planning for rotary phase converter - confused

[gheumann]

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?

 

[JimDawson]

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

 

[toolcrib]

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

 

[JohnG]

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.

 

[Ulma Doctor]

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.

 

[Alan Douglas]

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.

 

[DAN_IN_MN]

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

 

[ironmonger]

<<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.

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

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

 

[toolcrib]

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

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.

 

[Cactus Farmer]

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.