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Old 6th January 2007, 12:28 AM   #1
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Default How to measure transformer VA?

How would you accurately measure the VA of a transformer that you have no specs on?
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Old 6th January 2007, 01:00 AM   #2
BWRX is offline BWRX  United States
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You need to know at least one spec so you can measure the conditions at which that occurs then you can calculate the VA rating. If you know what the loaded down secondary voltage should be, load the transformer secondary down while monitoring the secondary current and voltage (make sure the primary voltage is close to what it should be or your numbers will contain some error). When the voltage drops to the spec as you increase the load on the secondary check to see how much current is being drawn and then multiply the two numbers. If there are two secondaries double that number.

Alternatively, you can get a rough estimate by using the old VA/lb trick. I don't know the numbers off hand but you can probably find some good ones if you search.
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Old 6th January 2007, 01:23 AM   #3
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If you are talking toroids, rough ballpark figures;

<500VA => 2.25lbs/100VA
- 500VA => 2.00lbs/100VA
>500VA => 1.75lbs/100VA
Transformers from a couple of manufacturers have higher numbers, some lower.
EI cores are too far spread for estimates.

An accurate way is to calculate the efficiency by measuring the ingoing and outgoing VA numbers.
A proper transformer will have the highest efficiency at the VA rating it is designed for.
I assume you know the design secondary voltage or the way of determining it.
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Old 6th January 2007, 03:29 AM   #4
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A more scientific way to determine VA capability is to power up the transformer on your bench and measure the no-load secondary voltage. Now add some resistance (big power resistors) across the secondary until the voltage drops by 30% for small transformers (<50 VA), 20% for medium sized transformers (50 to 100 VA) and 12-15% for big suckers (>>100VA). For high efficiency semi-toroidal transformers, all voltage drops will be roughly 1/2 of these listed. Make note of the voltage under load and use ohms law to discover VA rating:

* Rated secondary current = E/I = voltage under load/resistance in ohms
* VA rating in watts = I*R = current from above formula * voltage under load
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Old 6th January 2007, 04:09 AM   #5
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What does a "semi-toroid" look like? Is it half a toroid?
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Old 6th January 2007, 07:08 AM   #6
Tweeker is offline Tweeker  United States
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Rectangular in cross section rather than circular I'm assuming.
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Old 6th January 2007, 07:33 AM   #7
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Default Re: How to measure transformer VA?

Quote:
Originally posted by burnedfingers
How would you accurately measure the VA of a transformer that you have no specs on?
You could also load it and when the temperature is 105 deg C (221 F) in side, then you'll have the rating. 105 deg is pretty
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Old 6th January 2007, 03:22 PM   #8
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Quote:
A more scientific way to determine VA capability is to power up the transformer on your bench and measure the no-load secondary voltage. Now add some resistance (big power resistors) across the secondary until the voltage drops by 30% for small transformers (<50 VA), 20% for medium sized transformers (50 to 100 VA) and 12-15% for big suckers (>>100VA). For high efficiency semi-toroidal transformers, all voltage drops will be roughly 1/2 of these listed. Make note of the voltage under load and use ohms law to discover VA rating:
What resistor value would you recommend and what power rating?
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Old 6th January 2007, 04:17 PM   #9
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I think the easiest way for you is to describe the transformer in mind and I'm sure the community here can tell you the VA rating.
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Old 7th January 2007, 11:49 PM   #10
AndrewT is offline AndrewT  Scotland
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sizes and weight are good clues
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