Protecting Dynamic Braking Resistors

[LMAO]

I have seen these resistor banks fused and not fused. In high motor HP application where resistors are about 1000kW I have seen them fused; whereas on smaller VFDs with small braking module (10-50kW) I have not seen fused resistors; i.e., no fuse at the output of transistor module powering the resistor bank.

Recently, we have had problems with these resistor enclosures in the field; apparently, output of IGBT shorted into resistor enclosure (and not the resistor bank) melting the enclosure and destroying the entire VFD because resistor enclosure was mounted on the VFD.

So people are recommending adding fuse at the output of dynamic braking IGBTs but I don't know if that's going to be a good solution because the enclosure is pretty much the same material as the resistor (steel) with similar resistance - maybe even higher - so not sure what good would fuses do...? Not only that, braking transistors are supposed to trip at over-current; if no over-current was sensed by the braking module the chances are it won't be cleared by fuse either (right?).

I am thinking of using extra temperature switches "on" the enclosure; we currently have one inside but it did not trigger because the surface of enclosure got hot and not the air inside.

Your thoughts?

 

[petersonra]

The drive itself typically does an I^2T calculation similar to an electronic overload that protects the resistor. However, that is based on the duty cycle of the brake transistor and how long it is on. I don't believe any normal VFD actually measures brake current.

I doubt there is anything you can do to protect the brake transistor if it fails, which seems to be what happened to you.

Not sure a fuse serves any real purpose. Chances are you are going to have to make it large enough that even a shorted brake transistor won't trip it.

A transistor shorted to the frame is probably not going to produce enough heat to trip a thermal switch.

Brake transistors tend to be highly reliable. My suspicion is some kind of installation error led to the transistor shorting to the frame. I think it would be best to figure out what really went wrong and fix that rather than trying to put a bandaid on it.

 

[Jraef]

The drive itself typically does an I^2T calculation similar to an electronic overload that protects the resistor. However, that is based on the duty cycle of the brake transistor and how long it is on. I don't believe any normal VFD actually measures brake current. ...

I agree with everything Bob said here, except this sentence. Few drives provide protection for the braking circuit if it is entirely external, typically there is a thermal switch in the DB module itself to protect it, but based on temperature of the device, not the current flowing to it. The duty cycle protection scheme will only apply to drives with an INTERNAL DB braking transistor, where you only have to connect the resistor itself. Even then, a number of the small inexpensive drives have zero monitoring of even the braking transistor circuit, be it current or duty time. Brake transistor failures on those are actually quite common.

That said, those are the type of drives that use IPMs, where the braking transistor is one of the transistors embedded inside of the IPM with the diode bridge and output transistors, all ina sealed case. So on those, there would be no way to have the transistor short to the case in a way that you could tell that was what happened (when an IPM goes bad, it all goes bad). So given the described scenario, it must have been a larger drive where the braking chopper was an external module in it's own separate box containing the transistor and the resistor. Shorting to the enclosure would have meant the transistor cracked in its case and shorted to the heat sink. That is typically a high resistance short, so it makes sense that the DC current flowing through the failed transistor to the heat sink would keep going until things started melting. So as Bob mentioned, it would be unlikely that a fuse would have cleared in time to to any good. But it also sounds like that module didn't have a thermal cutout either, that's not a good design.

 

[LMAO]

I guess my explanation was not clear. Braking IGBT is a standalone module outside the drive. Resistor bank enclosure was the one that shorted the output of braking IGBT. We run the cables from braking IGBT outside to resistor bank enclosure and terminate them across braking resistor bank. In this case, resistor cables somehow shorted to resistor bank box and melted it.

 

[Tony S]

Sorry this is late.

We had a similar problem with a reasonably sized drive. If the braking wasn?t working due to yet another resistor failing the machine was downright dangerous.

It had got to the stage where stores stocked four of these resistors ?just in case?. I emptied the stores and made up a series parallel network and mounted the resistors in a mesh cage on top of the VSD panel.

It didn?t look pretty but it worked.

In the 15 years the machine was in action after the modification we never changed another resistor.

We were up to our eyeballs with resistors in stores ;-)

 

[Electric-Light]

What are you braking? It's not about the size of drive as it is what is on the shaft of the motor.


The resistors are like a heating element in a bucket of water.
If it doesn't brake too often, the bucket of water is enough to absorb the braking, then cool down in
between on its own. They're very much like your brake rotors. They can handle and hold 60 to 0 in reasonable intervals, but not coasting down a hill for miles. Suppose the heating element size is right but, if you brake too often or the load is too heavy for water to absorb at once, it will overheat.

Suppose you downshift. Now you're operating a compressor. The energy is used to drive a compressor which heats up the air which goes out the tail pipe and you have an infinite supply of cool air from the environment.

Another alternative is regenerative braking which converts the power back to AC power and dumps it back into the power grid.

 

[Electric-Light]

- kW determines the size of heating element.
-The temp rise determines how big the water heater tank is.

So, if you hook up a 4.5kW water heater filled with 70F water as a braking resistor, it's 4.5kW regardless of tank size, but if the limit is 140F, you can get away with running the heater longer with a 55gallon tank than a 35 gallon tank.

Once the water hits 140F, it will cool off on its own and eventually down to room temp.

-An auxiliary radiator to circulate and cool the water will make this happen much quicker or if it's large enough, you can run the heater indefinitely without overheating.

So, if your configuration allows, thermostatic cooling fan that turns on at certain surface temp, then continue to run until cut-out temp is reached could solve the issue. If you run the fan continuously, it would increase dust accumulation rate.

Ideally, an overheat on braking resistor would activate the emergency braking mechanism and bring the machinery to a complete stop safely. On a car, if I'm coasting down a hill and the transmission slips into neutral, I can use the regular brakes to bring the car to a stop safely.

If the regular brakes don't have enough capacity to do so, then expect to crash.