Dangers of Working with Inverters?

nineandtwothirds · Jul 12, 2019

Hey All,
I have a 1992 Fadal VMC40 with the mitsubishi Freqrol Z300 Model Z320-7.5k-UL. I often get a spindle fault upon starting the machine, but when I let it warm up and sit for half an hour or so it works no problem. I figured it was the inverted because I was getting an alarm on it and would have to reset.

Today, the inverter just started smoking.

I assume I'll have to replace it, but I'd like to take a look and see if there's something obvious that I or someone substantially more qualified could repair.

My question is this: What are the dangers of working on inverters that have previously been under power? I'll identify the capacitors and discharge them. Is that the only risk? Anyone have experience with these inverters in particular? Or have an idea of what might be going wrong?

Thanks as always.

ewlsey · Jul 12, 2019

Once the caps are discharged, it's pretty safe. Sounds like an IGBT module has shorted.

nineandtwothirds · Jul 12, 2019

Well, uh, I think I found the problem. The silver (aluminum) unit that mounts to the heat sink has...melted. Is this the IGBT module? Further input and advice always appreciated.

gustafson · Jul 12, 2019

Nah, they all look like that.......

kidding..........

pull the numbers off it and google it, some are discontinued, but there are guys who specialize in finding that type stuff

nineandtwothirds · Jul 12, 2019

Looks like it's the brake resistor. Is this a regen circuit issue? I have no idea what I'm doing...

bloomautomatic · Jul 12, 2019

nineandtwothirds said:
Looks like it's the brake resistor. Is this a regen circuit issue? I have no idea what I'm doing...

View attachment 260887

Based on your pic, it's probably the brake resistor. You could try replacing that by itself, but something else in the drive may have went that caused the resistor to go.

You may be able to disconnect it and try the drive. It may fault if it doesn't sense it. If that's the case, you should be able to change a parameter in the drive to disable the braking - unless it's controlled by the CNC.

OR - a quick ebay search turned up a few of that model for $300 or so. It may be better to buy another unit than to put money into one that's already been smoked and overheated.

Hodge · Jul 12, 2019

"Today, the inverter just started smoking."

You realize gum and patches are available....

ewlsey · Jul 12, 2019

Pretty common failure mode. The braking IGBT has likely shorted and dumped the DC bus voltage into the braking resistor continuously until something finally let go. This is so common that many machines have a thermistor on the braking resistor to shut down if the resistor gets too hot.

Check all of the transistor modules. I think you will find one has a dead short.

gustafson · Jul 12, 2019

Hodge said:
"Today, the inverter just started smoking."

You realize gum and patches are available....

Look, if you are going to post something, try to be useful

Like, tell the guy what kind of gum you are suggesting, I mean, is it juicy fruit, sugarless

People need to know

FredC · Jul 12, 2019

One thing to check is your incoming voltage. One of my invertor's braking resistor will get so hot you could fry eggs on it when the voltage gets over 248. 245 is OK, so a buck boost transformer dropped it to 236 as the power company was unwilling to adjust it this time.

awake · Jul 12, 2019

nineandtwothirds said:
Well, uh, I think I found the problem. The silver (aluminum) unit that mounts to the heat sink has...melted. Is this the IGBT module? Further input and advice always appreciated.

View attachment 260886

Not now, it isn't!

Hodge · Jul 12, 2019

gustafson said:
Look, if you are going to post something, try to be useful

Like, tell the guy what kind of gum you are suggesting, I mean, is it juicy fruit, sugarless

People need to know

My pardon, nicotine gum, of course

thermite · Jul 13, 2019

awake said:
Not now, it isn't!

And this is why the nation has so much college debt?

Usta bee we'd have just said:

"Not no more, it ain't"

... and had money left for beer.

rons · Jul 22, 2019

gustafson said:
Look, if you are going to post something, try to be useful

Like, tell the guy what kind of gum you are suggesting, I mean, is it juicy fruit, sugarless

People need to know

Yeah, like the last one. :icon_bs:

JST · Jul 22, 2019

FredC said:
One thing to check is your incoming voltage. One of my invertor's braking resistor will get so hot you could fry eggs on it when the voltage gets over 248. 245 is OK, so a buck boost transformer dropped it to 236 as the power company was unwilling to adjust it this time.

That, and while you are at it, check the trip point for the OV if you can. The "245 OK and 248 not OK" is typical of setting the OV point for the brake low enough that a "normal variation" of line voltage can trigger it.

usually the trip voltage is settable. Setting too high can cause a shutdown trip if the "brake resistor" does not drain enough charge off the bus capacitors fast enough, so it may be set fairly low, and you may want to be careful with re-setting it.

But, if the bus voltage is driven up by incoming line voltage, the brake resistor may be "on" at whatever its full allowable duty cycle is. If that is too much for the heatsinking it has, then you may burn it up, and in any case, all that you are doing is wasting power with no benefit, so a small increase in trip setting may fix it with no bad consequences. .

Rob F. · Jul 22, 2019

gustafson said:
Look, if you are going to post something, try to be useful

Like, tell the guy what kind of gum you are suggesting, I mean, is it juicy fruit, sugarless

People need to know

Everyone knows to chew and use Trident when working on three phase stuff, right?

thermite · Jul 23, 2019

Rob F. said:
Everyone knows to chew and use Trident when working on three phase stuff, right?

Chew. Use. That's only two.

What's for the third leg? Try to balance it on yer glans?

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