单项选择题
Which of the following types of motors is often designed for use in correcting power factor?（）
A.Polyphase B.Wound-rotor C.Induction D.Synchronous
单项选择题 Power transformers are rated in （）.
单项选择题 The purpose of DC generator brushes is to （）.
单项选择题 Which of the listed precautions should be taken when cleaning the internals of a motor with compressed air?（）
