Transformers - Help please

That depends on the efficency of the transformer

100 VA is 100W if the load is a pure resistance, like a heater or a filament lamp. It will be less than 100W if the load is not resistive in nature -- it depends on the nature of the load exactly what the value is. If you give a clue what the intended appliances are, we can probably guess the watts to VA ratio (which is known as the Power Factor).

If connected for 240 volts its max amp rating is 100 volt-amps / 240 volts which is about 0.42 amps. If connected for 120 volts it would be 100 / 110 or about 0.84 amps.

This is just the max amp rating - the transformer doesn't "output" amps. It supplies an alternating (AC) electrical potential or voltage so the actual current that would flow is dependent on amount and type of load that the transformer supplies.

Perion

in article Uzk1c.2904$zu.2079@newsfe1-win, Mack at snipped-for-privacy@yahoo.co.uk wrote on 3/3/04 4:55 AM:

Your initial statement does not ring true. I would expect a transformer changing 240 VAC to 120 BAC but not 110 VAC.

Bill

Thank you all for replying - Its a heater which is pure resistance. Can anyone tell me the formula for working this out please?, if their is a 'simple' rule of thumb, that will do nicely. Cheers. Mack.

snipped-for-privacy@yahoo.co.uk

240v to 110v is pretty much a standard step-down in the UK. Looking at the OP email, it looks like a UK post. Probably provide just under 1 amp on a resistive load.

Teddy Rubberford.. "The man in the rubber hood"

| Thank you all for replying - Its a heater which is pure resistance. Can | anyone tell me the formula for working this out please?, if their is a | 'simple' rule of thumb, that will do nicely. | Cheers. | Mack.

For a heater, it is most likely 100 VA = 100 W.

Watts is a measure of actual energy used or produced. Volt-amps is a meausre of the capacity to carry energy. When a load (such as a motor) shifts the phase of the current away from the phase of the voltage (this is called a reactive load), then it is effectively pulling more power than it needs during part of the cycle, and pushing the excess back during the other part of the cycle. In the worst case, nearly all the power drawn will be pushed back. This is a power factor of nearly zero. The current will be just about 90 degrees out of phase with the voltage. In such a situation, the transformer still has to be rated to that level because it still has the voltage being applied, and still carries the current. It still heats up due to resistive and core flux losses. So the ratings of transformers and other power carrying devices is not expressed in watts, but in volt-amps.

The term 'VA' comes from 'Volt-Ampere' which is one rating of transformers. It means, literally Volts times Amps.

It is useful for transformer ratings because the same number applies to either the primary or secondary. If you have a 240V to 110 V step down transformer that is rated 100 VA, then...

Secondary...

100 VA = 110V X 0.909 A

Primary...

100VA = 240V X 0.4166A

The power in Watts is from a formula... P = V X A X pf pf = Power factor. For resistive loads it is 1.00, so the power in watts is equal to Volts X Amps (which is 'VA').

100VA with a resistive load is 100 Watts

daestrom

So what would happen if you connect say an equipment 110V AC 200 VA to this transformer?

Sizzle