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
单项选择题 If a DC motor runs hot, the cause may be （）
单项选择题 The unit of electrical resistance is the （）
单项选择题 Some large A-Cmotors are equipped with heatersThese could be （） (1)resistance heaters bolted directly to the frame (2)low voltage windings embedded in the motor windings
