospace actuators, fuel valve actuation and starter functions.
Aircraft AC motors
Aircraft AC motors are most commonly of the `induction motor' type. Induction motors operate upon the principle that a rotating magnetic field is set up by the AC field current supplied to two or more stator windings (usually three-phase). A simple rotor, sometimes called a `squirrel cage', will rotate under the effects of this rotating magnetic field without the need for brushgear or slip rings; the motor is therefore simple in construction and reliable. The speed of rotation of induction motors depends upon the frequency of the applied voltage and the number of pairs of poles used. The advantage of the induction motor for airborne uses is that there is always a source of constant frequency AC power available and for constant rated applications it offers a very cost-effective solution. Single-phase induction motors also exist, however these require a second set of phase windings to be switched in during the start phase, as single phase windings can merely sustain and not start synchronous running.
AC motors are most likely to be used for continuous operation, i.e. those applications where motors are continuously operating during flight, such as fuel booster pumps, flight instrument gyroscopes and air-conditioning cooling fans.