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

[LarryFine]

Seems to me that the "two phase" camp is biased against negative numbers. If L1-N and L1-L2 are the same phase (multiple of 2), then if you believe in negative numbers, L1-N and L2-N are the same phase (multiple of -1).

Alas, the pitfalls of insisting that the grounded conductor be the only reference.

 

[david luchini]

Seems to me that the "two phase" camp is biased against negative numbers. If L1-N and L1-L2 are the same phase (multiple of 2), then if you believe in negative numbers, L1-N and L2-N are the same phase (multiple of -1).

Cheers, Wayne

Could you elaborate? I don't follow what you are saying.

 

[wwhitney]

1/180 = -1 + 0j
180 deg and a non positive qty are the same thing in this case

1/120 = -0.5 + (sqrt3)/2 j

what is the diference?
other than the phase angle

I may be misunderstanding your question, but if it is "what is the difference between the set of vectors (in polar form) {1/0, 1/180} and {1/0, 1/120}, then the answer is that the first two vectors are linearly dependent (they span a 1-dimensional real vector space) and the second two vectors are linearly independent (they span a 2-dimensional real vector space).

Cheers, Wayne

 

[Ingenieur]

I may be misunderstanding your question, but if it is "what is the difference between the set of vectors (in polar form) {1/0, 1/180} and {1/0, 1/120}, then the answer is that the first two vectors are linearly dependent (they span a 1-dimensional real vector space) and the second two vectors are linearly independent (they span a 2-dimensional real vector space).

Cheers, Wayne

the difference is 60 deg phase angle

 

[Ingenieur]

Alas, the pitfalls of insisting that the grounded conductor be the only reference.

isn't that WHY we ground it?
to have a reference and stabilize?

 

[wwhitney]

Could you elaborate? I don't follow what you are saying.

OK, take the typical 120V/240V center-tapped transformer with terminal L1, N, and L2. If I look at the L1-N voltage waveform (120V RMS) and the L1-L2 waveform (240V RMS) simultaneously, then if I multiply the first waveform by the constant function 2, I get exactly the second waveform (in the idealized case). V_L1-L2(t) = 2 * V_L1-N(t) as functions.

Likewise, if I look at L1-N and L2-N, then if I multiply the first waveform by the constant function -1, I get the second waveform. V_L1-N(t) = -1 * V_L2-N(t).

I think everyone would say that L1-N and L1-L2 are "in phase." So why should there be a terminological difference between "multiplying by 2" and "multiplying by -1"?

Cheers, Wayne

 

[wwhitney]

the difference is 60 deg phase angle

OK, and therefore?

Cheers, Wayne

 

[winnie]

Once more around the park, guv'nor?

Sure, why not

There are _two_ phase angles, 120V at 0 degrees and 120V and 180 degrees.

It is still 'single phase'. This has nothing to do with transformer theory, the internal connection of the transformer, weather it is an isolation transformer or an auto transformer, weather you can use it to generate rotating fields or the color of the sky in your universe. It is called single phase because that is what we call it.

Of course when you have residential service with 120V at 0 degrees and 120V at 120 degrees, we _still_ call it 'single phase'.

-Jon

 

[winnie]

If you mean a full wave bridge such as gets used as the input bridge on a VFD*, only one diodes conducts at a time so lower losses. But my point was the document call it six phase yet it is three centre tapped coils of the arrangement that is being referred to as single phase.

*If you mean some other arrangement, can you provide a diagram, please.

Agreed that it is called six phase, and I believe I stated a bunch of threads back that it would be called 'hexaphase'.

I just thought it interesting that it was simultaneously called six phase _and_ that the individual phases were labeled in pairs. The diagram recognizes both that there are six separate phase angles feeding the rectifiers and that there is a special relation between phases 180 degrees apart.

Thanks for the explanation of why it would be used. My guess: since you either have to make the transformer larger _or_ you reduce the transformer efficiency (because you need to pack more turns of wire into the same space) that this arrangement would be used for high current low voltage applications where the voltage drop in the diodes is a large factor.

-Jon

 

[jumper]

Sure, why not

There are _two_ phase angles, 120V at 0 degrees and 120V and 180 degrees.

It is still 'single phase'. This has nothing to do with transformer theory, the internal connection of the transformer, weather it is an isolation transformer or an auto transformer, weather you can use it to generate rotating fields or the color of the sky in your universe. It is called single phase because that is what we call it.

Of course when you have residential service with 120V at 0 degrees and 120V at 120 degrees, we _still_ call it 'single phase'.

-Jon

Yep.:thumbsup:

Both services are conventionally named/called single phase. 120/240V and 120/208V.

Not two phase, not split phase, not three phase, just single phase.

 

[jumper]

OK, take the typical 120V/240V center-tapped transformer with terminal L1, N, and L2. If I look at the L1-N voltage waveform (120V RMS) and the L1-L2 waveform (240V RMS) simultaneously, then if I multiply the first waveform by the constant function 2, I get exactly the second waveform (in the idealized case). V_L1-L2(t) = 2 * V_L1-N(t) as functions.

Likewise, if I look at L1-N and L2-N, then if I multiply the first waveform by the constant function -1, I get the second waveform. V_L1-N(t) = -1 * V_L2-N(t).

I think everyone would say that L1-N and L1-L2 are "in phase." So why should there be a terminological difference between "multiplying by 2" and "multiplying by -1"?

Cheers, Wayne

Seems to me like you are trying to mathematically show the wave forms we see when we plot all three wave forms.

If so, I have no problem with it. Just that I already know what exists, and this method is a bit complicated to explain to the average sparky.

 

[david luchini]

OK, take the typical 120V/240V center-tapped transformer with terminal L1, N, and L2. If I look at the L1-N voltage waveform (120V RMS) and the L1-L2 waveform (240V RMS) simultaneously, then if I multiply the first waveform by the constant function 2, I get exactly the second waveform (in the idealized case). V_L1-L2(t) = 2 * V_L1-N(t) as functions.

Likewise, if I look at L1-N and L2-N, then if I multiply the first waveform by the constant function -1, I get the second waveform. V_L1-N(t) = -1 * V_L2-N(t).

I think everyone would say that L1-N and L1-L2 are "in phase." So why should there be a terminological difference between "multiplying by 2" and "multiplying by -1"?

Cheers, Wayne

And, -1 * V_L2-N(t) = V_L2-N(t-180),

so V_L1-N(t) = V_L2-N(t-180)...That doesn't look to me like V_L1-N and V_L2-N are "in phase."

 

[Ingenieur]

And, -1 * V_L2-N(t) = V_L2-N(t-180),

so V_L1-N(t) = V_L2-N(t-180)...That doesn't look to me like V_L1-N and V_L2-N are "in phase."

niiiiice!!!

 

[Besoeker]

Agreed that it is called six phase, and I believe I stated a bunch of threads back that it would be called 'hexaphase'.

I just thought it interesting that it was simultaneously called six phase _and_ that the individual phases were labeled in pairs. The diagram recognizes both that there are six separate phase angles feeding the rectifiers and that there is a special relation between phases 180 degrees apart.

Thanks for the explanation of why it would be used. My guess: since you either have to make the transformer larger _or_ you reduce the transformer efficiency (because you need to pack more turns of wire into the same space) that this arrangement would be used for high current low voltage applications where the voltage drop in the diodes is a large factor.

-Jon

Yes, high current low voltage rectifiers is mostly where we used them. Typically, we made single units up to 10kA and systems up to 40kA with four units in parallel. The transformer primaries were phase displaced from each other to produce a 24-pulse system minimising harmonics back on the supply. Water cooling was used so the losses were dealt with remotely and/or used for process heating.

 

[Ingenieur]

up to number 3!!! :lol:

 

[wwhitney]

And, -1 * V_L2-N(t) = V_L2-N(t-180)

Only if V_L2-N(t) is a sine wave. So yes, in the special case of a sine wave, take your pick between "times -1" and "180 degrees out of phase." I'll pick the one that applies more generally, "times -1".

Cheers, Wayne

 

[jumper]

And, -1 * V_L2-N(t) = V_L2-N(t-180),

so V_L1-N(t) = V_L2-N(t-180)...That doesn't look to me like V_L1-N and V_L2-N are "in phase."

Yep.

I get the idea Wayne is presenting and do not mind, but I prefer to stick to standard classroom methods.

 

[buffalonymann]

What we, UK, call single phase is one live and one neutral. What you are calling single phase is two lives and one neutral.

We call it single phase because that's exactly what it is. One primary winding, one secondary winding - produces just one sine wave from L1 to L2. It doesn't stop being a single phase xfmr because we tap the midpoint to provide 120 VAC.

In fact, with perfectly balanced 120 VAC loads, we don't even need the center-tap; we could connect the loads in series across the 240. Everything but the imbalance is across 240VAC.

 

[jumper]

Only if V_L2-N(t) is a sine wave. So yes, in the special case of a sine wave, take your pick between "times -1" and "180 degrees out of phase." I'll pick the one that applies more generally, "times -1".

Cheers, Wayne

Cool.

But since we are specifically talking about sine waves, I will stick with with standard methodology of discussing 120/240V services and the general equations we use.

Simpler to explain.

 

[jumper]

We call it single phase because that's exactly what it is. One primary winding, one secondary winding - produces just one sine wave from L1 to L2. It doesn't stop being a single phase xfmr because we tap the midpoint to provide 120 VAC.

In fact, with perfectly balanced 120 VAC loads, we don't even need the center-tap; we could connect the loads in series across the 240. Everything but the imbalance is across 240VAC.

Can you show me a real life power distribution tranny with a single secondary winding and a center tap?

Everyone I find is two separate windings ones on a common core with x2 and x3 leads. Whether internal or external.