Because of their advantages, many types of aircraft motors are designed to operate on alternating current. In general, ac motors are less expensive than comparable dc motors. In many instances, ac motors do not use brushes and commutators and, therefore, sparking at the brushes is avoided. They are very reliable and very little maintenance is needed. Also, they are well suited for constant speed applications and certain types are manufactured that have, within limits, variable speed characteristics. Alternating current motors are designed to operate on polyphase or single phase lines and at several voltage ratings.
The subject of ac motors is very extensive, and no attempt has been made to cover the entire field. Only the types of ac motors most common to aircraft systems are discussed in detail.
The speed of rotation of an ac motor depends upon the number of poles and the frequency of the electrical source of power:
Since airplane electrical systems typically operate at 400 cycles, an electric motor at this frequency operates at about seven times the speed of a 60 cycle commercial motor with the same number of poles. Because of this high speed of rotation, 400 cycle ac motors are suitable for operating small high speed rotors, through reduction gears, in lifting and moving heavy loads, such as the wing flaps, the retractable landing gear, and the starting of engines. The 400 cycle induction type motor operates at speeds ranging from 6,000 rpm to 24,000 rpm.
Alternating current motors are rated in horsepower output, operating voltage, full load current, speed, number of phases, and frequency. Whether the motors operate continuously or intermittently (for short intervals) is also considered in the rating.