Over-Voltage on a VFD

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chedore

GANEX
Location
Quebec, Canada
Occupation
Electromecanical technician, Automation Technician
Hello there. I am currently working on a thermoforming machine with 2 mold platen, each drived by a 30hp motor controlled by a GS3-4030 VFD with GS-4030-BR-ENC brake resistors. We currently are trying to troubleshoot the lower platen, which triggers an Overvoltage Limit when going down at 10hz. Im thinking about motor regeneration issues. It ramps up to the selected speed and then triggers the limit. The platen supports an aluminium mold used for thermoforming, which lifts and push into the plastic sheets before going back down to the original position. So far, we have tried these solutions: Increasing the Overvoltage limit (and staying in the drive limits), Increasing the decel time, increase slip compensation, try different s-curves... so far no luck.

I was wondering if trying to connect the 2 GS-4030-BR-ENC brake resistor in serie would be a good way to try and find if that was the original problem. Any heads up? Anyone seen this kind of problems before? Thanks for the answers!

-Fred
 
Hello there. I am currently working on a thermoforming machine with 2 mold platen, each drived by a 30hp motor controlled by a GS3-4030 VFD with GS-4030-BR-ENC brake resistors. We currently are trying to troubleshoot the lower platen, which triggers an Overvoltage Limit when going down at 10hz. Im thinking about motor regeneration issues. It ramps up to the selected speed and then triggers the limit. The platen supports an aluminium mold used for thermoforming, which lifts and push into the plastic sheets before going back down to the original position. So far, we have tried these solutions: Increasing the Overvoltage limit (and staying in the drive limits), Increasing the decel time, increase slip compensation, try different s-curves... so far no luck.

I was wondering if trying to connect the 2 GS-4030-BR-ENC brake resistor in serie would be a good way to try and find if that was the original problem. Any heads up? Anyone seen this kind of problems before? Thanks for the answers!

-Fred
A brake resistor might the way to go. I have used this system a number of times. Mainly on sectional paper machine drives.
 
Usually a DC overvoltage trip occurs do to excess energy being generated in the motor that gets fed back into the drive and then to the DC bus. the brake resistors are supposed to bleed off that excess energy.

You could have a brake transistor that has failed so no energy is being bled off to the brake resistors. You also might have the braking setup configured wrong.

Putting two resistors in series is going to make the problem worse because less energy would be bled off from the DC bus. Putting them in parallel would double the rate of energy being used up but might cause the brake transistor to fail due to excess current.

It is also possible that the brake resistors are working as designed and that there is just too much energy being generated for the resistors to use up fast enough to keep the DC bus voltage at an acceptable level.

There is probably a way to watch the DC bus voltage on the keypad to verify what is happening.

There may be some parameters that affect how the braking works.

Best bet might be to just call tech support and ask them.
 
Hello there. I am currently working on a thermoforming machine with 2 mold platen, each drived by a 30hp motor controlled by a GS3-4030 VFD with GS-4030-BR-ENC brake resistors. We currently are trying to troubleshoot the lower platen, which triggers an Overvoltage Limit when going down at 10hz. Im thinking about motor regeneration issues. It ramps up to the selected speed and then triggers the limit. The platen supports an aluminium mold used for thermoforming, which lifts and push into the plastic sheets before going back down to the original position. So far, we have tried these solutions: Increasing the Overvoltage limit (and staying in the drive limits), Increasing the decel time, increase slip compensation, try different s-curves... so far no luck.

I was wondering if trying to connect the 2 GS-4030-BR-ENC brake resistor in serie would be a good way to try and find if that was the original problem. Any heads up? Anyone seen this kind of problems before? Thanks for the answers!

-Fred
My apologies - you already have a brake unit. Can you add another?
 
My quick calculations here tells me that you have to install another brake resistor (paralleled with the existing) of the same size as what you already have. Your brake resistor has a resistance of 27 ohms and making the total resistance to about half that value will probably solve your over-voltage tripping.
 
Thanks everyone for the answers. The machine is currently being serviced for another trouble, I will update this thread with the result when we try to fix the resistor problem.

I'm going with the parallel connection solution but before I'll monitor the DC bus to see exactly what's happening!
 
Your brake resistor must fit within a specific range of resistance values given by the VFD mfr, because if too low, you can burn out the braking transistor. Putting resistors in parallel LOWERS the resistance in the circuit, so be VERY careful with that.

One thing to try... do you NEED a controlled Decel profile at all? If you set the Decel to "Coast", there is no regeneration, because the transistors just turn off.
 
Your brake resistor must fit within a specific range of resistance values given by the VFD mfr, because if too low, you can burn out the braking transistor. Putting resistors in parallel LOWERS the resistance in the circuit, so be VERY careful with that.

One thing to try... do you NEED a controlled Decel profile at all? If you set the Decel to "Coast", there is no regeneration, because the transistors just turn off.
Hi everyone, sorry for the late reply, things got very busy at the shop and we used the machine molding with the top platen for a while! We started tackling this problem again today...

I just tried changing the decel to Coast, but since the load is pushing down, and our motor's rising the platen up, when it gets to its upper limit and coast to stop the platen lowers a bit. Also, when lowering the platen we still got the Overvolt error when the lower platen reached its preset jog HZ speed (10,0HZ). After that, I tried changing the Accel/Decel values, changing the slip, played a bit with the protectives functions without any luck so far. I tried setting the preset jog HZ speed to 25,0HZ and the Drive had an Overvolt error around 13/14hz.

It seems to me that this problem only arise when we stop varying the frequency of the drive or reach a set speed. Tomorrow our main electrician is scheduled to take a closer look at our resistor to see if they are OKAY, and I currently am exchanging emails with the Drive/Brake mfr to know if our setup is within the specific range of resistance.

Are we still in the regenerative margin of errors?

Thanks and as usual, have a great evening.

-Fred
 
Quick question this morning,
the brake resistor setup we have installed currently is 4800w @27.2ohms, it's rated for a 30hp motor. If we scale up to 6000w @20ohms, rated for 40hp, can there be any possible issues regarding lower resistance?
 
Quick question this morning,
the brake resistor setup we have installed currently is 4800w @27.2ohms, it's rated for a 30hp motor. If we scale up to 6000w @20ohms, rated for 40hp, can there be any possible issues regarding lower resistance?
Maybe. Can the braking transistor take the increase in current?

What does the manual say is the maximum braking load.
 
image_2022-04-06_091846.png
I am not sure how to interpret those values, the top highlighted braking resistor is what we currently have installed and the bottom one is what I'm suggesting to install. The load on our bottom platen is quite heavy so it would make sense that's its currently regenerating when going down if our resistor lose any% of efficiency.

Thanks for your input, and I'm open to know more about this foreign subject.

edit: I'm adding the dynamic braking unit spec sheet too. I was focused on troubleshooting the resistor but haven't thought one second about trying to see if our Dynamic Braking Unit is functioning as it should. image_2022-04-06_092934.png

I will be looking into this while we make a decision according to the infos we have.
 
Last edited:
Quick question this morning,
the brake resistor setup we have installed currently is 4800w @27.2ohms, it's rated for a 30hp motor. If we scale up to 6000w @20ohms, rated for 40hp, can there be any possible issues regarding lower resistance?
Is this something that once worked and is now giving this trouble?

Has the motor or load characteristics changed?

Is the load high inertia type of load - that can lead to requiring derating the drive used (going with larger drive).
 
We may have found part of the problem... The Dynamic Braking Unit was changed two years ago, and our technician may have forgot to switch back the Voltage selection jumper to our current motor setup.
DynamicBrakingUnit480vOriginal.PNG

We switched it to 460v, and so far, while using 2 DBU and 2 Resistors paralleled according to the mfr manual we were able to make everything work without any problems yet. The bus VDC stops at 787Vdc and we get no over-voltage errors!
DynamicBrakingUnit460vUpdate.PNG
We are currently putting it back on the original resistor/DBU unit and preparing for more tests, I will keep you all up to date with the results.
image_2022-04-06_135333.png
 
Update: Everything is working as it should be, even with 2000kg of raw plastic loaded on the bottom platen.

Thanks everyone for the inputs and tips, I learned a lot today regarding these devices!

To sum up the verification checklist:
  1. The resistor was @29.2ohms on my multi-meter which is decent, +/-2ohms shouldn't really affect the normal functioning of the setup
  2. Changing all types of parameters in the drive unit DOESN'T affect the problem: persisting
  3. Trying to add a resistor paralleled did not solve our issue
  4. We were reading way over 800VDC on the VFD Bus Voltage monitoring function before switching up the Voltage Selection jumper
If you run into this kind of issue in the future, the verification in this checklist are OK and the system use a Dynamic Braking Unit, this later might be the problem and the root of all your problems.

 
Hello there. I am currently working on a thermoforming machine with 2 mold platen, each drived by a 30hp motor controlled by a GS3-4030 VFD with GS-4030-BR-ENC brake resistors. We currently are trying to troubleshoot the lower platen, which triggers an Overvoltage Limit when going down at 10hz. Im thinking about motor regeneration issues. It ramps up to the selected speed and then triggers the limit. The platen supports an aluminium mold used for thermoforming, which lifts and push into the plastic sheets before going back down to the original position. So far, we have tried these solutions: Increasing the Overvoltage limit (and staying in the drive limits), Increasing the decel time, increase slip compensation, try different s-curves... so far no luck.

I was wondering if trying to connect the 2 GS-4030-BR-ENC brake resistor in serie would be a good way to try and find if that was the original problem. Any heads up? Anyone seen this kind of problems before? Thanks for the answers!

-Fred
We had several hundred VFD'S and when a problem arose that I could not figure out would either call the great & very talented tech that performed all the start ups on drives we replaced or installed or toll free manufacturer 1 - 800 # from the manufacturer. Our start up guy had a 1" thick start up & programming Manuel and even as talented as he was had to call the manufacturer service department up to remedy a problem. Was amazed that they told me to adjust a single parameter & all problems went away.
 
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