How to prevent back emf when forcing a motor

Thread Starter

Droneman

Joined Nov 11, 2020
1
Good day everyone,

I am designing a sort of winch system, where the user can pull the package freely, and once they are done, the winch reels back up the string around a drum. I would like to use a DC motor connected to the drum to reel back the string. This means that when the user is pulling the winch, the drum will spin in one direction (let's say clockwise), but when the motor is reeling back the string, the drum will rotate in the opposite direction (anti-clockwise). This also means that the motor - which is not engaged in the first part - will be forcefully spun clockwise, and then it will kick in and spin anti-clockwise afterwards.

I am worried that when we are forcefully spinning the motor in the clockwise direction, it will act as a generator. This means we are sending current back into the system - possibly damaging electronics. How do I overcome this? I was thinking we simply have a switch that disconnects the motor from the main system. This way we can forcefully turn it and allow it to act as a generator. Then we want to spin the motor anticlockwise, we just turn on the switch.

Thanks for your comments!
 

shortbus

Joined Sep 30, 2009
8,866
I am designing a sort of winch system, where the user can pull the package freely, and once they are done, the winch reels back up the string around a drum. I would like to use a DC motor connected to the drum to reel back the string. This means that when the user is pulling the winch, the drum will spin in one direction (let's say clockwise), but when the motor is reeling back the string, the drum will rotate in the opposite direction (anti-clockwise). This also means that the motor - which is not engaged in the first part - will be forcefully spun clockwise, and then it will kick in and spin anti-clockwise afterwards.
It can also be done by a mechanical means that isn't electronic. Look at something called by a couple of different names, one way bearing, sprag clutch or freewheel. The last name is probably the one most people are familiar with, it is the thing in a multi speed bicycle that allows the pedals to only move the bike when pedaled in the right direction. Not knowing the load on your winch you will need to pick the right size for it.
https://www.amazon.com/Bearing-Clutch-Freewheel-Backstop-Keyway/dp/B073HRCHBG
 

MrSoftware

Joined Oct 29, 2013
2,015
A sprag clutch would let it freewheel in the same direction that the motor will turn when running on battery power, but won't allow it to freewheel in reverse. Otherwise the motor would be freewheeling when it tries to reel the string back in. :)
 

shortbus

Joined Sep 30, 2009
8,866
A sprag clutch would let it freewheel in the same direction that the motor will turn when running on battery power, but won't allow it to freewheel in reverse. Otherwise the motor would be freewheeling when it tries to reel the string back in. :)
Not if you put it in the correct direction. There are many of them in you automatic transmission in your car, There is also one in the starter motor drive that keeps the starter motor from over speeding when the engine starts. They are used in washing machine agitators so it won't turn when the spin cycle is on, I can go on and on about them. The winch drum needs a bearing on each end so by putting a sprag/one way bearing on the drive end of the drum, it will free wheel when pulling the rope out but engage to allow the motor to wind it back up.
 

MrSoftware

Joined Oct 29, 2013
2,015
You are correct they are used lots of places, but they allow the load to overrun (go faster than) the motor. They don't allow the load to spin backwards against the motor. They couldn't, or the motor wouldn't be able to drive the load.
 

sbkenn

Joined Apr 15, 2017
23
Can you put a relay or electric clutch in the system in the circuit ? You could also use a non-permanent magnet motor. Note: you won't be able to force a worm gearbox, or even a high-ratio spur/planetary one. I recently bought an electric winch that has a manual clutch built in.
 
Last edited:

LesJones

Joined Jan 8, 2017
3,506
I am not clear as to how the winch behaves. Is it always attempting to pull the string back onto the drum but the pull is not so strong that it prevents the user pulling the string off the drum ? Or is there some push button or switch system that controls the direction to drive the drum ?

Les.
 

SamR

Joined Mar 19, 2019
3,531
An actual winch has a high mechanical advantage by using gears that make "freewheeling" difficult so they have a mechanical disconnect to allow freewheeling or a mechanical (or electrical for DC motors) reverse to feed line out. They also have a brake or ratchet cog that prevents the load from taking line off of the drum. Using a small DC motor as a line retriever without any gearing simply needs a diode to prevent back feed when the line is pulled off of the drum causing the motor to become a generator.
 

LesJones

Joined Jan 8, 2017
3,506
IF the winch is allways attempting to wind the string back on the drum a series diode will not solve the problem as the current will current will be in the same direction when manually pulling the string off the drum. (But it will be attempting to reverse the polarity of the supply.) I think what is needed is a constant current power supply that can tolerate the reverse polarity on it's output.

Les.
 

RPLaJeunesse

Joined Jul 29, 2018
183
Thinking about DC motor basics here. Say a positive voltage winds the cord. Apply a positive voltage to the motor, it winds up under load. Lessen the load and it runs faster, at less current. Why less current, while circuit resistance is constant? Because the counter-emf gets more positive. So while unwinding the (counter-) emf generated will be negative, below ground. Now flip things around, with the motor connected to the positive supply and the ground side switched by a single transistor (likely a FET). Switch on to wind, switch off to unwind. During unwinding the switched motor lead will rise above the supply, possibly to 3 or 4 or more times the supply. If the switch to ground transistor can handle this high voltage its not a problem. Still wise to add a transient clamp diode that will prevent the switch from seeing any narrow spikes near the switch voltage rating. For example use a 12V motor and supply, but a switch transistor rated for 100V and add an 82V clamp diode. Or maybe a 200V switch transistor and a 160V clamp diode.
 

shortbus

Joined Sep 30, 2009
8,866
You are correct they are used lots of places, but they allow the load to overrun (go faster than) the motor. They don't allow the load to spin backwards against the motor. They couldn't, or the motor wouldn't be able to drive the load.
What ever you say. If the company I worked for hadn't moved to China, I 'd get on the phone and tell the engineers there it wouldn't work. Even though many of the machine they designed had been using them for years and making money doing it.

He asked for something that allowed the drum to freewheel in one direction and motor in the other. If the one way is pressed into the drum(in the correct orientation) and the center hole is mounted to the gear box shaft or motor shaft, it will allow turning free in one direction, but when the motor/ gear box shaft moves in the other direction the one way locks and the drum revolves in that direction. Works the same as the rear hub in a multi speed bike. Which is also called a free wheel.

It was also how the first/original Warn winches were made to allow the cable to be pulled out and driven back in.
 

MrSoftware

Joined Oct 29, 2013
2,015
We're having a miscommunication. Think about your bicycle wheel scenario. The wheel is a spool and when you pedal the bike it winds the string in. The wheel can freewheel forward (winding the string in) faster than the pedals are going just fine, but if the wheel tries to spin backwards (to let the string out) the pedals will also spin backwards. i.e. it freewheels forward, but not backwards.
 

shortbus

Joined Sep 30, 2009
8,866
We're having a miscommunication. Think about your bicycle wheel scenario. The wheel is a spool and when you pedal the bike it winds the string in. The wheel can freewheel forward (winding the string in) faster than the pedals are going just fine, but if the wheel tries to spin backwards (to let the string out) the pedals will also spin backwards. i.e. it freewheels forward, but not backwards.
When a bike does that it's because of either a low quality freewheel of one that is on it's way to needing some work. The old type freewheel was bad for doing that. They used a spring loaded ratchet in them not a sprag, when dirt and got in them or the spring started to go bad or got weak, the freewheel stopped working correctly.

The new type is a true freewheel/sprag bearing. I see they have now changed the name for them, from freewheel to freehub. I didn't know of that change and always use the old freewheel term. My bad. If you look here; https://en.wikipedia.org/wiki/Freewheel they show a picture of the sprag type vs the freewheel ratchet type, though the ones in a bike rear hub use more than one ratchet.

Here is a video of what is inside the older ratchet freewheels -
 

Bernard

Joined Aug 7, 2008
5,788
Neither the over running clutch or diode will work. I assume that there is a switch to start pickup? Back EMF into open circuit is of no concern.
I examined polarities using small motor, diode, battery, & trusty Heathkit MM1 multi meter.
 

Bernard

Joined Aug 7, 2008
5,788
An interesting read on similar subject; Search Forums, Falconry Lure Machine Aug 4, 2014, by Icarus1977. Video near the end.
 

MisterBill2

Joined Jan 23, 2018
9,469
Good day everyone,

I am designing a sort of winch system, where the user can pull the package freely, and once they are done, the winch reels back up the string around a drum. I would like to use a DC motor connected to the drum to reel back the string. This means that when the user is pulling the winch, the drum will spin in one direction (let's say clockwise), but when the motor is reeling back the string, the drum will rotate in the opposite direction (anti-clockwise). This also means that the motor - which is not engaged in the first part - will be forcefully spun clockwise, and then it will kick in and spin anti-clockwise afterwards.

I am worried that when we are forcefully spinning the motor in the clockwise direction, it will act as a generator. This means we are sending current back into the system - possibly damaging electronics. How do I overcome this? I was thinking we simply have a switch that disconnects the motor from the main system. This way we can forcefully turn it and allow it to act as a generator. Then we want to spin the motor anticlockwise, we just turn on the switch.

Thanks for your comments!
First, it is a bad choice to power a motor and then force it to rotate in the opposite direction. In fact, that is a VERY bad choice. It seems like the intention is to provide the equivalent of a spring that keeps a constant tension on the line as the line is pulled out, and then reels it in as the line is released.
A DC motor is the wrong choice for such an application. The correct choice woulld be a very high slip induction motor. That would provide a constant torque in the wind up direction at all times.
BUT really, the whole idea and concept seems like a poor choice.
 
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