单项选择题
When would you ground an electric hand tool?（）
A.When it is a DC machine
B.When it is an AC machine
C.Any machine that operates on over 100 volts
D.Only when working on wet steel plates
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The load between two steam-driven alternators operating in parallel may be adjusted by varying （）
A.the speed of the alternators
B.the field strength of the machines
C.the power factor of the alternators
D.steam supply to the prime movers -
单项选择题
The purpose of the commutator and brushes on a DC generator is to（）
A.change AC to DCcurrent
B.change DC to AC current
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单项选择题
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A.they are used
B.the field windings are connected to the load
C.the armature circuit is connected to the load
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