单项选择题
The number of cycles per second occurring in AC voltage is known as the （）
A.phase angle B.frequency C.wave form D.half mode
单项选择题 Complete maintenance of electrical motors should include periodic checks for machine （）
单项选择题 The frequency of an alternator is controlled from the main switchboard by adjusting the（）
单项选择题 The frequency of an AC generator is controlled by the （）
