單項(xiàng)選擇題Which of the following statements represents the main difference between a relay and a contactor?（）

A.Contactors control current and relays control voltage
B.A relay is series connected and a contactor is parallel connected
C.Contactors can handle heavier loads than relays
D.Contactors are made from silver and relays are made from copper


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1.單項(xiàng)選擇題Any electric motor can be constructed to be（）.

A.short proof
B.ground proof
C.explosion proof
D.overload proof

2.單項(xiàng)選擇題Current changing in direction and rising and falling in value is （）.

A.AC
B.DC
C.generator
D.fresh water generator

3.單項(xiàng)選擇題The output voltage of a three-phase alternator is regulated by the （）.

A.AC voltage to the armature
B.AC voltage to the field
C.DC voltage to the armature
D.DC voltage to the field

4.單項(xiàng)選擇題To repair a small electrical motor that has been submerged in saltwater, you should （）.

A.wash it with fresh water and apply an external source of heat
B.renew the windings
C.send it ashore to an approved service facility
D.rinse all electrical parts with a carbon tetrachloride cleaning solvent and then blow dry the motor with compressed air

5.單項(xiàng)選擇題A fuse that blows often should be replaced only with a fuse of （）.

A.the recommended current and voltage rating
B.higher current and voltage rating
C.higher current and lower voltage rating
D.lower current and higher voltage rating