Hi, thanks for the question. Our best suggestion is to use a washing
 machine motor, and gear it down. As the speed is stepped down then
 the motor should be able to take easily the kinds of weights you are
 suggesting. It may even be that you could get the gearing from a
 washing machine as well, as in wash mode they often rotate at about
 that speed  (I think!). It may take a bit of tinkering, but it should
 work.

p.s. I am assuming that the weight of the curtain is supported by a
 rail rather than by the spindle of the motor (or gearing) direct