How to determine VA rating of Trans

[jtsaudio]

I've salvaged a transformer from an old non-working receiver I found and it tests ok, measuring 90vac across the two main secondaries(each is 45VAC measured to the center tap).

The rear panel states the power consumption is 490W/870Wmax, and the mains input fuse is 5A.

How do I determine the VA rating of this transformer?

-Joe

[ostripper]

Transformer capacity is rated in Volt-Amps (VA) which is generally the same as wattage (Watts). Most loads are labeled with their proper operational voltage (Volts), current (Amps), frequency (Hz), and wattage (Watts or VA).

This might explain it... 5(a) X 120v = at least 600va..
Inrush considerations also are a factor.. mine will blow a 10amp
fuse with no softstart.

A commercial 1500Va trafo is rated 1548W (below), so I suspect you have a 800VA trafo.
http://www.antekinc.com/AN-15450.pdf

In scale , my "unknown" 1500+VA toriod
uses a 10A fuse.
OS

[bear]

Well for starters we know that it is less than the max values shown on the back panel, and less than the 5A x 120v = 600w in all probability.

The proper method is to put a resistive dummy load on the secondary and watch for a 10% drop in the secondary voltage compared to the unloaded secondary voltage. This would be a good rating for "continuous duty" - assuming the transformer can do continuous duty...

The VA rating is then computed by figuring the ohms law given the value of the resistor and vdrop across it.

That's pretty much it.

You'll need some hefty power resistors to do the job.
You might be able to fashion something up using nichrome wire from a toaster or some other heater element... but they tend to vary resistance with temperature, but that might get you in the ballpark.

_-_-bear

[Wavebourn]

There are 2 options of designing transformers: or loss (usually 10 percents) or overheating that is the difference between maximal allowed temperature of the transformer and an environment temperature that your equipment may experience. Usually well optimized transformers have max overheating on the power corresponding to 10% loss, so you may load it gradually and measure.

[jtsaudio]

Thanks for your replies. I've been thinking about them.

Well for starters we know that it is...less than the 5A x 120v = 600w in all probability.

Is it common to use input fuses rated higher than the power transformer they supply? This seems counter-intuitive...

How could such a (higher rated) fuse protect the gear that follows?

-Joe

[AndrewT]

Originally posted by jtsaudio
Is it common to use input fuses rated higher than the power transformer they supply? This seems counter-intuitive...

How could such a (higher rated) fuse protect the gear that follows?

a fuse rated about the same as the maximum continuous primary current will rupture most times when one switches on.

To avoid this, it is normal practice the use a fuse about three times higher rating.
There are many downsides to this practice.

Using a soft start circuit allows much a smaller fuse to be used and gain the added protection available with close rating.


A fuse is there to prevent you setting fire to your house.
It is not there to save damaging an already faulty piece of equipment.

[DigitalJunkie]

http://www.diytube.com/phpBB2/viewto...t=1465&start=7

[AndrewT]

does someone have the data to update the graphs in that 1964 paper to cover toroidals