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technical question (motors)
Old 02-26-2015, 11:12 AM   #1
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technical question (motors)

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 )


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Old 02-26-2015, 11:27 AM   #2
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Could you joint the ends of the cables and have them pass through pulleys at the edge of the door, rather than being rigidly attached at that point?

This would force the cable tensions to be equal.

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Old 02-26-2015, 11:35 AM   #3
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Not easily, and the problem there is not only does it make it more difficult to detach the lines when you want them out of the way, it also removes the extra safety of having two lines and two brakes each capable of holding the 300 pound door on it's own.
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Old 02-26-2015, 12:42 PM   #4
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I think keeping the motors in series with a shunt across the faster motor is the simplest and best (cheapest) solution.

It may well be that in time the mechanical friction of the slower unit will decrease. Using a shunt with an adjustable clamp to vary the resistance may be needed to compensate.

Just for laughs you might want to check the lubrication of both motors' gearing.

By the way size the shunt resistor so that in the event the the other motor opens it can handle the additional current. Kirchhoffs's law applies.

Edit: change "opens" to "shorts" If the other motor opens nothing will happen. Ie open circuit. My fingers were faster than my brain.
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Old 02-26-2015, 01:10 PM   #5
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Can you put a weight on the underneath of the door to put it under load? Say a spare tire?
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Old 02-26-2015, 03:24 PM   #6
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Hard to tell from that video, but could you run an axle (black pipe) across from side-to-side to lock the winches together? They would have to turn the same distance then, and you still have one motor braking both sides if one fails (otherwise, I like travelover's idea).

A simple (or maybe not) logic control approach might just be some kind of tension switch on each side. If that side loses tension (it is going too slow in raising it), it shuts off the other side so it can catch up. Could be twitchy, and would you need to reverse the logic for lowering?

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Old 02-26-2015, 03:43 PM   #7
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Hey thanks for the ideas so far.

Connecting the two winches together with a bar would be nice, but I don't see an easy way to do that directly. The motor does not have dual shafts.

I *could* wrap a turn or two of each winch cable around a freewheeling bar that spans the top of the door frame. Sort of the way you do a few wraps on a boat winch to sheet in. That is an idea for sure and I think it would work. Adds some complexity and weight though. I am also not sure how well the winch line would spool on the winch if it is coming off of a bar set at close distance. It might tend to bunch up instead of spooling nicely.

I am going to try the resistor idea. I have been searching through my junk box for a bunch of 125 watt 5.1 ohm resistors I know I have. That should be a good size to make various parallel series combinations of values. Figure at 12V across the resistor (other motor shorted) you would have roughly 2.5 amps running through the resistor or 31 watts.

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