Single Phase Motor on 2 phase

[Kcarroll4787]

I am installing a new compressor for a customer. His shop is 2-phase 5 wire (4 hot 1 neutral) but the new compressor is single phase. I mainly do commercial and light industrial work, but I have never come across 2 phase. Even though I sort of picked up on the basics I'm still unfamiliar with it so hopefully someone can help me out. I need to get the 2 phase supply to the single phase compressor. Can I just use one leg or is there a transformer I need?


Thanks!

 

[kwired]

I am installing a new compressor for a customer. His shop is 2-phase 5 wire (4 hot 1 neutral) but the new compressor is single phase. I mainly do commercial and light industrial work, but I have never come across 2 phase. Even though I sort of picked up on the basics I'm still unfamiliar with it so hopefully someone can help me out. I need to get the 2 phase supply to the single phase compressor. Can I just use one leg or is there a transformer I need?


Thanks!

Just connect across one of the phases, assuming it is the correct voltage. It is just two single phase sources that are out of phase with one another all in one generating housing, with the 5th wire being connected to center tap of each phase. The use of both phases will give a true two phase motor rotation when connected properly, otherwise pretty much anything else ever connected to such a system is single phase.

 

[hardworkingstiff]

Just connect across one of the phases, assuming it is the correct voltage. It is just two single phase sources that are out of phase with one another all in one generating housing, with the 5th wire being connected to center tap of each phase. The use of both phases will give a true two phase motor rotation when connected properly, otherwise pretty much anything else ever connected to such a system is single phase.

How is this 2-phase system wired into panels? Is there a 2-phase panel that accepts 5 conductors for the feed?

If I understand what you said correctly, you have 4 "hots" to choose from to use with the neutral, but the hot to hot connection can only be between the phases from the same source?

 

[Smart $]

How is this 2-phase system wired into panels? Is there a 2-phase panel that accepts 5 conductors for the feed?

If I understand what you said correctly, you have 4 "hots" to choose from to use with the neutral, but the hot to hot connection can only be between the phases from the same source?

I believe they made (make?) a 4-hot-bus panel w/isolated neutral bus. Can also connect standard single-phase panels off the main.

Connection options include all L-N, adjacent L-L, and opposing L-L. Adjacent L-L voltage is square root of two times L-N voltage, or opposing L-L divided by square root of two. Not certain of proper terminology.

 

[kwired]

How is this 2-phase system wired into panels? Is there a 2-phase panel that accepts 5 conductors for the feed?

If I understand what you said correctly, you have 4 "hots" to choose from to use with the neutral, but the hot to hot connection can only be between the phases from the same source?

I can't tell you, not because it is classified but I just don't know

This kind of source is probably older than both of us put together, and maybe even some more. I don't really know how it was installed, but was more common before modern "panelboards" were around for certain. I do understand the principle of how they work, but don't know how distribution was handled.

Draw yourself two sets of winding coils with centertaps that cross one another perpendicular at both centertaps. That is your basic representation of how the source is connected. Voltage and current in each winding is out of phase from the other winding, the windings of a two phase motor are connected similarly so with the two resulting currents being out of phase we get rotation of their magnetic fields to drive the motor.

Your single phase induction motors are two phase motors, we just manufacture a second phase with things like capacitors, different reactance values in two different coils, etc. A real two phase motor just happens to have two voltage/currents input that are out of phase with one another.

 

[hardworkingstiff]

OK, if the 2-phase generation is with 2 sets of windings (both center tapped and the centers of the 2 windings are tied together) with the voltage generation being 90 degrees out of phase with the other.

If the L-N voltage is 120V, the for the L-L voltage of one winding would be just like a single-phase transformer, right? So it would be 240V L-L.

The L-L voltages of adjacent phases (one from each winding) would be 90 degrees out of phase so if we use A-squared plus B-squared is = to C-squared, then 120*120+120*120= 28800. Take the square root of that and you have 169.7 or the same thing as 120*sqrt2. Am I looking at this correctly, if not, where did I mess up?

 

[GoldDigger]

You are correct.
And if you look at the larger right triangle formed by two adjacent line/line pairs will be 169.7 times sqrt2, which is 240.
So far, so good.

Tapatalk!

 

[kwired]

OK, if the 2-phase generation is with 2 sets of windings (both center tapped and the centers of the 2 windings are tied together) with the voltage generation being 90 degrees out of phase with the other.

If the L-N voltage is 120V, the for the L-L voltage of one winding would be just like a single-phase transformer, right? So it would be 240V L-L.

The L-L voltages of adjacent phases (one from each winding) would be 90 degrees out of phase so if we use A-squared plus B-squared is = to C-squared, then 120*120+120*120= 28800. Take the square root of that and you have 169.7 or the same thing as 120*sqrt2. Am I looking at this correctly, if not, where did I mess up?

You are probably correct, but if you want to run a 240 volt single phase load just make sure to connect to two ends of the same phase of the source. If you want to run a 120 volt load pick any lead to the neutral.

I think there is also a two phase 3 wire system. Same two phases with no center taps, and one end of each phase is connected together for a common point between the two phases.

This one could be easier to confuse with a three phase system since there are three system conductors.

 

[Smart $]

OK, if the 2-phase generation is with 2 sets of windings (both center tapped and the centers of the 2 windings are tied together) with the voltage generation being 90 degrees out of phase with the other.

If the L-N voltage is 120V, the for the L-L voltage of one winding would be just like a single-phase transformer, right? So it would be 240V L-L.

The L-L voltages of adjacent phases (one from each winding) would be 90 degrees out of phase so if we use A-squared plus B-squared is = to C-squared, then 120*120+120*120= 28800. Take the square root of that and you have 169.7 or the same thing as 120*sqrt2. Am I looking at this correctly, if not, where did I mess up?

You are looking at it correctly... but I believe the original standard for two-phase voltage was 220/110.

 

[kwired]

According to Smart's information it was apparently common to use four wires with no interconnection between each phase as well, so we have two phase three wire, four wire and five wire systems.

 

[kwired]

Getting back to OP, is this shop supplied with two phase or does it have a "Scott T" transformer to derive two phase from utility power?

I thought that all utility supplied two phase had gone away, but you are in a location where it was fairly common from what I understand.

 

[hardworkingstiff]

According to Smart's information it was apparently common to use four wires with no interconnection between each phase as well, so we have two phase three wire, four wire and five wire systems.

I was just thinking, if you had a 2-phase 3-wire system connected to a single-phase panel, you would have a L-N voltage that was 70.7% of the L-L voltage. That would really throw someone for a loop.

 

[hardworkingstiff]

Getting back to OP, is this shop supplied with two phase or does it have a "Scott T" transformer to derive two phase from utility power?

I thought that all utility supplied two phase had gone away, but you are in a location where it was fairly common from what I understand.

Also, what are the voltages available?

 

[PetrosA]

Here's another illustration from Audel's Handy Book from 1944.

 

[zbang]

I thought that all utility supplied two phase had gone away, but you are in a location where it was fairly common from what I understand.

IIRC, the utility in Philadelphia (PeCo?) still supplies real 2-phase services, which they feed with a scott-t transformer. I think someone wrote about that here a year or so ago. Since PeCo wouldn't change the service to read 3-phase, he installed a scott-t transformer just off the service so the building could be rewired with 3-phase panels.

 

[kwired]

IIRC, the utility in Philadelphia (PeCo?) still supplies real 2-phase services, which they feed with a scott-t transformer. I think someone wrote about that here a year or so ago. Since PeCo wouldn't change the service to read 3-phase, he installed a scott-t transformer just off the service so the building could be rewired with 3-phase panels.

Would seem to me they would want to get rid of the 2 phase service as they need specialty equipment in order to provide such service (the scott-t transformer). The three phase is already there and supplied with what they would call everyday items for the most part. Most places if you wanted 2 phase you would need to derive it yourself from your own scott-t or a generator or something else.

 

[zbang]

Would seem to me they would want to get rid of the 2 phase service as they need specialty equipment in order to provide such service (the scott-t transformer). The three phase is already there and supplied with what they would call everyday items for the most part.

Why change something that's operating (and paid for)? They would have installed the 2-phase services many years ago, and simply installed the scott-t's when they stopped generating 2-phase; call it 30+ years ago. To change the service to the customer, they'd have to go into each customer and change out their service and anything that actually needs 2-phase power. This is pretty much what ConEd had to do in NYC as part of turning off the Edison DC system. ConEd paid for a lot of motors and rectifiers on customer premises. Not changing the services has the added advantage that if the customer really wants 3-phase, the utility gets the customer to pay for the installation.

 

[kwired]

Why change something that's operating (and paid for)? They would have installed the 2-phase services many years ago, and simply installed the scott-t's when they stopped generating 2-phase; call it 30+ years ago. To change the service to the customer, they'd have to go into each customer and change out their service and anything that actually needs 2-phase power. This is pretty much what ConEd had to do in NYC as part of turning off the Edison DC system. ConEd paid for a lot of motors and rectifiers on customer premises. Not changing the services has the added advantage that if the customer really wants 3-phase, the utility gets the customer to pay for the installation.

Why did they install Scott-t 30+ years ago is the question. I promise you if they were getting rid of two phase service today, they would tell customer you only get single or three phase, if you want anything else you need to derive it separately.

 

[zbang]

Why did they install Scott-t 30+ years ago is the question. I promise you if they were getting rid of two phase service today, they would tell customer you only get single or three phase, if you want anything else you need to derive it separately.

I'm guessing here, but let's assume that the 2-phase service was tarriffed back then. It's likely that they could stop installing new 2-phase, but would have to keep supplying what they'd agreed to supply as long as the customer wanted it. Telling the customer "we're changing our supply, so you have to change your service and equipment" just doesn't fly. (Just like some of the less-common 3-phase systems; you can keep the existing one, but we're not installing new ones.)

And, as I mentioned, I read on MH that Peco would not change out an existing 2-phase service unless the customer paid for it.

FWIW, Peco still lists 2-phase at 120/240v and 2400v in it's blue book.

 

[kwired]

I'm guessing here, but let's assume that the 2-phase service was tarriffed back then. It's likely that they could stop installing new 2-phase, but would have to keep supplying what they'd agreed to supply as long as the customer wanted it. Telling the customer "we're changing our supply, so you have to change your service and equipment" just doesn't fly. (Just like some of the less-common 3-phase systems; you can keep the existing one, but we're not installing new ones.)

And, as I mentioned, I read on MH that Peco would not change out an existing 2-phase service unless the customer paid for it.

FWIW, Peco still lists 2-phase at 120/240v and 2400v in it's blue book.

Understood to some degree. That doesn't mean they can't find ways around that, one being to make it costly enough to have two phase delivered that the customer will eventually opt for something else, even if that means customer ends up with their own two phase conversion equipment. Conversion equipment will cost less for limited load than it will to supply the entire premises if only limited load uses the two phase.