I was expecting to find that any 120VAC power source in our building would be in phase with one of the phases of our 480VAC. I was even basing a test fixture design for testing LED drivers on this. When I looked at the 120 and all three phases of our 480, I found the closest phase was approximately 45 degrees out of phase with the 120. Can someone explain why this is? My guess is the 120 was made with a 3-phase transformer using 480 on the primary and the 3 different 120 outputs produced on the secondary would not be in phase with either of the input phases. Is this correct??

 

It's because the 120V single-phase is from one phase to neutral, while you are measuring the three-phase from phase to phase.
Here's a simulation showing that.
The top traces are phase to neutral and the bottom traces are phase to phase.

 

Derived from 2 phases rather than a phase and a star neutral of the supply transformer.
Max.

 

It's because the 120V single-phase is from one phase to neutral, while you are measuring the three-phase from phase to phase.
Here's a simulation showing that.
The top traces are phase to neutral and the bottom traces are phase to phase.

View attachment 152962

I am looking on a scope with differential probes at each phase of the 277 (to neutral) and the 120 referenced to neutral. the closest 277 phase is approx. 45 degrees in relationship to the 120.... I am not looking at 480 line voltage on scope (ph-ph).

 

Somehow those numbers don't look right?
How is the 120v derived? Where is the 120v transformer connected?
You cannot obtain 120v directly off a 480v transformer whether Star or Delta.
Max.

 

Derived from 2 phases rather than a phase and a star neutral of the supply transformer.
Max.

This is what I am assuming... does the power distribution in a commercial building develop the 120 from the stepped down 480 transformer or is it made from a separate transformer off the main 3-phase bus voltage that enters the building (ie: 13.8kV)

 

Somehow those numbers don't look right?
How is the 120v derived? Where is the 120v transformer connected?
You cannot obtain 120v directly off a 480v transformer whether Star or Delta.
Max.

I don't know where it is derived... I am assuming in a commercial building you have a HV 3-ph feeder to a step down transformer outside the building and 3-ph 480-4kV enters the building depending on power requirements at the power distribution panels and that all lower voltages are stepped down using transformers including the 120V which may be derived from a center tapped 240VAC secondary?

 

It can be either source depending on the size of the Industrial installation.
If North America it would generally be a 240v C.T. type or there could be some 208v star secondary transformers for single voltage 120v supplies.
Max.

 

It can be either source depending on the size of the Industrial installation.
If North America it would generally be a 240v C.T. type or there could be some 208v star secondary transformers for single voltage 120v supplies.
Max.

what would be the phase angle difference you would expect to see between the closest phase of 277 and 120 derived from either a 240 CT or 208 star? I know it isn't zero degrees like I expected. I will have to go trigger the scope again and more properly look at the phase angle difference... it looked like about 2mS, so that is where I got the 45 degrees from.... 2/16.67(360)

 

What means of coupling are you using for each 'scope input?
Max,

 

What means of coupling are you using for each 'scope input?
Max,

Here is a screenshot of the 120VAC and closest phase of 277VAC. I used the horizontal cursors and set each one to the peak and came up with a 30 degree phase shift.

Both inputs are connected using a differential probe and both are dc coupled. 120 is ch2 (green) and 277 is ch4 (purple)

 

My simulation shows the same phase-shift you observed.

 

exzctly

 

My simulation shows the same phase-shift you observed.

I'm not exactly understanding your correlation between your simulation and my measurements. I am looking at the closest 277 phase to neutral waveform compared to 120 phase to neutral (assumption that our building has a wye 208 stepdown transformer rated around 100-300KVA). Delta time between waveforms is 1.39mS or 30 degrees. Your simulation shows a 1.7mS dt (36.7 degrees) between line voltage and a phase voltage. Help me understand because I am surely no expert. I do know the difference between line and phase voltage and the differences between wye (star) and delta configurations as it pertains to voltage and current. I assumed that a 3 phase stepdown transformer with 208 secondary would have it's 3 phases in phase with the higher primary voltage. Let me know what I'm missing

 

LV on a delta / wye stepdown is 30 out of phase with the HV source.

 

Isn't AC theory fun

 

Delta time between waveforms is 1.39mS or 30 degrees. Your simulation shows a 1.7mS dt (36.7 degrees) between line voltage and a phase voltage.

That's because my simulation was with 50Hz sources and your supply is 60Hz.
1.7ms @ 50Hz is 30°.
See the simulation below with 60Hz.

 

Been following this, but don't quite understand why it makes any difference what of the 3 phases it is in or out of phase with? Since it will only be used for single phase.

 

Been following this, but don't quite understand why it makes any difference what of the 3 phases it is in or out of phase with?

Because inquiring minds want to know.
I assume it's for edification of the TS.

 

Been following this, but don't quite understand why it makes any difference what of the 3 phases it is in or out of phase with? Since it will only be used for single phase.

My original design was two reference voltages at the comparator that could be set for 0-180 degrees (0-8.3mS). I had incorrectly thought the 120 line would be in phase with one of the 277 phases and therefore a driver test at 120/277 would use the same adjustment eventually fire a SSR for 120 or 277 at the peak voltage and a the second reference voltage was to adjust for peak voltage using 480VAC (L1 & L2 phases) which would have a different phase angle or delay based on only using the 120 as the reference waveform into my optoisolator. I found out that none of the 277 phases were in phase with my 120... the closest had a 30 degree phase angle difference. I am still struggling to see why there is a phase difference when 480 is on a 3-phase transformer and 208 is on the secondary why secondary 208 would be out of phase with the primary 480, or the secondary 120 phases would be out of phase with the 277 phases. I would think L1 primary and secondary phase angles would be in phase and L2 and L3 the same thing. Yet my 120 is not in phase with any 277 phase.

My solution is that I am going to have three separate input optocoupers, one for 120VAC, 277VAC and 480VAC. The phototransistor emitters will be connected through a rotary switch to ground. The three collectors will be connected to the base of ramp reset transistor. That way the timing will always be the same... approx. 4.15mS for peak and it won't matter what 120 phase is used or the two phases of 277V if the test fixture is used in another location.

I am appreciative of all the help... still waiting for my epiphany moment to occur...