# 3 phase 240 Delta Vs single phase 240

## Main Question or Discussion Point

Hello to all.

If I have a 240v 3 phase delta system, My phase to phase is 240V. there is no center tap on the phase for 120V. If I hook up a single phase 240V motor to 2 of the 3 phase delta legs, will the motor perform correctly??? I didnt know if the 120 deg phase shift would keep the motor from running correctly do to the 180 deg phase shift for single phase. Of course this would be a small motor so I dont un balance the system. THANKS Paul

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Averagesupernova
Gold Member
It should be no problem but I've never heard of not having a center tap on one of the transformers.

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GENIERE
You will be using one phase of a three phase transformer. If the kva of that winding can support the motor load there should be no problem except maybe some unanticipated mechanical stress. If it buzzes a lot shut it down.

Well I will be using 2 of the 3 phases to operate the single phase 240V motor. I am concerned about the phase shift of the 120 vs 180. As far as the center tap is concerned, This would go to ground on a delta system to achieve a 120v system.

Averagesupernova
Gold Member
I don't think you understand the phase shift issue. Two phases are 120 degrees apart from each other relative to the last phase. When you stick a voltmeter across one transformer and have 240 VAC, that is really all there is to it. The ends of a transformer are ALWAYS 180 degrees out of phase with each other relative to THAT transformer. I struggled with this at one time too. 3 phase can be somewhat confusing.

I assume the motor supply will be dual switched, and also that the motor chassis is grounded to earth.

Ive seen the sine wave for 3 phase. When one leg is at its peak the second is 2\3 on it way down and the last one is 1/3 on it way back up to peak. So each one is 120 out of phase with each other. So If I am going across 2 of these 3 legs I get the 240v but thought I would also get the 120 phase shift. Since the motor is single phase and operates on the 240 single phase with a phase shift of 180 I didnt know if this would work. I guess I still dont see the last phase issue. Isnt there still a phase shift of 120 between the 2 phases regardless of the third leg??? I am using the power company 3 phase. I didnt install a single phase transformer. The Delta Tranformer is on the power company side. It takes the 13.8kv down to 240v I am assuming its a Delta since I have 240 and not 208( Wye ). The breaker panel has 3 phase breakers in it and single phase breakers.(one leg to groung for the 110v) So I was going to put in a 3kva single phase transformer to correct the phase shift.(2 legs hook up to the primary from 2 of the 3 phase legs) and then I would have 240/110 on the seconday with a phase shift of 180.) I hope im not making this more confusing than what it is???

GENIERE
hawk7 said:
Ive seen the sine wave for 3 phase. When one leg is at its peak the second is 2\3 on it way down and the last one is 1/3 on it way back up to peak. So each one is 120 out of phase with each other.
Correct.
hawk7 said:
So If I am going across 2 of these 3 legs I get the 240v
Correct if you assign A,B, and C to the connection points and measure A-B, A-C, and B-C.
hawk7 said:
but thought I would also get the 120 phase shift.
Incorrect.
hawk7 said:
Since the motor is single phase and operates on the 240 single phase with a phase shift of 180
If you label the legs of the transformer A, B, and C you will measure 240vac from A to B, from A to C, and from B to C. If you choose to connect your motor from A to C, the motor will see single-phase 240vac.
hawk7 said:
I didnt know if this would work. I guess I still dont see the last phase issue. Isnt there still a phase shift of 120 between the 2 phases regardless of the third leg???
Yes but if you connect accross A to B you are using only one phase. If you connect 3 motors, one across A-B, one across B-C, and one across A-C, each of the motors will be across a single phase and each of the motors will operate approximately 120 degrees out of phase with the others.
hawk7 said:
I am using the power company 3 phase. I didnt install a single phase transformer. The Delta Tranformer is on the power company side. It takes the 13.8kv down to 240v I am assuming its a Delta since I have 240 and not 208( Wye ).
That is correct in a residential situation, but not universally true.
hawk7 said:
The breaker panel has 3 phase breakers in it and single phase breakers.(one leg to groung for the 110v)
You are referring to single pole breakers and double pole breakers, phase has nothing to do with it. The double pole breakers connect to both buss bars in your main breaker panel, thus when ON the screws on the breaker will measure 240vac across them and 120vac from each to neutral. From the utility delta transformer assume you are connected to the A and B terminals which will be connected to the double pole main breaker in your main breaker panel. The utility company will also tap into the center of the A-B transformer winding and connect that to the neutral buss in your main breaker panel. The NEC requires the neutral wire to be earthed within several feet of the connection. Since it is earthed, it is termed the groundED conductor. The safety groundING conductor AKA equipment groundING conductor is connected to the neutral buss as well as all metal plumbing. The groundING conductors are not relevant to your question other than to remember to connect the frame of the motor to the EGC to prevent it being energised due to insulation failure.

So you have single-phase power delivered to your home. I think your confusion arises from the fact that from leg A to neutral (N) measures 120vac and from B to neutral (N) measures 120vac. When measuring from A-B you measure 240vac thus indicating A-N is 180 degrees out of phase with B-N. Join the club, you have lots of company, the point is arguable but in practical terms it don’t mean a thing except don't connect one to the other. The important thing to know is that your residence receives single phase power.
hawk7 said:
So I was going to put in a 3kva single phase transformer to correct the phase shift.(2 legs hook up to the primary from 2 of the 3 phase legs) and then I would have 240/110 on the seconday with.so a phase shift of 180.
Totally unnecessary
hawk7 said:
I hope im not making this more confusing than what it is???
You are.

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Averagesupernova
Gold Member
Hawk, you say you have no center tap for 120 volts but you say you have single pole 120 volt breakers in your panel. I SERIOUSLY DOUBT that there is not a center tap on one of the transformers. Let me ask you this: When measuring the voltage from each leg to what is known as conduit ground what is the voltage?

GENIERE
Averagesupernova said:
Hawk, you say you have no center tap for 120 volts but you say you have single pole 120 volt breakers in your panel. I SERIOUSLY DOUBT that there is not a center tap on one of the transformers. Let me ask you this: When measuring the voltage from each leg to what is known as conduit ground what is the voltage?
I think he was referring to a transformer he was going to buy, but I'm not sure. You interpretatation may be correct.

you are correct Geniere. It was one I was going to buy. I havnt had to do to much on the 240 delta systems so it was kinda new to me. The plant I work in is 480 grounded 4 leg Y system. We do have 1 1000kva 240 delta transformer for our heat treat but everthing is 3 phase operated. any single phase come from the 480v side via single phase transformers installed thru out the plant. our other 5 transformers are 480V 1000kva with a 4160 primary. THe main thing I do is troubleshoot CNC machines.They are made up of servo drive,electronic circuits and plc programing. We also maintain the plants power distributuion, conduit installs and anything else electrical. Thanks to all of your input and a special thanks to Geniere for spelling it out for me. I look forward to future post to help expand my knowlede.

One More note on the "The breaker panel has 3 phase breakers in it and single phase breakers.(one leg to groung for the 110v) "

This box has single pole breakers and 3 pole breakers. No double pole breakers.(No 220) The 3 pole breakers operate 3 phase compresors on some freezers in a grocerie store. I wanted to make sure you all knew that this was a 3 phase panel and not a single phase panel. If this makes any differances please advise.

GENIERE
hawk7 said:
One More note on the "The breaker panel has 3 phase breakers in it and single phase breakers.(one leg to groung for the 110v) "

This box has single pole breakers and 3 pole breakers. No double pole breakers.(No 220) The 3 pole breakers operate 3 phase compresors on some freezers in a grocerie store. I wanted to make sure you all knew that this was a 3 phase panel and not a single phase panel. If this makes any differances please advise.
Whoa! I thought we were talking about residential wiring. It is possible that you may be looking at a Delta or Y configuration but since I’m not an electrician I have no experience to draw on. There are ways to achieve the voltages you speak of with either configuration and you can derive a neutral with a delta system I think it’s called a bastard leg but not sure. You originally mentioned 240vac, but now 220vac. It’s not that important for home use as most appliances and shop tools for Joe Fixit are designed for 220 to 240vac and often labeled 230vac. What ever you do must comply with local electrical codes. I’m afraid I can’t advise you further and sleep well.

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Averagesupernova
Gold Member
GENIERE, how can you go from seeming fairly informed about the subject to not wanting to touch it? Or is that just my strange perception? I believe he already posted he is working with a 3 phase delta system. One of the transformers in a delta system will have the center tapped for a neutral/ground. The voltage from the ends of that transformer will measure 120 volts to the tap and the voltage from the last leg known as the 'wild' leg in my experience will measure like 160 or something. Of course you can get the voltage Hawk7 is after on a delta system. Technically ANY 2 of the three legs would work, but I'd stay on the transformer that has the center tap for the neutral.

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GENIERE
hawk7 said:
…THe main thing I do is troubleshoot CNC machines…
I fixed one once many tears ago, I think in early 1960’s. Before I finished my degree I had to leave after my sophomore year and go to work. I worked for RCA Corp. servicing their computers at four locations. We had a contract to service CNC machines in a large machine shop. I didn’t know that at the time. The boss calls and sends me to do the service. I complained strongly because I had never seen one, let alone be able to fix it. Didn’t help and off I went. Turns out it was a simple fix to do with a mechanical problem on a paper tape reader. Found out later the operator usually fixed the tape reader himself except when he got ornery and wanted a few hours rest.
...

GENIERE
Averagesupernova said:
GENIERE, how can you go from seeming fairly informed about the subject to not wanting to touch it? Or is that just my strange perception? I believe he already posted he is working with a 3 phase delta system. One of the transformers in a delta system will have the center tapped for a neutral/ground. The voltage from the ends of that transformer will measure 120 volts to the tap and the voltage from the last leg known as the 'wild' leg in my experience will measure like 160 or something. Of course you can get the voltage Hawk7 is after on a delta system. Technically ANY 2 of the three legs would work, but I'd stay on the transformer that has the center tap for the neutral.
I’m aware, although I’ve never seen it, that what I called a bastard leg may be a 277vac lighting circuit (480vac delta), and that obviously cannot be used for the motor. Since the freezers are 3-phase, and he measured 240vac my concerns are not probably warranted but I can’t be certain. A homeowner, in most cities can do his own electrical work. I am reasonably certain that only a licensed electrician is permitted to do commercial work. If something should happen, the insurance companies are going to look for a scapegoat.

Overall I will be able to use 2 of the 3 pahses (which is going across one phase) to achieve the 220V. As far as me stating 220 and 240, Its a bad habiit of mine. Our shop has 480V 3 phase that I work on and somtimes other electricians wil state 440V when technicaly it not. Just bad habbits. We have several Forien machine that are tranformed to 396V. So one should be careful whne stating voltage values. You guys are also talking about the wild leg of a Delta??? Isnt that only a concern when working with a center tapped Delta?? which mine is. So im sure the power company has the side oppisite the center tap marked orange per NEC for the high leg (wild leg) here is a nice page on the subject. I hope I can do this
http://www.kilowattclassroom.com/Archive/DELTAWYEPhasors.pdf [Broken]

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Averagesupernova