The length of air gap is inversely proportional to power factor and hence result in poor operating power factor of an electrical machine. Larger air gap length means higher resistance to magnetic flux which causes undesirable increases in magnetizing current and the associated electrical loss. Why would that not mean that air gap length would be minimized in all types of electric machines?
Electrical – Why is the air gap in synchronous machines larger than in asynchronous machines
electric-machine
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Best Answer
The statement is true only for induction (asynchronous) machines. Reactive current is only required in the armature to magnetize the field. In permanent-magnet synchronous machines, no external magnetizing current is required. In wound-field synchronous machines, as much DC field excitation as is desired is supplied by a separate excitation system. For a synchronous generator, lagging reactive current requirement of the load can be supplied by increasing the excitation. For a synchronous motor, the DC excitation can be adjusted to keep the AC supplied to the motor at 1.0 pf. The motor can also be "over excited" to supply reactive current to other loads.
Induction machines are designed with the smallest air gap that mechanical construction will permit. Mechanical considerations require the air gap to be directly proportional to rotor diameter. Synchronous machines can have a larger air gap than induction machines and empirical data indicates that they do. That might be desirable for cooling or to make the machine more "rugged".
Source: John. H. Kuhlmann, Design of Electrical Apparatus, 1940
Source: Juha Pyrhonen, Tapani Jokinen , Valeria Hrabovcova, Design of Rotating Electrical Machines 1st Edition 2008