單項(xiàng)選擇題The main difference between a motor control circuit containing low voltage protection and low voltage release is that the latter contains （）.

A.a magnetic operating coil
B.normally open line contacts
C.thermal-overload protection
D.a momentary-contact start button


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2.單項(xiàng)選擇題The part of a fuse that melts and opens the circuit is made of （）.

A.copper and antimony
B.steel and babbitt
C.aluminum or beryllium alloy
D.zinc or an alloy of tin and lead

3.單項(xiàng)選擇題Time delayed or delayed action-type fuses are designed to （）.

A.prevent grounds in branch circuits
B.prevent opens in motor circuits
C.permit momentary overloads without melting
D.guard lighting and electronic circuits

4.單項(xiàng)選擇題Which of the following statements is true concerning all three-phase alternators?（）

A.Each has three separate but identical armature windings acted on by one system of rotating magnets
B.Each has one armature winding acted on by three identical but separate systems of rotating magnets
C.All three-phase alternators are designed to operate with a O8 leading power factor
D.The three phases always provide power to the load through three sets of slip rings and brushes

5.單項(xiàng)選擇題The function of damper windings in a synchronous motor is to （）.

A.eliminate slippage
B.provide excitation
C.provide starting torque
D.increase efficiency