frequency inverter running at power fail

[Ken]

Hello there,

I'm wondering how I could fix something that should not be fixed...
The problem is that several frequency inverters could be power cut (emergency stop) during operation.
So then the drives end up with a free spinning motor until standstill.
This will generate power that needs to be disposed off or blocked so it's disposed through the rotation itself.

I've had two situations where the traces of the mains burned holes through the PCB and I'm guessing this is the root cause.
At this point it is nearly impossible to set this all up another (yes the right) way.

I can't possibly believe that I’m the only one with this issue, how does one go about this?

Thanks in advance!

[SeanB]

Are they all on the same supply, single phase, or 3 phase? If so, and they are all fused with a single block of overcurrent protection, and are the same manufacturer and model, then look into connecting the DC bus inside each of them with a commoning DC bus, and add in a brake resistor as well to this common bus. Rating of brake resistor is that of the largest supply. this will also allow energy recovery from motors stopping, with this being used by the other inverters, or, if too large, in the brake resistor to dump the energy safely. Very easy to do and also a big saving in energy, plus any emergency stop operation, that disconnects power input, will result in all drives shutting down safely, and not having an overvolt trip out as they brake the motors to a stop. typically you have a large VDFD, and a few smaller ones, so the large one operates most of the time, and the small ones stop and start as needed, but having the DC bus commoning means you can go for much faster speed change rates, as the individual inverters wheel power amongst them, and can ramp down to stop from full speed in as little as a single revolution, and not have an overvolt or overcurrent trip, even doing it multiple times a minute. Seen that in a big motor control station, where the main motors were 100kVA variable speed drives on hydraulic pumps, and the small motors were 22kVA motors running at a ramp up from zero to 100Hx in a half turn, and the same back down. Had nearly 1F of DC bus capacitor, at 800VDC, and a brake chopper that was a separate cabinet above it, connected with a massive bundle of cables that allowed the controller to vary the dump power smoothly, and a set of 10 115mm fans running all the time to cool them. 40mm power cables to feed it, and it was 40 tons for the controller cabinet alone.

If they are separate drives contact the local supplier, and get individual brake chopper modules for each drive, one per drive, sized for the drive power. As well, if the drives are on a very stiff supply (short circuit power capacity exceeding 10kA) you probably want to get some line reactor chokes on the incoming supply per inverter as well.

[Ken]

Thank you for your insightful reply!
I’m going to look into the right braking resistors, the drives are in pairs of 3 with two being 22kW and one 11kW on separate fuses.
To me the breaking resistors sound like a solid solution for dissipation of power.
It sounds obvious, but I just haven’t come across this type of situation.

Thanks again cheers! 

[BeBuLamar]

If you free wheeling the motor you wouldn't need the braking resistor. Braking resistor is when you want the drive to quickly decelerate the motor to a stop. I never had problem with burning PCB if I program it for coast to stop (free wheeling).

[Ken]

I definitely don't need to stop the motor fast, it's just the power that gets interrupted. At that point, I can throw any pair of settings on the controller, but it is off then. It is very weird to me that this happened, but it's the 3rd. I did change the type of controller I'm using because I really disliked the old ones (KIMO I'm just going to say it because that is how much I dislike their controllers). It's a weird one, and for the breaking resistors, well, if they consume the energy that was burning the PCB, I don't mind what the actual function was in this case.



The proposal is still with the customer, but for now I'll wait and see how the other controllers hold up.

[BeBuLamar]

I generally don't put a contactor between the output of the drive and the motor but many people do. So if you have this contactor open when you have power failure then it can't burn the drive regarless. Also if the power interuption is because of the emergengy stop I would not power down the drive but send a disable command to the drive and open this contactor

[Ken]

The contactor would work regardless, so that is a good solution.
I'm not allowed to keep the drive powered because of EX regulations... That really messes up a lot.

[WattsThat]

There is a lack a very important information in the original post.

Once a variable frequency drive stops generating output current, a standard squirrel cage asynchronous motor does not generate voltage, it’s just a spinning mass of metal.

The only way the motor can generate voltage without power applied is if the motor is a permanent magnet synchronous machine.

Please clarify what type of motor you have so we can have an informed and useful discussion.

[Ken]

Okay got yah

It's an asynchronous motor. To me, permanent magnet motors are controlled by a motor controller.

Personally, I tend to like an asking approach more than calling out, but that might be a matter of taste.

I'm still grateful for your technical input.

[WattsThat]

With an asynchronous motor, it is not voltage from the motor causing the arc. Are the varistors in the photos on the input (line/grid) side of the drive? If so, it’s more likely the excess voltage is from an input reactor or perhaps an inductor in a harmonic filter on the input side of the drive dumping its energy when the line drops suddenly.

[Ken]

There is nothing in the mains of the drive that I know off, at least not in the whole machine I have control over. It might be the filter of the drive itself dumping it's energy. I've only had this issue with these drives and I've worked with a lot of them.