How to wire up a Buck-Boost Transformer?

[JasonCo]

Have a question about buck-boost transformers. I'm not sure how they work. But I have a question that says a Supply Circuit is from a 2-pole breaker 277/480v, 3-ph, 4-w panelboard. The load is rated to operate at 24 volts. The transformer is a 120 x 240 - 12/24 volt.

Okay so for the Primary side of the transformer, do I wire this up in series or parallel? And same question for the secondary side.

The question asked me which diagram is correct, and there are 4 choices and each one is wired either in parallel or series for the primary and secondary sides. There is only one answer, not sure... I just don't get buck-boost and how to wire them up.. Specially if the 2-pole supply circuit is 480 volt and the transformer is 120 x 240 - 12/24... Any help is greatly appreciated, thanks a bunch for your time!

 

[Smart $]

That's a fudged up question. The truest answer is, back up and punt.

Are you sure you re-wrote that correctly with the load being rated for 24 volts? I would have expected it to be 240 volts. At 24 volts, the best you could do is hook it up as a step-down (isolation) transformer and run a hot 277 volt primary to hot 27.7 volt secondary. In many cases the 24 volt rated load will tolerate the excess voltage... some cases, not so much and let out the smoke.

 

[JasonCo]

The exact question is: Which connection diagram, shown in Figure 224.108, should be used where the Supply Circuit is from a 2-pole breaker 277/480-volt, 3-ph, 4-W panelboard? The load is rated to operate at 24 volts. The transformer is a 120 x 240 -- 12/24 volt.

 

[kwired]

That's a fudged up question. The truest answer is, back up and punt.

Are you sure you re-wrote that correctly with the load being rated for 24 volts? I would have expected it to be 240 volts. At 24 volts, the best you could do is hook it up as a step-down (isolation) transformer and run a hot 277 volt primary to hot 27.7 volt secondary. In many cases the 24 volt rated load will tolerate the excess voltage... some cases, not so much and let out the smoke.

Most cases here the 277 will operate closer to 285 and push the smoke out a little sooner.

Maybe they wanted to connect it to buck 277 by 24 volts in the question and this was misunderstood?

 

[JasonCo]

Oh one thing I didn't really take into consideration but there is a answer choice E. None of these... So I guess that is the answer haha... sorry about that. It was really confusing me too, guess that's got to be the answer

 

[kwired]

Oh one thing I didn't really take into consideration but there is a answer choice E. None of these... So I guess that is the answer haha... sorry about that. It was really confusing me too, guess that's got to be the answer

Sounds like a good possibility based on what you have given us.

 

[JasonCo]

Sense the 2-pole is putting out 480 (240 on each conductor). Couldn't you wire it in parallel to get an even 240 volts across the primary side. And secondary would be series to get the 24 volts. Am I wrong?

Edit: hmm... nvm because each conductor would be 277. Hmmmm

 

[ptonsparky]

Sense the 2-pole is putting out 480 (240 on each conductor). Couldn't you wire it in parallel to get an even 240 volts across the primary side. And secondary would be series to get the 24 volts. Am I wrong?

Edit: hmm... nvm because each conductor would be 277. Hmmmm

Sounds like you need to go back a couple chapters and review.

 

[Smart $]

Sense the 2-pole is putting out 480 (240 on each conductor). Couldn't you wire it in parallel to get an even 240 volts across the primary side. And secondary would be series to get the 24 volts. Am I wrong?

Edit: hmm... nvm because each conductor would be 277. Hmmmm

There is no voltage on each conductor. Voltage is the potential difference between two points (conductors or points thereof).

The two primary windings would be connected in series for a 240 volt primary. The closest you could get to 240 is one wire connected to one pole of the breaker and the other wire connected to neutral... for 277 volts.

If you had two transformers, you could connect the four primary windings in series and put 480 volts across the combined primaries to achieve 240 volts per primary. Question only supplies one transformer.

 

[kwired]

Sense the 2-pole is putting out 480 (240 on each conductor). Couldn't you wire it in parallel to get an even 240 volts across the primary side. And secondary would be series to get the 24 volts. Am I wrong?

Edit: hmm... nvm because each conductor would be 277. Hmmmm

If the neutral is not involved there is no 277 to work with, it is simply 480 volts between the two conductors.

Your only chance of doing much of anything with the equipment you mentioned is if you are actually applying one line to neutral @277 volts and using the 12/24 coils to reduce voltage - but you can only buck it 10% because rating of transformer is 240/24 which that ratio is 10. So if input were 277 that 24 secondary becomes 27.7 so you could buck the 277 down to 249.3. But it is going to be a little hard on the transformer that is only rated for 240 volts but it is only about 15% over voltage so will likely take a long time to smoke it as long as load current is within the rating of the unit.

 

[JasonCo]

There is no voltage on each conductor. Voltage is the potential difference between two points (conductors or points thereof).

The two primary windings would be connected in series for a 240 volt primary. The closest you could get to 240 is one wire connected to one pole of the breaker and the other wire connected to neutral... for 277 volts.

If you had two transformers, you could connect the four primary windings in series and put 480 volts across the combined primaries to achieve 240 volts per primary. Question only supplies one transformer.

Yes sorry I worded that wrong! Meant to say that each conductor to Neutral on the Y system would be 277, and any phase to phase would be 480 volt.

 

[JasonCo]

If the neutral is not involved there is no 277 to work with, it is simply 480 volts between the two conductors.

Your only chance of doing much of anything with the equipment you mentioned is if you are actually applying one line to neutral @277 volts and using the 12/24 coils to reduce voltage - but you can only buck it 10% because rating of transformer is 240/24 which that ratio is 10. So if input were 277 that 24 secondary becomes 27.7 so you could buck the 277 down to 249.3. But it is going to be a little hard on the transformer that is only rated for 240 volts but it is only about 15% over voltage so will likely take a long time to smoke it as long as load current is within the rating of the unit.

Okay I see how you did that, so it is possible to buck a transformer from 277v down to 249.3v on the primary side? Is that part of how a buck-boost transformer works?

24 x 277 / 240 = 27.7
277 - 27.7 = 249.3

PS: I don't want to get off topic but this might better help me understand. If I have a load that is 240v at 150 amps, and the circuit available is 208 volts. The rating for a Buck Boost transformer that can operate the load at its rating voltage is how many KVA?
I'm just confused about all this.. How you can have a available circuit of 208 volts but the load is 240v at 150 amps...

 

[kwired]

Okay I see how you did that, so it is possible to buck a transformer from 277v down to 249.3v on the primary side? Is that part of how a buck-boost transformer works?

24 x 277 / 240 = 27.7
277 - 27.7 = 249.3

When used as a buck -boost there isn't really a primary or secondary, you connect the two coils in series to create a single coil with different "taps". I probably was off a little bit on the numbers though. The rating of the transformer is 240 volts on one portion and 24 volts on the other - so there is 10 percent of turns in the 24 volt segment then there is in the 240 volt segment. Place them in series and apply 24+240 = 264 volts across the two ends and you will have 240 across the 240 segment and 24 across the 24 segment. Raise or lower the applied voltage and those values will change in proportion to what you changed the input voltage. So to correct what I said before if you put 277 across something rated for 264 your input is about 105% of rating. Apply 277 across the series coils and voltage across the 240 volt rated segment becomes about 252, and the remainder of about 25 will be across the rest of the coil. Not as much over voltage as I mentioned before.

 

[don_resqcapt19]

...
PS: I don't want to get off topic but this might better help me understand. If I have a load that is 240v at 150 amps, and the circuit available is 208 volts. The rating for a Buck Boost transformer that can operate the load at its rating voltage is how many KVA?
I'm just confused about all this.. How you can have a available circuit of 208 volts but the load is 240v at 150 amps...

They are asking you to use a buck boost transformer to increase the voltage from 208 to 240 and they want to know the required kva of the buck boost transformer.

The kva of the buck boost transformer is based on the load current and the difference between the two voltages. Your actual load kva is much larger than the kva you need for the buck boost transformer. (that should be enough for you to get the correct kva)

 

[iwire]

Guys, maybe I am missing something but I do not believe this thread has anything to do with using a transformer as an auto-transformer (AKA a buck boost)

Supply Circuit is from a 2-pole breaker 277/480v, 3-ph, 4-w panelboard. The load is rated to operate at 24 volts. The transformer is a 120 x 240 - 12/24 volt.

the Supply Circuit is from a 2-pole breaker 277/480-volt, 3-ph, 4-W panelboard? The load is rated to operate at 24 volts. The transformer is a 120 x 240 -- 12/24 volt.

Jason Co,

You may be using a buck boost style transformer but you will not be wiring it as a buck boost.

If I am understanding you correctly you cannot do what you are trying to do with that transformer you have.

You will need a 480 volt primary and a 24 volt secondary.

Many combinations are available, 240/480-24/48 etc. https://www.platt.com/platt-electri...24-48/Acme/T-1-37923/product.aspx?zpid=187071

 

[K8MHZ]

Guys, maybe I am missing something but I do not believe this thread has anything to do with using a transformer as an auto-transformer (AKA a buck boost)






Jason Co,

You may be using a buck boost style transformer but you will not be wiring it as a buck boost.

If I am understanding you correctly you cannot do what you are trying to do with that transformer you have.

You will need a 480 volt primary and a 24 volt secondary.

Many combinations are available, 240/480-24/48 etc. https://www.platt.com/platt-electri...24-48/Acme/T-1-37923/product.aspx?zpid=187071

Would the above be 240 in, 24 out or 480 in, 48 out? I don't see how to go from 480 to 24 with the above. Am I missing something?

 

[iwire]

Would the above be 240 in, 24 out or 480 in, 48 out? I don't see how to go from 480 to 24 with the above. Am I missing something?

Typically they have split primary and secondary wingdings so you can do 480 or 240 in with 24 or 48 out.

 

[K8MHZ]

Typically they have split primary and secondary wingdings so you can do 480 or 240 in with 24 or 48 out.

Thanks. I never had to use one before.

 

[Smart $]

Thanks. I never had to use one before.

48VAC supplies are most common in the instrumentation field AFAIK.

 

[K8MHZ]

48VAC supplies are most common in the instrumentation field AFAIK.

For example? I'm not trying to debate you, I am just curious.