May I ask a question about the single vs two phase stuff

[jaggedben]

Mivey, what's the 'canonical' definition?

 

[GoldDigger]

Mivey, what's the 'canonical' definition?

It depends on which canon you are reading.

 

[jaggedben]

It depends on which canon you are reading.

I know. I want to know what he means by it.

 

[mivey]

Mivey, what's the 'canonical' definition?

A very old definition used by Steinmetz, Fitzgerald, and such old founding fathers:

An n-phase system will have n equal magnitude voltages displaced by 360/n degrees.

 

[jaggedben]

A very old definition used by Steinmetz, Fitzgerald, and such old founding fathers:

An n-phase system will have n equal magnitude voltages displaced by 360/n degrees.

Thanks.

Interesting how this definition wouldn't resolve the discussion about the 120/240 system. But it seems to preclude calling a 3-wire 120/208 service a polyphase system.

 

[Besoeker]

Thanks.

Interesting how this definition wouldn't resolve the discussion about the 120/240 system. But it seems to preclude calling a 3-wire 120/208 service a polyphase system.

Then where does the √3 come from if it isn't the relationship between 120V and 208V?

 

[jaggedben]

Then where does the √3 come from if it isn't the relationship between 120V and 208V?

I'm not sure what your point is. My point is that the displacement of the two line to neutral voltages is not 360/n.

 

[Besoeker]

I'm not sure what your point is. My point is that the displacement of the two line to neutral voltages is not 360/n.

The √3 comes from the 120 deg displacement.

 

[Carultch]

Then where does the √3 come from if it isn't the relationship between 120V and 208V?

Draw a triangle with the first side 12 inches, and the second side of 12 inches at 120 degrees from the first side. The length of the side that closes this triangle, opposite the 120 degree angle, will be 20.8 inches. The first and second sides represent two phases to neutral of a 3-phase system, from which the 120/208V single phase system is derived. The remaining side length represents the voltage between the two phases.

In exact numbers, the ratio between the long side and either of the two shorter sides, will be sqrt(3). You can prove it with the law of cosines.

 

[mivey]

Thanks.

Interesting how this definition wouldn't resolve the discussion about the 120/240 system.

I think resolving is a lost cause.

It does indicate that two equal magnitude voltages separated by 180d would be a system with voltages separated 360d/2. A 120/240 system has a set of 120 volt signals that fit that definition. Taking the signals away from the source using the 180d displacement gives you a system of two phases that match the canonical definition.

But it seems to preclude calling a 3-wire 120/208 service a polyphase system.

It is a subset of a 3-phase system, the same as quadrature two-phase is a subset of a four phase system.

Think about it: The 3-wire quadrature two-phase we get from a transformer bank is just the 120/208 system shifted from 120d to 90d. We could also call 120/208 two-phase except that the name has already been taken by the quadrature two-phase system.

 

[ggunn]

I think resolving is a lost cause.

FWIW, I don't think resolving it is necessary. What you call it is inconsequential as long as you know how to deal with it.

 

[Besoeker]

FWIW, I don't think resolving it is necessary. What you call it is inconsequential as long as you know how to deal with it.

Agreed.

 

[Sahib]

I think resolving is a lost cause.

It does indicate that two equal magnitude voltages separated by 180d would be a system with voltages separated 360d/2. A 120/240 system has a set of 120 volt signals that fit that definition. Taking the signals away from the source using the 180d displacement gives you a system of two phases that match the canonical definition.

It is a subset of a 3-phase system, the same as quadrature two-phase is a subset of a four phase system.

Think about it: The 3-wire quadrature two-phase we get from a transformer bank is just the 120/208 system shifted from 120d to 90d. We could also call 120/208 two-phase except that the name has already been taken by the quadrature two-phase system.

For 120/208V, you have to give up the concept that it is a balanced system because the two phasors sum is not zero. So to classify a system as a poly phase system, we better rely on an unambiguous concept. We better take the inherent ability of poly phase systems to produce a rotating magnetic field as the criterion for classification into poly phase system. The 240/120V or 208/120V does not meet this criterion and so not a poly phase system. It must be single phase system!

 

[mivey]

FWIW, I don't think resolving it is necessary. What you call it is inconsequential as long as you know how to deal with it.

Yes, a better way to look at it.

 

[mivey]

For 120/208V, you have to give up the concept that it is a balanced system because the two phasors sum is not zero. So to classify a system as a poly phase system, we better rely on an unambiguous concept. We better take the inherent ability of poly phase systems to produce a rotating magnetic field as the criterion for classification into poly phase system. The 240/120V or 208/120V does not meet this criterion and so not a poly phase system. It must be single phase system!

You have made a mistake. Read what you wrote again.

 

[Sahib]

You have made a mistake. Read what you wrote again.

Please point out the mistake: I could not find any.

 

[mivey]

Please point out the mistake: I could not find any.

This:

The 240/120V or 208/120V does not meet this criterion and so not a poly phase system.

 

[mivey]

So to classify a system as a poly phase system, we better rely on an unambiguous concept.

Pick whatever makes you happy. Nothing will change.

 

[GoldDigger]

Two hots and a neutral of 120/208 will produce a rotating magnetic field, just not balanced. If you add the condition of not using the wye point conductor, then the statement is correct, but there is no point in mentioning the 120 part.

Sent from my XT1585 using Tapatalk

 

[Sahib]

Two hots and a neutral of 120/208 will produce a rotating magnetic field, just not balanced. If you add the condition of not using the wye point conductor, then the statement is correct, but there is no point in mentioning the 120 part.

Sent from my XT1585 using Tapatalk

So the only 240/120V with or without neutral is incapable of producing a rotating magnetic field by itself and so must be a single phase system.