单项选择题
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 （）
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
单项选择题 In order to change the direction of rotation of a D-Cmotor （） (1) the field leads must be changed (2) the input leads must be changed