OT: Need 110V from 220V 3ph

Hello,
I'm a one man shop, low tech and low budget. I've just installed a new low cost PC based control on a small CNC metal lathe. The AC drive/spindle motor needs 240 3ph, the PC, servo power supply, and controller boards need 120V 1ph.

I'm wanting to get my 120V from my 4 conductor 240 3ph power cord. This way I don't have the need for 120V power cords or big and hot step down transformers etc...

When I measure the V between pairs of the 3 hot wires on the 3ph power cord, I can get the ~120V on a certain pair. Can I steal off of these 2 wires and then use the Green (GND in power box) from my 3ph power cord to get my 120V 1ph and ground wire?

I can cheat a bit more than you guys can at your big shops, however, I do want to be *smart* and *safe* for me and the equipment. Any ideas or advice?

Yes, I already have a couple 240 -> 120 transformers, 1kva and 2kva, but they are HUGE, HEAVY and put out some unbelievable heat in a matter of minutes. Trying to get out of using these after seeing how hot they get.

IF your incoming power supply is 240vac WYE then it will have a neutral, any phase to neutral will be 120vac.



If your system is Delta grounded then only 2 phase to neutral will allow you to obtain 120vac, the other leg to neutral is called the "wild leg" and will give you in excess of 120vac which can be detrimental to devices.

If not of this applies or there may be a need for "isolation" then use a transformer properly sized for the load you have.

If you're reading 120 V AC between two phases, you don't have a 3 phase circuit. You have a single (2) 240 service, with a neutral. Check voltage on all 3 lines that you think are hot to ground. One should come back as 0 volts.

If it does, the 120 V AC you said is exactly that.

Thanks for responses. I've heard alot about the Delta and Wye stuff, but I have no clue what I have at my shop or how to tell.

I would have to check my reading again to be sure, but I think I get somewhere around ~120-125 on 2 of the pairs and then ~240+ on another pair.

I do definitely have 3ph, as I have 5 metal working machines all with 3ph motors running just fine for sometime now.

Somebody was telling me about how some electricians bond the ground and neutral 'bus bars' together in the junction box. The 3ph machines I wired up in my shop have the green wire going to the ground bus bar (with the other green wires). However, I noticed that an electrician I'd hired a while back to wire up my 3ph air compressor, hooked the ground to the white bus bar in the junction box rather than the green bus bar. The air compressor works just fine...

About my transformers, they are Dongan industrial control transformers. They are big and square. Is it normal for a 1kva transformer to get finger burning hot within minutes? Even with little to no load connected to it?

Thanks for the help.

CroCop,

I re-read your post. I'm getting ~125, 125 and 240 measuring between the 3 hot wires, or the 3 phases. I've not checked each of the 3 hots to ground as of yet.

Back to my last post, if I get V from a hot to the ground wire, then that must mean that my ground wire is also a neutral (bonded together in my box)? If I don't get any V from any hot to my ground wire, then...? Sorry for my stupidity, I'm trying to learn, but I just don't know this stuff and don't want to kill myself or fry my equipment.

From your description I suspect that what you have is a 240V delta with a center tap - once common it is becoming much less common in the US.

What this means is that your transformer secondary has four taps
X1, X2, X3, and X4. Between any two of taps x1,x2,x3 you get 240 volts.

X4 is a center tap between X1 and X2. Between X1 and X4 or X2 and X4 you get 120volts, but between X3 and X4 you'll measure around 208 volts (not to be confused with a 208V Wye transformer) . Never make a connection between X3 and X4.

Each of the 120V segments, X1-X4 and X2-X4 are in phase on this kind of a system as each is generated in each half of the same winding.

edit:
Look at this PDF - bottom right figure 12-12.
http://www.mikeholt.com/instructor2/img/product/pdf/1036681800sample.pdf

And Randy's advice in the next post is good advice. Worth much more than you paid for it.

Randy,

Point taken, and I know you are right. I'm definitely a novice with electricity. However, one thing I have learned is to not take chances with electricity. I've also learned that a few measurements with the mulitmeter is all it takes to eliminate guesses. I guess I was hoping that I could get some information on what measurements to take so I could confirm what I had, and how to proceed from there.

Electricians here in the south (where there is no industry) are a bit different than those found in the north. I will call one if I have to as a last resort. However, I would still like to have some extra opinions (that I can confirm/deny with actual measurments) so that;

1. I can learn for myself, my ultimate goal.

2. I'm educated enough to ask the right questions and confirm the job was done right when I do get an electrician to come out.

just a suggestion ... label EVERY wire in your service cord ... something like X, Y, Z, A, B, H, anything - it doesn't really matter what letters you use for each one ... then make careful measurements between each pair - and between each wire and ground ... then post a chart of your readings ... (example: X-Y=120, etc. etc.) ... we can tell a lot more from that systematic approach than from the somewhat (no offense intended) ambiguous descriptions that you're giving us ...

that having been said, I'll go along with the others who suggested that you call in some qualified reinforcements ...

good luck ...

Snag..I am like you..once i have a hold of something i just wont let it go!!!

However here you are playing with fire..(Literaly!!)

take Ron B's post to heart..Post the findings you have and you might be suprised..I dont know how many time i hear a customer tell me they have 2 phase power..(Becouse the have 240v) Reality is they have single phase.. From what you discribe it could be as Alaric suggests an old system or could be you just have a machine that has a neatral in it..

Post the readings and we will see..

snaggletto said:

Is it normal for a 1kva transformer to get finger burning hot within minutes? Even with little to no load connected to it?

Click to expand...

Absolutely NOT!!
If you don't have primary & secondary mixed up, are applying the correct volts to the primary, truly have no load, then there may be shorted windings in the secondary. With no load, they should get slightly warm over time, but burning your fingers indicates overload.
The only other possibility is they're rated for some line frequency higher than your supply. We get that occassionally down here with transformers designed for 60 Hz melting when connected to our 50 Hz supply (lack of iron in the core).

Snaggletto,

The first thing to do is to find out What your voltage you have from the plant power supply. I have found that often the easiest way is to read the nameplate on the main supply transformer. The nameplate should tell you how many phases, 1, 2, or 3, and the primary incomming voltage, and most importantly, the secondary winding voltage(s).

240 volt 3-phase power is becoming obsolete in the U.S. There are not many transformers available in the U.S. that can supply 240 volts 3-phase and 120 volts 1-phase as a balanced 3-phase circuit. As has been stated by others, there are many ways to get an unbalanced 3-phase 240 volt/120 volt circuit. I suspect that this is what you have. However, if you are measuring 120 volts line-to-line on the circuit, but this is power to a 3-phase motor of any voltage, then somebody has probably screwed up!!

There is one other possibility that should be considered. Your transformer may be actually supplying 208/120 volt 3-phase power, with a grounded neutral. That would mean that you have 208 volts phase-to-phase and 120 volts any phase-to-neutral and 120 volts phase-to-ground. I have often used a 208/120 volt transformer to run 240 volt 3-phase motors. Many of them have several voltage taps, and if you select the proper tap, from the same balanced 3-phase 208/120 volt transformer you can get 218 volts 3-phase to run the motors, and 126 volts 1-phase line-to-neutral to operate the controls, lights, and receptacles in the plant.

By the way, just because your plant uses 3-phase motors, do not ASSUME that the power supply is 3-phase. There are several ways and many devices available to convert 1-phase to 3-phase power.

When I measure the V between pairs of the 3 hot wires on the 3ph power cord, I can get the ~120V on a certain pair. Can I steal off of these 2 wires and then use the Green (GND in power box) from my 3ph power cord to get my 120V 1ph and ground wire?

Click to expand...

The above question makes me scared. Whatever you do, please DO NOT use the green ground wire as a current conductor.

Certainly it will conduct current, but it is illegal to use it to conduct current, and if an accident occurs, you could be held libel in a court of lawyers, even though the ground conductor did not cause the accident. Do not think because it is a small shop that no one will ever find out. When someone is killed or severely maimed, they will track you down, even 30 years later.

See Article 250 of the U.S. National Electrical Code. Article 250 plainly spells out that the ground wire is to be "non-current-carrying", and that efforts must be made to see that it does not conduct current under normal operating conditions. The NEC has the force of law in most locations in the US and many other countries, becauses it has been "adopted" by most state, county, and municipal governments. The short answer is NO, DO NOT USE THE GREEN GND TO GET 120 VOLTS. The proper way to do this is to replace the power cord with a 5-conductor cord, 3 black line conductors, a white neutral conductor, and a green ground wire. Then you can tap power off between any line conductor and the neutral conductor. BUT when you tap power off, you still must have proper overcurrent protection (fuse or circuit breaker) for ALL circuits. There are many special rules in the N.E.C. that apply to power tap overcurrent protection.

To chime in.

Yes the readings as previously suggested will help others figure out what you have. DO them for the learning experience.

How many sets of utility wires do you have outside? Better yet how many meters and what do they say. You may have singls and three phase service. OR you may have 3 phase and from that a "derived" single phase. Do you have 480??

NO transformer properly connected properly loaded properly loaded should get too hot to touch.

LAST get a competent electrician. Yes you can do it yourself yes you can figure it out. BUT at what cost if you screw up??

Dan Bentler

Hey,
Thanks for the tips people. I have 4 big wires coming into my service box (main electrical entrance) and a smaller wire attatched to the ground bus bar. I will label and record my measurements for all the wire pairs. I'll get back this afternoon/evening with my results. Thanks again.