单项选择题
Electrical failures in motors are caused by the breakdown of insulation, which may be caused by （）
A.penetration of moisture B.accumulation of dirt C.overheating D.All of the above
单项选择题 A three-phase alternator operates at 450 volts with a power factor of 0~8.If the ammeter indicates 250 amperes, what should be the KW meter reading （）.
单项选择题 Sparking at the brushes of a running motor could be an indication of （）.
单项选择题 If a DC motor runs hot, the cause may be （）
