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I have a 3ph 600 volt 5hp motor running a grinder. It takes roughly 5 minutes to stop when the stop button is pressed. I'm looking to wire up dynamic braking to stop the motor much quicker. What's size resisters should I use? Which wattage?

After power is removed the voltage drops to 20 vac after 2 seconds while it coasts to a stop.
 

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Do you already have a VFD on it? If not, you can't just hook up resistors and expect it to stop very much faster. Dynamic Braking involves keeping the motor energized to make it into a generator, then pulling the kinetic energy of the spinning mass out of that generator and dumping it into resistors as heat. a VFD is capable of doing that. If you just open the contactor and then close another contactor afterward that connects the resistors, the motor magnetic fields have collapsed by then and all you are left with is a little residual magnetism. That's where you are getting the 20V you see, but the energy that this represents that would be useful toward slowing the motor is minuscule, it would likely not even be noticeable.

If you don't already have a VFD, you can't do dynamic braking. If you have one, it might already be capable of it, but you MUST get the resistor information from THAT drive mfr, otherwise you can burn up the braking transistor. If you have a VFD that is not capable of DB, then there are retrofit units from Bonitron that can be added, and they will give you the resistors or the values.
 
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A frequency converter with a a resistor maybe?
If it has to be stopped with emergency buttons - a mechanical brake, controled from some SRS controller.
 

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you can setup DC injection braking on one winding of the motor, you could try it first with a 12v battery
Do you care to draw the wiring? And sign it, for full responsibility if anything blows.
That is amazing how people suggest crap, without knowing ANY details at all. :censored:
 

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Do you care to draw the wiring? And sign it, for full responsibility if anything blows.
That is amazing how people suggest crap, without knowing ANY details at all. :censored:
YOu're right, you spout off a lot of crap. :thumbup: Entertaining to read at least. Keep going, you're about to make a lot of new "friends" on this site with your attitude and necro posting. ;)
 
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