1336-WB110 Brake Chopper Cooking Resistors

I have a client that has burnt up their brake chopper resistors twice in the last few weeks with the drives idle but the disconnects on. The difference is that their co-gen turbine has been having regulation problems. My theory is that the co-gen is outputting a high ac line which is being converted to a high DC bus. I think that the high DC bus is then causing the choppers to pump the energy to the resistors and cooking them.


These are around 100hp drives, so they have the SCR pre-charge, but I don't think that a high line would inhibit the pre-charge from turning full on. I don't think that there is a way to stop this from happening except to make sure the disconnects are open when the turbine is fired up. They want to wire the resistor thermos to the drives, but I don't think that this will do anything either as I don't think the drive has the capacity to turn off the pre-charge on demand.


These are really old drives and I'm having a hard time just finding documentation on them. Fun stuff.

Most manuals show a thermal switch or overload relay wired to a line contactor to prevent just such a problem.

Most Manufacturer's choppers are enabled regardless of the status of the drive. Some of the separate choppers have an enable input, so you can enable the chopper only when the drive is running. If its running with a decent load, you may avoid the problem of the chopper turning on, but then again, maybe not... depends how high the line is.

It is surely a high line problem. I've seen it many times.

Yes the chopper is fused, but the resistors cooked before the fuses blew (drawings show contactor/ol but really only has fuses).


They just need to keep the drives disconnected when firing up the co-gen. I'm surprised that the high volts didn't take out other drives and equipment in the paper mill.

Thanks for the reply.

They REALLY need to adjust the generator's regulator!

Gene Bond said:

They REALLY need to adjust the generator's regulator!

Click to expand...

+ 1. Ya think?

I think that the two incidents occurred BECAUSE they are trying to stabilize it. I'd offer to help but I'm not going anywhere near that monster jet engine. Far too loud and way too dangerous.

Yes, the Chopper circuit on a 1336 DB unit is triggered strictly based on the DC bus voltage. The only way to avoid this is, as was said, a DC contactor and a thermal switch.


Modern A-B drives now have a thermal manager for the DB resistors, but the 1336 is too old for that level of sophistication.