medium size DC motor control via resistor or resistance in wiring? - Page 1
Forum section: electronics / projects

author: KTP
author_rank: Frequent Contributor
timestamp: February 14, 2015, 02:53:39 pm
content: I am doing some work on the garage pod portion of our homebuilt RV. The ramp door we built is about 250 to 300 pounds and too heavy for one person to safely lift. As a first go, I hooked up a cheap/noisy 2500 pound rating winch in the center of the roof and used a Y harness to pull on the door. This worked, but I have decided I am less than thrilled with the noise and Y harness. I am going to go with dual 3500 pound Superwinch Terra series with synthetic line. Very much overkill for the door but we may use one of the winches for other purposes if I make it easy to remove. The Superwinch is whisper quiet compared to the cheapy one. I don't want to use springs or other methods because I want the door to act as a platform with no side obstructions. For redundancy and safety, I am going with two winches instead of one winch and a multiple pulley cable system. Each winch will pull on one side of the door.So....I want the two winches to work together. The drum speed does change with load but considering the door is 300 pounds and the winch is rated at 3500 pounds, this may not be enough to keep them in sync. I am thinking an easy way to do this is by having some resistance in the wiring, such that the voltage drop across the wiring to the winch taking more load would tend to cause the motor to slow down, letting the other winch catch up.The chart for the winch claims it draws 19 amps at 12V at no load and 46 amps with a 500 pound load. I would guess about 35 amps with a 300 pound load.I would think 0.02 ohms of wire resistance would be a reasonable resistance. This would be a 0.7 volt drop with a 35 amp draw (one winch taking entire door load) while the other winch would have a 0.38 volt drop. The 19 amps no load seems high (I don't have the winch in hand yet)...series wound and not permanent magnet?I can get 0.02 ohms by using 10AWG wire and a 20 foot path (10 foot run). Alternatively, I could use a power resistor and wire the winches with a more reasonable 6AWG. They should never run for more than 30 seconds and never more than about 500 pound load.Any ideas or gotcha's?Here is a video of the single noisy winch pulling up the door:https://www.youtube.com/watch?v=a5E6MLQA8EQ&feature=youtu.be

author: G7PSK
author_rank: Super Contributor
timestamp: February 14, 2015, 03:05:13 pm
content: As the winches are more powerful than needed you could try putting them in series. If I was doing that conversion though I would have used an under floor hydraulic ram or 2 easier to control can be fitted with anti hose rupture valves to prevent catastrophic failure, if the winch cable breaks the ramp comes crashing down, not nice if you are underneath.

author: KTP
author_rank: Frequent Contributor
timestamp: February 14, 2015, 03:29:00 pm
content: Quote from: G7PSK on February 14, 2015, 03:05:13 pmPutting them in series...interesting. I need to think on that one.Can't have anything under the garage pod or door as it sits on a flatbed truck. I have thought about winch or cable failure and this is one reason I want to go with two winches, each with it's own brake and rated to hold the door weight by itself (along with appropriate cables and mounts). The Amsteel synthetic winch cable is rated at 6000 pounds.

author: KTP
author_rank: Frequent Contributor
timestamp: February 14, 2015, 03:46:29 pm
content: Thinking about it more, I really like the series wiring idea.It would need to be done post solenoid as I am guessing the solenoid would not like just 6V on its electromagnet. I could use one solenoid to control both winches and have a spare since they are rated up to 200 amps or so.When one motor came under load, the voltage drop across it would decrease, sending more voltage to the other motor. Perfect, and no resistors needed.I do not know how these motors perform on 6V though. That question remains. They are plenty fast at zero load on 12V (double the speed of the winch in the video which was fast enough!)Edit: I did just read online that the Terra series uses permanent magnet DC motors.

author: SeanB
author_rank: Super Contributor
timestamp: February 14, 2015, 03:48:17 pm
content: Simply place a pulley each side of the door and thread the winch cables through, then join in the middle with a coupler that is too large to go through the pulleys ( or use a ring eye to catch it if not) so that the differential is going to be done by the joint moving from side to side. Still safe ( broken cable means only one lift cable lifts while the other winds up freely) and with a short throw and equal length power cables ( very important to have all power cables to the winches same length to make the motor voltage applied to each equal) it should handle the small differences without running into lock.

author: KTP
author_rank: Frequent Contributor
timestamp: February 20, 2015, 05:24:11 pm
content: Update:The two 3500 pound winches wired in series works perfectly! I mean the door raises and lowers with EXACTLY the same tension on each side (each winch is now connected to a single side of the door allowing the ramp to be clear of cables up the middle). Either 3500 pound winch and 7000 pound rated synthetic cable can support the 300 pound door on it's own so the safety factor went up quite a bit. I am going to measure the current draw and install a marine breaker at 20% or so above the peak current with normal raise/lower. This along with limit switches should protect the door hardware from someone trying to keep the winch running while the door is already closed.The noise level has dropped by 90% with the new winches and the series wiring.

author: KTP
author_rank: Frequent Contributor
timestamp: February 20, 2015, 11:34:36 pm
content: Might have jumped the gun here a little in my praise of the series wiring.It does track perfectly going up, but sometimes when lowering one winch will slow and slack the cable while the other winch continues to lower the door. It seems when the winch is feeding out cable and lowering the door the motors are not really under load (duh).I don't have a great solution for this but maybe it isn't so big of an issue. I will need to think on this a bit.