Choosing an ac Motor

Feb. 6, 1998
Small ac motors, typically under 200 W, usually fall into one of eight different classifications

Small ac motors, typically under 200 W, usually fall into one of eight different classifications. Each type boasts a list of qualities that make it suitable for a particular group of applications. A few motor characteristics are illustrated in the following guide.

Induction motors have relatively simple construction based on a squirrel-cage rotor. It is one of the most widely used power units. Its special features include a high efficiency/ power factor with low noise and vibration, high starting torque, and continuous rating. Applications include electronic copying machines, automatic vending machines, electronic computers, machine tools, graphic arts and optical equipment, and pumps.

Reversable motors are similar in construction and performance to induction motors. This type is best suited to applications that require frequent on/off and forward/reverse control. The direction of rotation is switchable under any load condition. Overrun is minimal, achieved by using a simple friction brake, which maintains some grip after the motor stops. They are widely used for power units in medical apparatus, cash dispensers, automatic vending machines, and various types of remote- control units.

Reaction synchronous motors employ a salient rotor that eliminates a dc exciter. It is characterized by its constantly synchronized rotation regardless of changes in the line voltage frequency or load factor. Common applications include power sources for floppy disk drives, teletype machines, facsimile equipment, electromagnetic oscillographs, timers, paper feeders, and various types of recorders.

Torque motors employ a solid iron rotor. Rotational speed varies in inverse proportion to the load, so stable operation occurs through the full speed range. Its special features include wide dynamic range, low speed operation, and locked-rotor torque. Also, changing the applied voltage easily controls torque or speed. The additional input required for locked rotor operation is small, and continuous locked rotor operation is permitted up to a specific voltage. Continuous ratings are specified at normal operating voltages, and intermittent ratings are given for higher voltages. Applications include constant tension winders, opening and closing devices, winder brakes, machine-tool feeders, dividers, clutch devices, and fasteners.

Speed-control motors are voltage-controlled induction or reversible designs with built-in rate generators and are typically used with speed-control circuits. The rate generator produces a signal voltage proportional to motor speed and feeds back to the speed-control circuit. Because equipment speed can be adjusted steplessly by turning a knob, these motors are ideal for applications that require manual speed variations such as belt conveyors and turntables.

Clutch-brake motors are small precision motors with built-in clutches and brakes. Combined with a gearhead, they can be easily used for rapid on/off positioning and cycling, indexing, inching, and intermittent operation. Their compact design makes these motors easy to incorporate in the drive unit of a machine, while requiring minimal installation space. They are widely used in various automated equipment.

Electromagnetic brake motors contain electromagnetically actuated brakes. One type of EM brake motor is fail safe. The armature of the electromagnet opposes the force of a spring when voltage is applied. This releases the brake and lets the rotor shaft freely rotate. When power is removed, the spring energizes the brake and holds the load. These motors are suited to applications requiring frequent, repeated, instantaneous reverse rotation and load holding.

Dynamic brake motors are compact ac motors that contain a braking circuit and phase capacitor. By simply connecting a start/stop switch, the rotors can be stopped within one revolution. Also, a single switch can reverse rotational direction. The fully electronic braking system eliminates abrasion, thus ensuring high reliability. The motors are suited for frequent, repeated on/off and reversing applications.

Information provided by Oriental Motor U.S.A. Corp., 2510 W. 237th St., Torrance, CA 90505.

© 2010 Penton Media, Inc.

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