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VFD drive blown with no load ??

VFD drive blown with no load ??

I'm totally new to VFD's

I just bought one to try to learn more and it looks like I've blown it before even starting!

I wanted to learn how to navigate the menus etc before actually linking it up to my three phase washing machine motor.

I connected it to my single phase supply and after setting the parameter to set it to factory defaults, I began to experiment with ramp up speed etc, and everything went fine.

Once I was confident that I knew how to control the basic manual operation, via the integral keypad, I hooked up the motor.

When I switched it on at the mains there wasn't even a puff of smoke, a crack or anything, but there was also absolutely no sign of life from the previously functioning drive.

The LED's are totally unlit, no sign of life at all, while there is still mains voltage measured at the input terminals.

Can it be dead already?

I'm just curious could I have killed it by experimenting with no load attached, as opposed to my three phase motor causing the damage once I connected it.

If the former is true then I just by another drive and try again, whereas if the motor is the culprit I would need to take a different course of action.

I do believe the motor is fine.

I trust the person I got it from who said it was fine. (9.2 ohms on each winding, and no path to ground).

The drum bearings were scrap, hence the motor was removed as a spare.

The drive is a Poweflex 4, single phase input.

I will take a look inside for obvious damage, but thought I would first mention it on here in case someone can suggest another possible cause?


RE: VFD drive blown with no load ??

Running it unloaded/unconnected would not piff it.

And unless you'd set it to some sort of auto-on it should not have "powered up black". Even if you shorted all the outputs if you powered it up it should light-up and be happy until you actually gave it a move command.

You've unwired the output again and it's still black?

I'd unwire the power input and rewire it just to be sure.

With what you've described I think it's infant mortality but let's see what others think.

Oh and one other thing. Since you're running the three phase input with single phase there is some possibility you've toasted one of the rectifiers. With no connected load you could move one single input to the unused third input and see if it comes-to. If not then move the other input over to the just unused connection and see if it wakes up. If it does then it would confirm a blown rectifier. I'm not sure why it would blow unloaded...

Keith Cress
kcress -

RE: VFD drive blown with no load ??

Thanks for all of that.

I did already try un-wiring and rewiring it, with no joy.

This drive is a single phase input model, so as such there is no unused three phase input. I even have it dismantled already and there is no actual unused three phase input on the board. Just RL1 and SL2 terminals. There is nothing visibly faulty inside. I just remembered one other thing I did before powering it off. The unit is rated from 48Hz to 63Hz. Once I changed the reset parameter, the default frequency went to 60Hz. I modified the parameter which changes the drive to 50Hz as per the UK. I jut did that to ensure the displayed inverter output frequency would be correct. That's all I changed.

RE: VFD drive blown with no load ??

A very basic question (excuse me for being so basic): Do you still have power? Most simple single phase VFD:s do not have any current limiting circuit and it is very well possible that the fuse tripped. Then, you would have what you see - i.e. nothing.

Gunnar Englund
Half full - Half empty? I don't mind. It's what in it that counts.

RE: VFD drive blown with no load ??

Yes, That was the first thing I checked.

"The LED's are totally unlit, no sign of life at all, while there is still mains voltage measured at the input terminals."

RE: VFD drive blown with no load ??

All VFDs have a DC bus terminals near the input voltage. Check to see if you have the buss voltage their? Typical 3 phase 480vac bus voltage is like 600v.

Also, I would check the connections or how you put it together from your experimenting. If your display is not functioning that typically is a sign the 24vdc is not functioning on the drive, check the 24vdc internally on the drive. Review how the display snaps together to the drive, sometimes connections gets bent and you never notice it till you have it powered up.

I have seen the same stuff over the years when I jump to a different model of VFD. I do not know how many times I have done the same thing your describing. Getting familiar with the vfd is what will make you not freak out when stuff like this happens.

RE: VFD drive blown with no load ??

I presume the DC bus voltage is the voltage output from the large bridge rectifier? MP158W in this instance.

RE: VFD drive blown with no load ??

Yes. It will typically be √2 x line voltage unless you mistakenly got a voltage doubling unit.

Single phase units often can come as voltage doubler units. In Europe that would get you a lot of DC bus voltage.

Keith Cress
kcress -

RE: VFD drive blown with no load ??

330 DC on the rectifier output. From where is the control panel voltage sourced? Would it be via its own transformer / rectifier? Thanks

RE: VFD drive blown with no load ??

Now check for a lower voltage. The manual should give voltages for an external pot or control. It could be the low voltage converter died an early death.

RE: VFD drive blown with no load ??

"It could be the low voltage converter died an early death."

That's what I've been thinking.

Not too sure what I'm doing, so maybe I better quit before I get lit !

RE: VFD drive blown with no load ??

Um yeah.. I was going to say messing with that 330VDC could get you to an early death too.

With the drive disconnected and measuring nothing on the 300V bus, (probably 5 minutes), you could probably ohm out the 24VDC output that the drive provides for it's own digital inputs. Look at the manual and find the 24V+ screw terminal then follow the actual traces back to find the section and regulator it comes from.

Or, don't waste your time and buy another drive NOT of the same brand. Probably less than 100GBP.

Or EBAY it and pay 50GBP.

Keith Cress
kcress -

RE: VFD drive blown with no load ??

Rather than engage in 20 questions back and forth, I'll jump to my theory and you can confirm or deny.

I'm thinking you bought this VFD off of Fleabay or some other scrap/surplus site, or obtained it free from a friend. In any case it's likely that you don't know how long it has been left on a shelf somewhere without power on it. When a VFD, which uses electrolytic capacitors for the DC bus, sits on a shelf for a year or more with no power applied to it, the caps go through a process called "deforming" which has nothing to do with shape. For caps to work, you need plates of conductors separated by insulation. In aluminum film capacitors, the plates are thin aluminum films wound around each other in an electrolytic liquid or gel. The insulation layers are a thin coating of aluminum oxide that forms in a chemical reaction with electricity. That AlOx is in a constant state of slowly dissolving back into the electrolte, but every time the caps are energized, it reforms itself. Leaving it with no power for a year allows that AlOx layer to dissolve back so much that it becomes dangerous to the caps, left for 2 years or more and it is completely gone in some areas and after 3 years it's all gone. When you energize it at full voltage, the caps burn through and if they don't actually explode (which is a possibility) they at least cease to function and either go open circuit, or they short through and put the raw unfiltered DC onto the transistors in the inverter, which immediately fail.

In the case of a PF4, the control power is derived from a DC/DC power supply off of that DC bus and it too is blown, rendering the drive inoperable. That's what it sound like happened here. There is no connection to the load involved in this process, it is a cascade process initiated immediately upon energizing the drive at full voltage. Sometimes it takes a few minutes for the failure process to complete itself, giving you the false sense of security that the drive was OK, but in reality the cascade was irreversible about 1 second after you applied power.

For future reference, old drives must be treated to a "re-forming procedure" prior to being energized after a year on the shelf or on a machine that has been ignored that long.

Your chances of finding a repairable part are less than 1%, and then the chances of that repair costing more than the price of a new drive are 99.99%. You have what is referred to as a door stop drive.

And to the earlier comments/suggestions, there are no DC bus terminals on a PF4.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: VFD drive blown with no load ??

@ itsmoked / jraef -

Thanks for all that useful information.

It all makes sense.

I had spoken to a helpful support engineer at Rockwell also, and she explained the exact same thing about a PF4 that had been shelved for a while, and the capacitor dying as a result.

Unfortunately I had bought another model the same, before coming back to this forum.

I like the format with the pot on the panel, so if it would work I would be happy.

This is just a means for me to learn about drives, no commercial gain, so I can't justify spending a fortune on a brand new reputable brand drive.

Once again, same as last time, I did buy it on ebay.

My question now is, how to I "reform" it?

Is it some procedure I can do, or is it beyond my scope?

One other factor which might be relevant, is that, as you can see in the attached photo, I applied my crocodile clips to the rectifier DC output to help prevent an early grave.

What proved interesting is how slowly it de-energised.

It didn't take 3 minutes like the label on the drive suggested, it took 9 hours !!

It really did creep down.

After I did the initial DC test, I went shopping and came back in about 90 minutes and it was still well up in the high 200 to 300V range.

I can't remember the exact value, but I did keep an eye on it and it really has crept down slowly.

Even now it still shows 2V.

I haven't received my next drive yet, but is there a reforming procedure I can carry out, lest I kill it also?


RE: VFD drive blown with no load ??

Go to, click on Support and navigate to the "Knowledge Base", then search for articles on "Powerflex capacitor reforming", there are specific instructions on how to do it. It's not a simple thing to do, thats why people sell old units cheap on ePrey. But with a little enginuity you can make it work with cobbled together parts. What you want to do is to slowly increase voltage over a long period, the length depending on how long it was left unpowered. If you don't know how long that was, play it safe and take 24 hrs to get to full voltage. Current is unimportant, there is no load other than the parasitic resistance of the caps, but at low aluminum oxide density, that's resistance is low so current can start off relatively high, as in a few amps on a small drive like that. The tricky part is opening it up and disconnecting the PC board power supply connection to the DC bus so you don't give the PCBs that low voltage.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: VFD drive blown with no load ??

That's great. I will take a look there. I'm just slightly concerned that the seller says it powers up and works fine. I'm thinking has he possibly already lit the fuse of the self destruct system, by powering it up after a period of non-use. Thanks

RE: VFD drive blown with no load ??

The capacitor story is a little exaggerated. If stored for a long time it can be an issue if you use a motor at full power just after powering up. I have capacitors stored for 30 years I put back into service. Reforming takes current. The fact that these capacitors are holding a charge for so long indicates reforming is not an issue. It is disturbing for another reason. The control electronics should have continued to draw power and dropped the voltage fairly quickly. That would tend to indicate the power supply for the control electronics is defective or the power connection to it is open. The seller did what you first did, applied power and it lit up, sell it as working.

RE: VFD drive blown with no load ??

I see this a lot with VFDs, in my experience the average is about 15 minutes of up time before failure, 60 minutes at most. Load definitely hastens and/or exacerbates it (the times I've seen the caps actually explode were under load), but is definitely not necessary. Maybe DC bus caps as used in VFDs is different from other types of caps, maybe yours were not electrolytic caps, I don't know. I'm expressing my experience with this issue over a 30 year span of dealing with VFDs in the field, virtually every brand sold in the US. This issue is universal.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: VFD drive blown with no load ??

If your worried about DC bus measurements. Just make sure you have the correct PPE for that voltage level. I believe this is like the qualified electrical worker training most companies put you thru so you can die of heat exhaustion instead of getting electrocuted or arced flashed to death. If you have no idea what i am talking about just slowly back away from your vfd and turn off the 120vac power.

RE: VFD drive blown with no load ??

I have no argument that an electrolytic can short or blow up in 15 minutes if exposed to high ripple currents after being stored for a while. This doesn't seem to be the case. It hasn't deformed and it maintains a charge for quite a while. Even with no capacitance, the control electronics should have worked. Another thing I find interesting is this cap should have a bleed resistor to insure it discharges in a reasonable time for safety reasons. I don't see one on that board and all this testing may have been done with the boards separated. Second hand reports are always suspect, but I would be looking at the HV current paths for an open.

RE: VFD drive blown with no load ??

There is a Power Board on the right side that he has apparently already de-soldered and removed. That PCB holds the capacitors too. It contains the Pre-Charge circuitry and bleed off resistor as well. And as I aid, the control power for this drive is tapped off of the DC bus with a little DC/DC chopper based SMPS. Bad caps kill that little SMPS circuit too.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: VFD drive blown with no load ??

So then, does this electrolytic capacitor degradation apply to all appliances?

Does it also suggest that electrolytic capacitors have a shell life, after which they need reformed?

And if not, why not?

Why should it be peculiar to VFD's ?


RE: VFD drive blown with no load ??

It's a problem with all electrolytic capacitors, but the effect is worst on high voltage types typically found in power electronics such as VSD's, switchmode power supplies, UPS, valve amplifiers, etc.

RE: VFD drive blown with no load ??

Thanks Scotty.

At least I learned some things from my £60, 0.1 second investment smiledazed

RE: VFD drive blown with no load ??

Ha-ha, I had many similar 'learning opportunities' when I used to build amps as a hobby. I wonder how many times I've seen the expensive semiconductors sacrifce themselves to protect the DC rail fuses...

RE: VFD drive blown with no load ??

I was building Applied Material's glass sheet heater controllers and had to put in a semiconductor fuse for each channel. Thirteen of them in each controller!! I often thought it would be cheaper to have the pass elements fry instead of the fuses.

Keith Cress
kcress -

RE: VFD drive blown with no load ??

BTW the replacement drive works perfect.

It had only been decommissioned recently.

I did notice however that the frequency on the drive display is slightly different from the actual rotational frequency of the motor, as measured on the oscilloscope, via a digital sensor I fitted on the armature.

I'm just wondering is there possibly,

a). a discrepancy between the drive output frequency and the drive display, or

b). some kind of de-synchronization between the actual output frequency and how the armature rotates

I suppose I can measure one of the output phases on channel 1 and the digital sensor output on channel 2 and see if they have common frequencies.

If not then I can presume there is some kind of slip between the output frequency and what the armature does.

Slip probably isn't a good term, but I don't know the correct name for what I'm trying to say.

RE: VFD drive blown with no load ??

The PowerFlex 4 is not a vector drive, it is V/Hz only. That means the VFD puts out a frequency and voltage to make the drive motor capable of running at a given speed, but has no feedback mechanism to tell it whether it does or not. Vector drives add that capability. So in A-B world, that would be a PowerFlex 40 in the older models like that, or the current one would be a PowerFlex 523. Those will be +-1% without an encoder as the feedback device, .01% with it.

5/1 made a minor edit...

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: VFD drive blown with no load ??

@jraef Excellent.

I'm actually glad I blew it up now!

It's been an education.

Thanks everyone.

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