True RMS & Averaging very different readings

[POLISHBEER]

Hi,

I'm a technician for an air compressor company. A customer had me look at a 55KW VFD compressor that has been faulting out. Fault code corresponds to missing phase.

He swears they have good power. Fuses tested good at disconnect.

The following readings are taken with our load completely disconnected from incoming power and readings were taken ABOVE Z the fuses so it's just their BUS power to their disconnect l.

My Fluke reads the following:
L1-L2=337v
L2-L3=337v
L3-L1=470v

L1-G=467v
L2-G=110v
L3-G=470v

The customer had a Klein DMM not listed as RMS

His meter saw:

L1-L2=0v
L2-L3=0v
L3-L1=470v

He then got his equipment guy over with his KLEIN DMM and repeated the same results as his KLEIN got.

I think it is grounded B phase and I think there's a problem with their BUS/BUS fuses

He claimed my Fluke was broken so I got my other fluke 87 a Greenlee RMS and a Menard's RMS meter and all of them repeated my weird readings.

Please help me understand what's going on with their power and why do or meters read so differently?

Thanks

Jordan

 

[don_resqcapt19]

Sounds like your meter is a high impedance meter and the Klein is a low impedance meter.
It appears the L2 is open.

 

[synchro]

The L3-L1 voltage readings are the same with both meters.

Just a guess, but perhapsthe L2 phase is open and the transformer secondary is ungrounded. And the readings on the Klein might be lower because the transformer secondary is ungrounded and the input impedance of the Klein meter is lower.
I just see that Don suspects the same thing.

I suggest having both a Klein and a Fluke put across the terminals at the same time and see if the results are the same. And then perhaps the Fluke will read close to the original Klein measurements

 

[texie]

As others have alluded to, this does not sound like a RMS vs average meter issue but rather a high impedance vs low impedance meter issue.

 

[ptonsparky]

The L3-L1 voltage readings are the same with both meters.

Just a guess, but perhapsthe L2 phase is open and the transformer secondary is ungrounded. And the readings on the Klein might be lower because the transformer secondary is ungrounded and the input impedance of the Klein meter is lower.
I just see that Don suspects the same thing.

I suggest having both a Klein and a Fluke put across the terminals at the same time and see if the results are the same. And then perhaps the Fluke will read close to the original Klein measurements

Yes

 

[LarryFine]

Welcome to the forum.

You're doing it wrong. To troubleshoot fuses, test across each fuse (line to load) one at a time.

 

[ptonsparky]

Welcome to the forum.

You're doing it wrong. To troubleshoot fuses, test across each fuse (line to load) one at a time.

With an open switch and in ohms.
Or
We know it doesn't take a true RMS meter to read FOP across a fuse. A Wiggy would work fine.

(Leaving out any safety precautions)

 

[LarryFine]

We know it doesn't take a true RMS meter to read FOP across a fuse. A Wiggy would work fine.

That's what I use, a Knopp K60.

 

[ptonsparky]

OP.
There should be no fuses in the grounded conductor of a purposely Corner Grounded Delta. Ungrounded Delta should have Fault detection of some sort.
VFDs typically do not like Delta of either kind.

 

[don_resqcapt19]

With an open switch and in ohms.
Or
We know it doesn't take a true RMS meter to read FOP across a fuse. A Wiggy would work fine.

(Leaving out any safety precautions)

Given that the disconnect is on the line side of the fuses, you can see read ohms across the open fuse, of course it would typically be more ohms than the good fuses.
However with the power on, reading more than a few millivolts across the fuse always means the fuse is open.