3 Phase Power Question

5611 Views 14 Replies 10 Participants Last post by ddawg16, Dec 18, 2015

Ralph III

Discussion Starter · Dec 16, 2015

Hello All,
I hired into a maintenance position last year and have a question in regards to 208v, 3 phase power as I'm not real familiar with such.

Anyhow we have pump, maintained by another company, that we are having issues with and it seems improperly wired to me as I research. BTW, they've sent two technicians out in the last month with the last stating the voltage is to low. He says it needs to be 240v to the pump....

Please confirm the following:

1) 208v 3 phase will show 120v on each leg (phase) but it will only read 208v across two legs; because the phases are never at peak power at the same time.

This seems correct but my Supervisor, who is intimately familiar with the system doesn't think so, and he spoke with an electrician who concluded the power company has a low voltage issue. That doesn't seem the case to me though.

I think they replaced the pump with an improper one or didn't wire it properly. I also noticed something "seemingly" odd with the electrical hookup for the pump.

The pump is protected by a breaker that has three hot wires (A, B, C). It is located in a panel which has various modules (contactors, float switches, timers). Each leg or phase on the breaker has 120v respectfully with A + B = 208 volts, B + C = 208 volts; However A + C = 0 volts.

That would indicate to me that A and C legs are the same phase. I would assume that breaker is meant to have three independent phases coming to it but I cannot say for certain because I don't know about the wiring requirements. I know the heat strip on my AC unit has a switch that separates two hot legs so they are used independently of each other.

2) I don't know if this breaker is serving some similar purpose but I cannot fathom why that would be the case? I cannot see any reason why you would have three legs on a breaker with two of them being the same? Maybe you can enlighten me.

3) How can you get 240v from 208v? Does the motor have to be wired for such or have a transformer of some sort? Otherwise, how can you get 240v as questioned above?

Thanks and Merry Christmas,
Ralph

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jbfan

· #2 · Dec 16, 2015

Ralph III said:
Hello All,
I hired into a maintenance position last year and have a question in regards to 208v, 3 phase power as I'm not real familiar with such.

Anyhow we have pump, maintained by another company, that we are having issues with and it seems improperly wired to me as I research. BTW, they've sent two technicians out in the last month with the last stating the voltage is to low. He says it needs to be 240v to the pump....

Please confirm the following:

1) 208v 3 phase will show 120v on each leg (phase) but it will only read 208v across two legs; because the phases are never at peak power at the same time.

This seems correct but my Supervisor, who is intimately familiar with the system doesn't think so, and he spoke with an electrician who concluded the power company has a low voltage issue. That doesn't seem the case to me though.

I think they replaced the pump with an improper one or didn't wire it properly. I also noticed something "seemingly" odd with the electrical hookup for the pump.

The pump is protected by a breaker that has three hot wires (A, B, C). It is located in a panel which has various modules (contactors, float switches, timers). Each leg or phase on the breaker has 120v respectfully with A + B = 208 volts, B + C = 208 volts; However A + C = 0 volts.

That would indicate to me that A and C legs are the same phase. I would assume that breaker is meant to have three independent phases coming to it but I cannot say for certain because I don't know about the wiring requirements. I know the heat strip on my AC unit has a switch that separates two hot legs so they are used independently of each other.

2) I don't know if this breaker is serving some similar purpose but I cannot fathom why that would be the case? I cannot see any reason why you would have three legs on a breaker with two of them being the same? Maybe you can enlighten me.

3) How can you get 240v from 208v? Does the motor have to be wired for such or have a transformer of some sort? Otherwise, how can you get 240v as questioned above?

Thanks and Merry Christmas,
Ralph

If , in fact, you do not have a reading between A and C, then you have an open fuse somewhere.
you need to trace back from the cabinet to the power source and see what is missing.

With your limited knowledge, this is really something you should not be messing with.

Reactions: 1

joed

· #3 · Dec 16, 2015

You can't get 240 from a 208 3 phase system.
Your reading of 0 volts is not correct. Did the electrician see that reading? What did he have to say. Sounds like one phase is missing.

Daniel Holzman

· #4 · Dec 16, 2015

Three phase power can be complex to understand, and you probably need a lot more basic background before you can assist in troubleshooting the problem. As others have noted, you need a qualified electrician to define, diagnose and solve the problem.

Just in terms of terminology, there are a few things that would help you better describe the situation. Three phase in the United States comes in two common form, delta and wye. A delta service has three different hot legs each 120 degrees out of phase with the other two phases. A delta service has no neutral, so the voltage reading between any two phases is the same. So for example, a delta 208V service would have three legs each showing 208V between any two legs. The voltage reading is the difference between the voltages on any two legs, not the sum of the voltage. In a delta service, you get single phase service by connecting a breaker between two legs on the panel. If you install a three pole breaker across all three phases, you get three phase service at 208V.

A wye service has three hot phases plus a neutral. You get two different single phase voltages off a wye service. Phase to phase is the same as the delta service, so in a 208V wye service you get 208V phase to phase. Phase to neutral voltage equals the phase to phase voltage divided by 1.71, so in a 208V wye service, phase to neutral is 120V. If you connect a breaker between any phase and the neutral, you get 120V single phase service. If you connect a breaker phase to phase, you get 208V single phase service. And if you connect a breaker across all three phases, you get 208V three phase service.

Some three phase services are more complicated, including corner grounded and high leg service, you really need a thorough understanding of electrical theory to understand what is going on with those types of service.

Conclusion: Leave this one to the pros.

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AllanJ

· #5 · Dec 17, 2015 (Edited)

Did this pump ever work correctly?

Whether or not you have a 3 phase system, you need to do more homework to find the dead leg. Shut off all of the breakers. Do you still get A to B 208 volts, B to C 208 volts, and A to C 0 volts?

If things are connected and turned on, parts of the wiring that might be expected to be dead could be live.

By the way it is possible to have a 120/208 system that is given two of the 3 legs of a 3 phase Y system but behaves approximately the same as a 120/240 volt single phase system.

So you need to find out what kind of system you have, and then find out what kind of system the pump motor wants.

Ralph III

Discussion Starter · #6 · Dec 17, 2015 (Edited)

Thanks for all the replies and let me clarify.

1) As noted, this piece of equipment is under contract by company "Z", so it's not something I plan on messing with.

2) The previous pump gave out after many years of service and it was recently replaced. We however have been getting an error light since the new pump was installed. I'm not familiar with the problem but I think the pump isn't running properly or keeping up with the system.

3) My Supervisor asked me to take a look at it for an opinion because I'm very good at troubleshooting and I have excellent reasoning ability. BTW, we've had two different "supposed" technician/electricians from company "Z" look at it with no solution as yet.

The first technician, who installed the pump, stated it was running correctly and had 240v on it. As he told my Supervisor. The last technician stated the power voltage was to low at 208v, that it needed 240v.

My Supervisor stated we have 208v 3 phase service. That indeed is the readings I got when I checked it. He however is under the impression we can get 240v from the system. He then called an electrician he uses a lot for an opinion. That electrician said it sounded like the POCO had a low voltage issue coming into our facility.

All of the above seems to be absolute bunk to me! The incoming power seems to be correct to me. However, as stated; I'm unfamiliar with 3 phase power but that is still my take on it.

----------------------------------------------------------------------------------

The only issue I discovered was with the 3 wire breaker that is in the panel which supplies the pump.

Each leg (A, B, C) has 120 volts on it. If you measure A/B and B/C you get the expected 208 volts.

However, if you measure A/C you get 0 volts. That tells me that A and C legs (phases) are one and the same.

The circuit isn't dead or a fuse issue as some Posters stated. There is power on all three breaker legs. The problem, as I see it, is that there is really only (A, B, A) on the breaker. Two of the legs are the same line.

A) I think who ever wired the breaker/pump mistakenly used the same leg on the breaker. He should have found the third leg of power and ran it to the breaker. I can see how he could have missed it though as the power runs to this pump panel, from the main panel, which has lots of timers, switches, terminals, etc. The power comes off the terminals and feeds the breaker, which then protects the pump.

B) Otherwise, they just used the wrong pump.

C) Please verify that there is no way to get 240v out of a 208v 3 phase system!!!! I thought that I read somewhere that you can do such but it depended on your wiring setup or that the equipment had to have some sort of transformer to convert it???

God Bless,
Ralph

P.S. This is just for my benefit but I would like to speak confidently to my Supervisor about it.

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joed

· #7 · Dec 17, 2015

The 120 volt reading could be deceiving. One of them could be coming back through the pump.
The BC 0 volt reading is another indication of this.
I don't work 3 phase panels but it could be possible the breaker is installed to have two phases the same. I know it's possible in FPE home panels to do it. If that's true then moving the breaker up or down one slot will fix it.

Arrow3030

· #8 · Dec 17, 2015

Sounds like a bad breaker. Remove the wires from the breaker and test voltage coming out of it. You should get 120v from each leg and 208 between each leg (three combinations). If you don't, this undoubtedly narrows the problem to the breaker or panel.

47_47

· #9 · Dec 17, 2015

Have similar set-ups at my shop and posting to get notifications out of curiosity.

carmusic

· #10 · Dec 17, 2015

are you sure the pump is 3 phase? maybe it is a 240v 1 phase pump running on 208v

Ralph III

Discussion Starter · #11 · Dec 17, 2015 (Edited)

carmusic said:
are you sure the pump is 3 phase? maybe it is a 240v 1 phase pump running on 208v

I don't know anything about the pump. All I know is what I have been told by my Supervisor or as I've noted.

**We have 208v 3 phase power -and- the last technician stated the voltage was too low for the pump at 208v. It was stated that we need 240v at the pump.

Ok, so how do you get 240v out of a 208v 3 phase system???

1) If you can't get 240v out of 208v 3 phase system, THEN they have obviously installed an incompatible pump that cannot operate properly at 208 volts. It's not within tolerance given we are having issues.

2) If you can get 240v out of a 208v 3 phase system, THEN it would be a wiring/setup issue. If I were troubleshooting the system I would start with the breaker. I've given my opinion to my Supervisor, so what he does with it is up to him.

I would like to definitively know though, whether you can setup something with 240v off of a 208v 3 phase system?

God Bless,
Ralph

P.S. I may check out the breaker just to satisfy my own curiosity at some point.

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joed

· #12 · Dec 17, 2015

Look at the name plate on the pump. That will tell you if it is single phase or 3 phase. Post an image if you don't know what you are seeing.
If the pump truly won't operate at 208volts then you have the wrong pump. Many devices are dual rated 208/240.

jbfan

· #13 · Dec 18, 2015

Ralph III said:
I don't know anything about the pump. All I know is what I have been told by my Supervisor or as I've noted.

**We have 208v 3 phase power -and- the last technician stated the voltage was too low for the pump at 208v. It was stated that we need 240v at the pump.

Ok, so how do you get 240v out of a 208v 3 phase system???

1) If you can't get 240v out of 208v 3 phase system, THEN they have obviously installed an incompatible pump that cannot operate properly at 208 volts. It's not within tolerance given we are having issues.

2) If you can get 240v out of a 208v 3 phase system, THEN it would be a wiring/setup issue. If I were troubleshooting the system I would start with the breaker. I've given my opinion to my Supervisor, so what he does with it is up to him.

I would like to definitively know though, whether you can setup something with 240v off of a 208v 3 phase system?

God Bless,
Ralph

P.S. I may check out the breaker just to satisfy my own curiosity at some point.

With 3 phase you will get between AB, BC, AC.

If you do not have a reading between the 3, you are missing a phase.

Reading from any phase to neutral will give you 120 volts, but like joed said, you could also be be reading, voltage through the pump.

The only way to get 240 volts from 208 is with buck/boost transformer

A 240 volt motor can run on 208, but may have issues with heat, speed and other things.

If the motor can start, it can run with a leg missing, but with issues.

Can you post a picture of the name plate?

What other 3 phase loads do you have in the facility?

If you are reading 208, you do not have issues with the poco.

I'm sure I can come up with more ideas when the pain meds wear off.

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micromind

· #14 · Dec 18, 2015

It's entirely possible to get 240 from 208, single or 3 phase.

You'll need buck-boost transformers, one if single phase, 2 if 3 phase.

Single phase is really easy to connect, 3 phase is a bit more difficult, but not all that hard.

To properly size them, we'll need to know what the actual 208 voltage is and what you need to boost it up to. Plus, we'll need to know the amps of the motor.

It's fairly common for a 230 volt pump motor to need more than 208; a lot of these motors are built as cheap as possible and they have no room for low voltage.

Rob.

P.S. If the motor is indeed 3 phase and it's being fed with single phase power (one dead leg), it most certainly will trip its overloads. If it doesn't, it'll burn up.

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ddawg16

· #15 · Dec 18, 2015

Put a clamp on amp probe on each leg. All 3 legs should read within about 10% of each other.

While you are at it.....look at the motor name plate. It will tell you exactly what it is rated for and how to wire it.

If you want, take a pic of it and post it.

Be prepared to pull the cover off the pecker head and look at the wires. It would not be the first time someone wired a motor backwards, especially when it has more than 3 poles.

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