Buck Boost / Step-up Transformer European vrs. USA standard question

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mark.richardson

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Salt Lake City Utah
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HVAC program director
Where can I find a 115v to 230v step-up or buck boost transformer that is USA standard with two 115 hot legs and not one leg 230v and the other leg common.
This is for students learning how to troubleshoot and even though it would work on the motors it would mess up the students when reading voltages working in USA vrs other parts of the world. (make sense?)
I've looked everywhere and can't find anyone who sells the USA standard 230v output like the school needs.
Thanks in advance for your help.
Mark
 
If you only have a 20A 120V supply circuit, you can't get a 20A 240V circuit.

What exactly are you trying to accomplish? Running a 240V condenser unit with a 120V supply?

-Jon
 
retirede said:
https://www.platt.com/p/0078067/acm...-120-x-240-120-240vac-1ph-nema-3r/acmt353041s

None in stock here, but they do exist.
Click to expand...
Mr. Retirede
Can I ask a question.
This will show my ignorance about buck boost transformers.
If I want to use a transformer primary 120v secondary 240v to run a 20 amp load which RLA/FLA are likely about 13 amps. Would a 5000Watt or 5kva transformer do the job? Considering the LRA that would spike for a split second before going down to FLA.

Also, if the primary amperage is typically twice the secondary amperage would I need to make sure the circuit breaker for the 120 primary is a 40 amp breaker since the secondary 240v would be 20 amps?

The last question is, will the buck boost transformer secondary have two hot legs of 120 volts each or will it have one hot leg of 240 v and the other leg neutral/common. I've noticed that the only step-up transformers I can find are with secondary of one hot leg 240v and the other common.
I need L1 and L2 to be 120v each for the 240volts.

Such is my lack of knowledge with buck boost transformers.
Thanks in advance.
Marik
winnie said:
If you only have a 20A 120V supply circuit, you can't get a 20A 240V circuit.

What exactly are you trying to accomplish? Running a 240V condenser unit with a 120V supply?

-Jon
Click to expand...

winnie said:
If you only have a 20A 120V supply circuit, you can't get a 20A 240V circuit.

What exactly are you trying to accomplish? Running a 240V condenser unit with a 120V supply?

-Jon
Click to expand...
Thanks for responding.
You guessed it.
I'm trying to run a portable A/C unit.
I'm putting together a roll around A/C unit that I can roll from site to site, classroom to classroom which makes it hard to find 230v power since most rooms only have 120v receptacles.

I'm guessing the circuit breaker for the 120v primary would have to be a 40amp circuit breaker to make the 240v secondary 20amp of the transformer work. Is that correct?

Is it also correct that for a 20amp secondary I would need about a 5kva transformer?
And would this transformer handle the LRA if the FLA/RLA were around 13 amps?

My other question is, would the 240 secondary have L1 and L2 each having 120v or would one leg have 240v and the other be common?
I need the USA standard of two hot legs.

I'm afraid this will be too expensive and I would be better off to find a 230 power supply.
This is for teaching purposes for HVAC students and as you can guess 230 volts isn't easy to find in a classroom.

I want your feedback even if it makes me look like an idiot. :)
Thanks
Mark
 
If the classroom doesn't have 240v available it is highly unlikely you will find a 120v circuit adequate for your transformer.
 
There are tricks you could play to make a transformer with a nominal 2.5 KVA rating do the job. And if the transformer has a 120/240V secondary you could wire it with an appropriate center tap as neutral. But you should ignore those details because no easy transformer setup will do what you need which is generate the necessary current at 240V from an ordinary 120V receptacle.

Your probably better off moving the students to a single classroom with the equipment.

If you can find small scale 240V demo equipment (RLA of 6A @240V or less), you might make what you are envisioning work.

-Jon
 
mark.richardson said:
Mr. Retirede
Can I ask a question.
This will show my ignorance about buck boost transformers.
If I want to use a transformer primary 120v secondary 240v to run a 20 amp load which RLA/FLA are likely about 13 amps. Would a 5000Watt or 5kva transformer do the job? Considering the LRA that would spike for a split second before going down to FLA.

Also, if the primary amperage is typically twice the secondary amperage would I need to make sure the circuit breaker for the 120 primary is a 40 amp breaker since the secondary 240v would be 20 amps?

The last question is, will the buck boost transformer secondary have two hot legs of 120 volts each or will it have one hot leg of 240 v and the other leg neutral/common. I've noticed that the only step-up transformers I can find are with secondary of one hot leg 240v and the other common.
I need L1 and L2 to be 120v each for the 240volts.

Such is my lack of knowledge with buck boost transformers.
Thanks in advance.
Marik



Thanks for responding.
You guessed it.
I'm trying to run a portable A/C unit.
I'm putting together a roll around A/C unit that I can roll from site to site, classroom to classroom which makes it hard to find 230v power since most rooms only have 120v receptacles.

I'm guessing the circuit breaker for the 120v primary would have to be a 40amp circuit breaker to make the 240v secondary 20amp of the transformer work. Is that correct?

Is it also correct that for a 20amp secondary I would need about a 5kva transformer?
And would this transformer handle the LRA if the FLA/RLA were around 13 amps?

My other question is, would the 240 secondary have L1 and L2 each having 120v or would one leg have 240v and the other be common?
I need the USA standard of two hot legs.

I'm afraid this will be too expensive and I would be better off to find a 230 power supply.
This is for teaching purposes for HVAC students and as you can guess 230 volts isn't easy to find in a classroom.

I want your feedback even if it makes me look like an idiot. :)
Thanks
Mark
Click to expand...
A few comments:. First, terms. "Buck/boost" usually refers to an autotransformer. This is opposed to a conventional or isolation transformer. An auto transformer does not provide isolation, so whatever system grounding arrangement the primary has will also exist on the secondary. With an isolation transformer,. The secondary will have no ground reference until you give it one by grounding a conductor. This if you want 240v with one "side" grounded, you would want an isolation transformer.
 
winnie said:
There are tricks you could play to make a transformer with a nominal 2.5 KVA rating do the job. And if the transformer has a 120/240V secondary you could wire it with an appropriate center tap as neutral. But you should ignore those details because no easy transformer setup will do what you need which is generate the necessary current at 240V from an ordinary 120V receptacle.

Your probably better off moving the students to a single classroom with the equipment.

If you can find small scale 240V demo equipment (RLA of 6A @240V or less), you might make what you are envisioning work.

-Jon
Click to expand...
Thank you. Fortunately our shop has the necessary 230volts But I'm afraid you and others are correct, it's not going to work the way I was hoping with the classrooms.
Thanks to you and all the others to clear this up for me.
Mark
 
230327-1632 EDT

mark:

In the United States many user electrical systems are supplied from a 240 V center tapped transformer that provides 120 V on either side of the center tap ( neutral in this case ). Ones supplied from a 3 phase source many provide two 120 V sources relative to neutral ( which is really not a neutral when only 2 phases are used ). And line-to-line voltage will be 208 V.

If your classroom has more than 1 circuit, then there is the possiblity that two circuits could be out of phase. Thus, either 240 or 208 would be available in the room. Therefore, two extension cords could give you the power you need with no transformer. This would be ideal because it would eliminate the internal impedance of a transformer.

It is also possible that an outlet in the hall or another room could provide the other phase.

Suppose you can not easily find two different phase sources, then can you find two separate 20 A sources? If so, then use a 1 to 1 isolation transformer to obtain your other phase. You can also use an auto transformer, but they are harder to find.

In any event if you use a transformer it should be oversized to reduce internal impedance.

.
 
Using two circuits as @gar describes is a bit of a hack, and I'd avoid demonstrating such a practice to students.

It also runs significant risks if the circuits are not wired properly.

There are devices that combine circuits in this fashion. Most are cheap unlisted Y cords with no protection at all. Some are fancier and are supposed to provide the necessary protection to make this connection safely, but I know of none that are fully UL listed.

On the other hand, it is likely that you already have multiple 120V circuits from different phases in each classroom. It might be inexpensive for an electrician to convert some of these to have proper 120/240V receptacles.

Jon
 
Correction: this product claims to meet UL standards and to be tested by a recognized agency.

I would be very leery, and investigate carefully with your school's safety/ compliance officer. But if this product does what it claims, you could likely run a 13A 240 (well, probably 208V in a school) system from two 120V 20A circuits.

quick220.com

Model A220-20L 20 Amp Power Supply Twist Locking Outlet

Portable, easy to use. A220-20L voltage converter using two 72-inch power cords with straight-blade, outlets for US/Canadian 220-240 volt straight blade (A220-20D), and locking plugs (A220-20L).
quick220.com
 
winnie said:
Correction: this product claims to meet UL standards and to be tested by a recognized agency.

I would be very leery, and investigate carefully with your school's safety/ compliance officer. But if this product does what it claims, you could likely run a 13A 240 (well, probably 208V in a school) system from two 120V 20A circuits.

quick220.com

Model A220-20L 20 Amp Power Supply Twist Locking Outlet

Portable, easy to use. A220-20L voltage converter using two 72-inch power cords with straight-blade, outlets for US/Canadian 220-240 volt straight blade (A220-20D), and locking plugs (A220-20L).
quick220.com
Click to expand...

It looks to me that this product has circuitry to keep it relatively safe in operation. It appears that it connects the ungrounded conductor on each of two 120V circuits on different phases directly to the output. The input neutral would be used for sensing and control purposes, but not as a return for any of the load current. And so the "hot" conductor in each input cord would be the only wire in that cord conducting significant current. There are several things objectionable about this, but the product might be exploiting a gap that exists between the NEC and UL requirements, since they felt a need to make the following statement:
"The NEC (National Electrical Code) addresses the permanent wiring of buildings and other facilities. In general, if it is in the walls or a permanent part of the facility, NEC defines how to do it safely. If you can plug it in, then UL, CSA, OSHA, and the testing laboratories handle product safety."

By using one or two isolation transformers, an arrangement could be made so that at each outlet the line and neutral currents are the same. It would also allow the power to be drawn from outlets that are on the same phase if desired. But it would cost more and also be larger and heavier.
 
Last edited:
gar said:
230327-1632 EDT

mark:

In the United States many user electrical systems are supplied from a 240 V center tapped transformer that provides 120 V on either side of the center tap ( neutral in this case ). Ones supplied from a 3 phase source many provide two 120 V sources relative to neutral ( which is really not a neutral when only 2 phases are used ). And line-to-line voltage will be 208 V.

If your classroom has more than 1 circuit, then there is the possiblity that two circuits could be out of phase. Thus, either 240 or 208 would be available in the room. Therefore, two extension cords could give you the power you need with no transformer. This would be ideal because it would eliminate the internal impedance of a transformer.

It is also possible that an outlet in the hall or another room could provide the other phase.

Suppose you can not easily find two different phase sources, then can you find two separate 20 A sources? If so, then use a 1 to 1 isolation transformer to obtain your other phase. You can also use an auto transformer, but they are harder to find.

In any event if you use a transformer it should be oversized to reduce internal impedance.

.
Click to expand...
Yes!
Thank you for that reminder. We have in the past done what you've suggested and it's a great solution. I'll have to mark the plugs going forward to recognize which two 120 v receptacles are on different phases. Thats going to have to be our solution for our classrooms.
Thanks again.
Mark
 
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