Which of the following method of speed control of D.C. machine will offer minimum efficiency ?

Which of the following method of speed control of D.C. machine will offer minimum efficiency ?

Q. Which of the following method of speed control of D.C. machine will offer minimum efficiency ?

A.            Voltage control method

B.            Field control method

C.            Armature control method

D.            All above methods

Answer: Armature control method

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