Can simple transformers be used in both "step-up" and "step-down&

[jfriddell]

I am strictly dealing with 1-phase electricity. I have an application where I have a 120 VAC only output generator (2000 W) and I wish to run a very small load (~ 400 W) which requires 240 VAC. Can a 240 VAC primary/120 VAC secondary transformer be used to "step-up" 120 VAC to 240 VAC? If this is true, then would you simply bring the 120 VAC neutral input around the transformer to serve as the neutral on the 240 VAC side of the transformer? Thanks for any help.

 

[ScottyUK]

Hi jfriddell,

Yes, in principle any plain transformer works equally well in either direction. So if you feed 120V in, you will get 240V out.

For this load the simplest thing to use would be an auto-transformer which is a special type of transformer which has only one winding but can step up or step down voltage. A 400VA auto-transformer would be quite inexpensive and certainly smaller and cheaper than a double winding (isolating) type. The neutral would then be common to both the 240V and 120V systems.

This link shows the principle:

http://en.wikipedia.org/wiki/Autotransformer

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I don't suffer from insanity. I enjoy it...

 

[waross]

Yes and no or maybe.
Does your 240 volt winding have a neutral? If yes then connect it to the 120 volt neutral and to ground.
If the 240 volt winding doesn't have a neutral then you don't have a neutral on the 240 volt system. It will be prudent to ground all exposed non-current-carrying parts.
An alternative. If your 240 volt winding has a neutral, run it as an autotransformer. Connect the neutral of the generator to the neutral of the 240 volt winding. Connect 120 from the generator to one of the hot leads of the 240 volt winding. The other hot lead of the 240 volt winding will produce 240 volts.
Ignore the 120 volt winding on the transformer. (Tape the leads)
yours

 

[ScottyUK]

Hi waross,

Are you talking about centre-tapped 240V supplies which would be described more properly as 120/0/120V? Yes, that would cause some confusion! More information is required about the 240V arrangement.

If the 240V winding is a two-wire type and the transformer is an isolating type then it doesn't matter which terminal is grounded and called 'neutral'. They are interchangeable. It would be sensible to keep the phasing so that the maximum voltage present is 240V rather than having the 120V primary in antiphase with the 240V secondary and creating a 360V shock hazard.

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I don't suffer from insanity. I enjoy it...

 

[davidbeach]

Scotty, as I read the OP, he is looking for a 120/240V 1-phase system (using the north American nomenclature). If that is in fact true, the answer to the OP's question is an emphatic NO. Going from 120 to 240 would work, but not to get 120/240 out. An autotransformer would not work, at least in the NEC realm, because the secondary (high side) neutral would have to be electrically connected to the primary (low side) neutral, but the primary neutral is not at the mid-point of a 120V winding.

If the OP needs 240V, the transformer described could be used, best to ground one leg coming out of the transformer.

If the OP needs 120/240V, he needs a different, two winding, transformer.

 

[waross]

Hello Scotty and David.
Scotty, I understand that in the UK the grounded conductor is called the neutral. Is this correct? In North America the wire which carries the unbalanced currrent from a 120/0/120 is called the neutral. I thought it was a definition until I embarassed myself in discussions with a UK engineer a decade or so ago when installing some equipment out of Ireland.
David, re the grounding. I think it can go either way if the voltage to ground will be over 150 volts. A GFCI (Ground Fault Circuit Interupter) is probably well advised whatever the grounding convention. Re the autotransformer. I don't see aproblem feeding one line and the center tap of a 120/0/120 transformer and grounding the center point instead of one of the end points. Is this a code issue and if so would the code apply if the voltage available in the event of an open winding would be equal or less than the applied voltage?
respectfully

 

[ScottyUK]

Hi waross,

Yes, the neutral in UK parlance is the current-carrying conductor which is bonded to the system earth. Normally this occurs at one, and only one, location. The exception to this rule is the technique employed by the utilities which is protective multiple earthing or PME where a distributed earthing system is employed alnog a distribution feeder.


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I don't suffer from insanity. I enjoy it...

 

[waross]

Hi Scotty.
We would call that the grounded circuit conductor, and it is invariably also the the center point or 0 in a 120/0/120, or the common point of a star system (neutral in our parlance. The exception is a corner grounded 3 wire delta system which has no common wire.
What do you call the conductor which carries the unbalance in the UK?
respectfully

 

[jfriddell]

Electrical Professionals:

As I read the posts, I see there is some discussion about my actual voltage needs. My 240 VAC load is a three wire system of two small motors. My 120 VAC supply is a simple, off the shelf gasoline powered generator made by Honda, Colemand, DeWalt, etc. with standard North America 120 VAC outlet receptacles.

In this case, is it acceptable to have the 120 VAC side neutral and the 240 VAC side neutral be common to each other?

Thank you all for a good, enlightening discussion.

 

[waross]

I understand that your transformer has a neutral connection in the 240 volt winding. If so the answer is yes.
If your transformer has only a 240 volt winding with no center tap availablble, wait for davidbeaches next post. I advise buying a 240 volt GFCI and installing it in the 240 volt circuit whatever way you connect the neutral.
yours

 

[oldmanpower]

In looking at your original post you don't state that you have a 120/240 transformer. If you do, hopefully it has a three wires on the 240 volt side. Simply connect the secondary center tap to ground to create a seperately derived system. If it does not have three secondary wires connect one side of the transformer to the hot lead of the primary side (phasing is important) to create a second phase. If you cannot figure out which way to phase the transformer then just hook it up one way and check the voltages with a meter. You should measure 120 volts from neutral to the new phase. If you measure 360 volts reverse the wires. Alternatively if you do not have a transformer. you can use 120/120 volt transformer. Hook it up as describe above and check the voltages as describe above. You will measure either 0 or 240 volts between the two hot legs. These two solutions are autotransformer schemes.

 

[CJCPE]

jfriddell:

You must determine if your load requires a neutral connection or just a ground. Also make certain that you have single phase motors. Most single phase 240 volt loads require only 2-wire 240v, not 3-wire 120v - 0/N - 120v. The only reason a load would need a neutral connection as opposed to a ground is if there are some parts of the load that require 120v or if the load voltage is cut in half for some operating mode. Single phase motors likely wouldn't require a neutral, but there may be 120v motor starter coils or pilot lights.

If you need a neutral, you need a transformer with a tap connection to create the 120-0-120. If you just need a ground, use the ground of the 120v circuit.

 

[waross]

Can you describe your transformer for us. That will eliminate a lot of speculation on our part and we can probably give you a better answer.
yours

 

[ScottyUK]

Hi waross,

Somewhat off-thread:

UK low voltage power, and also Europe so far as I'm aware, uses a four-wire distribution system comprising three phases connected in star with the neutral conductor being connected to the star point. The star point is solidly earthed. Single phase loads are connected line-neutral, three phase loads may be connected line-line or line-neutral depending on whether they're star or delta connected. The only imbalance current which could exist would be due to imperfect balancing of single phase loads, and this imbalance current flows in the neutral back to the distribution transformer. There are only two basic voltage levels: 230V (nominal) which exists line-neutral, and 400V (nominal) which exists line-line.

We have no equivalent to the corner-grounded delta or the centre-tapped 120/0/120V systems over here.


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I don't suffer from insanity. I enjoy it...

 

[jfriddell]

Professionals:

I do not have a transformer yet. I wanted to research before proceeding with the project. In all likely hood, the supply will be the 120 VAC generator as described and the load will be a 240 VAC motor circuit. Since my load is not large, only 400 W, I wanted to see if I could use the smaller generators, which commonly do not have 240 VAC outputs, thereby saving the customer from purchasing larger than needed equipment soley for the sake of having 240 VAC generator output.

I hope this helps. The responses to this thread have been much more than I ever expected and I thank all who have contributed. Thank you!

 

[waross]

Hello jfriddell
The transformer should be no problem. As you have seen there are several good ways to connect the transformer.
One warning, the generator has to be oversized. Generator sizing should be three times the largest motor plus all other loads.
If the motors must start simultaneously then three times both motors.
good luck
yours