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Unknown transformer VA ratings

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Alex M

Mon Jun 03 2013, 07:47PM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Hi,

So I have this mains step down transformer that I have no idea what its VA rating it is, since I got it out of some old junk years ago. It puts out around 18vac unloaded and then about 17vac with a 1 amp resistive load (22vdc when rectified and smoothed).

It weights about 850 grams and I would say that the secondary winding looks to be around 26-28 SWG.

Here it is next to a PP3 battery for size comparison.

On the internet I have read that 15-20VA per lb is a rule of thumb for E I cored transformers, so that would be roughly 30VA? I put it on a load test the other day and at around 1.2 amps is where it started to warm up a bit, but it seemed to cope with a 1.4amp load just fine (these measurement's were taken with a non-true RMS multimeter on a resistive dummy load).

The only thing printed on the transformer is "P0265TA" and "1527", but couldn't find anything online though.

Does the 30VA figure sound about right for something this size and weight? or am I way off with that estimate, as for some reason I think this thing could probably push a little bit more power before it starts complaining.

Thanks

Sulaiman

Mon Jun 03 2013, 09:57PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

The simplest method is to
find a similar construction transformer of similar weight so similar VA

HV Enthusiast

Mon Jun 03 2013, 10:44PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

I would guess thats good for about 10-20VA depending on size.

My guess is about 18VAC output at 1.5A.

klugesmith

Tue Jun 04 2013, 04:03AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Please share any experimental data you get, e.g. temperature rise vs. load.

Some things I have learned along the way:

1. Transformer temperature rise and VA rating depend on the RMS current.
That can be significantly higher in circuits with a rectifier, filter, and DC load,
compared to an AC resistive load with the same load current.
It's because of the crest factor in current waveform that periodically boosts the capacitor voltage.
The VA derating factor depends on the type of rectifier and filter.
Some transformers come with a rectifier rating (not to be confused with rectifier transformers).
Here's a reference to start with:

2. For a given transformer weight, VA ratings will of course depend on acceptable temperature rise and operating duty cycle. Yours looks like it's from a consumer electronics or appliance product, so it's probably intended to be energized continuously, and should never get hot enough to feel or smell like it's about to start a fire.

3. Some transformer applications (e.g. MOT's, certain UPS units) spend most of their "on" time at loads near maximum. They can get more VA per pound by using thicker wire and fewer turns, to reduce copper loss at the expense of higher core loss (which is practically independent of load current). I guess they could be distinguished by having lower primary winding resistance than other transformers of the same size and voltage.

Sulaiman

Tue Jun 04 2013, 12:20PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

The 'rule of thumb' that I use for continuous current rating is, Idc = 0.62 x Iac.

e.g. 18Vac 18 VA transformer is rated for 1 A rms, with a bridge rectifier and capacitor, Idc = 620 mA.

For 4HV use you can run the transformer hortter !

Dr. Dark Current

Wed Jun 12 2013, 12:48PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

For 230 V mains the power is around 1200/R_pri (for small transformers - tens of VA, the constant is larger - around 2000, for very big transformer in the kVA range, it is smaller, like 1000.)

Shrad

Wed Jun 12 2013, 05:12PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

1527 could be a charted code meaning something like 15VAC 27VA, and keep in mind those transformers are usually a couple volts higher than specs

Experimentonomen

Wed Jun 12 2013, 06:58PM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

And that specs are almost always highly overrated. Can sometimes be overspecced as much as twice actual power, as was the case with a 800W rated transformer i have from an amplifier, it turned out to be about 400VA in real life.

teravolt

Wed Jun 12 2013, 07:18PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

it looks to me like about 10w

Thomas W

Wed Jun 12 2013, 08:32PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

Experimentonomen wrote ...

And that specs are almost always highly overrated. Can sometimes be overspecced as much as twice actual power, as was the case with a 800W rated transformer i have from an amplifier, it turned out to be about 400VA in real life.

Not always, many of my older transformers are rated e.g.
1100VA 24-0-24vout 170vin
can run happly over 2200VA

and my variac can be run at way over 40A for about 30 mins id say when its rated for 25A

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