# Powerflex external brake resistor



## Brad765 (Feb 9, 2010)

A couple times now, we have had full DC voltage on BR1 and BR2 to the brake resistor on drive power up. Catching the resistor elements on fire. Does anyone have an idea why this happens.


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## jmsmith (Sep 10, 2011)

I can't remember the exact parameters, but there are several that apply to dynamic braking. One of these, if not set right, will leave brake voltage on until the stop button on the drive itself is pressed. If done with auto set, I think that this will remove braking voltage once MOTOR stop has been sensed. Most applications that I used these drives on we were required to have a separate brake relay that the resistor was fed thru that was controlled by an end-of-travel switch. Apparently the government and railroads don't fully trust soft switching! :blink:
- Jim

Sent from my iPhone using ET Forum


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## John Valdes (May 17, 2007)

Check the braking transistor (IGBT). I bet its burned closed, or faulty. This transistor bleeds DC voltage to the resistor bank when the voltage exceeds the bus voltage threshold. This happens during braking and during high input voltage conditions. Whenever the control sees the bus voltage exceed the threshold. If it happens on start, it sounds like the transistor is bad or the transistor is receiving an input to turn on. While braking this transistor pulses voltage to the resistor bank. It could be fryed.

Find the braking transistor. It could be on the drive (control) or be part of the braking resistor assembly. Usually small drives under 5-10 HP have one on the control itself and larger ones may have it located on the control and on the resistor assembly.
Using your multimeter select "diode" check. Measure it (IGBT) forwards and backwards. You should have spec sheets that will tell you what the reading should be. Then make sure it's not shorted.
If you are not sure, just replace it. Also check to see if it is receiving an signal to turn on when it should not.


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## JRaef (Mar 23, 2009)

John Valdes said:


> Check the braking transistor (IGBT). I bet its burned closed, or faulty. This transistor bleeds DC voltage to the resistor bank when the voltage exceeds the bus voltage threshold. This happens during braking and during high input voltage conditions. Whenever the control sees the bus voltage exceed the threshold. If it happens on start, it sounds like the transistor is bad or the transistor is receiving an input to turn on. While braking this transistor pulses voltage to the resistor bank. It could be fryed.
> 
> Find the braking transistor. It could be on the drive (control) or be part of the braking resistor assembly. Usually small drives under 5-10 HP have one on the control itself and larger ones may have it located on the control and on the resistor assembly.
> Using your multimeter select "diode" check. Measure it (IGBT) forwards and backwards. You should have spec sheets that will tell you what the reading should be. Then make sure it's not shorted.
> If you are not sure, just replace it. Also check to see if it is receiving an signal to turn on when it should not.


Bingo.
If you have a small drive, you will not be able to repair it, the braking transistor is integral to the power module that has all of the diodes and transistors in one unit. If it's larger then the transistor is in the external module. If you connected resistors on your own to a larger drive that required a brake MODULE, then that right there is the problem. But assuming you did not, it sounds like your braking transistor (chopper) is about to fail.


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## JRaef (Mar 23, 2009)

Brad765 said:


> A couple times now, we have had full DC voltage on BR1 and BR2 to the brake resistor on drive power up. Catching the resistor elements on fire. Does anyone have an idea why this happens.


New thought from noticing that you are in Canada...

Any chance you have a 480V drive and are feeding it 600V? That would look to the VFD as though the DC bus voltage was too high and it would immediately try to bleed off voltage to the resistor bank. If the voltage is PERMANENTLY high, it would never stop.


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## Netree (Sep 3, 2011)

The brake chopper also is my thoughts; I have seen this happen, as the picture below. This was a Lenze 8200 inverter. The resistor got very hot, and also melted the cabinet fan and document holder from the door.


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## Brad765 (Feb 9, 2010)

Thanks all for help, but as I came to work this morning to find the drive has already been sent out. 
The drive had been working fine for a few years until yesturday morning. A 96A powerflex 700 drive controlling a 480vac 3 phase 50hp belt conveyor. 

Thanks again. 
Hopefully find out when drive comes back


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## John Valdes (May 17, 2007)

Brad765 said:


> Thanks all for help, but as I came to work this morning to find the drive has already been sent out.
> The drive had been working fine for a few years until yesturday morning. A 96A powerflex 700 drive controlling a 480vac 3 phase 50hp belt conveyor.
> 
> Thanks again.
> Hopefully find out when drive comes back


Make sure the repair facility provides a description of the failure and lists the replacement parts. And make sure they have the ability to load test the control.
It is always good to know what they did and what caused the problem to start with. Without that information, you might not have the issue corrected at all.


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