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Summary:
"The Importance of
'Slot Combination' in A-C Motor Design" With
trilingual summary
Since the earliest years of a-c
machinery, motor designers have struggled with prediction of -- and
cures for -- electromagnetic noise, vibration, and torque anomalies
arising from the interaction between stator and rotor slots. The
number of stator slots, N1, is chosen mainly for flexibility in
winding configuration. Heat dissipation is also a concern. Ideally,
N1 is evenly divisible by the product of phases and poles.
To minimize noise, vibration, and
torque irregularities, the number of rotor slots N2 must then avoid
relationships with N1 which theory or experience indicate will
produce objectionable harmonic force waves in the magnetic field of
the machine. Many published guidelines, some of them contradictory,
have appeared over the past century.
For example, designers often contend
that N1 should always be larger than N2, and that the two numbers
should be separated by at least 15 percent, but no more than 25 to
30 percent. Others claim that N2 should always be an even number,
not divisible by the number of poles.
Complying with all the rules is seldom
possible. Even when an N1 and N2 combination does produce a
frequency that excites stator core resonance, the effect may not be
objectionable because of structural damping in the assembly. That
will vary with motor construction. Thus, a slot combination
successful in one frame size, for one manufacturer, may be
unacceptable elsewhere.
Wound-rotor motors pose special
problems because, unlike the squirrel-cage, the rotor must contain a
balanced polyphase winding. Synchronous machines, in which damper
winding configuration cannot be greatly varied, also may require
values of N1 not used for induction machines.
Slotting used at one polarity may not
suit two or more speeds. Service shops asked to convert a
single-speed motor to a two-speed rating can seldom assure that the
slot combination will not cause operating problems at one speed or
the other.
In large machines, when stator core
diameters exceed the available sheet steel width, each lamination
circle is made up of segments. That further restricts N1 (best
chosen to be integrally divisible by the number of segments) and
therefore N2 as well.
At higher motor voltages, particularly
above 4000, thicker insulation dictates fewer (and larger) stator
slots. If N2 remains smaller than N1, the rotor cage bars may become
unacceptably large. In both stator and rotor, heat transfer areas
relative to heat-producing conductor material will decrease, and the
designer is likely to make N2 larger than N1 -- at the price of
higher stray load loss.
Thus, choosing the best slot
combination for any motor is no simple task. Accurate predictions of
the effect of even slight changes continue to be difficult.
From "The
Importance of 'Slot Combination' in A-C Motor Design"
...by
Richard L. Nailen, EA Engineering Editor - published in
Electrical Apparatus
June 2005
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