Synchronous machines may be either inner or external pole machines (Cp Figure 9). As direct current power required for excitation is relatively small as compared to alternating current energy, it is more economical to feed the rotors via slip rings with direct voltage. Alternating voltage can then be fed through permanent terminals, resp. tapped off. For this reason inner pole machines are generally manufactured.
Synchronous machines are mainly inner pole machines.
The stand of the inner pole machine comprises a steel casing containing a lamella pack with magnetically high-grade iron.
The windings are housed in the inner-positioned grooves. In the case of the single-phase machine these are distributed around some two thirds of the total circumference whereas, in the case of a three phase synchronous machine, three coils have each been displaced spatially within 120 degrees.
L1; L2; L3 Three-phase conductor
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