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

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单项选择题

A.90~0 kw
B.127~27 kw
C.155~70 kw
D.194~85 kw

单项选择题

A.normal operation
B.a dirty commutator
C.increased brush capacity
D.water vapor absorption

单项选择题

A.high mica condition
B.low ambient temperature
C.clogged ventilation ducts
D.an open in the shunt field