# vfd ,plc , breaking issue?



## the resistance (Jan 26, 2013)

I think the plc is telling the brake to disengauge and then the motor wont jog because of the lockout causing its weight to distribute accordingly and the platform does a quater turn then will not move


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## Big John (May 23, 2010)

Feed the brake circuit from a small transformer on the load-side of your motor disconnect so it's not possible to release the brake with the disconnect off. If this is a motor-mounted brake it also removes the hazard of multiple sources of power to a single piece of equipment.

My only concern is it sounds like you might be relying on the brake as an effective means of lockout. I'm not sure that with stored energy behind that brake it would be considered rendered safe even with electrical power removed.


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## Jabberwoky (Sep 2, 2012)

Could you use an output contact from the VFD such as a N.O. that closes when the drive is 'ready'. Run that to the PLC input and modify the program so the brake output can only turn on when the VFD is ready.


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## JRaef (Mar 23, 2009)

I see the problem as being that you have now added a condition that the PLC program does not know about and is not programmed to deal with. By adding the disconnect between the drive and the motor, there is now a condition wherein the motor power can be cut off, so no jogging, but the brake can be released because it is already powered separately. I don't see why someone would be trying to jog the motor with the disconnect switch open, so I'm assuming that is a mistake.

All you need to do is add an aux contact to your disconnect switch so that when the disconnect is opened for safety, the brake coil circuit is opened as well, so the PLC can no longer release the brake if someone forget to re-close the disconnect. I would add a 2nd aux contact in the switch to kill power to the VFD Run command signal too by the way. You don't want the VFD to be running when someone opens that switch, it can damage the VFD transistors.


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## the resistance (Jan 26, 2013)

Thanks everyone for the input .i did decide to tie a nearby interlock circuit into the a auxillary contact i ordered for the disconect 
when the interlock circuit is broken the brake holds and the vessell is stable


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## Jabberwoky (Sep 2, 2012)

I didn't catch that you put the disconnect between the motor and the drive. You definitely do not want that drive running when someone pulls that disconnect. When I used to do service calls I would relocate it to the line side of the VFD.


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## Trigger_442A (Sep 15, 2012)

What about when the drive is interlocked to operate two motors because your company cheaper out on two drives


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## Peewee0413 (Oct 18, 2012)

,........


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