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The quantity that ultimately limits the size of a dc machine is

  • A.the flux the field winding can produce
  • B.the mechanical strength of the shaft
  • C.the insulation class of the windings
  • D.the commutation duty at the brushes

Answer: D

Reactance voltage grows with current and speed, and beyond a certain product no arrangement of interpoles and brush grades will commutate cleanly. That ceiling is a large part of why very large variable-speed drives are ac rather than dc.

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