单项选择题

Sparking at the brushes of a running motor could be an indication of （）.

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

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单项选择题
If a DC motor runs hot, the cause may be （）

A.high mica condition
B.low ambient temperature
C.clogged ventilation ducts
D.an open in the shunt field
单项选择题
The unit of electrical resistance is the （）

A.ampere
B.volt
C.watt
D.ohm
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