单项选择题
When current and voltage reach maximum values at the same time, the power factor is（）
A.minimum B.maximum C.leading D.lagging
单项选择题 Which is an acceptable means of correcting high mica between commutator bars?（）
单项选择题 Generator brushes are staggered to（）
单项选择题 The voltage output of an AC generator is controlled by（）
