volt-amps [VA] conversion

[croyleje]

hey everyone i am working on a building a bench top power supply and am trying to select the transformer for the supply.  i have several from old surplus lots but all the ratings are in [VA] volt-amps how can i convert this into amps or watts?  i have read alot of web pages and there are alot of formulas are any of these correct?

volt-amps X 0.8 power factor = watts ?

volt-amps = watts ?

thanks Jason

[Stephen Hill]

The rule of thumb is that 1VA = 1W

[Zero999]

The rule of thumb is that 1VA = 1W

Only with resistive loads. If there's a rectifier and filter capacitor on the secondary, the transformer won't be driving a resistive load. The power factor will be below unity, so will need derating accordingly.

[Flávio V]

Do not forget than the VA can get outside the actual desired valours.

My custom 250VA 50V transformer at heavy load(8A)gets a lower voltage(50V)(these valours are after the capacitors)

For regular use the capacitors,since they increase the max voltage voltage,will decrease the max current....

+-0,7A per transformer A...

So my transformer will work without audible noise and normally at 70-72V max after the capacitors at +- 3.5-4A...

Also,the capacitors might draw more than the transformer can and will reduce the max voltage for +-0.3s...and after they are almost charged they won't need many A and will incriase the transformer possible supplying voltage....


I think than this is a reason than transformers should't be rated to W...they are similar but not equal....

[croyleje]

why don't they just rate all the transformers in amps i understand VA but just don't understand why they use VA?

[T4P]

why don't they just rate all the transformers in amps i understand VA but just don't understand why they use VA?

VA is before power factor. Watts is a useless measure for transformers as explained above
And amps .. at what voltage? A Transformer does not put out perfect amount of voltage at no load you know

[Architect_1077]

OK, here's a question: how does one know what power factor to use?

[alm]

The power factor depends on the load. It is 1 for an ideal resistor, 0 for an idea capacitor or inductor, and something in between for most real components. You can calculate it by measuring real power and apparent power, and dividing those two. Apparent power is just the product of RMS voltage and current. Real power is the average of the instantaneous product of voltage and current, as measured by decent energy meters (eg. Kill-A-Watt), a DMM like the Gossen Metrahit energy, or an oscilloscope measuring both voltage and current. Some test gear is also able to measure power factor directly.

[ejeffrey]

why don't they just rate all the transformers in amps i understand VA but just don't understand why they use VA?

volt-amps is just a funny way of writing amps.  It is just the current times the voltage.  The reason transformers are rated in volt-amps instead of current is because transformers of similar size have similar ratings, and because volt-amps are basically the same on the primary and secondary side.

[Mechatrommer]

why don't they just rate all the transformers in amps i understand VA but just don't understand why they use VA?

volt-amps is just a funny way of writing amps.

i believe its not funny. whats funny is instead of stating it as "VA" why dont just simply use "W" for watt. expressing such, VA or W we'll get another parm.... imaginary "R"... ohms law. or vice versa.. by measuring R (effective or real), we can work out the A if we apply certain V, and rework back if it complies VA rating. maybe the VA (instead of W) is to get rid people from memorizing (W) P=VI (VA).

[ejeffrey]

i believe its not funny. whats funny is instead of stating it as "VA" why dont just simply use "W" for watt. expressing such, VA or W we'll get another parm.... imaginary "R"... ohms law. or vice versa.. by measuring R (effective or real), we can work out the A if we apply certain V, and rework back if it complies VA rating. maybe the VA (instead of W) is to get rid people from memorizing (W) P=VI (VA).

Because watts is wrong, and watts are not the same as volt amps.

[Mechatrommer]

Because watts is wrong, and watts are not the same as volt amps.

any useful link? to differentiate? maybe i got semantics error here.

[Zero999]

Because watts is wrong, and watts are not the same as volt amps.

any useful link? to differentiate? maybe i got semantics error here.

Real power is measured in Watts.

Apparent power is measured in Volt Amps.

P = VA*PF

E.g.

V = 10V
I = 2A
PF = 0.25

VA = 10*2 =  20VA
P = 20*0.25 = 5W

[jimmc]

If a purely resistive load is  connected to the drawing A amps at V volts then W watts will be dissipated in the load where W = V x A.
Imagine now that a perfect capacitor is connected across the resistor, additional current will be drawn from the transformer but no extra power will be dissipated in the capacitor.
The losses in the transformer will increase although no extra power is being drawn.

The Wikipedia article is quite good at explaining the difference if you remember that actual power is measured in watts and apparent power in VA.
http://en.wikipedia.org/wiki/Power_factor

With non linear loads such as rectifiers, the calculations are complex but crib sheets such as the one below are a good starting point.
http://www.hammondmfg.com/pdf/5c007.pdf

Jim