Discussion
A permanent magnet dc motor differs from a separately excited one in that its flux
Answer: C
Correct answer: cannot be varied
The flux of a permanent magnet machine is fixed by its magnets, so there is no field winding to adjust and no field weakening in the ordinary sense - speed above base can only be obtained by injecting a demagnetising armature current, which is inefficient. The compensations are real: no field copper loss, no excitation supply, higher efficiency and a smaller frame for the same output. The magnets do weaken somewhat with temperature, but reversibly, not steadily.
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