| Every service technician
should have at least one multi-speed motor in his or her truck to help in
making acceptable substitutions in the field.
Multi-speed motors come in two basic varieties. The first variety has
an extra set of windings called a booster winding that behaves like a
transformer. The second variety comes with two distinct separate sets
of windings.
So how do these motors work? Remember from last issue how you determine
motor speed by the number of poles (poles divided by the constant 7,200
gives you revolutions per minute). When the motor is under load, however,
the rotating part of the motor slows down or “slips.”
If the load is constant, you can increase the slip by weakening the strength
of the spinning magnetic field. One way is to decrease the voltage to
the magnet wire that makes up the poles.
You can decrease the voltage externally by using a speed control or internally
through the use of the booster winding in a multi-speed motor. In other
words, the booster winding acts like a transformer, changing incoming
line voltage to a lower voltage at the windings.
The booster winding may come with taps that allow you to apply different
voltages to the poles, creating different speeds in the motor. Remember
that “speed” taps just affect the strength of the spinning
field—not the actual speed—meaning you can only affect motor
speed with a load on the shaft. This is because slip occurs when the load
works against the weakened magnetic field. Consequently, if you bench
test a multi-speed tapped motor using a tachometer or strobe, you will
detect little variation between speed taps since there’s no load.
Since the booster winding method of creating multiple speeds involves
using a motor with just a single set of pole windings, you’ll find
that the horsepower is always lower as the speed (voltage) is reduced.
Consequently, this design is generally unsuitable for loads other than
fans.
The second type of multi-speed motor with two completely separate sets
of windings allows you to use one or the other speed at a given time.
Having two pole sets wound independently offers you more flexibility to
produce constant horsepower in mechanical applications since you are energizing
just one set of poles at a time.
We also refer to a multi-speed motor “weakened” by speed taps
as a multi-horsepower motor. For example, if you have a 1/3 horsepower
three-speed motor, it generally would deliver 1/3 horsepower when connected
to its high-speed tap, ¼ horsepower at its middle-speed tap, and
1/6 horsepower on low speed.
Knowing this, you can begin to appreciate the versatility of multi-speed
motors in the field. You could use the above multi-speed motor to replace
single-speed 1/3 horsepower, ¼ horsepower, or 1/6 horsepower motors
with the same number of poles. To achieve the correct results, simply
select the correct tap and carefully insulate the two unused taps. The
result would be a motor that produces the same performance, similar fan
noise characteristics, and the same static pressure as the original single-speed
model.
Multi-speed motors give the service technician another versatile tool
in the field. That's why is always good to have some in stock for emergency
substitutions.
by Neil Simon, aka the Motor Doctor - regional sales
manager for A. O. Smith Electrical Products Company.
|
