A necessary evil: the air gap in rotating machines

Electrical Apparatus, Sep 2005 by Nailen, Richard L

How does the air gap influence locked-rotor current (and torque)? For gap values within a range that's practical in view of the other limitations discussed here, the answer is "not much."

The relationship is not simple. Several components of both stator and rotor reactance are inversely proportional to the air gap, while other components are unaffected. But the amount of change is highly variable depending upon slot dimensions and stator winding configuration. In any event, an air gap variation of even 20% is unlikely to make a significant change in locked-rotor performance.

"Increasing the air gap" often implies simply machining an existing rotor to a smaller diameter. That decreases the rotor slot reactance (and, as we've seen, several other consequences can be expected as well). If, however, a machine is designed and built for a larger gap, the slot configuration-and the associated reactance-can remain unchanged despite a smaller rotor diameter.

A common limit on measured air gap variation in an assembled motor is ±10% of the average. However, close examination of that requirement reveals that it is not as simple as it seems. To see why, consider some readings for a hypothetical machine. Suppose the gap intended by design is 0.06 inch. Now suppose a series of measurements yields these actual values:

0.050, 0.052, 0.061, and 0.064

What is the "average"? Adding all four readings together and dividing by 4, we get 0.059 (quite close to the design figure). The 10% variation allows readings to be anywhere from 0.054 to 0.065. Two of the measurements fall below the minimum, which is unacceptable.

An obvious solution is machining, to enlarge the gap. Assume the possibility of an increase by 0.006. We would then get this new set of measurements:

0.056, 0.058, 0.067, and 0.070

Now, the average is 0.063. The 10% limits become 0.057 and 0.069. One reading is still below the minimum, and now one other reading exceeds the maximum.

What's happening is that any action taken to bring readings closer to a numerical average will also change that average. To get past this problem, we can go to the extreme of increasing the gap by 0.015. The measurements then become:

0.065, 0.067, 0.076, and 0.079

The average is 0.072; limits are 0.065 and 0.079, and all readings are now acceptable (barely)-except that the average gap has been increased by 25%, which is unacceptable regardless of eccentricity.

Additional complications

A further complication is the variation in gap that may occur from one end of the machine to the other. Still another is the way measurements are made. Actual feeler gauge readings of the gap in an assembled machine are possible only for the larger horsepower ratings, and only at certain locations.

An out-of-tolerance value may exist over only a small area, and therefore have little influence. No single set of four or eight measurements can establish the condition of the total peripheral gap area.

In a large, high-speed machine, with an air gap of 1/8 inch or more, slight changes are seldom troublesome-but neither are they greatly effective in achieving some desired performance change.