Also, here's a pic of a printer I wired for PHI (Packhard
Humanities Institute) a few years ago, one of the rare times in the last few years I did the hardware on
This is the bit of wiring for the variacs to control camera and
projector tension motors.
The 4 CRYDOM solid state relays at the top-left are under OPCS
to switch the tension power from forward to reverse for cam +
The idea is in "forward" mode, the variac sets the takeup tension
set by the variac,
and the 'feed' tension is leakage current through the power
resistors at the top right.
When the motor direction is changed to 'reverse', the relays swap
The tension motors keep tension on the film, both camera and projector.
The type of motors most commonly used for tension motors on optical printers and downshooters are from "Bodine Electric".. "high slip" AC induction motors, such as the K-2 series "KCI-24", model #0712 (1550 rpm, 1/300 HP, 115 VAC/60V), #0714)
Here's a current link to the product from their website store:
..or the product page:
They call them "High Slip" motors because they spend most of their time NOT turning,
just keeping pressure on the film. The fact these motors can remain "stalled" almost all of the time they're operating is what makes these motors special.
Here's a few wiring diagrams I've provided in the past: