I have been messing around with using winch motors to raise and lower the ramp door on the RV we are building. The door is 250 to 300 pounds and we want it to be free (no springs or cables you can't easily unhook). This is so we can plug in our jackstands (the same ones that remove the camper and garage pod from the flatbed) to convert the ramp into a stable porch or work surface with no obstructions (cable springs to clothesline you).
Anyway, I decided to try wiring the winch motors in series. They are plenty fast at ~6V as seen in the video and the series connection would cause them to share the load equally. This works very well when the door is being raised. One winch line can start with 2 feet of slack and it will rapidly catch up to the other winch and the door will come up evenly.
The problem is when the winches are lowering the door (powering out). Because the motors are not really under a load, they don't lower at the same rate. One motor seems a little faster than the other and the winch line on that side ends up taking all of the weight of the door on the way down. Probably slight differences in windings on the two 12V DC permanent magnet motors.
I did try the motors in parallel and the faster motor is a problem both on the way up and the way down (I had some thought of using another relay to have the motors be in series on the raise and parallel on the lowering).
One idea I just had while writing this is perhaps a high power resistor across the faster motor to shunt some of the current. I am not sure if this would work very well though. It might be worth a shot.
Any other tricks other than putting encoders on the winch motors and running a control loop? I think I would live with the unequal lowering before going that far. Each winch is rated for 3500 pounds and has a mechanical brake.
Here is a short video of the winch motors in series, raising the door.
(not nearly as loud as the video implies