KVA Rating

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MOD85

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Hello to everybody

Recently co-worker and i were talking about transformers. And we were debating about two things

1.- If I have a reducing transformer, lets say 240VAC to 120VAC, can it be used to elevate 120 to 240???

2.- If i do this, he says that the KVA rating of the transformer will be reduced by the half, so if the transformer is rated for 10KVA, instead its load capacity will be 5KVA

how can this be true?

Thanks in advance to all.
 
MOD85 said:
Hello to everybody

Recently co-worker and i were talking about transformers. And we were debating about two things

1.- If I have a reducing transformer, lets say 240VAC to 120VAC, can it be used to elevate 120 to 240???

2.- If i do this, he says that the KVA rating of the transformer will be reduced by the half, so if the transformer is rated for 10KVA, instead its load capacity will be 5KVA

how can this be true?

Thanks in advance to all.
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Transformer deals only with voltage and current not with kVA.
 
Yes, and it can't be true, in that order.

Disregarding any internal losses for the moment, the power that is provided to the primary of a transformer must necessarily equal the power that is taken away from the secondary. The KVA rating of a tranformer (or any other device, for that matter) is based on the ability of the equipment to remove the heat that is generated from within. It does not matter which side gets power applied, and which side has power taken away, the ability of the transformer to reject heat is the same. Therefore, the KVA rating is the same.

Welcome to the forum.
 
KVA is KVA

Step-up or Step-down, doesn't really make any difference.

Application is usually important factor in best transformer design for the situation.
 
charlie b said:
Yes, and it can't be true, in that order....
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Exactly.
Hameedulla-Ekhlas said:
Transformer deals only with voltage and current not with kVA.
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I know that you know that VA are Volt-Amps, or the product of volts times amps, of course, so what is it that you mean?


MOD85 said:
... can it be used to elevate 120 to 240?
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When using a step-down transformer in reverse, you will get nearly the expected voltage and current, but the inrush current is often higher, and can lead to tripping of the primary OCP simply because of the relative positions of the windings to each other.

Welcome to the forum.
 
charlie b said:
Therefore, the KVA rating is the same.
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From what I understand, that's almost true. I believe there's a slight difference due to the primary being wound against the core, and the secondary is wound over the primary, but the capacity loss should be small. I've also heard this leads to excess inrush current.

I've also heard that a 240-to-120v tansformer, for example, isn't wound exactly 2:1, but is slightly higher (say, 1.9:1) to account for losses, so using it backwards would give you a slightly lower ratio, say 1:2.1 instead if 1:2, so your output may be lower than 240v.

Added: Don't forget that, due to the current ratio being opposite the voltage ratio, your primary current would be double the secondary.
 
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Volta said:
Exactly.

I know that you know that VA are Volt-Amps, or the product of volts times amps, of course, so what is it that you mean?



Welcome to the forum.
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I said transformer deals with current and voltage. It means when you decrease voltage in a specific amount and current increases and kVA is the same and Vise versa.

So, it doesnt affect the kVA except some small internal loss.
 
LarryFine said:
I've also heard that a 240-to-120v tansformer, for example, isn't wound exactly 2:1, but is slightly higher (say, 1.9:1) to account for losses, so using it backwards would give you a slightly lower ratio, say 1:2.1 instead if 1:2, so your output may be lower than 240v.
Click to expand...

This is more true of small (<15kVA) power and almost all 'control' transformers. For larger units it is all but a non-issue. You can notice this the most with 'doorbell' transformers where their no-load, open circuit voltage, is quite a bit higher than their full load voltage.
 
Transformer uses

Transformer uses

I just wanted to add my 2 cents by agreeing that power is power and any losses for reverse transformer use would be minimal, if any.
 
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