Best way to brake a DC motor

shortbus

Joined Sep 30, 2009
7,600
I would like your thoughts on why this should be done electronically and not like it is usually done, mechanically? Do you really think you are saving energy by shutting the motor on and off with every coupon dispensed? Do you understand it takes more energy to start a motor than to let it run at no load?
 

mvas

Joined Jun 19, 2017
537
How quickly does your motor need to decelerate, once the STOP Signal is detected?
How much "over-shoot" vs "under-shoot" can the coupon perforation tolerate?
Is this an issue or a non-issue?

An SSR design is 10 times easier than raw MOSFET - given this thread is already 40 replies long and growing.
I would use an SSR, Solid State Relay.
I would add a TSV or Reverse Diode across the motor.
If the SSR fails, the screw connectors make them very easy to replace.
They also have smaller form factor solder-in modules, if so desired.
CMX200D3
3 amps rms, 30 amps surge for 10 ms, 1 ms turn off, 3-10 vdc input, 200 vdc output
http://www.crydom.com/en/products/catalog/cmx-series-dc-pcb-mount.pdf

If you don't already know how to design a MOSFET Motor Driver Circuit,
then now is not the time for you to be experimenting with MOSFET's.
 
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cmartinez

Joined Jan 17, 2007
6,886
I would very much appreciate your thoughts whenever you find some time. I am wondering if the relay would be a simpler option?
A relay won't let you control speed. If what you want is a quick fix, then you could buy an off the shelf speed controller. Assuming that you do not need to reverse direction.
 

Thread Starter

gerstley

Joined Nov 20, 2015
58
I do not need to control the speed or reverse direction. The gear motor I have been working with rotates about 1/2 revolution after the motor is turned off. That is too much. I am not necessarily looking for a quick fix, I am looking for something that is reliable and hopefully somewhat simple too. The circuits I showed at the beginning of this thread both worked for me. It was only when the shoot-through problem was brought up that my bubble popped! The idea of a continuously running motor with a clutch mechanism will not work for me for a variety of reasons that I should not get into here.
 

cmartinez

Joined Jan 17, 2007
6,886
I do not need to control the speed or reverse direction. The gear motor I have been working with rotates about 1/2 revolution after the motor is turned off. That is too much. I am not necessarily looking for a quick fix, I am looking for something that is reliable and hopefully somewhat simple too. The circuits I showed at the beginning of this thread both worked for me. It was only when the shoot-through problem was brought up that my bubble popped! The idea of a continuously running motor with a clutch mechanism will not work for me for a variety of reasons that I should not get into here.
Then you could use a couple of ssr, inverse-logic connected. One in series with the supply, and the other one in parallel with the motor so as to short-circuit it and help more quickly stop it.
 

Tonyr1084

Joined Sep 24, 2015
4,375
more quickly stop it.
An INSTANT stop requires infinite energy dispersal. So far we know that half a rotation is intolerable. The question becomes "How short do you want to stop it?" That will be followed by "How massive is the motor?" Stopping it in a quarter turn - assuming you said geared drive output, the motor - depending on the gear ratio - will dictate how quickly it can stop. That along with the armature mass and RPM at the time of braking.

All along I've wanted to answer the question "Best way to brake a DC motor" is "With a hammer."

My DeWalt battery powered drill has instant stop. It does so by engaging some sort of a pawl to lock the motor from rotating any further. I have to wonder how long that will last before it succumbs to the forces of physics and it itself breaks.

Transmission parking pawl
 

cmartinez

Joined Jan 17, 2007
6,886
I would use this motor or something similar https://www.servocity.com/60-rpm-hd-premium-planetary-gear-motor
If I was to use a pair of SSR's, there wouldn't be an issue with shoot-through? I think a quarter revolution or less would be acceptable.
With such a small motor, and with the proper circuitry, you can have the luxury of applying reverse current for a very short moment so as to stop it almost instantly. What's the duty cycle of your application? That is, how often do you plan to stop it?
 

djsfantasi

Joined Apr 11, 2010
6,338
Can you power the motor for a shorter period of time? Such that momentum carries the drive wheel to where you want it? Of course, this technique may not be reliable enough. Or is it? While the particular motor states that the gear train can be damaged if it’s locked, it appears as if the pawl is out. But a slow speed stall, without power, may allow you to use a pawl to guarantee final position.
 

Thread Starter

gerstley

Joined Nov 20, 2015
58
Typically, the motor would run for a period between 1/2 second and 1 1/2 seconds depending on the material being dispensed. It would be stopped for maybe 2 to 4 seconds before restarting.
 

cmartinez

Joined Jan 17, 2007
6,886
Typically, the motor would run for a period between 1/2 second and 1 1/2 seconds depending on the material being dispensed. It would be stopped for maybe 2 to 4 seconds before restarting.
And how much current do you think it would be consuming? The datasheet says that it draws about 1/2 an amp at 12VDC, that's 6W just to keep it running. It has a stall current of about 20A, so it seems rather powerful for its size.
 

cmartinez

Joined Jan 17, 2007
6,886
I am not sure I have an accurate idea. I do know that the coupon paper would likely tear before the motor stalled so it should never see 20 amps. Maybe 5 amps on startup?
... and another 5 to stop it, probably 10 due to accumulated inertia... not sure, but it looks quite manageable. The circuit might be an itsy-bit complicated though. Since it's for a coupon dispenser, I assume you're using a light sensor of sorts to stop the motor?
 

mvas

Joined Jun 19, 2017
537
How far does the Motor travel, after power off, with a high amp Schottky Reverse Bias Diode attached to the motor?
If too much braking ( is there such a thing ? ) , then add a low ohm resistor in series with diode?
Is that enough Dynamic Braking?
 
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