A necessary evil: the air gap in rotating machines

Electrical Apparatus, Sep 2005 by Nailen, Richard L

But it often gets overlooked when it should be getting close attention

NOT TOO BIG, NOT TOO SMALL. THE ESSENtial air gap separating rotor from stator in any electric motor or generator has several important influences on machine performance-nevertheless, in any design discussion, few essential details get less attention than the air gap.

Air offers much higher resistance to the passage of magnetic flux than the so-called "magnetic materials." Such materials allow a far more intense field to pass through a given space with a relatively lower driving potential (what we call magnetomotive force or mmf). That force is measured in ampere-turns within the electrical circuit causing the magnetization.

Naturally, then, we want to minimize the intervention of air into the magnetic field path. Yet it can't be omitted. It can't be too small. Discontinuities or joints in the magnetic core material are necessary also in a transformer or other "solid-state" electromagnetic apparatus, but are kept small or in staggered locations, to minimize their influence.

Special demands of motors and generators

The electric motor (or generator) presents the unique necessity for an air gap fully separating a rotating magnetic structure from a stationary one. Not only must the separation be complete; it must be large enough so that manufacturing tolerances will not allow the separated components to come into damaging contact during machine operation.

As a further complication, these components exert a strong magnetic attraction on each other. The mechanical structure-including the shaft of the rotating component-must be stiff enough to maintain separation despite that pull. In large machines, the magnetic pull is calculated, then carefully compared to the restraining stiffness, to ensure the assembly's "magnetic stability." (see Figures 1 and 2.)

The high "reluctance" of air means that for every unit length of magnetic flux path, the mmf required to drive flux through the air portion of the path will far exceed (as much as tenfold) what's needed for the magnetic (steel) portion. Consequently, the machine's magnetizing current (and the associated PR loss in the winding) will be determined largely by the size of the air gap. The larger that gap in an induction motor, the lower the power factor.

That creates an unavoidable contradiction in the design process. For mechanical reasons, we want to avoid too small an air gap; for electrical reasons, we don't want it too large.

Attempts to generalize about this are liable to be wide of the mark, by failing to account for all the influences at work. For example, when "energy efficient" motors were being introduced, the claim was often made that they used "smaller air gaps" to reduce losses. In its 1992 report (TR-101290, Vol. 2) on energy efficient motors, the Electric Power Research Institute cited a decreased air gap as one of seven basic ways to increase motor efficiency.

The reasoning is that a smaller gap meant less magnetizing current and therefore a lower stator copper loss. That's true. However, in the theoretical calculation of stray load loss for an induction motor, some component losses vary as the inverse square of magnetizing current. By increasing that current, a larger gap reduces those loss components. Some "energy efficient" motors have therefore used larger air gaps for the best overall effect. In this, as in many other design issues, the answer to the question "what's best?" is, "that depends."

A larger air gap can also reduce electromagnetic noise. Although slot combination and winding configuration exert major influences on noise production, they're seldom subject to change after the machine is built. But studies have shown that doubling the air gap can reduce overall sound pressure level by at least 10 decibels. Although that large a gap increase is seldom possible, significant noise reduction can occur with much smaller gap increases.

A necessary compromise

The resulting compromise is a familiar one. For a given rotor diameter, the slower an a-c machine speed (meaning the larger the number of magnetic poles in the winding), the smaller the gap. Horsepower output will be lower, and the power factor lower as well, so that the electrical effect of a large gap is relatively less acceptable than for a high-speed, higher-horsepower machine having that same rotor diameter. As polarity decreases and both speed and horsepower go up, the trend reverses.

In published material on basic design, the most common guidelines relate power and speed to two basic dimensions: core diameter squared, and core stack length, or DL in shorthand notation. There are a couple of variations. Theoretically, the diameter should be the stator inside diameter, what's often called the bore or gap diameter. Some designers, however, have used the core outside diameter instead. Also, the exponent (the power to which the diameter is raised) has been quoted as either 2.0 or 2.5.

Given those properties, the design practices usually go on to recommend various approaches to selecting the number and size of slots, the winding configuration, and arriving at estimated heat dissipation and performance. Seldom, however, is any firm guidance offered on the selection of air gap.