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Two-Phase Systems

07/27/2009 2:27 PM

Is there anything such as two phase system used now a days? If it is, can you explain it with the vector diagram? We have a UPS and it has 240V output with two hot wires? Some body of my group would like to call it as 'two phase' system. I don't agree to this. To me, it should be called single phase with 2 hot wire.

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user-deleted-1105
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#1

Re: Two phase vs single phase

07/27/2009 3:21 PM

There was a thread about AC sysems around the world a while back..it seems the US have some half arsed system of 240v which is split into 2 anti phase 120v supplies in the same house, by means of a centre tapping...
I still have no idea why they have this bonkers system .
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JRaef
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#2

Re: Two-Phase Systems

07/27/2009 5:46 PM

True "two phase" polyphase power systems, where the phase angle relationship is 180 degrees from each other, were the first AC systems ever installed. That's what Tesla's generators at Niagara Falls were. The earliest adopters of polyphase power in the US therefore used two phase because the very first applications were electric lights, which couldn't care less. These installations were generally around Niagara and Philadelphia. Because 3 phase needs fewer wires and provides more continuous torque in a motor however, true two phase power never advanced much further and has been largely displaced with 3 phase. There still are a few buildings left in those areas that use it however.

BUT...

In any other part of the world where they NEVER had true two phase polyphase power distribution, they will refer to 2-out-of-three phases as "2-phase" power. The phase relationship between them is NOT truly two phase, because the phase angle for 2-out-of-three phases is still 120 degrees, not 180. Because we in the US had this legacy to deal with, we refer to 2-out-of-three phases as "single phase" power.

Single Phase power in the rest of the world is Phase to Neutral. There is NO Line-to-Line phasor relationship in a 230V single phase system in the UK for instance, because there is only one hot line.

Residential is a completely different thing altogether. In residential systems, we take the 2-out-of-three phases from a HV grid distribution system and transform it into a split-phase single phase system for the house. So what you really have is 240V single phase, with a center tapped secondary that provides 120V Phase-to-Neutral on either side.

So the answer to your original question about "2 phase" and what it means is entirely subjective to where you are; in an international forum such as this, answers can be confusing. I hope this helped.

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CJMcGill
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#4
In reply to #2

Re: Two-Phase Systems

07/27/2009 6:29 PM

Jraef, Actually it was 900, not 1800 as I understand it. CJM

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JRaef
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#5
In reply to #4

Re: Two-Phase Systems

07/27/2009 9:08 PM

Yes, each 1/2 phase is 90 deg from the next, but if you look at it from a complete phase relationship, they are 180 deg. Semantics really.

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CJMcGill
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#3

Re: Two-Phase Systems

07/27/2009 6:26 PM

Dear Guest,

"Phase" is a reference to time. If current flows through both terminals at the same moment, it is single phase and not two phase.

One thing you will find as you search this subject out, is that terminology will vary from place to place around the world. It is always good to check into the specific meaning when such a situation arises, commendable.

Early in Alternating current usage, motors could not start when 2 hot legs of single phase were applied. They needed a capacitor to offset the phase or a small DC starter motor.

Alternators were then contrived with 2 different phases as output, which then could develop torque from a stopped position without other means to start the rotation.

I mention this as a means to understand the difference between single phase and two phase. Two hot wires of single phase cannot start a motor without help. Two hot wires of 2 phase can start a motor without help. The reason for this is that there is no "phase" difference in single phase, but there is a "phase" difference in two phase and three phase.

Vector-wise, you are familiar with the vector diagram for Wye circuits, and can understand the 'vee' that would be left after removing one of the 3 phase vectors, leaving a 2 phase vector, separate in timing by 1200 relative to the common point (neutral).

There is the original 2 phase arrangement that held the the 2 phases at 900 separation in timing, resulting in a "T" shaped vector. These were either 4 wire, 2 for each single phase, or 3 wire with one common phase (common wire had to over-sized).

Now as for the output of your UPS, if it is DC from 2 hot wires, it is single phase by definition. If you have AC from a typical UPS, it will be single phase unless there are 2 secondary transformers in a special configuration.

Remember, different phases differ in time. The peak voltage will occur at a different time in 2 phase circuits. Peak voltage occurs at the same time in single phase circuits.

Regards, CJM

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#6

Re: Two-Phase Systems

07/25/2010 3:50 PM

I can remember studying in 1960's something like SCOTT connection and another type where 3ph is transformed into 2ph

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