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A dc generator is largely obsolete today because

  • A.no machine can be built to generate direct current economically
  • B.alternating current machines cannot be built at large ratings
  • C.rectifiers produce direct current more cheaply and reliably
  • D.direct current is no longer used for any purpose

Answer: C

A transformer and a solid-state rectifier have no commutator to maintain and are cheaper for the same output, so dc is now almost always made from ac. The dc machine survives in traction and industrial installations that are still in service.

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