单项选择题
A.Reverse current protection B.Reverse power protection C.Synchronising device D.Frequency meter
To parallel an incoming machine to a running machine th...
单项选择题To parallel an incoming machine to a running machine therefore it is necessary to ensure firstly that （）.
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
The magnetic field is provided by electromagnets so arr...
单项选择题The magnetic field is provided by electromagnets so arranged that adjacent poles have （）.
A.opposite polarity B.the same polarity C.negative polarity D.positive polarity
An a-cswitchboard is usually（） the corresponding d-cboa...
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.
A.larger and heavier than B.as much as C.smaller and lighter than D.more dangerous than
