# planning for rotary phase converter - confused



## gheumann (May 30, 2014)

I've been planning for the install of an RPC. The schematics I see on google show bringing in the single-phase, and the third phase magically appears on a 3rd wire with the single phase wires representing the other legs. I have been taking it for granted. But an electrician (not familiar with RPC's)  just made me think - he said "with single phase the 2 wires are 180 apart in phase. In 3 phase the 3 are 120 degrees apart. How can you use the same single phase wires as 2 of the 3 legs?

Good question. Can anyone shed any light?


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## JimDawson (May 30, 2014)

gheumann said:


> I've been planning for the install of an RPC. The schematics I see on google show bringing in the single-phase, and the third phase magically appears on a 3rd wire with the single phase wires representing the other legs. I have been taking it for granted. But an electrician (not familiar with RPC's)  just made me think - he said "with single phase the 2 wires are 180 apart in phase. In 3 phase the 3 are 120 degrees apart. How can you use the same single phase wires as 2 of the 3 legs?
> 
> Good question. Can anyone shed any light?




This is the best explanation I found:  http://en.wikipedia.org/wiki/Rotary_phase_converter


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## toolcrib (May 30, 2014)

The answer is not straight forward, but generally consists of ground/neutral is how we define it.  A transformer does not require any incoming leg to be connected to an output leg, so the output voltage can have any offset you want.  (as long as the offset does not exceed the breakdown voltage of the insulation in the transformer)

At my house the two power lines coming in are both 120V to neutral/ground as measured by a volt meter, but are 208V line to line when measured with the same volt meter.  If the phase angle of the two lines were 180 degrees apart I would measure 240V between these lines. 
With them being 120 degrees apart the line to line voltage is 120V*((1+sin(30))^2+(cos(30))^2)^(1/2) = 207.85V.

The power at my house comes from a single three phase transformer at the end of the block that powers about a dozen homes.  The two legs going to each house varies as you go along the block.  My neighbor gets A and B, I get B and C, and the neighbor on the other side of me gets C and A.  This pattern continues down the block, and explains why my power legs are 120 degrees apart.

Even if your two input legs are 180 degrees apart a balanced rotary phase converter will induce a third leg that will be 120 degrees from the other two.  It is just the center of the two incoming legs and the third induced leg will have a voltage relative to the center of the two incoming legs.  A similar power service from a transformer is referred to as "High-leg delta".  If you look up this and understand it you should have no trouble understanding the output of a RPC.

Without some diagrams, this is quite difficult to explain.

I hope that helps,
Andrew


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## JohnG (May 31, 2014)

If you start with single phase power (lets call it 1 phase power to do arithmetic cleanly) and need 3 phase power, you need 2 additional phases since:
       1 phase+1 phase=2 phases
       1 phase+2 phases=3 phases

Electric power comes from a voltage across a pair of terminals.

You have a supplied voltage across a pair of terminals, call them A and B.  That's 1 voltage, call it AB.

3 phase has a third terminal, call it C.  One new terminal creates 2 new terminal pairs for a total of 3 pairs.  Since power comes from voltage across a pair of terminals, you now have:
     1)  the original pair AB with supplied voltage
     2)  a new pair AC with induced voltage
     3)  a new pair BC with induced voltage

Get that much clear in your mind before you launch into the mechanics of how the convertor does this.  The induced voltage are out of phase with the supply voltage and with each other.  

You electrician makes it sound like you start with 2 voltages and add 1.  You start with 1 and add 2.


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## Ulma Doctor (May 31, 2014)

gheumann said:


> I've been planning for the install of an RPC. The schematics I see on google show bringing in the single-phase, and the third phase magically appears on a 3rd wire with the single phase wires representing the other legs. I have been taking it for granted. But an electrician (not familiar with RPC's)  just made me think - he said "with single phase the 2 wires are 180 apart in phase. In 3 phase the 3 are 120 degrees apart. How can you use the same single phase wires as 2 of the 3 legs?
> 
> Good question. Can anyone shed any light?




the third phase has nothing to do with magic, the third leg is created by induction by the spinning rotor inside the windings much like an alternator.
there is a downside, there are losses. it takes more power to run than it produces.
but it is a cheap and effective way to get 3 phase power without draggin' in another conductor.


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## Alan Douglas (May 31, 2014)

> How can you use the same single phase wires as 2 of the 3 legs?


Because (as already noted) the ground reference is not the same for the single- and the three-phase.


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## DAN_IN_MN (May 31, 2014)

gheumann said:


> I've been planning for the install of an RPC. The schematics I see on google show bringing in the single-phase, and the third phase magically appears on a 3rd wire with the single phase wires representing the other legs. I have been taking it for granted. But an electrician (not familiar with RPC's)  just made me think - he said "with single phase the 2 wires are 180 apart in phase. In 3 phase the 3 are 120 degrees apart. How can you use the same single phase wires as 2 of the 3 legs?
> 
> Good question. Can anyone shed any light?



A couple of thoughts.

I have a "homemade" RPC.  3 phase motor powered by 220.  I kick the shaft of the motor to get it up to speed and then turn the 220 on.  What ever I'm powering is connected with another switch using the 220 wires plus the 3 leg of the motor.  I power my car lift, mill, and lathe with this.  If my lift doesn't have enough power, I can run my mill which improves the quality of the 3rd leg.

The other thing I'd suggest is a VFD (Variable frequency Drive)  

Here's a write up about them.

http://en.wikipedia.org/wiki/Variable-frequency_drive


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## ironmonger (Jun 1, 2014)

toolcrib said:


> <<snip>>
> 
> The power at my house comes from a single three phase transformer at the end of the block that powers about a dozen homes. The two legs going to each house varies as you go along the block. My neighbor gets A and B, I get B and C, and the neighbor on the other side of me gets C and A. This pattern continues down the block, and explains why my power legs are 120 degrees apart.
> 
> ...




There is no phase angle in a single phase supply. You cannot, by definition, have a phase angle DIFFERENCE until you have something to compare it to. 

Your power legs are not 120 degrees apart, the phase angle of your house is 120 degrees different than your neighbors. 

Do not confuse polarity with phase angle. Using that logic a battery would have a phase angle of 180 degrees... It is just that the polarity of the two legs are opposite.

paul


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## toolcrib (Jun 2, 2014)

ironmonger said:


> There is no phase angle in a single phase supply. You cannot, by definition, have a phase angle DIFFERENCE until you have something to compare it to.
> 
> Your power legs are not 120 degrees apart, the phase angle of your house is 120 degrees different than your neighbors.
> 
> ...



Paul,

There are two power legs coming to my house, and I have earth ground/neutral to compare them to.  Both power lines coming into my house show a voltage of 120 with respect to earth ground/neutral.  I stated above that the line to line voltage is measured at 208V.  This is only possible if there is a phase angle of 120 degrees between the two power legs with respect to earth ground/neutral.

I agree that absent the reference point of earth ground I would not be able to measure a phase angle between the two power legs, because I would only have those two points to measure.  The addition of earth ground to the circuit gives me the third point for my phase angle measurement.

I am not sure where the reference to polarity comes from, I did not use that term in my response.  Please clarify the logic you see in my post that states a battery has a phase angle of 180 degrees.  I was not trying to state this, and if I did will want to correct my wording.  Since a DC circuit is by definition not changing with time there can be no phase angle.

Thanks,
Andrew

I will also note that for my circuits earth ground and neutral are tied together at my panel and represent the same line in any circuit calculations.  I will also state that on the 208V supply to my air conditioner there are two hot lines, not the hot line neutral line pair going to my 120V outlets.


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## Cactus Farmer (Jun 2, 2014)

I am as confused as the OP> I have a 8-12 HP add-a-phase and a 10HP 3 phase motor in my shop. I've been using it to power my 3ph  motor for ten plus years. It runs from 1/2 to a 5 HP motor with apparent easy. The 10 HP "idler" motor just hums and has a very slight temperature rise after 3-4 hours of use. It ran bigger motors for the two previous owners. I turn on the Add-a-phase and the 10hp is running. There are three wires running around the wall of the shop in their own conduit. Every 10-12 feet there is a drop box with a cover and when I need to have 3 phase power I remove the cover plate and cut the wires that are there and splice in the new wire. A strain connector is added to the bottom of the box and the cover reinstalled. Is this the  wrong way to do this? 'Cause it sure seems to work well. I'm not an EE major so some of the lingo is unknown to me. I just know that it works and when I wired the shop the electrician (helper, not licensed) was sure we were going to blow/smoke something.......I just said ,I know it works. He was amazed and I find this is still the case. A lot of "electricians"  don't know this trick. Phase converters work if wired direct but you loose a lot of the rated HP. Adding the "idler" motor seems to fix the problems to a great extent.  The third phase is indeed generated by the windings in the" idler" motor. What the wave forms look like would be a fun thing to know, I sure they aren't nice sine waves but it doesn't seem to make a hill of beans to the 3 phase things I run off the homemade rotary converter.


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## xalky (Jun 2, 2014)

Cactus Farmer said:


> I am as confused as the OP> I have a 8-12 HP add-a-phase and a 10HP 3 phase motor in my shop. I've been using it to power my 3ph  motor for ten plus years. It runs from 1/2 to a 5 HP motor with apparent easy. The 10 HP "idler" motor just hums and has a very slight temperature rise after 3-4 hours of use. It ran bigger motors for the two previous owners. I turn on the Add-a-phase and the 10hp is running. There are three wires running around the wall of the shop in their own conduit. Every 10-12 feet there is a drop box with a cover and when I need to have 3 phase power I remove the cover plate and cut the wires that are there and splice in the new wire. A strain connector is added to the bottom of the box and the cover reinstalled. Is this the  wrong way to do this? 'Cause it sure seems to work well. I'm not an EE major so some of the lingo is unknown to me. I just know that it works and when I wired the shop the electrician (helper, not licensed) was sure we were going to blow/smoke something.......I just said ,I know it works. He was amazed and I find this is still the case. A lot of "electricians"  don't know this trick. Phase converters work if wired direct but you loose a lot of the rated HP. Adding the "idler" motor seems to fix the problems to a great extent.  The third phase is indeed generated by the windings in the" idler" motor. What the wave forms look like would be a fun thing to know, I sure they aren't nice sine waves but it doesn't seem to make a hill of beans to the 3 phase things I run off the homemade rotary converter.


I was just gonna say " just hook it up already...It just works"  You beat me to it.)


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## Cactus Farmer (Jun 2, 2014)

I'm just fast like that! ;>)


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## ironmonger (Jun 2, 2014)

toolcrib said:


> Paul,
> 
> There are two power legs coming to my house, and I have earth ground/neutral to compare them to. Both power lines coming into my house show a voltage of 120 with respect to earth ground/neutral. I stated above that the line to line voltage is measured at 208V. This is only possible if there is a phase angle of 120 degrees between the two power legs with respect to earth ground/neutral.
> 
> ...



    The two legs that you are referring to are the start and end winding leads from a transformer. The center tap merely allows a lower voltage to be supplied. Nothing to do with phases. Neutral is a code definition and has nothing to do with legs or transformers. The transformer secondary that serves you is two coils with the end winding of one connected to the start winding of the other. The common wire is called neutral and is grounded in your panel. Electrical safety requirement not electrically necessary. 

    The distribution transformer that serves you has a center tapped winding that supplies the low and high voltage to you. It is one of three phases. Regardless of what your voltmeter tells you, phase (more correctly phases angle) represents the different times in relation to a reference phase waveform that the voltage begins climbing towards its maximum. This occurs twice in each cycle, once for the positive part of the AC cycle and once during the negative parts of the cycle. 

    Phase angle is expressed in degrees, but represents time. The time at which the voltage of a phase begins to rise as compared to the preceding phase. One cycle takes 1/60 of a second, 0.016 second. The second phases begins to rise 0.0055 second later, one third of 1/60th of a second. Using phase angle eliminates time as a unit, thus making the phase discussion independent of 50 cycle, 60 cycle or 400 cycle supplies.

    If you observe a single phase waveform on an oscilloscope, and place one lead on one leg of the 220 volt buss, and were to observe the positive and negative traces of the neutral and the other 220v leg you will observe that they travel together. On a dual trace scope this will be demonstrated beyond dispute. 

    The three phases are in reality three separately excited windings, and while the voltage is delivered to the load on three wires (disregard neutral or ground it has nothing to do with the phases) think of each of those legs as a PAIR of wires. each of those legs share a common wire from each of the transformer coils. It is a matter of economy rather that electrical necessity.

    In reference to the voltages that you read. I offer no explanation for the difference that you have observed. There is only one phase of the three phases at the distribution transformer available for your service. Therefor only one phase in your house/service. I speak only to the confusion regarding phases. 

    Perhaps there is a bad ground in your house or service and some leakage between your house and a neighbors is occurring, with those leakage currents resulting in your readings. I would suggest that you wire two light bulbs in series and measure the voltages at the lamps when the series pair is wired to 220. If it is stray grounding or leakage voltages that you are measuring, applying a load and then measuring the voltage would reduce the false readings. Voltmeters are very sensitive and will read a voltage even if it is being supplied through a large resistance, hence very little current. Presenting a large load or resistance would cause enough voltage drop to the milliamp stray voltages to eliminate or vastly reduce them. 


    If the leakage readings are the result of very high leakage currents, it's time to call in your local electrical utility.

    The reference to the battery was to point out that it is impossible to measure phase angles with a voltmeter. Just because the voltages are different does not have anything to do with identifying phases.

    In closing, and in the words of Red Green, "It's only temporary unless it works" 

paul


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## toolcrib (Jun 2, 2014)

I found a diagram that matches the secondary power distribution for my neighborhood.  Apparently the power I have is derived from three phase.  This agrees with what I see which is a single transformer supplying over a dozen houses.  The transformer has all three primary lines attached and three secondary lines leaving. 

Each house is connected by two insulated wires and a bare grounded steel cable that also provides support to the power leads.  This is referred to as 120/208v and is the three phase shown in the diagram with one leg missing from each house.  The missing leg alternates down the street in what I am guessing is a crude method to load balance.




I do not know if this is still common practice in new wiring, although I have found references to it in electrician forums.  Mostly in reference to power distribution to an apartment building with a single three phase transformer.

If anyone living around me has an oscilloscope I would love to test this and show the phases clearly.  I may end up buying one, as the USB style costs less than $100.

Andrew


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## John Hasler (Jun 2, 2014)

toolcrib said:


> I found a diagram that matches the secondary power distribution for my neighborhood.  Apparently the power I have is derived from three phase.  This agrees with what I see which is a single transformer supplying over a dozen houses.  The transformer has all three primary lines attached and three secondary lines leaving.
> 
> Each house is connected by two insulated wires and a bare grounded steel cable that also provides support to the power leads.  This is referred to as 120/208v and is the three phase shown in the diagram with one leg missing from each house.  The missing leg alternates down the street in what I am guessing is a crude method to load balance.
> 
> ...



This is exactly how my house in Minneapolis was supplied.  The farm I live on now, on the other hand, is supplied from one center-tapped single-phase transformer.  The overhead 7200V line that feeds that transformer is just two wires (i.e., one phase plus neutral), the other two phases having branched off onto side roads miles back.


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## gheumann (Jun 4, 2014)

Thanks to everyone. This has been very illuminating!


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