Sizing a VFD Brake Resistor

remlapw

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Oct 2013
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Hi Guys,

I'm wondering if anyone can help listing the considerations when looking to size a braking resistor for a VFD. It's typical for our clients to provide complete MCCs to interface with (we supply motor list, VFD/starter requirement, feedback if required and brake if required). I've found a manual from AB regarding sizing for a PF700 (Publication PFLEX-AT001J-EN-P – March 2007) which goes through quite a detailed process. It seems to me that one could quickly pick an oversized resistor to deal with the job and disregard the calculations involving load inertia (how is one to calculate these values?). Is this correct? If so - how is one to know what is adequate?

What is the typical protection type circuitry required for using an external resistor? I have seen different ideas on how this can be done, from using the DB thermal switch to open a contactor on the line power side of the VFD. Is it possible to rather simply wire the thermal switch direct into an input and configure it as an AUX Fault (fault action coast to stop)?
 
The easy answer is to do what the manual tells you to do. If you need help calculating inertia and total system energy your friendly neighborhood mechanical engineer should be able to help with that. This is the best way to insure you get the "optimal" wattage for the resistor set.

A quick and dirty that has held reasonably true for me in the past with general applications is use about 100 watts of resistor per drive horsepower. This is for "general" applications, where normal decel doesn't require much if anything from the resistor and you really only need the resistor for fast stops and e-stops.

As for thermal protection there are many different ideas on this. I kind of like your thought of running the thermal into the aux fault input to the drive. The two downfalls to this are the drive will not run even if you don't need the resistor and it doesn't protect the resistor in case the braking transistor/controls goes gonzo and tries to continuously dump the buss into the resistor. Parker/SSD uses MPCBs in the resistor line on the 890's and opens up the resistor if the MPCB trips. Back in the 690 days they would use a free-standing motor overload section and run the status into the drive to shut down the drive. My personal feeling is that the MPCB in the resistor circuit is the best way to go but that is just a personal opinion.

Keith
 
Your first question to answer is how much braking hp do I need. Just because the motoring hp is 100, doesn't mean that the braking hp is 100. It could be 10 or 200. Once that question is answered and you are sure the VFD has the capacity to handle the braking wattage, then you need to determine if the drive has a built-in brake chopper or not. The chopper is the voltage-sensing unit that determines when the excess DC bus energy needs to be diverted to a resistor package. While checking out the chopper situation, you also need to determine what the maximum current capacity of the intended chopper is. (It could be specified as the minimum allowable brake resistance---same thing)
Generally, the resistor is specified at the minimum allowable value for the chopper although, if you only need a little braking, you could increase the resistance value a bit, maybe 25% max.
The most subtle part of the resistor sizing is determining the wattage. For that, you need to determine the duty cycle. If continuous, you need to pick a resistor wattage at or above the actual braking wattage. If the duty cycle is so low that the resistor is at ambient temperature every time it is energized, then you can use a wattage 1/10th of the actual wattage. For duty cycles in between those two extremes, the brake manufacturer publishes charts to assist you. They are usually set up as 10seconds braking in 60seconds, 10seconds braking every 30seconds, etc. If you must err, err on the high side with wattage.
Finally remember to enclose the resistors in a ventilated enclosure. They can get red hot! Also, consider the dust that might gather on the resistors. It will ignite if it gets hot enough!

Good luck. Hope this helps.
 
Brake resistor bimetallic protection relay

Have anyone found a bimetallic overload relay that doesn't open the power circuits? It is stated clearly in the SEW Movidrive MDX61B system manual that the brake resistor circuit cannot be opened in the event of a thermal trip. (See attached pic.)In this particular application (hoist) the following DC-bus overvoltage would be a pretty unpleasant way to fault and stop the drive...
:confused:

Capture.jpg
 
Better VFDs include a brake resistor circuit monitor that will help to protect it.

The problem with doing it using an OL relay and contactor is that the brake power on a 480V drive is around 700-800VDC, so you cannot use standard off-the-shelf AC rated components for that, and DC contactors for any significant amount of energy are going to be a lot bigger and very expensive.
 
Hijacking this thread again to answer my own question, just for reference. A Schneider ZB32-10 in conjuction with ZB32-XEZ solved the problem. However I didn't manage to find any information regarding if it cuts the measured circuit or not. Schneider were not as communicative as one could wish, but practical tests showed that the measured circuit remains closed.
 
Hijacking this thread again to answer my own question, just for reference. A Schneider ZB32-10 in conjuction with ZB32-XEZ solved the problem. However I didn't manage to find any information regarding if it cuts the measured circuit or not. Schneider were not as communicative as one could wish, but practical tests showed that the measured circuit remains closed.
Well, considering that those are Moeller / Eaton overload relays, I can see why Schneider would not be forthcoming with help... o_O

An OL relay like that does NOT open the power circuit itself, it just senses it. So the aux contact (97/98) that changes state could be connected to a digital input of the VFD that is programmed to shut it down, or it could be wired to a line input contactor if all of your digital inputs are consumed elsewhere.

Technically that OL relay is only rated for 690VAC max, no mention of DC ratings, so that may be problematic because on a 480V drive, the DC bus potential is as high as 850VDC (the trip point of most drives).
 
Just a quick thought
most VFD's can brake at 100% torque but the braking transistor (DBU) can only handle about 10% duty cycle. if you need above that then you need to use an external DBU or even multiple units to handle the power.
you may want to consider a line regen VFD not a braking resister they can handle the same braking load as the motoring load and you have the benefit that all the enegy I sent back into the ac line for use elsewhere.
as stated most vfd'd monitor the current in the brake resister and cut it off on an overload. also what you are trying to protect is not the resister but the brake transistor in the vfd
if you need to brake a 200hp motor at 100% then you will need an external DBU to do it.
 
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