AC Motors

Today AC motors are more widely used in industrial applications than DC motors. They are available to operate on single-phase or three-phase supply voltage systems.

This allows the motor control designer to choose the type of motor to fit the application. Most single-phase motors are less than 3 hp; although some larger ones are available, they are not as common. Three-phase motors are available up to several thousand horsepower, although most of the motors that one will be working with will be less than 50 hp.

The AC motor provides several advantages over DC motors. One advantage the AC motor has is its design eliminates the need for brushes and commutators, The second advantage is that its rotating member is made of laminated steel rather than wire that is pressed on a core, which reduces maintenance. The AC motor does not need brushes and commutators since it creates the flux lines in its rotating member by induction. The induction process that is used to get the current into the rotating member is similar to the induction that occurs between the primary and secondary windings of a transformer. This is possible in an AC motor because supply voltage is sinusoidal.

The rotating field in the AC motor is called the rotor, and the stationary field is called the stator. The design of the rotor is different from the rotating armature in the DC motor because it's made completely of laminated steel rather than having copper coils pressed on a laminated steel core.

This allows the AC motor to operate longer than the DC motor with less periodic maintenance, which means that more AC motors are used in industry than DC motors. One should realize that the main reason DC motors were used in industry in the 1940s through the 1960s is that their speed could be controlled more easily than controlling the speed of AC motors. With the advent of variable- frequency drives, the speed of all AC motors can be adjusted more easily than DC motors, and the AC motor requires less maintenance since it does not have brushes.

In this section we introduce each of the different types of AC motors and explain their basic parts, theory of operation, methods of controlling their speed and torque, changing the direction of rotation, and procedures for installation and troubleshooting. This basic information will also allow one to understand methods that motor controls use to take advantage of the motor’s design to provide control.

Also introduced will be the basic parts that are found in all AC motors and explain their operation and function. After the operation of a basic three-phase motor is explained, each type of AC motor is introduced and its special design features and applications for which it's best suited are discussed. One will be able to use this information in recognizing the type of AC motor one is working with: understand the theory of its operation, which will permit one to install and interface it to motor controls; and be able to troubleshoot the motor and quickly determine what faults it has. The three-phase motor is presented first, since some of the parts of a single-phase motor are designed specifically to compensate for the differences between three-phase and single-phase voltage. If one fully understands the characteristics of three-phase voltage and how three-phase motors take advantage of them, one will quickly understand single-phase motors.

Suggested reading: Basic Polyphase Devices and AC Drives: An Introduction