Types of Motors

Direct Current (DC) Motors

Picture of a DC motor. Illustration recreated for web by Technologists Inc. using graphic supplied by Dr. R. Naves, Department of Physics and Astronomy, Georgia State University to PNL, for use with FEMP O&M Best Practices, as a model. - Click image to enlarge

DC motor—Click image to enlarge and view reference

Direct-current (DC) motors are often used in variable speed applications. The DC motor can be designed to run at any speed within the limits imposed by centrifugal forces and commutation considerations. Many machine tools also use DC motors because of the ease with which speed can be adjusted.

All DC motors, other than the relatively small brushless types, use a commutator assembly on the rotor. This requires periodic maintenance and is partly responsible for the added cost of a DC motor when compared to an alternate-current (AC) squirrel-cage induction motor of the same power. The speed adjustment flexibility often justifies the extra cost.

Alternate Current (AC) Motors

Picture of an AC motor. Illustration recreated for web by Technologists Inc. using graphic supplied by Dr. R. Naves, Department of Physics and Astronomy, Georgia State University to PNL, for use with FEMP O&M Best Practices, as a model. - Click image to enlarge

AC motor
Click image to enlarge and view reference

Alternate-current (AC) motors - As in the DC motor case, an AC motor has a current passed through the coil, generating a torque on the coil. The design of an AC motor is considerably more involved than the design of a DC motor. The magnetic field is produced by an electromagnet powered by the same AC voltage as the motor coil. The coils that produce the magnetic field are traditionally called the "field coils" while the coils and the solid core that rotates is called the "armature."

Two basic types of AC motors:

Induction motor—The induction motor is a three-phase AC motor and is the most widely used machine. Its characteristic features are:

Simple and rugged construction.
Low cost and minimum maintenance.
High reliability and sufficiently high efficiency.
Needs no extra starting motor and does not need to be synchronized.

Two basic assemblies make up an induction motor: the stator assembly and the rotor assembly. The motor "induces" current into the rotor by the rotating magnetic flux produced in the stator. Motor torque is developed from interaction of currents flowing in the rotor bars and the stator's rotating magnetic field. When a three-phase voltage is applied to the stator winding, a rotating magnetic field of constant magnitude is produced. This rotating field is produced by the contributions of space-displaced phase windings carrying appropriate time displaced currents. The rotating field induces an electromotive force (emf).

Synchronous motor—The most obvious characteristic of a synchronous motor is its strict synchronism with the power line frequency. The reason the industrial user is likely to prefer a synchronous motor is its higher efficiency and the opportunity for the user to adjust the motor's power factor.

A specially designed motor controller performs these operations in the proper sequence and at the proper times during the starting process.