Multi speed squirrel-cage motors may be of the single-winding or two-winding type. The former have a stator winding, which can be connected to give either one of two speeds having a ratio of 2:1. The method of connection is usually furnished by the controller manufacturer.

The frame of the two-speed single-winding motor is about the same as that of the single-speed motor. The two-winding motor has two separate stator windings, which can be wound for any number of poles so that any two synchronous speeds can be obtained.

In addition one or both of the stator windings may be arranged for reconnection as in a single winding motor, giving a total of three or four speeds, but the two speeds obtained on a single winding must have a ratio of 2:1. Thus, a four-speed two-winding motor might have speeds of 1800, 900, 1200, and 600 r/min.

Multispeed motors are designed as (1) variabletorque motors, (2) constant-torque motors, and (3) constant- horsepower motors. The rated torque at four speed points for each type is shown in Fig. 20 35.

Variable torque motors have 1200/600 r/min, and are used on loads, such as in centrifugal pumps and fans whose horsepower requirement decreases more rapidly than the square of the reduction in speed.

Constant-torque motors have horsepower ratings at each speed directly proportional to the speed, for example, 20/10 hp and 1200/600 r/min, and are used on conveyors, mixers, reciprocating compressors, printing presses, and other “constanttorque” loads.

Constant-horsepower motors have the same horsepower rating at all speeds. They are used principally on machine tools, such as lathes, boring mills, planers, and radial drills.

Multispeed motors of the constant-torque or variable-torque type are usually given a standard horsepower rating at the top speed but may have odd horsepower ratings at the lower speeds, since the latter are fixed by the speed ratios.

FIGURE 20-35 Basic load characteristics of multispeed motors having a 4:1 maximum speed ratio: (a) power; (b) torque.

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