DC Motor

To calculate the current that will flow through the switch you need to determine the actual value of resistance of the circuit including the brushes, brush to commutator resistance, switch contact resistance and interconnecting conductor resistance. You should not assume it to be zero. If you make that assumption, you might as well assume the current to be 11.5V/0.5 ohm = 23 amps. Each of those items has an actual value of resistance that may be low but when added can be 0.1 ohms which would impact the answer by greater than 10%.

P. S. Motors windings, brushes, commutators and switches don't like that kind of treatment forever and ever.

Surprisingly, if the motor was correctly engineered in the first place, there seems to be little damage.

I have several battery hand tools which stop almost immediately using this principle, some have lead very hard working lives with me....and NONE of the motors has, up to now, given up!! A battery or two sure.....

I have also used the principle for several home made projects over the years, always with reasonable quality motors, no problems....

I suspect that very possibly cheap motors, used time and time again in this way over long periods may suffer eventually....personally no experience though....

I feel that the sheer abruptness of the short, stopping the motor in my experience in well under half a second (I have done no proper timing checks!) generally, probably does not generate the necessary heat that might damage the commutator. If I could get a temperature probe on a commutator, I would check for you, but the motors I have at his time are not accessible in this manner, sorry....

I have also done this with a certain 3 phase Brushless DC motors also with no negative effects...over 5 or 6 years with maybe 1000 stops per day......

Running overloaded with a lot of sparking for even a minute, probably does more longterm damage than stopping in this way to a well designed motor in my opinion, as it gives the commutator time to really get hot and allow any soldered/glued connections to get loose.....I have seen that happen many times on large DC motors.

My conclusion is that if a motor was reasonably constructed for a reasonable working life, no problems will be seen...

One other comment I would like to make is that the motors and the driven loads that I have my experience with are such that the weight of the armature is mostly similar or even more than the weight of the ancillary equipment it is driving.

Remember, that it is generally not possible to stop a motor any faster than it takes to start.....

If there are very heavy weights attached, my timings and comments may not (will not) be true, say for a motor that takes 10 seconds to reach its full speed...... (assuming no "starter" that reduces starting currents in circuit of course!).

I hope that you find the comments helpful.

Andy,

Excellent experience based feedback!

Surprisingly, if the motor was correctly engineered in the first place, there seems to be little damage.

I agree. General purpose motors with brushes are not usually engineered that well, not to say that many are not, or that switches can't take the punishment.

Three-phase milling machine motors are designed for instant (for all practical purposes) reversing, for example.

I have a couple of battery-powered hand drills and one miter saw which shunt the armature when the trigger is released, and both seem to be holding up O. K. The switches and brushes are the components which have to take the beating, but if they are designed for the task, then there's no reason to think they won't last a long time.

And when you tear apart the kids' PowerWheels ride on battery operated toys, you will find a ballast resistor that is placed across the motor terminals by the accelerator (?) switch pedal when you lift your foot that provides dynamic braking of those ubiquitous machines. And my kids were brutal with theirs. The motors held up great even when my ambitious son, the boy that he is, figured out how to hook up multiple batteries in series and more than double the speed of those plastic toys. Seems like he couldn't do power slides with just the stock set up. He's eligible for his driver's learner permit in a month. Valium anyone? I've kept those DC motors for some yet to be determined project. (Along with all my other "good stuff" )

If you wanted to reduce the motor heating you could put a 0.5 ohm resistor in series with the shorting switch to share the load. Your braking would be reduced though.

Dear Guest

Every windscreen wiper motor in every automobile is stopped in this way. If you can, get hold of the motor's performance curves and look up the starting current (this can, for a windscreen wiper wiper motor, be as high as 50Amps). Select your switch to be 1.5 to 2 times that, then you should be safe. A similar peak instantaneous current goes through the motor when it is switched on (when direct on line switching is used) and the motor is still at a standstill (no back EMF), so any switch that can handle a startup and running under load can also handle a short-circuit stop.

Have a look at www.valeo-swf-motoren.de. There are performance curves in the catalogue section to have a look at. Look up and read the "news" section in www.innovatechsolutions.com.au to get more info on how to read performance curves.

Good luck!

Rolf

" Every windscreen wiper motor in every automobile is stopped in this way."

Wind-Screen Wiper motor will stop at one end not in-between. So it is not "Instant-Stop" anyhow.

Sorry to rain on your parade, but if you took one apart, you would find out that you are completely wrong in your last comment.......

Yes, every windscreen wiper motor IS stopped in this manner as it has to stop EVERY time in exactly the same place!!! The only way to do this is to short out the motor at the "parking spot".....thats what the parking spot is for, when the motor must run continually, this spot is taken to +12 volts, the other connection of course being ground, motor runs. When the motor must be stopped, this spot it is taken to ground. A ground on either side of the motor is a short!! Motor stops instantaneously....

Otherwise, a new motor being relatively stiff, would need another adjustment to an older motor where everything runs very free.

Think about it. If you are still not sure, go and get such a motor from a junkyard and take it apart!! Then you will know for sure......

Thanks for details

Overhead plant cranes use an even more severe form of braking of the hoist motors called "Plugging." Plugging is when reverse current is supplied to the motor for nearly instantantaneous stopping.

It depends on whether or not the supply has been disconnected at that instant, which has been withheld from the forum.