- - Estimating Motor Load -

*Re: “Resolving Voltage Problems with AC Induction Motors,” Doug Dorr and Philip Lim, March 2001, page 41* .

This article convincingly explains the influence of voltage quality factors on induction motors (IMs) and proposes a helpful procedure for resolving potential problems. I would like to add a few suggestions.

First, the abbreviation AC should not be placed before “induction motors” because induction motors can only be AC.

Second, in Step 6 under “Estimating Motor Load” (page 45), the authors suggest calculating the motor load by dividing the real slip by the full-load slip. This assumes that the working part of the curve (induction motor load vs. slip) is linear. However, this part is only close to linear, not exactly linear.

In the above graph, the working part of the induction motor load is shown as a function of slip. Full-load slip equals 2.61%. You can derive the motor load values by two methods: slip division (as proposed in the article) or the characteristics shown in the graph. In the second case, you determine the load difference (ÄP) as follows:

• ÄP = (P1 - P2) * 2/(P1 + P2) * 100, %

As you can see, the 1.89% slip value is equal to a motor load value of 75%. If you use the slip division method, the motor load value would be 72% with a load difference of 4%. This motor load value is approximately 4% smaller than the one derived from the graph's characteristics.

This demonstrates that slip division calculations may be unacceptably large and must be decreased. Calculations will always be approximate due to inaccuracies in the data, but we should be more conservative and figure the allowable decreased motor load. I propose to multiply the motor load values obtained in Step 6 by a 1.05 correction factor. This will provide real (not lowered) values, which will help power quality personnel avoid electrical insulation overheating and prolong the life of induction motors.

Gennady Grevnin Consolidated Edison Co. of N.Y.

Author's responseMr. Grevnin is correct in saying that induction motors can only be AC. The more common terms are “induction machines” and “AC motors,” but it is not technically incorrect to use the term “AC induction motor.”

I also agree that his proposed calculation is more precise. When writing the article, Philip Lim and I were clearly aware of the slight nonlinearity in the load vs. slip curve. We also knew the steady-state voltage applied at the motor terminals would impact the slip. Our aim was to develop a useful means of estimating motor load with the nameplate as the only available data. In the field, the motor impedance data needed to perform the more accurate calculation is not readily available, but the nameplate data should always be attached to the motor.

I do like Mr. Grevnin's proposal to multiply the resultant value obtained in Step 6 by 1.05 to get a more conservative load estimate, especially when the calculation puts you in the gray area between needing to reduce the percent imbalance or motor load.

Doug Dorr EPRI PEAC Corp.

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