Open Delta and frequency

[Strathead]

So, one of my journeymen was asked to read the frequency of an open delta (with neutral stinger) system at the panel. He read the following with a Klein meter that has a frequency switch setting.

A-N 60hz
B-N 60hz
A-B 60hz

however...
C-N 320hz
C-A 420hz
C-B 520hz

Is there a theory reason for this? Or is there are problem with the system?

I don't think I have ever been asked to read frequency.

 

[Ingenieur]

Either the meter is bad/misused or a ton of harmonics from vfd’s or something and it is reading the highest rather than the fundemental

 

[jumper]

Either the meter is bad/misused or a ton of harmonics from vfd’s or something and it is reading the highest rather than the fundemental

Question:

Aren't harmonics multiples of the fundamental ie,

60Hz, 120Hz, 180Hz, etc?

 

[Ingenieur]

Question:

Aren't harmonics multiples of the fundamental ie,

60Hz, 120Hz, 180Hz, etc?

Yep
but who knows the accuracy
if they are valid readings what else could it be?
you would expect mostly triplen 180, 540 etc

 

[jumper]

Yep
but who knows the accuracy
if they are valid readings what else could it be?
you would expect mostly triplen 180, 540 etc

Thanks, the readings were throwing me off and I needed make sure.

 

[Ingenieur]

Thanks, the readings were throwing me off and I needed make sure.

Maybe arcing?
welding, furnace or something?
you could see 300 and 420 at times

 

[Besoeker]

Question:

Aren't harmonics multiples of the fundamental ie,

60Hz, 120Hz, 180Hz, etc?

Thy are. But typically just odd multiples.
My experience with 3-phase systems is mostly variable speed drives where the harmonic frequencies are 5th, 7th, 11th, 13th etc. (6n±1).
The figures given by the OP don't seem plausible for a 60 Hz system and can't really be attributed to harmonics. A frequency of 520 Hz is not a harmonic of 60Hz.

 

[Besoeker]

Yep
but who knows the accuracy
if they are valid readings what else could it be?
you would expect mostly triplen 180, 540 etc

I'd expect triplen mostly on single phase loads.
An open delta is three phase. Of course we don't know what the loads were or even if the system was actually loaded.

You question whether they were valid readings. I'm inclined to think not.

 

[Jraef]

So, one of my journeymen was asked to read the frequency of an open delta (with neutral stinger) system at the panel. ...

I've seen High Leg Delta (aka Stinger leg), I've seen Open Delta, but I've never seen High Leg Open Delta. I've only seen it in textbooks, never in real life.

But if that's what you have, it would look like this:

I see no point in measuring anything from L3 (assuming that matches your "C" phase) to Neutral, it's not a valid connection for anything. If you have anything with a Switch Mode Power Supply or VFD connected to this system that might be producing harmonics, someone should rethink that; this is NOT going to be good for anything electronic.

Frequencies don't change through transformers. How your meter READS frequency might be affected by the oddball connection however. I would pay no attention to that.

 

[steve66]

Did you measure voltage from C to B or to ground or neutral?

If C is open, and its just picking up noise, no telling what the frequency meter is going to read.

Also, if I remember right, I believe some frequency meters often count "0" crossings to determine the frequency. So when the waveform gets close to zero, it can cross back and forth several times due to harmonics, even if the harmonics aren't very large compared to the voltage. The meter can misread this as a higher frequency.

So that could explain the higher frequency, and the fact that the frequency doesn't measure as an exact harmonic.

 

[Ingenieur]

I assume a probe type meter vs clamp on due to the measuring points
what are the served loads?

 

[GoldDigger]

You do not have voltage readings taken with a low impedance meter?
The most likely possibility for me is that C is actually open and all that is being measured is capacitively coupled noise.

 

[mivey]

I've seen High Leg Delta (aka Stinger leg), I've seen Open Delta, but I've never seen High Leg Open Delta. I've only seen it in textbooks, never in real life.

But if that's what you have, it would look like this:

They are quite common in a lot of commercial locations where you have mostly single-phase load and a small amount of 3-phase like a motor or small piece of equipment or A/C unit. There are many on residential as well where 3-phase A/C used to be all the rage.

 

[Besoeker]

Frequencies don't change through transformers.

Good, valid point if I may say so.

 

[mivey]

You question whether they were valid readings. I'm inclined to think not.

Me too. A bit Denmarkian.

 

[gar]

171031-1741 EDT

Put a scope on it and see what the waveform looks like. Syncing the scope to AC line gives you a good phase and frequency reference.

.

 

[Besoeker]

Me too. A bit Denmarkian.

Denmarkian? Not a term I'm familiar with.

 

[Besoeker]

171031-1741 EDT

Put a scope on it and see what the waveform looks like. Syncing the scope to AC line gives you a good phase and frequency reference.

It's a good suggestion and probably what you or I could and would do.
Techniques we have access to and experience with. Not all do.

 

[Ingenieur]

How fast are motors on the system turning? Lol

 

[Besoeker]

How fast are motors on the system turning? Lol

We don't know if there are any nor any of the load details. Or even if the system was loaded at all.
Unless I missed something......