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Until the 1980s the conventional d.c. machine was the automatic choice where speed or torque control is called for, and large numbers remain in service despite a declining market share that reflects the general move to inverter-fed a.c. motors.

First, I think 'call for' means require. So it can be re-written "where machine calls for speed or torque control."

I could understand the sentence, like above. But I can't understand grammatically. How preposition can be used without noun?

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    This is a "prepositional passive": the object of a preposition phrase which is affected by the verb may be cast as the passive subject of the verb, but is still understood as the object of the preposition. – StoneyB Oct 14 '17 at 17:29
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You are correct that to call for means "to require" here. The sentence is using the passive voice, which is why you see "is called for" - remember that the passive uses to be and then the past participle of the verb, which is "called for" here.

Because this is the passive, remember that the subject may not even be stated in the sentence. It is not the machine doing the calling for! We don't know what is doing the requiring; it might not even matter.

Remember that when we make a passive sentence, we put the object first, then to be, then the past participle. So the object is "speed or torque control", the be-verb is "is", and the past participle is "called for". There's no object after the preposition, because the object is already speed or torque control.

Yes, it might look strange because of the preposition with nothing following it, but it's very common with phrasal verbs in the passive voice. Some examples:

The box was opened up.
The rotten orange was thrown out.
The animals will be seen to.

  • Thank you. I think up and out is not prepositions, though. – Ting Choe Oct 16 '17 at 23:17

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