单项选择题
A.two frequencies must be brought into phase
B.the incoming machine accepts a small amount of load
C.the running machine must be unload
D.both voltages are equal
单项选择题 The magnetic field is provided by electromagnets so arranged that adjacent poles have （）.
单项选择题 An a-cswitchboard is usually（） the corresponding d-cboard because of the higher voltage and lower currents and smaller bus-bars.
单项选择题 The star formation is most commonly used and requires（） on the alternator.