Brake Resistors Overheating 2

[JASGripen]

Hello, what mechanical issues can create VFD brake resistors to overheat and short that is not related to the actual programming or VFD.

Had to replace the resistors twice recently on an industrial 480V washing machine motor, before I go ahead and replace the VFD it would be helpful to get some input on any mechanical issues or sensors that could create the brake resistors to overheat and short.

Thank you.

 

[davidbeach]

Some combination of lack of airflow over the resistors and too much energy dissipated in the resistors.

 

[JASGripen]

Air flow is okay, fan in compartment is working. Could try to cheat by adding an additional brake resistor in series to the ones that are there to reduce the heat, but I would prefer not to do so as that is not the original spec.

 

[davidbeach]

Two things - heat in and heat out. Problem exists only when heat in exceeds heat out. Heat out is a function of air flow across the resistor. Heat in is a function of the amount of braking. You either don’t have enough heat out or you have too much heat in. Your problem statement was failing resistors, not excessive braking. Ergo, you aren’t removing enough heat. More resistors in parallel, not series, or more air flow. Your choice. Next time, if there is a next time, grossly over estimate the braking energy to be dissipated. If it’s close, it’s wrong.

 

[JASGripen]

You are correct parallel for adding resistors, the overheat safety switch was in series - mixed it up. Good advice on grossly over estimating the braking energy to be dissipated.
Thank you.

 

[waross]

Hello, what mechanical issues can create VFD brake resistors to overheat and short that is not related to the actual programming or VFD.

So the workload increased and someone was told to speed up the wash cycles.
Now someone is trying to slow the spin cycle down too fast.
I'm sorry. I know that you said to don't go there, but this has all the symptoms of too much braking.
This is also one of the symptoms: "that is not related to the actual programming or VFD."
If the spin cycle has also been shortened and the clothes are not completely dry that will add to the resistor overheating issue.
I may be wrong. Have you measured the current to the brake resistors?
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Braking resistors essentially must absorb ALL of the kinetic energy in the spinning mass as electrical energy pulled off of the motor, then dissipate it as heat. So bottom line: if you are burning out resistors you are trying to make them dissipate more energy than they can handle in a given time period. So you either must reduce the kinetic energy, extend the time period or increase the energy dissipation capacity.

But there are two parts to a dynamic braking system; the resistors AND the DC chopper circuit that controls the energy flow into them. So you cannot simply add on resistors, because you change the resistance in the circuit and thus the amount of current flow through the DC chopper transistor, so you move from burning up the resistors to burning up the transistor in your drive, which is much more expensive.

There is a somewhat complex formula for determining the maximum braking capability of the drive that only the drive mfr can help you with, because they know what kind of “duty cycle” their chopper transistor was designed to handle. Then you will also need a good value of the amount of inertia in your washing machine when fully loaded. From that, using their formula, you determine; A) IF the system can do what you are asking, and B) what wattage and ohmic value of resistor(s) it will require.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[JASGripen]

Okay located two issues on the operation of the washer unit.

1) Bad speed sensor for the drum occasionally stopping wash cycle.
2) Users resetting unit and skipping cycles to get to the assumed last wash cycle after unit shutdown.

Point 1 is what jraef mentioned which I assume could create problems with the brake resistors, but not sure. Point 2 is what waross mentioned which is most probably the reason for the brake resistors overheating.

 

[waross]

Thank you for reporting back to us. Not everyone does. lps

Bill
--------------------
"Why not the best?"
Jimmy Carter