i've bought a step down transformer recently. it's 1000va - does that mean watts?

Question

i've been told that va means watts when it comes to the step down tranformer. i presume that va means voltage amps but how does that mean watts?

tks

Answer 1

Volt-Amps (VA) are a little more general than Watts.

The VA rating describes the sum of the reactive and the resistive power. "Watts" is a unit to describe resistive power. "VARs" (Volt-Amps Reactive) is a unit to describe the reactive power.

Reactive power is the power that goes into storage effects (capacitance and inductance) whereas resistive power is the power that actually gets bled away from the line as, for example, heat or light.

Imagine a pure inductor. The inductor stores energy in the surrounding magnetic field, and so when you drive current through the inductor, you also are accessing the magnetic field around it. That is, it takes POWER to maintain that magnetic field. This power is not resistive at all. That is, power is never RADIATED AWAY from the inductor. Instead, the power shows itself by putting a LAG between the voltage and the current. That is, for an AC sine tone, the current through an inductor will always be a quarter wavelength behind the voltage. (whereas with a resistor, they would always be synchronized)

Similarly with a capacitor, energy has to go into storage in the electric field between the plates. (some people refer to this as "displacement current" -- it's the current rushing in and out of the plates maintaining the electric field) The effect of a capacitor on a pure AC sine tone is that the current will "lead" the voltage rather than being in sync. (again, it will lead by a quarter wavelength)

To put it another way, with pure inductive and capacitive loads, initial changes in voltage do NOT immediately result in the appropriate changes in current. Imagine a turntable with wheels. If you attach a rod to one edge of the turn table, as you pull on the cart, you will first have to rotate the turntable in order to pull the cart. If you were attached directly to the non-rotating part of the cart, you could have pulled the cart immediately. However, now you have to rotate the turntable first. If you want to push the cart back and forth, your cart will still be moving toward you when you are moving back toward it. Due to the turntable, its motion lags yours. Finally, when you want the cart to stop moving, the inertia of the turntable will cause it to still want to turn so even though you'll stop, it will pull you along before it finally stops. In fact, this "extra" inertia is reflects the extra energy you had to put in at the beginning to get the whole thing started. That is, there is always a lag between what the cart and turntable are doing and what you want them to do. This "extra" work you have to do per unit time to deal with these leading and lagging currents is your reactive power.

When you run something like a motor, you have these reactive loads intrinsic to the device. That means your voltage and current will not be in sync. Thus, there is reactive power.

So when you are transmitting power, you have to count on both of those. Your loads will both be reactive and resistive. The sum of the reactive and resistive power gives you the total power, which is expressed in units of "VA" or "Volt-Amps."

Answer 2

The measurement of electrical power that is computed by multiplying volts times amps. In a DC circuit, volt-amps (VA) and watts are the same because DC circuits do not add inductance and capacitance that take away power. In AC devices, volt-amps ratings are higher than watts. For example, in an uninterruptible power supply (UPS), the volt-amps rating is approximately 60% higher than the actual wattage.

One watt is one ampere of current flowing at one volt. Watts are typically rated as AMPS x VOLTS or VOLT-AMP (V-A). However, this rating is only equivalent to watts when it applies to devices that absorb all the energy, such as electric heating coils or incandescent light bulbs. With computer power supplies, the actual watt rating is only 60% to 70% of the VOLT-AMP rating.

Answer 3

P = VIcosφ, where φ is the phase angle between voltage and current.
If the load you connect to the transformer is purely resistive, cosφ = 1, and P=VI = 1,000 W. If there are inductance or capacitance in the load, then not all the current flowing to the load will produce "real" power, so P < VI.

Answer 4

Watts is volts multiplied by amps.

Answer 5

See Ohms Law

Answer 6

volts*amps=watts so yes VA=W.