Electrical Apparatus magazine, June 2005 The Importance of 'Slot Combination' in A-C Motor Design

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.

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Featured Technical Article
June 2005

On this page is a summary of the Electrical Apparatus June 2005 featured technical article, by Richard L. Nailen, P.E. , "The Importance of 'Slot Combination' in A-C Motor Design"
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