Short Stop Brake - not working - help

[Mitch Bray]

I recently purchased a 3 phase table saw that came with a short stop unit (dc braking) that was not connected. I am running the saw from a rotary phase converter. When I connect the short stop the breaker (on the panel) for the RPC trips upon powering down. I don't understand what is going on. The RPC feeds into a 3 phase panel, so the saw has it's own 20 amp breaker between itself & the RPC. Why is it tripping the RPC's 50 amp breaker? I called Short Stop & they were no help at all. They said my unit was old (1980) & that they were not responsible for troubleshooting.
Any help would be appreciated. The saw takes a couple of minutes to come to a stop after powering down so braking would be very useful.

Thanks - Mitch

[Rod Sheridan]

Sounds like a wiring issue, do you have a drawing?

Regards, Rod.

[Paul F Franklin]

It sounds like the braking current is being applied to the motor before the AC contactor opens...can't imagine any other reason for the brake to affect the RPC. Normally there would be a time delay on the brake to allow the AC contactor to open.

This would perhaps explain why the main breaker is tripping...the DC from the brake is attempting to stop the RPC as well as the motor and the RPC tries to correct and draws too much current??? Just a guess really.

How is the brake being triggered? Can you set it up so you manually trigger the brake after opening the main contactor?

[Mitch Bray]

It sounds like the braking current is being applied to the motor before the AC contactor opens...can't imagine any other reason for the brake to affect the RPC. Normally there would be a time delay on the brake to allow the AC contactor to open.

This would perhaps explain why the main breaker is tripping...the DC from the brake is attempting to stop the RPC as well as the motor and the RPC tries to correct and draws too much current??? Just a guess really.

How is the brake being triggered? Can you set it up so you manually trigger the brake after opening the main contactor?

Thanks for the response. That is the kind of troubleshooting I was hoping for. I have been unable to figure out why or how the RPC was getting feedback from the brake when the contactor is disconnected, but maybe the brake is triggering too quickly. The trip on the RPC's breaker happens instantly when the saw is switched off . That also explains why the RPC's breaker is tripping rather than the saw's. Thanks.

[David L Morse]

Another thought here. The DC brake generates DC by rectifying the voltage supplied by two of the three phase legs. The three phase power synthesized by a RPC has two "hard" legs (the ones connected directly to the 240V single phase supply) and one synthesized leg ("wild" leg). If the brake is connected to one hard leg and the wild leg (2 out of three chance if wired randomly) it presents a severely unbalanced load to the RPC. It's plausible that this produces enough phase shift to cause one of the idler motor's winding voltage to increase enough to cause Iron saturation. If the Iron saturates the magnetizing current goes up several orders of magnitude, likely enough to cause the breaker to instantly trip.

It might be worthwhile to look at which legs are supplying the brake power and if not the two hard legs then change to that and try again.