diyAudio

diyAudio (http://www.diyaudio.com/forums/)
-   Solid State (http://www.diyaudio.com/forums/solid-state/)
-   -   transformer ratings... pls help (http://www.diyaudio.com/forums/solid-state/14984-transformer-ratings-pls-help.html)

JojoD818 10th May 2003 05:07 PM

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

moamps 10th May 2003 05:22 PM

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

Regards

Circlotron 11th May 2003 10:34 AM

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.

JojoD818 12th May 2003 02:08 PM

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

trigon 12th May 2003 02:10 PM

Yes it is.

And you may say as well 1 kW

Trigon;)

dhaen 12th May 2003 02:20 PM

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,

JojoD818 12th May 2003 02:22 PM

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. :nod:

Jojo

peranders 12th May 2003 02:26 PM

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.....

moamps 12th May 2003 02:29 PM

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.

JojoD818 12th May 2003 03:26 PM

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


All times are GMT. The time now is 04:09 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio


Content Relevant URLs by vBSEO 3.3.2