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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