208/120V system 2-pole load a 2 phase or single phase?

gar

Senior Member
210804-1735 EST

Now I am going to start a sequence of statements that are repetitions of what has been said already. But there is a reason behind repeating some of these questions.

First, can a two wire source be anything other than DC or single phase assuming a single sine wave source for the AC case?

I believe most will answer only single phase. I want to repeat that I really consider DC to be a 0 frequency AC source.

Second, if I have 3 wires as a source, then how many phases do I have? My answer is 1 or more. This is where many of you will have differing opinions. So define some of these with your assumptions. Then I will proceed with additional questions, and comments.

.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
210804-1731 EDT

Frank DuVal:

Where did I say the load was single phase, other than when that was what I was talking about?

.
The OP did. Confusion like this can result when the discussion grows beyond the original question.
 

gar

Senior Member
210804-2020 EDT

LarryFine:

In post #4 he is talking about it being 2 phase.

One can not tell what kind of load the load is until it is opened up and analyzed.

The load might be two separate single phase loads, or even three single phase loads. Or it could be a 2 phase load, or even a 3 phase load.

.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Second, if I have 3 wires as a source, then how many phases do I have? .
See post 35 for my answer. : - )

I now agree with you that a 120/208V system has two phases. Yet we call it a single phase system as it is only used for single phase loads.

Cheers, Wayne
 

gar

Senior Member
210804-2145 EDT

wwhitney:

I wasn't going to mention it yet, but the building my son is part owner of is supplied from 2 phases of a 3 phase distribution system, I am able to see the primary and secondary wiring on the pole. So I know the source is 2 hot wires and 1 neutral going into two transformers. If you were to call this a delta it would be a very squashed delta. The output of the two transformers is a balanced open delta at the 120/240 V level. So input is 2 phase and output is 3 phase.

.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
I now agree with you that a 120/208V system has two phases. Yet we call it a single phase system as it is only used for single phase loads.
If I gave you a black box source with three hot leads coming out of it how would you decide how many phase are provided?

I count the number of Line-Line voltages that can be measured. I use the same methodology for systems with or without an accessible neutral conductor. I have looked at 45 year old engineering texts and 55 year old reference standards, not one of them said a single Line-Line voltage was 2 phase(s). Of course back in those days 2-phase was still in existence.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
If I gave you a black box source with three hot leads coming out of it how would you decide how many phase are provided?
I answered this directly back in post 35.

https://xenforo.mikeholt.com/thread...e-or-single-phase.2562631/page-2#post-2689894

I count the number of Line-Line voltages that can be measured. I use the same methodology for systems with or without an accessible neutral conductor.
With just 3 leads coming out, you can't say ahead of time if one is a neutral or not. You have to measure the relative voltages and phase shifts. You could also measure the voltages to earth, but I think that shouldn't matter for deciding on the phase terminology.

And then when you measure the 3 voltage waveforms, you'll decide that there's 1, 2, or 3 phases, depending on the various possibilities.

Cheers, Wayne
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
I wasn't going to mention it yet, but the building my son is part owner of is supplied from 2 phases of a 3 phase distribution system, I am able to see the primary and secondary wiring on the pole. So I know the source is 2 hot wires and 1 neutral going into two transformers. If you were to call this a delta it would be a very squashed delta. The output of the two transformers is a balanced open delta at the 120/240 V level. So input is 2 phase and output is 3 phase.
Very good. You have pointed out that to be precise I should say that 120/208V single phase is used only to directly supply (without transformers) single phase utilization equipment. It certainly can supply a transformer to create a three phase system that can supply three phase utilization equipment.

Cheers, Wayne
 

gar

Senior Member
210805-0442 EDT

How to view a 2 phase, 90 degree difference, motor torque. Note: instantaneous torque times RPM is proportional instantaneous power.

Using trig identities we have sin^2 x = ( 1 - cos 2*x ) / 2 and cos^2 x = ( 1 + cos 2*x ) /2 . Add the two together and we have the 2 phase motor, and the result is sin^2 x + cos^2 x = ( 1 - cos 2*x ) / 2 + ( 1 + cos 2*x ) / 2 = 1/2 + 1/2 = 1 , and a constant torque.

.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
With just 3 leads coming out, you can't say ahead of time if one is a neutral or not. You have to measure the relative voltages and phase shifts. You could also measure the voltages to earth, but I think that shouldn't matter for deciding on the phase terminology.

And then when you measure the 3 voltage waveforms, you'll decide that there's 1, 2, or 3 phases, depending on the various possibilities.
Voltages to earth are meaningless unless you know the system is actually referenced to it.

Simply measuring the voltages allows you to mathematically determine if the voltages are either Line-Line or Line-Neutral. Generally we work with nominal voltages and nominal phase angles, especially when first analyzing a basic power system (i.e. delta at 60°, wye at 120°, or series winding at 0°). This simplified method may not be applicable to non power systems where any voltage angles is possible.

For example can you determine the system simply given:
Vxy =120, Vyw=120, Vwx=0
Vab=120, Vbc=120, Vca=240
Vrs=120, Vst=120, Vtr=208
Va1a2=120, Va2b2=85, Vb2a1=85
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I think one can differentiate between balanced and non balanced poly phase sets.

Starting with three wires and any 'non-linear' phase difference between the AC on the conductors, one can use transformers to derive a complete poly phase set. Eg. starting with two l-n 'phases' of a 3 phase wye system, one can derive the third 'phase'. Or starting with true 2 phase balanced power, one can derive true 3 phase power using a Scott T transformer configuration.

But if you start with an unbalanced system, and derive a balanced system, and then place a balanced load on that derived system, then you will see unbalanced loading on the original system.

Consider the '2 legs of a wye being used to derive all 3 legs' and then place a 100A resistance load on the 3 leg system. If I did my math right, you get 173A supplying the two original legs, with a leading power factor on one leg, lagging on the other.

You can similarly differentiate other polyphase but not complete sets.

Jon
 

Frank DuVal

Senior Member
Location
Fredericksburg, VA 21 Hours from Winged Horses wi
Occupation
Electrical Contractor, Electrical Engineer
In post #4 he is talking about it being 2 phase.

One can not tell what kind of load the load is until it is opened up and analyzed.

The load might be two separate single phase loads, or even three single phase loads. Or it could be a 2 phase load, or even a 3 phase load.
I can tell, it is a single phase load! Show me a two phase load that is not using two different sine waves. Oh, you say, modern dryers use both hot to hot and hot to neutral. Yep, and each of those loads is single phase! No square root of two in there. There are no motors or other equipment inside that load that requires two different sine waves.

The source could be 12 phases, but the electrician will just hook up single phases to make it work. ;)
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Voltages to earth are meaningless unless you know the system is actually referenced to it.
Agreed.

Simply measuring the voltages allows you to mathematically determine if the voltages are either Line-Line or Line-Neutral.
I agree that if you know the voltage waveforms are all sine waves of a common frequency, then it is enough to measure the RMS voltage between each pair. Given |Vab|, |Vbc|, |Vac|, then the law of cosines determines all the pairwise phase shifts between those 3 waveforms.

But the presence or absence of a neutral is something you determine after characterizing the system. I don't think the "neutral or not neutral" status of any given conductor has any bearing on determining how many phases there are.

Cheers, Wayne
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Starting with three wires and any 'non-linear' phase difference between the AC on the conductors, one can use transformers to derive a complete poly phase set.
I'm not so familiar with the intricacies of transformers: can one introduce an arbitrary additional phase shift with a customized transformer? I.e. can I get 5-phase or 7-phase starting with just 120/208V 3-wire?

Cheers, Wayne
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I'm not so familiar with the intricacies of transformers: can one introduce an arbitrary additional phase shift with a customized transformer? I.e. can I get 5-phase or 7-phase starting with just 120/208V 3-wire?

Cheers, Wayne


Yes, but it might require some funny turns ratios and multiple phases connected in series.

If you look at the state of a common three phase motor, you will likely see 12 or 18 different phases of net slot current

Jon
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
So if I start with a single phase source, and use a capacitor to provide a phase shifted voltage, (as capacitor run motors do), and then use a transformer to produce a 3 phase system from those two out of phase sources, what happens? Do I get a 3 phase voltage system that meets nominal when unloaded, but with poor voltage stability upon loading?

Cheers, Wayne
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
I'm not so familiar with the intricacies of transformers: can one introduce an arbitrary additional phase shift with a customized transformer? I.e. can I get 5-phase or 7-phase starting with just 120/208V 3-wire?

Cheers, WayneA
With transformers you can scale voltages and then add or subtract them. And so you can generate an AC voltage at any given phase angle with respect to the 120/208V voltages provided, if there are no restrictions on the number of transformers, efficiency, cost, etc. Or more generally, from two sources with different phases other than 180 degrees, as gar mentioned above.

The easiest way for me to think about this is that if you create two AC voltages that are 90 degrees from each other and equal magnitudes, then if you scale them by cos (θ) and sin(θ) respectively and add them, you can create a voltage at any arbitrary angle θ . This is exactly what they do in modulators and demodulators (modems) for digital modulations (WI-Fi, 4G, 5G, etc.). Of course this will probably not be the most efficient way for creating multiples of 30 degrees with transformers when starting out with two AC voltages which are 120 degrees apart.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
So if I start with a single phase source, and use a capacitor to provide a phase shifted voltage, (as capacitor run motors do), and then use a transformer to produce a 3 phase system from those two out of phase sources, what happens? Do I get a 3 phase voltage system that meets nominal when unloaded, but with poor voltage stability upon loading?

Cheers, Wayne

In theory, yes.

In practice, the closest you get are 'static phase converters' which use capacitors and transformers to run three phase motors from single phase sources, but depend upon the spinning motor as part of the circuit.

-Jon
 
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