Most of the power-generating systems on aircraft produce AC. For this reason, a majority of the motors used throughout the aircraft are designed to operate on AC. There are other advantages in the use of AC motors besides the wide availability of AC power. In general, AC motors cost less than DC motors. Some types of AC motors do not use brushes and commutators. This eliminates many problems of maintenance and wear. It also eliminates the problem of dangerous sparking.
An AC motor is particularly well suited for constant-speed applications. This is because its speed is determined by the frequency of the AC voltage applied to the motor terminals.
The DC motor is better suited than an AC motor for some uses, such as those that require variable-speeds. An AC motor can also be made with variable speed characteristics but only within certain limits.
Industry builds AC motors in different sizes shapes and ratings for many different types of jobs. These motors are designed for use with either polyphase or single-phase power systems.
In this chapter, AC motors will be divided into series, synchronous and induction motors. Synchronous motors may be considered as polyphase motors of constant speed, whose rotors are energized with DC voltage. Induction motors, single-phase or polyphase, whose rotors are energized by induction, are the most commonly used AC motor. The series AC motor is a familiar type of motor.