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Generators & Electric Motors General Discussion

Transformer Question 3pahse to Single Phase


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  #1  
Old 10-30-2006, 08:07:05 PM
richar22
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Default Transformer Question 3pahse to Single Phase

This might sound like a stupid question but I thought I would ask. I have an Onan 12.5JC, rated 12.5kw 3 phase and 8.6kw Single Phase. Is it possible to use a transformer so that the generator can be set in 3phase for the full 12.5kw rating, and get 240v single phase for my home use. In other words can a transforme be used to convert 3phase to single phase without a power loss. Again I know this is probally not possible but thought I would ask. Thanks Richard
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Old 10-30-2006, 08:37:54 PM
Jim Rankin Jim Rankin is offline
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Default Re: Transformer Question 3pahse to Single Phase

I could be wrong, but I don't believe it's possible to improve on reconnecting the generator end. The transformer would add losses and have the same problems with asymetric loading of the coils inside.

If you couldn't reconnect, then a 208 to 240 transformer or buck and boost transformers might make sense. One of our magneciter Onans is a 120/208 wye and backs up a 240 delta 3 phase 4 wire service. To match the voltages, we bought a transformer, but then we found that the manufacturer would not warranty it if there was more than 5% single phase loading on it. Ended up running all the single phase directly off the generator and all the 3 phase off the transformer. Two transfer switches instead of one, a transformer and a rats nest of wiring, in other words a big mess.
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Old 10-30-2006, 11:36:12 PM
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Default Re: Transformer Question 3pahse to Single Phase

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Originally Posted by richar22 View Post
...In other words can a transformer be used to convert 3phase to single phase without a power loss.
In a word: No.

Transformers allow one voltage to be stepped up or down to another voltage. They have no capacity whatsoever to change single phase to 3 phase or vice versa. None at all. Forget it...
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Old 10-31-2006, 08:08:44 AM
armandh armandh is offline
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Default Re: Transformer Question 3pahse to Single Phase

3 phase is 3 seperate supplies that are offset in time from each other.
the successive peaks in voltage [and current] provide greater effeciency turning motors.
using windings that have an extra tap will boost 208 up to 240 but a third of the gen goes unused. ok for stoves and driers providing the same rms power
far better in a 208 3 phase system is to break the load in to 3 parts.
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Old 10-31-2006, 09:38:24 AM
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Default Re: Transformer Question 3pahse to Single Phase

That answers the question. As I understand it can be used to adjust supplied voltage but not to change the phase. Just something I always wondered because I thought that was what utility co was using of the pole to the home. Thanks Richard
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Old 10-31-2006, 01:17:24 PM
armandh armandh is offline
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Default Re: Transformer Question 3phase to Single Phase

that brings the 7200 volt down to 120/240. only on poles where you see 3 or 4 wires on big insulators is 3 phase available. the usual residential is single phase. 3 phase can be transformed down with 3 separate pole transformers or a single casing with 3 inside. if there are 3 HV lines going to a pole transformer and 4 heavy wires coming out to feed a building it is probably 208Y [117 from each leg to the center] if there are 3 hv lines going in and just 3 wires out and one is grounded probably 240 delta. [most common in my area.
if there are 3 transformers and one has a grounded center tap could be 240 delta where two legs are 120 to gnd with one wild phase. lots of other configurations. 240/380Y etc
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Old 10-31-2006, 07:42:00 PM
Mike Dennis Mike Dennis is offline
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Default Re: Transformer Question 3phase to Single Phase

Quote:
Originally Posted by armandh View Post
if there are 3 HV lines going to a pole transformer and 4 heavy wires coming out to feed a building it is probably 208Y [117 from each leg to the center]
Actually 120/208. The relationship is the square root of 3 (or 1.73). As in 120 X 1.73 = 208. With a multiple transformer bank, this system can be ID by EACH transformer having ONE of the secondary terminals connected to the neutral. Each leg will measure 120 volts to ground or 208 volts from any line to any other line.

Quote:
if there are 3 hv lines going in and just 3 wires out and one is grounded probably 240 delta.
This one commonly called "grounded corner delta". No 120 volt service is possible from this configuration. From any leg to any other leg is 240 volts. Commonly, one of those legs is at ground potential - but not always.

Quote:
if there are 3 transformers and one has a grounded center tap could be 240 delta where two legs are 120 to gnd with one wild phase.
Three transformers on the pole (sometimes only two for OPEN DELTA) with the center secondary lug of ONE (usally the larger) transformer connected to neutral and its other two secondary lugs connected to the other two transformer secondarys and another lead between those two transformers (the "wild" leg) is in fact delta. Between any two legs will be 240. Between each of two of the legs to neutral will be 120 volts. The third, or wild, leg will then be 208 to neutral.

Of course, there are other variations such as 480/277 etc but those are for larger installations.

To the original question, no, a transformer can not change between one and three phase. It TRANSFORMS or changes voltages and currents. A 480 to 120/240 transformer will yield the lower voltages. For every amp on the 480 side, you get almost 2 amps of power out the low side. (Transformer losses eat a little of it.) Of course, it can work the other way too; 120 v in, 480 volts out - at one fourth the amps.

Motors work better on 3 phase. A single phase motor must create a "phase shift" in order to start so single phase motors are a bit more complicated.
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Old 10-31-2006, 09:43:44 PM
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Bill Geyer Bill Geyer is offline
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Default Re: Transformer Question 3pahse to Single Phase

Hello Richard and all,

We had a device at work, a "phase converter". It may not have been a transformer, but it looked like one, windings and laminations. I think it said something like phase converter transformer, but really don't remember. It allowed us to run the shop lathe which had 3 phase motor from single phase line. Would it convert the other way?
Even if it did, it was very inefficient, used a lot of amps, and the 3 phase motor on the lathe was de-rated some in horse power. More loss than using one leg of the 3 phase genset of Richard's. Perhaps that is what Richard was thinking about.

Bill
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Old 11-01-2006, 01:29:33 AM
armandh armandh is offline
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Default Re: Transformer Question 3phase to Single Phase

today rotary converters and 3 phase output inverters will do the one to three conversion with loss. 3 to one could be done with differently designed equipment but given the loss just forget the third phase. the best bet with a gen that is 208/120 is dividing the 120 volt load in to 3 parts. splitting the 240 volt load in to 3 parts if possible and some transformers to boost up the voltage if needed. the only big bugaboo is the AC and I would get 3 120 volt window units for emergency use.
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Old 11-01-2006, 01:35:45 AM
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Default Re: Transformer Question 3pahse to Single Phase

ps the phase converter....
a load through a cap has the current laging the voltage [time to charge]
it will fool the motor by shifting the other 2 phases but not enough to run efficiently this will not work in reverse.
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Old 11-01-2006, 09:14:08 AM
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Default Re: Transformer Question 3phase to Single Phase

dividing the load
if you have a sub panel or install one in your lighting distribution put a separate transfer switch on the sub panel and feed both legs from the third phase of the 208/120.
caution
be sure none of the feeds going out of the sub panel are split 240
where a black/red/white feed two circuits with the common white as a return. this could overload the white with both the red and black on one phase. [where the load is balanced there is little current in the white wire,] here the load is series between the 240, an open white here will cause typical loose neutral problems but only on those two circuits [as opposed to whole house problems]
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Old 11-01-2006, 01:33:25 PM
Fred M. Fred M. is offline
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Default Re: Transformer Question 3pahse to Single Phase

How practical would it be to modify a 3-phase generator to produce 1-phase?

Would the resulting generator only put out as much as one of the original 3 phases, or would there be an advantage?

Fred
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Old 11-01-2006, 09:48:17 PM
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Default Re: Transformer Question 3pahse to Single Phase

Not entirely sure what you mean by 'modify'. But I assume you mean to disassemble the generator, and rewind the stator in a single phase configuration.

In my book, this falls under the chapter of "impractical".

In the first place, your 3 phase machine is good for at least 2/3 of it's rating when operating in single phase (i.e your 15 kW machine will make at least 10 kW single phase). So there's not a heck of a lot to gain. In the second place, since you're asking about this, I'd guess you won't be doing the rewinding work yourself. Last I checked, motor rewind shops don't work for cheap, so unless you're talking about a really large machine it'll probably be cheaper to buy a new single phase head.
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Old 11-01-2006, 10:57:08 PM
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Default Re: Transformer Question 3pahse to Single Phase

Almost all AC compressors will run just fine on 208. About 200 volts is the point of no return. The correct range is always on the data plate of the compressor; not necessarily on the condensing unit.
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Old 11-01-2006, 11:25:23 PM
Fred M. Fred M. is offline
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Default Re: Transformer Question 3pahse to Single Phase

Quote:
Originally Posted by Jim McIntyre View Post
Not entirely sure what you mean by 'modify'. But I assume you mean to disassemble the generator, and rewind the stator in a single phase configuration.
Jim-

I'm a bit ignorant about how these things are constructed.

I was under the impression that in a 3-phase generator, there were three separate stator windings spaced 120 degrees apart, and that you could unbolt their associated pole pieces in order to replace, remove or move them. If that were true it might be a relatively simple matter to remove one and move another so the remaining two were 180 degrees apart.

As you can tell I've never had one apart!

Fred
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Old 11-02-2006, 08:28:37 AM
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Default Re: Transformer Question 3phase to Single Phase

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Originally Posted by Fred M. View Post
Jim-

As you can tell I've never had one apart!

Fred
A 3 phase generator's stationary stator is much like a 3 phase motor with overlapping windings all around. the difference is that in place of a Cu ladened rotating part the generator has a rotating field coil. this gets its excitation power from slip-rings and brushes or [in a brush-less design] 3 diodes connected to a rotating excitation stator elsewhere on the same shaft turning in a stationary field.
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Old 11-02-2006, 08:32:36 AM
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Default Re: Transformer Question 3phase to Single Phase

PS they may be shown like 3 poles in a diagram but they are not built like that since the beginning of the last century
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Old 12-02-2006, 10:30:25 AM
Phillip Hutchinson Phillip Hutchinson is offline
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Default Re: Transformer Question 3pahse to Single Phase

just sitting here,1-25 am sunday morning (cant sleep) reading some old posts,someone says here you cant convert 3 phase to single phase,well i pose a question what about a 3 phase welder not a mig coz it has diodes and uses 3 phases but an ac arc welder it only has an earth and an electrode wire,any ideas
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Old 12-02-2006, 10:55:33 AM
Jim Rankin Jim Rankin is offline
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Default Re: Transformer Question 3pahse to Single Phase

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Originally Posted by Phillip Hutchinson View Post
just sitting here,1-25 am sunday morning (cant sleep) reading some old posts,someone says here you cant convert 3 phase to single phase,well i pose a question what about a 3 phase welder not a mig coz it has diodes and uses 3 phases but an ac arc welder it only has an earth and an electrode wire,any ideas
to start with, most big old 3 phase supplies are rectified to DC or maybe are AC/DC like you say. The 3 phase creates a smoother DC with less ripple when it's rectified. With the AC welder, generally, the higher the frequency-the smoother the arc/the better the weld. So now you can get high frequency and other waveform options on some of the newer advanced welders. The electrical supply transformer on the other hand has to maintain the input frequency and wave form on the output AC wave.

I have an old 400 amp Lincoln Ideal Arc AC only welding supply and it is single phase input. I got a big Hobart DC supply about the same age (40 years old) and it is 3 phase.

I do know if it could be done, it would be used everywhere to make it easier to balance single phase loads on a 3 phase supply.
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Old 12-02-2006, 11:38:16 AM
Fred M. Fred M. is offline
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Default Re: Transformer Question 3pahse to Single Phase

Quote:
Originally Posted by Jim Rankin View Post
I do know if it could be done, it would be used everywhere to make it easier to balance single phase loads on a 3 phase supply.
Jim-

It isn't exactly a simple transformer, but there are at least three companies producing generators with inverters. They generate 3 phase, rectify and electronicly generate single phase power.

I have a Yamaha and a small Honda that work that way. It is impressive how rock-solid the voltage and frequency are, regardless of load. Even with a temporary overload, the output level doesn't budge.

It may be the use of a 3 phase generator that makes them lighter than non-inverters for the same power. The 65 pound Yamaha has the same continuous power rating as a 95 pound Powermate. The Yamaha can support a temporary overload while the Powermate barely makes it to rated power.

If it wasn't for a significant price difference, there would probably be many more on the market.

Fred
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