Chuck Yung
EASA Senior Technical Support Specialist
Anyone who has spent much time reading the EASA AC Motor Redesign book, or motor design textbooks, will recall that there are certain stator-rotor slot combinations that can detract from the performance of an electric motor.
Rotor skew: The rotor bars are angled from end-to-end to distribute the bars across more than one stator slot. This reduces noise and smooths out the torque curve. See Figure 1.
Cusp: A dip in the motor torque curve, during acceleration. Sometimes called a “saddle torque.” See Figure 2 A and B.
Cogging: When the stator-rotor slot combination is such that several alignment combinations are possible, the motor may develop different torque for various o'clock positions of the rotor.
Most of these are readily identified by counting the number of stator slots and rotor bars, then comparing the difference to Table 1 below. The number of poles is a factor, which is why we should always consult the table when changing the motor speed.
While the columns for cogging and cusp are clear, the causal factors for noise cannot always be predicted mathematically. Occasionally a manufacturer builds a motor that, according to our table, ought to be fine, but the motor is noisy. Then the designer adds that to his/her list of exceptions to the rule, and they try to avoid that combination in the future.
Benefits of skew
It is understandable that manufacturers will sometimes skew the rotor bars to avoid all those possibilities. The skew is a simple way to smooth out anomalies and improve the torque curve. But there is a trade-off when a rotor is skewed. The motor efficiency decreases because more heat is generated in the rotor. With the emphasis on motor efficiency, a manufacturer is less likely to skew the rotor bars. And since torque is perpendicular to the rotor bars, it creates an axial component to the torque. That is rarely a problem with a ball bearing machine, but in a sleeve bearing machine, too great a skew angle will move the shaft axially.
Because manufacturers have so much experience to draw upon, sometimes they recognize that a cusp is fairly slight (as illustrated in Figure 2A), and – knowing the application for which the motor is built – elect to supply the motor as is. As long as the bottom of the cusp is above the torque curve demanded by the load, it should not hurt performance.
It is that latter case when a motor is designed to meet performance requirements, which can suddenly become interesting. The motor might run for years, even decades, with no discernible trouble. But then the customer decides to install an electronic soft-start, a wye-delta starter, or an autotransformer to reduce starting current. As a result, the torque curve shifts down (Figure 2B) and the motor is unable to accelerate the load.
One recent call was a case where the customer had shorted the rotor leads of a wound rotor motor and ran it using direct-on-line starting for years. A wound rotor is very likely to have a “bad” stator-rotor slot combination, as both the stator and rotor require a slot count divisible by 3 phases, and compatible with the number of poles.
Eventually, the customer decided that a soft-start would be helpful. Afterward, the motor would only start about half the time. An evaluation confirmed that, sure enough, the stator-rotor slot combination indicated cogging.
With reduced voltage
The result is illustrated in Figure 2B where the cusp now dips below the load torque curve. Under this condition, the motor would no longer accelerate, and – if the problem was not recognized – the motor would fail due to thermal overload. (It would burn up.) The motor is still the same robust motor that provided decades of service. But by adding a reduced-voltage starter (soft-starter), the user has created a situation in which the motor will fail.
Other consequences
Cogging is another possible consequence of reduced voltage starting. Depending on the stator-rotor slot combination, the motor might start numerous times successfully, and only fail to start when the rotor coasts to a stop at just the wrong position.
As Figure 3 illustrates, the relative position of stator and rotor slots results in several possible torque curves. If anyone of them crosses the torque curve of the load, the motor will stop accelerating, and fail.
So when helping a customer trouble-shoot a starting issue, don’t rule out a bad stator-rotor slot combination just because the motor was in service for years. A weak bus – perhaps because the customer added more load, and the transformer is inadequate for the total kVA; the addition of a soft-start method (Y-delta, electronic soft starter, etc.), one or more open rotor bars; any of these could be the underlying reason for a motor that will no longer start. If the nameplate is not original, someone less skilled might have redesigned it for a different speed, resulting in the bad combination. And yes, while very uncommon, I have seen bad stator-rotor slot combinations from manufacturers.
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