Air Gap

If the air gap around a rotor is not uniform the motor may not start in certain positions. Because the rotor is not centred, probably due to worn bearings, there is an out of balance magnetic pull. Radial play in between the shaft and the housing should be detected by hand and bearing wear detected by feeler gauge between the rotor and the stator, or armature and field poles may be measured at three or four fairly equidistant points around the machine. If possible one measurement should be made at the bottom of the machine and another in line with the drive. Compare with previous records to check wear. At minimum air gap. Clearance of the bearings should be renewed to avoid the possibility of the rotor rubbing on the stator.
On small machines two feelers on opposite sides of the rotor should be used to avoid error caused by rotor movement from normal position when only one feeler gauge is used.
In synchronous motors and D.C. motors sparking may occur if the radial air gaps between the armature and the field poles are unequal. If necessary renew bearings or add or remove soft iron shims from under the pole shoes. Unequal field strength has a similar effect of sparking at the brushes. This might be due to short circuit or earth fault on the field coils, or a short circuit on the shunt and field coils.
An increase of air gap gives an increase in 'reluctance'.
In a salient pole A.C. generator this fact may be used to produce a sinusoidal flux density curve by gradually increasing the length if the air gap towards the pole tips.
In the induction motor the air gap should be as small as possible if the motor is to act with a high power factor. An increase in air gap increases the reactance of the motor and lowers its power factor. Small motors are accurately machined and centring of the rotor is very important so ball or roller bearings are fitted.

Air gap	Motor size
0.25mm	1kW
0.75mm	10kW
2.0mm	100kW