Vfd

At my job, we have a 5 conveyor line, each using a Power Flex 4 VFD......The incoming voltage to all Motors is 480V........However the Output voltage line to ground for 2 of the motors is 320 and the other 3 at 230..........From Phase to Phase, the 2 motors that were 320 to ground are 430 Phase to Phase and the other 3 motors that were 230 to ground are 520 Phase to Phase..........Could anybody give me any clues to what is going on here? What solutions are tips could I use to solve this problem.......

VFD you mean inverter? if you mean inverter then you can not measure the output without an osciloscope because the output has no stady freq.

By VFD , he means VFD , An inverter is something different.

Where did the guy mention a 'scope ? I can't see it - it is one of the things he can use to measure the outputs .
It sounds like you have a horrible grounding problem . I would check very carefully the drive and equipment grounds - be aware that 600VDC can very easily ruin your day .

I don't believe any voltages you read to ground are valid. The outputs are digitally synthesized, AC-to-DC-to-AC.

What problems are you having with your vfd/motors?

jstolaruk said:

I don't believe any voltages you read to ground are valid. The outputs are digitally synthesized, AC-to-DC-to-AC.

What problems are you having with your vfd/motors?

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Correct! The output of a VFD is not measurable with reference to ground. Of course the gold standard here for answering these kinds of questions is if DickDV will chime in, then we will know!

I hope this helps a little.

You don't need a scope to measure voltage on a VFD output, but you do need a meter that reads true RMS. Older or cheap multimeters may not do this. You should be reading line to line, though. What are these readindgs?

You didn't mention speed. The output voltage of a VFD drops as the speed drops - at 50% speed the output voltage should be 50% of the maximum full speed voltage.

Are the motors performing adequately?

Golly, randylud, now my hat won't fit!

The general rule is to never (yup, I said never) measure anything on the drive output leads to the motor. The signals are so complex and so fast that even true rms reading meters will usually not read accurately.

Your best bet is to use the drive keypad to display just what is going on on the motor leads. That would typically be output voltage, frequency, and current. The drive knows just what it is synthesizing with all those high frequency pulses and can display it for you.

As for voltage to ground, it is quite difficult, nearly impossible actually, to get any meaningful data on this.

Those of you who know me, know how much I dislike floating delta power networks. I would suggest that you measure the INPUT voltage to ground. It should be very close to balanced--approx. 277V to ground on each phase. If you find that the supply is out-of-balance by a lot, you will surely have trouble with ground fault detection on the drive. You may even get nuisance fuse releasing on your drive input fuses due to the fact that the drive attempts to creat its own virtual neutral. This can result in plant-wide ground leakage currents passing thru the drive input leads and releasing the fuses--even when the drive is powered up but not running the motor.

If you have any of these problems, the only sure fix I know of is to install a drive isolation transformer with a delta primary and wye (star) secondary. Ground the center of the wye. That way the transformer takes the leakage currents and the drive ground fault detection works better.

Unfortunately, a transformer is often a difficult solution to swallow both for cost and space reasons but it does pretty much end ground reference issues.

When you use an analog meter to measure any VFD output, it must be 20K ohms per volt impedence. Some of the newer ones are 200K ohms and will NOT read VFD outputs any better than a digital meter.

I have found by taking 3 1M ohm 2 watt resistors and connecting them to the VFD outputs in a WYE type configuration gives the best method of measuring the outputs. Use the WYE for the scope common and you will see the wave the motor lead actually sees.

When using a clamp on to measure VFD output currents, try moving the loop around and watch how much the value changes. I have found that if I put the power lead closest to the opening of the jaws while holding it so the lead is perpendicular to the clamp jaws gives the most stable and accurate readings. At least that is the way my Yokogawa works.

It seem s that you are new and dont know some thinks ,inverter and VFD is the same "The DC bus feeds the final section of the drive(VFD): the inverter. As the name implies, this section inverts the DC voltage back to AC. But, it does so in a variable voltage and frequency output."

"New" ? - I don't think so !! "Inverter" is an old and much misused term - "inverting" DC-AC is not what goes on in these drives , PWM is a more accurate term . I'm not sure that I have seen any drive manufacturer refer to the term "inverter" in any manual recently , though I'm sure someone here will know differently - I would agree , that the term may be used to identify a section of the drive .

I worked with a guy some years aga - now sadly deceased , and he a magical piece of test equipment known as a "testerscope" - which was in fact a neon test screwdriver - I wonder if this is what you refer to when you mention "'scope" .

In reference to the original post , I would agree that if the incoming supply to the drive is unbalanced , then there maybe something horrible seen at the motor , however if the incoming supply is balanced , I would expect to see balanced voltages at the motor , I see the multimeter in this case as more of a comparator , as mentioned , there are too many things going on with this , as to be practically impossible to measure without resorting to an oscilloscope or other analyzer .

sorry samios1 i agree with 10baseT on this one! calling a drive an inverter is like calling an alternator a generator..yep they both do the same thing in the end but how they do is totly different..But DickDv has spoken!! i would listen to what he says..he seems to be the forum drive Guru here! But Matt as others have said..do you have a problem or were you just testing? The only reading i will take from a drive with a meter is the incoming voltage and the outgoing current..i am still not convinced that the out going current is correct either..perhaps Dick could jump in here? i usualy have more current out that in..that would make drives over 100% efficiant in my book!

OK...lets see if I remember this right.... The output IGBT's are switches that turn on and off at a very high frequency (4K Hz or higher)as compared to the motor base frequency. The output is pulse width modulated because you want the IGBT's to either be off which means full DC voltage across them and no current = 0 power...or on wtih full current that the load can draw and 0 volts across them which = 0 power dissapated in the semiconductors.

This is an assumption that I have always made and you guys can tell me if I'm right or wrong.... The complex voltage wave out of the drive is trying to generate a current sine wave to feed the motor. Obviously you can't run a 60 Hz motor at 1 Hz and 480v so the drive tries to regulate the current down by this complex switching.

If you say the input voltages are 480 vac Phase to Phase, then I suspect that your meter is not new enough to measure the output voltages on VFD's. When you're running the VFD at any frequency, the output votages should be balanced within 2 to 3 volts, Phase to Phase. If they are very unbalanced, then you might have gate firing issues from the control board of your VFD. Small fluctuations on the input voltage side will not effect the output voltage, because the DC bus Capacitor bank usually has enough capacity for small imbalances. Most VFD's have input voltage imbalance detection, and phase loss detection. I would try measuring the output voltage from the drive, at the drive terminal blocks, from phase to phase. How long is your lead length from drive to motor?

I forgot to mention in my last post that Allen Bradley makes software to monitor what's going on in the drives. The software is called Drive Explorer and Drive Tools. There is an interface required to connect to a laptop also. That's probably the best way of determining what's going on inside the drive.

Inverter" is an old and much misused term - "inverting" DC-AC is not what goes on in these drives , PWM is a more accurate term .

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If that is the case then why do they call them "inverter rated motors"?

Using a glossary of terms provided by Reliance Electric:
http://www.reliance.com/pdf/catalogs/imdc_2005/drives/dxvii_dxxii.pdf

Inverter - [font=Helvetica LT Std,Helvetica LT Std]1. An ac adjustable-frequency drive

2. A particular section of an ac drive. This section uses the dc voltage from a previous circuit stage (intermediate dc circuit) to produce a pulse-width modulated or stepped ac current or voltage waveform that has characteristics similar to the desired sine-wave frequency.

3. A circuit whose output signal is the inverse of its input. [/font]

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Do not over complicate the issue trying to use your own definitions for something when the industry itself uses the terms continuosly.

The basic construction of a VFD consists of 4 major components.
1. Rectifier: This converts our 3-phase AC voltage electrical supply into a constant DC voltage. For a 600 VAC supply, the DC voltage would approximately be 850 VDC, known as the DC Bus.
2. DC Bus: This is an inductive and capacitive circuit to maintain a constant and smooth DC Bus voltage that tries to resist changes from the main AC supply.
3. Inverter: Also known as IGBTs, this section converts the DC Bus voltage by pulsing it by a transistor network to form a variable voltage and variable frequency supply for a 3-phase electric motor.
4. Controller: Controls the pulses and calculates the magnitude of the voltage, current and frequency to obtain optimum motor performance under all conditions.
As you can see an inverter is part of it but it is also used to describe it by the industry in general.

As DickDV mentioned most drives (inverters) have displays that can display; voltage, current, rpm, frequency, and more. This is usually the most accurate tool that is readily available.

Again it is possible to use a True RMS meter to obtain fairly accurate readings and some of these have been available for over 20 years. Example: I have used a Fluke 87 III, which is True RMS, for over 20 years. If it was not possible to obtain accurate readings with a meter then how would they be able to obtain them to display? If you want to verify the V/HZ relationship you may need more than the display.

To get back to the ""inverter" issue I will give another example. Siemens Simovert is short for Siemens Motor Inverter.

Another point worth mentioning is there are 3 types of drives (inverters):
1. Variable voltage inverter
2. Current source inverter
3. Pulse width modulation
At this time I will not go into the cycloconverter.

To the original poster, or anyone interested, I have a whitepaper that offers some detailed information on VFD's and motors;
http://www.patchn.com/mtrwhtpaper.htm