Difference between single phase 2 hot 240 and 1 hot 240

[Bigrick31]

Lets say I have an air conditioner or a lighting ballast and both say they will operate at 240v. Normally in a residence you would just run 2 hot wires (1a phase and 1b phase) But what if instead of 2 hots you used a step up transformer to turn a single 120v circuit into a 240v circuit so instead of 2 hot wires you have 1 hot and a neutral? Do both scenarios work the same and if not what is the difference between the two (besides a hot wire and a neutral wire )

 

[hurk27]

Lets say I have an air conditioner or a lighting ballast and both say they will operate at 240v. Normally in a residence you would just run 2 hot wires (1a phase and 1b phase) But what if instead of 2 hots you used a step up transformer to turn a single 120v circuit into a 240v circuit so instead of 2 hot wires you have 1 hot and a neutral? Do both scenarios work the same and if not what is the difference between the two (besides a hot wire and a neutral wire )

Nothing except the amperage of the 120 volts will be twice the amperage of the 240 volts, a 25 amp 240 volt central AC unit will draw 50 amps on the 120 volt side of the transformer.
had a home owner once try to get 240 this way from his 120 volt service so he could put in central air, but what he didn't realize he only had a 30 amp service, he hooked the transformer to the service conductors coming in from the meter, after about a day that service drop was laying on the ground nice and crisp, also transformers are isolating, so grounding and bonding will have to take place on the secondary side, or there will be no fault path.

 

[iwire]

Lets say I have an air conditioner or a lighting ballast and both say they will operate at 240v. Normally in a residence you would just run 2 hot wires (1a phase and 1b phase) But what if instead of 2 hots you used a step up transformer to turn a single 120v circuit into a 240v circuit so instead of 2 hot wires you have 1 hot and a neutral?

You left a lot out but if the transformer you are talking about has just two primary connections at 120 volts and two secondary connections at 240 volts you would never get a 'neutral' on the secondary side.

What you would have is 240 line to line and nothing line to ground.

Most likely the NEC would require this transformers output to be grounded so you would take one of the secondary conductors and bond it to ground creating a 'grounded conductor' that would have to be white but it would not be a 'neutral'.



Now you would still have 240 line to line but now you would have 240 line to ground as well.

Is this all just a thinking thing or are you thinking about doing this?

As Hurk pointed out that will mean double the current on the 120 line.



Do both scenarios work the same and if not what is the difference between the two (besides a hot wire and a neutral wire )[/QUOTE]

 

[gar]

110305-1005 EST

Bigrick31:

I going to simplify your question and work back to your question as stated.

Consider a 240 ohm power resistor center tapped in the middle at the 120 ohm point.

For a 120-0-120 single phase source the transformer center tap will be grounded and also connected to a wire called neutral. This provides two output voltages relative to ground or neutral. Each is 120 V, but they are 180 degrees out of phase with respect to each other.

Connect the 240 ohm resistor across the two hot leads. This puts 240 volts across the 240 ohm resistor and 1 A flows thru the resistor. What is the voltage difference between the resistor center tap and the transformer center tap?

Next disconnect the transformer center tap from ground, and connect a 100 V battery between the transformer center tap and ground. What is the voltage between the resistor center tap and ground? What is the voltage difference between the resistor center tap and the transformer center tap?

Disconnect the battery, leave nothing connected to either the transformer center tap or the resistor center tap. Now connect one end of the 240 V supply to the neutral wire and ground. What is the voltage from ground or neutral to the transformer center tap, or to the resistor center tap?

During all of these gyrations has the current thru the resistor changed?

Now consider your airconditioner.

Suppose it has a 4 wire plug with EGC, neutral, and two hot wires for use in a single phase application with 120-0-120 as the supply. If internally the neutral wire is not used, meaning no current flows thru this wire under any normal operating conditions, and that the EGC only serves its intended purpose, then this airconditioner could be connected to a 240-0 power source.

On the other hand if the airconditioner were designed with some 120 V circuitry connected between one hot wire and neutral, then you could not run this from a 240-0 supply without the addition of a 120 source.

An isolated lighting ballast would only be a two terminal device and could be operated across either a 120-0-120 or 240-0 supply.

.

 

[Bigrick31]

Thanks for the replys everyone! This is a hypothetical situation using a center tap transformer to turn 3 wire (4 if you count the ground) 120/240 into 3 wire 240/480. Should have been more specific sorry about that. So if I understand correctly both ways of wiring a single phase 240v motor or ballast will work unless either one of them have something that runs off of 120v in which case they need the two 120v legs so that they have both 240v and 120v available?

Another question again hypothetical If I had say a European 240v appliance that worked on both 50hz and 60hz would there be an easy way to convert a single US 120v 60hz circuit to a 240v 60hz circuit without using 2 phases of power IE only A phase and a neutral . This would be a case were maybe you only had 1 available circuit in your panel and you need 240v for something and your trying to obtain 240v without somehow freeing up another circuit on the opposite phase.

 

[mivey]

Thanks for the replys everyone! This is a hypothetical situation using a center tap transformer to turn 3 wire (4 if you count the ground) 120/240 into 3 wire 240/480. Should have been more specific sorry about that. So if I understand correctly both ways of wiring a single phase 240v motor or ballast will work unless either one of them have something that runs off of 120v in which case they need the two 120v legs so that they have both 240v and 120v available?

yes

Another question again hypothetical If I had say a European 240v appliance that worked on both 50hz and 60hz would there be an easy way to convert a single US 120v 60hz circuit to a 240v 60hz circuit without using 2 phases of power IE only A phase and a neutral . This would be a case were maybe you only had 1 available circuit in your panel and you need 240v for something and your trying to obtain 240v without somehow freeing up another circuit on the opposite phase.

A 120 to 240 volt transformer would do it.

The neutral is a conductor just like a "hot" is. The only thing really special about a neutral is that it is usually tied to ground.

 

[LarryFine]

This is a hypothetical situation using a center tap transformer to turn 3 wire (4 if you count the ground) 120/240 into 3 wire 240/480.

You don't need (or want) three primary wires for a single-phase transformer primary, even if it has two windings or a center tap. A 240-to-240/480 (or 480 w/ center tap) transformer will work.

So if I understand correctly both ways of wiring a single phase 240v motor or ballast will work unless either one of them have something that runs off of 120v in which case they need the two 120v legs so that they have both 240v and 120v available?

What matters to most loads is the voltage between conductors, not the voltage to ground from any of them. If a load requires 240v between lines and 120v between either and the neutral, either a grounded center tap or a grounded line will work.

The big issue with the original question was about using an EGC as a line conductor. That has been asked many times, and the answer is always the same. There's more to proper wiring than fooling an inspector and/or an receptacle tester.

If I had say a European 240v appliance that worked on both 50hz and 60hz would there be an easy way to convert a single US 120v 60hz circuit to a 240v 60hz circuit without using 2 phases of power IE only A phase and a neutral.

Absolutely. A 120-to-240 (or a correctly-connected 120/240-to-120/240) transformer will work, or even an auto-transformer, if you could find one with the correct connections. Remember that the primary current will be double the load current.

In theory, you could even use the primary only of a 120/240v transformer as an auto-transfornmer, but I don't know how well it would work, or whether it would be within any operation restrictions set by the manufacturer.

 

[mivey]

In theory, you could even use the primary only of a 120/240v transformer as an auto-transfornmer, but I don't know how well it would work, or whether it would be within any operation restrictions set by the manufacturer.

Hmmm. Never thought of that.

 

[dbuckley]

An issue you need to be wary of is phase to ground voltage.

Say you have a "thing" designed to run on the standard resi 120V supply with 240V between the hots, the phase to ground voltage is 120V. Put the 120 to 240 transfomer in there, ground one side, you still have a 240V supply to the thing, but the phase to ground voltage is now 240V rather than 120V.

This may exceed the design parameters (or the listing) of the thing.

 

[LarryFine]

Say you have a "thing" designed to run on the standard resi 120V supply with 240V between the hots, the phase to ground voltage is 120V. Put the 120 to 240 transfomer in there, ground one side, you still have a 240V supply to the thing, but the phase to ground voltage is now 240V rather than 120V.

All the more reason to use a CT-equipped secondary and ground the CT. If the line-side grounded terminal is brought through, as with an auto-transformer setup, there are no bonding issues, either.

Added: For that matter, a 1:1 transformer could be used as a BB with the secondary in series with the source (properly phased, of course) to convert the 120v to 240v, and the grounded conductor still brought through to the load to provide 120/240v.