Three phase confusion

[Barbqranch]

I mostly deal w/ single phase 120/208 power (or is it 208/120, I can never keep them straight) in a small industrial setting. Another warehouse facility has a high leg delta service.

Ok, so here is the question. If I wanted to look at the 208 three phase w/ a 3 input scope, I would just measure all the wave forms from neutral, and the three sine waves would be separated by 120 degrees. No problem. However, I am having trouble picturing in my mind what the scope pattern would be for the high leg delta, and what would be the logical place to measure from (I am guessing neutral). And what the phase angles would be from that perspective.

Thanks.

 

[GoldDigger]

I mostly deal w/ single phase 120/208 power (or is it 208/120, I can never keep them straight) in a small industrial setting. Another warehouse facility has a high leg delta service.

Ok, so here is the question. If I wanted to look at the 208 three phase w/ a 3 input scope, I would just measure all the wave forms from neutral, and the three sine waves would be separated by 120 degrees. No problem. However, I am having trouble picturing in my mind what the scope pattern would be for the high leg delta, and what would be the logical place to measure from (I am guessing neutral). And what the phase angles would be from that perspective.

Thanks.

It's 208Y/120 if you use the NEC description.

If you take A-C as the zero phase reference, A-N will be in phase with A-C and C-N will be at 180 degrees.
A-B and B-C will be offset by 120 degrees in opposite directions, while
B-N will be at a 90 degree angle.

 

[Smart $]

I mostly deal w/ single phase 120/208 power (or is it 208/120, I can never keep them straight) in a small industrial setting. Another warehouse facility has a high leg delta service.

Ok, so here is the question. If I wanted to look at the 208 three phase w/ a 3 input scope, I would just measure all the wave forms from neutral, and the three sine waves would be separated by 120 degrees. No problem. However, I am having trouble picturing in my mind what the scope pattern would be for the high leg delta, and what would be the logical place to measure from (I am guessing neutral). And what the phase angles would be from that perspective.

Thanks.

If you measured a high leg delta using the neutral as your reference for all three inputs, you'd get 0, 90, and 180 degree waveform separations, sorta like it is missing a fourth at 270 degrees... but your 90 degree waveform (the high leg) will obviously have greater vertical width if using the same scale.

One method to get the 120 degree separation is to create a virtual true neutral using three high-ohm resistors connected as a wye network and each open end connected to a line, and use the common node as the reference.

 

[Smart $]

It's 208Y/120 if you use the NEC description...

The NEC doesn't distinguish designations for 1Ø and 3Ø.

I believe IEEE short designations 120/208 for 1Ø and 208/120 for 3Ø. Full designations are 120/208V 1Ø 3W and 208Y/120 3Ø 3W.

 

[gar]

151015-2207 EDT

Barbqranch:

If you actually use a scope to make measurements, then be very careful.

Almost all scopes will have the scope chassis directly connected one side of the signal input of all channels. This also means that the chassis and commons of all the signal inputs are connected to the EGC pin of the power plug.

If you are not bypassing the EGC pin (I am not at all suggesting you bypass the EGC), and you connect your scope probe common lead to an electrical system neutral, then there is the possibility of a large current thru that small scope probe common lead that might burn up the lead.

If you were to connect the probe common to a hot wire you have even worse problems. This would raise the scope chassis above earth ground some large or small amount until the scope lead burned up.

The way to make a voltage measurement between two hot leads is with matched probes and amplifiers phased for a difference measurement. This requires two input channels for every difference measurement.

Where you to want to make rough measurements relative to the EGC at the scope power plug (meaning there will be some unwanted error), then don't use the scope proble common leads. In fact remove them from the probes.

Typical scopes have all signal channels referenced to the scope common and to its chassis.

A differential isolation amplifier could be used at a scope input to make hot-to-hot measurements. There are possibly ones like this available.

To see how things work I would suggest using small stepdown transformers to provide a three phase circuit at possibly a 6 V level. A few watt filament transformers would be appropriate.

.

 

[Barbqranch]

Gar, this was a thought experiment. I used to work w/ the physics dept. at the local university, and so am quite familiar w/ oscilloscopes, so I do appreciate your caution regarding the common ground on the input. So, no, I was just trying to wrap my head around it, in my mind I can "see" the 208Y/120 pattern, but I was having trouble w/ the high leg delta. I did realize I could make a pseudo wye configuration, but was having trouble imagining the scope trace using one of the 4 existing wires.

Thanks!

 

[Phil Corso]

Both NEC (220.5(A) and ANSI/IEEE designate the 'Y' for 208Y/120, 480Y/277, and 600Y,347!

One, I had been associated with, 831Y/480, is not officialy listed!

Regards, Phil Corso

 

[GoldDigger]

The NEC doesn't distinguish designations for 1Ø and 3Ø.

How about this?

220.5 Calculations.
(A) Voltages. Unless other voltages are specified, for purposes of calculating branch-circuit and feeder loads, nominal system voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347, and 600 volts shall be used.

There is no designation listed there for high leg delta though.
It is true that, for example, 480 could be 480 single phase two wire or 480 three phase three wire delta, since for load calculations it is either not critical or would be clear from context.

 

[Smart $]

How about this?
There is no designation listed there for high leg delta though.
It is true that, for example, 480 could be 480 single phase two wire or 480 three phase three wire delta, since for load calculations it is either not critical or would be clear from context.

Well it is obviously 3Ø if there is a "Y" in the designation. So where is the designation for 120/208V 1Ø 3W.

And as far as that section goes, note it starts with, "Unless other voltages are specified..."