Buck Boost Xfrmr

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Sparky Joe

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Had to size a buck boost transformer last week and really bothered me that I wasn't confident enough with my knowledge on how to do it, and finally had to resort to a stinkin chart.

The customer prints t-shirts and has a heater for the shirts to set the ink. He just moved to a new place, the old place has 240V, the new one 208V. The heater nameplate reads 13.5 amps at 230 volts.

Just want to know the math that I'm forgetting for sizing this. Oh and the chart had me buy a 0.75kVA

Thanks -Joe
 
Crap!... yep, you're right.

You can even do that with a .5 kva autotransformer connected one. The .75 Joe proposes will be fine.
 
It's only that I have scratched my head before while boosting a 110 amp 3 phase load with two transformers the size of 1/2 a loaf of bread.

Connecting my 1 AWG to 12 AWG transformer leads....
 
mdshunk said:
The single phase boost calc is:

240-208 = 32 volts

(32 * 13.5) / 1000 = .432 KVA
Click to expand...

Now wouldn't you size the primary off the size breaker you would be connected the buck-boost to, in this case 20 amp, so;

(32x20)/1000=.64kVA

or a .75 xfmr




Now for a new question; we now need a fused disconnect(or some sort of OC device), because of the transformer? Something to do with the tap rules right? Whereas the machine used to just be cord and plug connected.
 
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Sparky Joe said:
Now wouldn't you size the primary off the size breaker you would be connected the buck-boost to, in this case 20 amp,
Click to expand...

No.

You never size a transformer based on it's primary OCP.

Size the transformer to the load, size the OCP to the transformer.
 
iwire said:
No.

You never size a transformer based on it's primary OCP.

Size the transformer to the load, size the OCP to the transformer.
Click to expand...
I was just formulating a similar reply. I'm not sure that I'd want to size my transformer so that it would hold up under a fault up to the OCPD trip point. That would be wasteful. Most transformers are 150 - 200% rated for short periods of time anyhow.
 
Sparky Joe said:
Now for a new question; we now need a fused disconnect(or some sort of OC device), because of the transformer? Something to do with the tap rules right? Whereas the machine used to just be cord and plug connected.
Click to expand...

Joe take a look at 450.4, it is all about auto transformers which is what a buck boost is.

It has the over current requirements.
 
Boosting 208 using a 240:32V buck boost.
Have 208V available so boost is 208/240*32 = 27.73V
( 240:32 => 208:27.7 )

Total boost is 208 + 27.73 = 235.73V

Total load is 13.5A x 230V = 3105VA (given)

Current in secondary coil is 3105/235.73 = 13.17A.

Buck/Boost VA = 13.17 * 27.73 = 365.2VA

Current in primary coil is 365.2/208 = 1.76A

The currents add in this case for primary 13.17 + 1.76 = 14.93A

14.93A x 208V = 3105VA (input VA = load VA)
 
Carry on ladies and gentlemen, you've got my un-divided attention.
Anyone care to explain the theory behind a buck/boost transformer?
How does it work exactly?
I know what they are and what they do, I just don't know how they do it.
I really want to know, so thanks in advance.
steve
 
hillbilly said:
Carry on ladies and gentlemen, you've got my un-divided attention.
Anyone care to explain the theory behind a buck/boost transformer?
How does it work exactly?
I know what they are and what they do, I just don't know how they do it.
I really want to know, so thanks in advance.
steve
Click to expand...

I don't know about theory, but a simple explanation seems to work for me.

Buck boost transformers are kinda like regular transformers in that they have a primary and a secondary winding. For example a primary winding of 208 volts and a secondary winding of 32 volts. If you measured across the secondary winding you'd get 32 volts just like in a regular transformer.

They are also kinda different in that the secondary winding is wired in series with the primary winding. Most transformers don't have primary and secondary connected at all. Because the secondary is connected in series with the primary, the secondary voltage will either add to, boost, or subtract from, buck, the primary voltage. Whether it adds or subtracts is based on the polarity of the secondary coil relative to the primary.

Hope that helps.

Jim T
 
From the factory, a BB transformer is just like a standard transformer, except the primary and secondary are usually dual-windings. Each pair can be connected in parallel or series, depending on the voltage applied or needed.

When we connect them according to the proper diagram, they effectively become auto-transformers. The secondary is connected in series with the primary, and either boost or buck (reduce) the applied voltage, depending on phasing.

The secondary voltage depends on two things: the voltage applied to the primary and how the secondary is connected. If we apply 208 to a series-connected 120/240v primary, the secondaries each output 10.4v. which can poroduce a 20.8v change in the output.

Uh-oh. I have to run. More later.
 
iwire said:
Joe take a look at 450.4, it is all about auto transformers which is what a buck boost is.

It has the over current requirements.
Click to expand...

Without reading I'll assume it doesn't need any secondary OC protection, seeing as they are used in sodium lights and other such multi tap ballasts all the time.
 
wasasparky said:
Boosting 208 using a 240:32V buck boost.
Have 208V available so boost is 208/240*32 = 27.73V
( 240:32 => 208:27.7 )

Total boost is 208 + 27.73 = 235.73V

Total load is 13.5A x 230V = 3105VA (given)

Current in secondary coil is 3105/235.73 = 13.17A.

Buck/Boost VA = 13.17 * 27.73 = 365.2VA

Current in primary coil is 365.2/208 = 1.76A

The currents add in this case for primary 13.17 + 1.76 = 14.93A

14.93A x 208V = 3105VA (input VA = load VA)
Click to expand...

Thanks for all the info wasa. So a .365kVA is what it needs and a .5kVA (off the shelf) would've worked.
I knew the "16/32" (one coil/both coils) on the transformer would've meant something though the salesman thought it meant 16volt supply/32 volt load, though he was also quick to say he had no clue and all the ones on his shelf were 12/24


Still wanting to hear the rest of 'Larry's reply though.....?

Thanks everyone
 
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