we used to use an ac motor with a bridge rectifier in series with the motor
and psu  on a take-up reel. (ac pins of the bridge were used)

basically, we used to control the power to the motor by applying a bias
current to a transistor which was connected across the + and - pins of the
bridge.
as the transistor pulls more current through the bridge .. the current also
passes through the motor. seeing as the bridge also acts like a rectifier,
you dont need back to back protection diodes either ( like if you had back to
back trannies in the motor circuit) and just need one big boy ( mj15001 etc)

use it .... dont use it ;)
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Thank-you for your time.

John Ward

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