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

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wwhitney

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Gar:

Now do the same thing with a 3 wire corner grounded delta. Displayed on the scope will be two sine waves with a phase difference between them of about 60 degrees.

So why do you wish to call your original example two phase, while the above example is universally called three phase?

Cheers, Wayne
 

gar

Senior Member
210802-2114 EDT

wwhitney:

What you described with one corner as a reference point is a 2 phase system, and it does have a 60 degree difference as you stated.

The delta supply you described can self start and run a 2 phase motor. So can 2 phases of a wye supply self start and run a 2 phase motor.

Neither of these two connections will provide the constant output torque of a three phase motor driven from a 3 phase source of 120 degree spacing, or a 2 phase motor on a 90 degree two phase source.

.
 

DrSparks

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210802-2114 EDT

wwhitney:

What you described with one corner as a reference point is a 2 phase system, and it does have a 60 degree difference as you stated.

The delta supply you described can self start and run a 2 phase motor. So can 2 phases of a wye supply self start and run a 2 phase motor.

Neither of these two connections will provide the constant output torque of a three phase motor driven from a 3 phase source of 120 degree spacing, or a 2 phase motor on a 90 degree two phase source.

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Yeah maybe. But the windings of the motor will be out of phase with the 120 degree 3 phase power. Also 3 phase motors will run, but not start on 2 phases but will overload.

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wwhitney

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Neither of these two connections will provide the constant output torque of a three phase motor driven from a 3 phase source
The 3 phase corner grounded delta I described certainly will. It is a 3 phase source, not a 2 phase source.

Cheers, Wayne
 

Besoeker3

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Oh I see
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Thanks for all your answers, it has been really helpful. Yes, im from Europe and very seldom i work with 208/120v systems here.
Same for me - that 208/120 is the crunch. The phase shift.
Of interest which part of Europe were yo/are you from?
 

jim dungar

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Of course it is.

If you said, for example, you need to pull a blue phase, then blue would be the adjective and phase would be the noun.

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And that would also be a wrong use of the word phase as a noun.
A single conductor is not a phase.

I did not say the word phase could not be used as a noun, I said it is commonly misused as a noun when it should being used as an adjective.
 
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Besoeker3

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And that would also be a wrong use of the word phase as a noun.
A single conductor is not a phase.

I did not say the word phase could not be used as a noun, I said it is commonly missed as a noun when it should being used as an adjective.
The red phase - red adjective, phase noun.
 

gar

Senior Member
210803-0922 EDT

wwhitney:

It is clear I did not make a clear statement of my point. I will try again.

A corner grounded delta when viewed from that corner ground point is a 2 phase source

From each hot wire ( there are two ungrounded wires ) connect a diode, each with its anode to its ungrounded wire, connect the two cathodes together and to a load resistor. The other end of the load resistor connects to the corner grounded conductor.

What does the rectified output signal look like? Two pulses per cycle, and a missing pulse. Thus, a two phase source when viewed with the grounded conductor as the reference.

The 3 phase delta source has no real life physical neutral point. There is an imaginary one out in space. However, you can connect a 3 phase delta load to this corner grounded source and receive 3 phase power with the three phase load floating other than at the corner grounded point. You can also put a 3 phase wye load on this corner grounded delta, leave the neutral point of the 3 phase wye floating, and provide 3 phase power to the load.

Two phases from a 3 phase system constitute a 2 phase system.

A two or more phase system with adequate spacing between the two phases can produce a sufficiently large rotating magnetic vector that a motor will self start rotation. This does not mean that you have full torque capability. Both a 90 degree 2 phase, and 3 phase 120 degree system can with suitable design produce an almost constant amplitude rotating torque vector.

A two phase capacitor run motor fed from a single phase source can produce a better power factor than a 3 phase motor. This kind of motor was invented and/or developed by a U of M professor, Bailey, that wrote the AC Machinery book that I studied from. This came from a request from Detroit Edison. Bailey was still alive at the time I took the course, but I never meet him. I did know the wife of his coauthor. His coauthor, Gault, was younger and had died before I came to the U of M. However, I did have an excellent teacher for the course, J. G. Tarboux.

.
 

wwhitney

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A corner grounded delta when viewed from that corner ground point is a 2 phase source
I'm going to disagree with that. I don't believe the terminology should depend on the choice of zero voltage reference point, nor on whether a circuit conductor is earthed or not. The delta is always 3 phase.

Your comments on calling 120/208V 2 phase are intriguing and bear more thought. In practice, I understand it is only used for single phase, 2 wire loads (or conglomerations thereof supplied by all 3 wires but internally divided into single phase, 2 wire loads), so it is commonly called single phase. If I could buy an off-the-shelf induction motor, with two windings adapted to run on 120/208V (akin to a capacitor run motor but without the capacitor), that would be a different matter.

Cheers, Wayne
 

wwhitney

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OK, if presented with an AC electrical supply of fixed frequency consisting of some number of wires, here's my proposal for the terminology on how many phases there are:

1) Consider all pairwise voltage waveforms, and group them by (approximate) magnitude. Restrict further consideration to the group of minimum magnitude.
2) Arbitrarily choose one of those waveforms for a baseline, and determine the phase shift angle for all the other waveforms.
3) Plot those angles as vectors of fixed length (say, the voltage magnitude) in the plane (polar coordinates).
4) The minimum number of lines through the origin required to cover all the vectors is the number of phases.

Phrasing Step 4 in terms of lines through the origin means that for a pair of wires A and B, it's enough to include the waveform from A to B, you don't have to include the waveform from B to A (which is just the negative of the former waveform).

In Step 1, the minimum magnitude group may not include waveforms involving all the wires, in which case I would say that more than one system is present. An example is a 4-wire center-tapped delta. So in that case the procedure should be repeated with the next higher magnitude group of waveforms.

This does give an answer of 2 phases for 3-wire 120/208V, which I think is reasonable, as it could run a polyphase load. Common usage is still to call that single phase, based on the characteristics of the loads typically supplied.

Cheers, Wayne
 

GoldDigger

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One distinction that you can make is that 120/240 can only directly drive single phase loads without using some form of active converter.
A two out of three 208Y/120 three wire source can produce two (90 degree) or three (120 degree) phase power using only passive components, i.e. transformers.


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drcampbell

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The Motor City, Michigan USA
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Engineer
Two hots and one neutral from a 3 phase system are a 2 phase source. Any 2 wires from this source are a single phase source. ...
Strictly speaking, that's true. There's nothing requiring the two phases in a two-phase circuit to be 90° apart.

But it's completely meaningless unless there's a device out there that requires two phases separated by 120°.
Otherwise it's just a multi-wire branch circuit that can supply both/either 120v single-phase and/or 208v single-phase power.
 

Frank DuVal

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210802-2051 EDT

DrSparks:

Take a two channel scope ( supply it from an isolation transformer to avoid a grounding problem thru the scope ) and connect the scope chassis to the power system neutral wire for the electrical reference point.

Connect channel A scope probe to one of the hot phase conductors. Get scope sync from channel A. Connect channel B to the second phase conductor.

Displayed on the scope are two sine waves with a phase difference between them of about 120 degrees.

.
Ah, there is YOUR error! The load is connected to the two phase wires, so it sees just ONE sine wave, therefore, single phase load from a single phase source! Just because you hook up the scope in another way does not show what the load is using.

Same oscilloscope error made by those that say the 120/240 typical residential service is two phase. "But if I hook up the two scope leads to the two hots and ground the scope to neutral/ground/center tap, I see two sine waves 180 degrees apart!" . But, again, that is NOT what the load sees! The load is single phase and sees just one sine wave.
 
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