How to prevent back emf when forcing a motor

MisterBill2

Joined Jan 23, 2018
7,709
@MisterBill2 Back to not taking time to read the whole thread are we?
I read the whole thread again and I find nothing stated by the TS to imply anything else than the motor is used to rewind a string winch after a package is pulled out. There may be a possibility that the motor gets switched on to do the retract, but I did not see that mentioned. Certainly a system that gets switched on and otherwise is allowed to turn freely is reasonable, but at that point there is the question of what switches it on and off.
 
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shortbus

Joined Sep 30, 2009
8,179
There is also nothing that says he wants to do this -
Consider that the TS is proposing the use of a stalled motor in place of a spring.
Did you happen to see this?
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.
 

MisterBill2

Joined Jan 23, 2018
7,709
OK, possibly I got it wrong. I probably missed the "freely" part. The very simple way to avoid any problem with the voltage generated when the string is pulled is to use a physical switch for the motor. With the switch off the motor voltage can not cause any problem. With the switch on the motor can return the lift to the home position. Just avoid using sensitive electronics for the motor control. A resistor across the motor in the off position could be used to provide a bit of drag to prevent coasting and tangling of the string.
 

strantor

Joined Oct 3, 2010
5,382
Would anyone care to take a trip back to the OP with me and determine if the problem we're trying to solve is actually a problem?

I am worried that when we are forcefully spinning the motor in the clockwise direction, it will act as a generator.
Yes, it will act as a generator. Why does this worry you?

This means we are sending current back into the system
I would say it means that you are presenting a voltage at the output of your system. If your system is poorly designed, then maybe it will allow some current into it's circuitry.


- possibly damaging electronics. How do I overcome this?
What are these electronics? What is this "system?" Is it a DC motor drive? If so, it should have no problem with back EMF. If it's something that you've designed, maybe design it to handle this.

Just think about every DC motor drive scenario ever conceived, if you're running a DC motor and then you cut power, it will keep spinning and generating back EMF. If this were damaging to the drive circuitry then broken DC motor drives would be filling up landfills faster than soda cans and plastic bags.
 

MisterBill2

Joined Jan 23, 2018
7,709
"What Electronics"is a very good question. It rather seems that a whole lot of details are not provided. Keeping the reversed polarity out of a simple drive circuit is as easy as opening a switch.
So there must be a lot more to the system than has been mentioned by the TS.
Amazingly, most of the contributors are not mind readers who would be able to immediately know what the TS has in mind.
What is certain is that there are indeed many motor control systems that work perfectly while the motor is being driven by either inertia or a load .
 

MisterBill2

Joined Jan 23, 2018
7,709
A DC motor with permanent magnets produces magnetic "cogging" when it is disconnected and is spinned.
That effect is primarily with permanent magnet DC motors. Wound field motors tend to have much less cog effect when turned with the power off.
And we have been told nothing at all about the motor size or voltage or what powers the system or any mention at all as to what causes the winch to decide to retract the string.

And it has been 11 days since the only post by Droneman, with not a single additional comment or response.
 

shortbus

Joined Sep 30, 2009
8,179
A DC motor with permanent magnets produces magnetic "cogging" when it is disconnected and is spinned.
Maybe a very little one from a standard PMDC motor that most would never notice. But a BLDC motor is a different situation, it does have a real "cog" feeling to it. This is due to the poles salient poles of a BLDC verses the distributed poles of a PMDC motor.
 

MisterBill2

Joined Jan 23, 2018
7,709
Maybe a very little one from a standard PMDC motor that most would never notice. But a BLDC motor is a different situation, it does have a real "cog" feeling to it. This is due to the poles salient poles of a BLDC verses the distributed poles of a PMDC motor.
I just did a check and our TS claims to be an electrical engineer. Perhaps the definition has changed a bit, maybe.
 

MisterBill2

Joined Jan 23, 2018
7,709
Is that important to you?
Odd that the OP has not come back, in spite of 22+ posts? :confused:
Max.
Max, YES! if an individual claims an educational background then I do not need to be concerned about confusing them when I reply, while if they are a new-be, then I need to provide a much more detailed explanation. Also, if an individual claims to possess a specific skill set then why do they ask such uninformed questions???
 

MisterBill2

Joined Jan 23, 2018
7,709
There are too many NOOBS on these electronics forums.
I never said or implied that I was down on Noobs. I am always willing to share knowledge and insight with those wanting to learn. And a long career has given me a lot of experience beyond what I learned in college.
My complaint is about those who represent that they have the education when it is very obvious that they do not have it.
 
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