We discovered a very similar state ofaffairs when we studied the d.c. machine (Chapter 3): its induced motional or ‘back’e.m.f. (E) turned out to be proportional to the field current, and to the speed ofrotation of the armature. The main difference between the d.c. machine and thesynchronous machine is that in the d.c. machine the field is stationary and thearmature rotates, whereas in the synchronous machine the field system rotates whilethe stator windings are at rest: in other words, one could describe the synchronousmachine, loosely, as an ‘inside-out’ d.c. machine.We also saw in Chapter 3 that when the unloaded d.c. machine was connectedto a constant-voltage d.c. supply, it ran at a speed such that the induced e.m.f. was(almost) equal to the supply voltage, so that the no-load current was almost zero.
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