Buck-Boost Transformer Selection

I have a new piece of equipment I'm powering up, and the incoming voltage is too high. The equipment's rating is 220VAC +/- 10%, and the incoming single phase voltage is 245VAC. So I've landed on using a buck-boost transformer to knock down the voltage.

After looking through various sites, I landed on needing a 1.3KVA rated transformer (24VAC x 55A = 1.3KVA) to go from 245VAC to ~220VAC. I ordered a step down transformer from McMaster(McMaster-Carr) as it says it is rated at 1.5KVA at the required input / output voltages. The nameplate KVA rating is 0.15KVA, though I understand because the primary and secondary windings are tied together for buck-boost operation (autotransformer), the ampacity increases. This transformer (ACME electric) weighs roughly 15 lbs.

After double checking a few other sites using their calculators, the recommended transformer is a 1.5KVA rated, weighing in at ~30 lbs. This is a calculator provided by ACME electric for the given input, required voltage drop, and load (55A).

Is the transformer I purchased through McMaster adequate, or are the ratings they provide bogus? Perhaps the ACME-recommended 30 lb transformer is isolated, whereas the one purchased through McMaster is not?

Thanks,

Ben

Is this kind of thing something you have done in the past or is this piece of equipment a special case.

The McMaster transformer page does not show the weight. How did you get that figure?

rons said:

Is this kind of thing something you have done in the past or is this piece of equipment a special case.

The McMaster transformer page does not show the weight. How did you get that figure?

Click to expand...

I haven't needed to use a buck/boost transformer in the past as all of my other equipment will handle 250VAC input.

My mistake, the transformer weight is 7 lbs.

Here are the McMaster transformer specs: T18149 | Brand | Acme Electric

zipfactor said:

I have a new piece of equipment I'm powering up, and the incoming voltage is too high. The equipment's rating is 220VAC +/- 10%, and the incoming single phase voltage is 245VAC.Ben

Click to expand...

Make sure you are using a meter that reads true RMS, many cheap digitals do not, analog meters with a needle should be OK. You might just be getting a bad reading.

zipfactor said:

I landed on needing a 1.3KVA rated transformer (24VAC x 55A = 1.3KVA) to go from 245VAC to ~220VAC. I ordered a step down transformer from McMaster(McMaster-Carr) as it says it is rated at 1.5KVA at the required input / output voltages. The nameplate KVA rating is 0.15KVA, though I understand because the primary and secondary windings are tied together for buck-boost operation (autotransformer), the ampacity increases.
Ben

Click to expand...

The .15 KVA rating of the transformer is for the delta, the 1.5 KVA rating is for the load, you need 1.3 KVA for the delta and 12.1 KVA for the load. The McMaster transformer is inadequate.

CarlBoyd

CarlBoyd said:

Make sure you are using a meter that reads true RMS, many cheap digitals do not, analog meters with a needle should be OK. You might just be getting a bad reading.





The .15 KVA rating of the transformer is for the delta, the 1.5 KVA rating is for the load, you need 1.3 KVA for the delta and 12.1 KVA for the load. The McMaster transformer is inadequate.

CarlBoyd

Click to expand...

I measured the voltage with a Fluke 87-V. I'm confident with the readings. But a good point, nonetheless.

I thought the load for buck/boost transformers was the voltage rise or drop multiplied by the load current. So in this instance, my voltage drop is 24V and the load current (FLA) is 55A.

What your saying is the name plate KVA is for the voltage change load (in this case drop)? And then within the specifications for the transformer they list the full line voltage load capacity (220VAC x 55A = 12.1 KVA) for the given input voltage and buck voltage?

Best to IGNORE the kVA, and focus on the current and voltage required. The power requirements can easily be misleading as you have discoverd.

You have 245V, you want 220V. Difference is 25V, so a 24V dropping transformer will work, if of adequate current capacity,

Your transformer is single phase, 240V input and since you want 24V output, it is rated at 6.25A. Page 4 here: https://hubbellcdn.com/brochure/BuckBoostTransformerBrochure.pdf

You have a load that wants 220V at 55A (unless you missed a decimal point). You have not even close to the required current capability, so, yes, your transformer is way short of what you need per what you wrote.

If it comes to power, assuming your 55A figure is correct, that would be on the order of 1500 VA (55A x 24V). The transformer produces 6.25 x 24, or 150VA

JST said:

Best to IGNORE the kVA, and focus on the current and voltage required. The power requirements can easily be misleading as you have discoverd.

You have 245V, you want 220V. Difference is 25V, so a 24V dropping transformer will work, if of adequate current capacity,

Your transformer is single phase, 240V input and since you want 24V output, it is rated at 6.25A. Page 4 here: https://hubbellcdn.com/brochure/BuckBoostTransformerBrochure.pdf

You have a load that wants 220V at 55A (unless you missed a decimal point). You have not even close to the required current capability, so, yes, your transformer is way short of what you need per what you wrote.

If it comes to power, assuming your 55A figure is correct, that would be on the order of 1500 VA (55A x 24V). The transformer produces 6.25 x 24, or 150VA

Click to expand...

The transformer specs are a bit misleading indeed. I kept looking at the transformer thinking it looked way too small for the given load. I'll send the undersized transformer back to McMaster, so no big deal.

No decimal point missed, 55.0A for the load at 220V. KVA is misleading as well, but now I understand the nameplate is for the load change (Delta) and not the full load at line voltage. Live and learn, and luckily it was a very inexpensive mistake with no magic smoke release .

zipfactor said:

I measured the voltage with a Fluke 87-V. I'm confident with the readings.

Click to expand...

That meter is designed to read true RMS, so a 60Hz periodic waveform is going to be accurate. Last night I had a reading of 248 VAC. This morning it was 246 VAC.

I just installed a pair of buck boost transformers on a 30 amp circuit powering my machine. We're on old-school 3 phase 240v Delta (no neutral) that measures 238-244v on any given day.
We use these Larson Electronics Transformers:
3-Phase Delta Buck/Boost Step-Down Transformer - 244V Primary - 222V Secondary - 34.35 Amps- 5/6Hz - Larson Electronics


Primary Amps: 31.25A @ 244 (this is your input rating)

Secondary Amps: 34.35A @ 222 (this is what comes out)

So basically you get a 9% drop on whatever your input voltage is.

After installation (one low leg hooked up to each transformer, high leg passes through) the incoming voltage of 243v dropped to 218v. The transformers are virtually silent too, but you can feel the vibration if you put your palm on the enclosure.

I use a 2kva Dongan buck/boost before my PT330. Gets the 240 down to 224 which is within 5% recomended tolerance.

I fried a board after a few months in before I bought the tramsformer. I learned the hard way what a few extra volts can do.

zipfactor said:

What your saying is the name plate KVA is

Click to expand...

.. off by one decimal point. Simple as that.

Dunno how or WHY, but you ordered just about exactly ten percent of what you need.

McMaster didn't reach out and challenge your selection. How would they even know what you were up to?

Meant to say the 2kva Dongan 85-y045 transformer when using to step down 240 to approx 220 it is good for 55kva.

Good luck.