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The Air Gap in Rotating Machines: A Necessary Evil

Efficient operation of electromagnetic devices requires that the magnetic circuit contain materials (such as iron) offering low resistance to the passage of magnetic flux. That minimizes the amount of electrical energy needed to create the magnetic field.

However, gaps in the magnetic circuit are normally unavoidable. They contain air. Because that offers high resistance to magnetic flux, such gaps require undesirable increases in magnetizing current and the associated electrical loss.

This is particularly true in motors and generators. The air gap needed to separate the revolving rotor from the stator should be as small as possible to reduce the magnetizing power requirement, yet large enough to prevent contact between the two despite manufacturing tolerances on their dimensions, or movement resulting from mechanical deflection and looseness in supporting bearings.

Few design guidelines exist for selecting the air gap size best suited to any rotating machine. For induction motors, in power ratings from 3/4 to 750 kilowatts, practical values from 0.2 to 5 mm are typical; the higher the motor speed, the larger the gap. A common empirical calculation involves rotor peripheral speed, core stack length, and rotor diameter. Although increasing the air gap will increase magnetizing current, it will also tend to decrease stray load loss.

In any event, the gap must be large enough to assure that eccentricity of the rotor with respect to the stator will not cause the shaft stiffness to be overcome by unbalanced magnetic pull, which could allow the rotor to strike the stator.

Whatever gap is used, it must be uniform. A non-uniform gap causes increased noise and vibration. Variation is commonly limited to plus or minus 10 percent of the average value.

In synchronous and d-c machines, two separate fields interact in the air gap. The a-c field created by the armature (stationary in the synchronous machine, rotating in the d-c machine) distorts that supplied by the d-c field, reducing its effectiveness and degrading machine performance. Increasing the air gap lessens the effect of that "armature reaction." Hence, these machines will have air gaps several times larger than those in induction motors.

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