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Old 10th May 2003, 05:07 PM   #1
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Default transformer ratings... pls help

Hi,
This may sound stupid to others but I have been thinking of this for the p[ast 3 days.

How do I compute a transformers VA value?

Example:
1. If I have a 48-0-48/10A transformer, how do I express this in VA?

2. If I have a 48-0-48/1000VA transformer, is there a way to know it's current rating?

Is there a formula for this?

Thanks in advance,
Jojo
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Old 10th May 2003, 05:22 PM   #2
moamps is offline moamps  Croatia
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Hi,
P=V*I, for V=2*48V, I=10A, then P=96*10=960VA
or
if P=1000VA, V=2*48V, then I=1000/(2*48)=10,42A

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Old 11th May 2003, 10:34 AM   #3
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Default The answer to the next logical question (maybe)

If you feed your 1000VA transformer into a bridge rectifier and then straight into a big capacitor like you would in most amplifier power supplies, you will be able to get about 600 watts dc with the transformer fully loaded.
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Old 12th May 2003, 02:08 PM   #4
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Default Thanks!

Thanks for the formula!

Is "VA" value same as the wattage?

If a transformer is rated 1000VA, is it also considered 1000watts?

Sorry for this questions but I'm so confused.

Jojo
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Old 12th May 2003, 02:10 PM   #5
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Yes it is.

And you may say as well 1 kW

Trigon
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Old 12th May 2003, 02:20 PM   #6
dhaen is offline dhaen  Europe
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Default Re: Thanks!

Quote:
Originally posted by JojoD818
Thanks for the formula!

Is "VA" value same as the wattage?

If a transformer is rated 1000VA, is it also considered 1000watts?

Sorry for this questions but I'm so confused.

Jojo
VA =P (Watts) when the power factor is unity. That is, when the voltage and current are in phase.
For most things we attempt in DIY the power factor will be unity.
There are other losses, primarily to do with rectification, which cause our useable wattage to be less than the rated wattage or VA.

Cheers,
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Old 12th May 2003, 02:22 PM   #7
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Default Wheew!

Trigon,

Thanks! Now things are getting clear.

I ask this questions because here in the Philippines I can only order transformers using their secondary voltage ratings and current rating. However, almost all projects I find in the net (especially amps) give the transformers' ratings in VA. Now I know how to convert them ratings.

Jojo
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Old 12th May 2003, 02:26 PM   #8
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Default Re: Thanks!

Quote:
Originally posted by JojoD818
If a transformer is rated 1000VA, is it also considered 1000watts?
Yes, but only if the load is purely resistive!

A heavy power supply will reduce the VA rating to half. A "normal" power supply reduce less but this is continuously power. If we talk music power I think you could answer yes to your question.

A rectifier bridge and caps creates reactive power which warms the transformer.

EDIT: dhaen was faster.....
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Old 12th May 2003, 02:29 PM   #9
moamps is offline moamps  Croatia
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Default Re: Thanks!

Quote:
Originally posted by JojoD818
Thanks for the formula!
Is "VA" value same as the wattage?
If a transformer is rated 1000VA, is it also considered 1000watts?
Sorry for this questions but I'm so confused.
Jojo

Hi,

1000VA is 1000W only when your transformer is loaded with resistive load. Current and voltage are then in phase (fi is zero, cosinus fi is 1), and you can get 1KW from transformer.
If you have bridges and caps (complex load), then you can get smaller power (Watts) from your transformer.

Regards

P.S. Circlotron, what is next Q&A ?

Edit; I'm slooow.
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Old 12th May 2003, 03:26 PM   #10
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Hey guys thanks for the info.

My test transformer is a 44-0-44/4A. I used 4 6A/200V diodes in bridge mode and 6,800uF per rail. I left it plugged without any load, when I came back it was very warm, not hot but my fingers can't stand the temp. Why?

If it heats up without load, what more if I load it?

Jojo
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