As far as I can understand it, this is an add-on motor unit for a counterweight system.

I don't think 'winch' would be right, since that usually suggests a drum round which a cable runs; in the theatre, still often hand-operated.

I would tend to call this a hoist, as in for example a 'spot hoist' — but as we know that this is in fact in relation to counterweight bars, that woldn't be right either.

I am unaware of an equivalent EN term, but would like to suggest the above as a good way of covering it, without its being a literal translation.

You might consider 'electric winch assist', but that seems to me unnecessarily clumsy, for the sake of sticking (too) slavishly to the original FR structure.

I would avoid any use of 'support', both because that's not really a term I think I'd use in this sort of engineering context, and also, because there is a possible ambiguity with to support = to hold up.

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Note added at 11 hrs (2013-12-03 19:39:03 GMT)
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Here's a good Wikipedia article that mentions this very thing:

http://en.wikipedia.org/wiki/Fly_system