Saturday, August 24, 2013

The Difference Between VA and Watts

What is the difference in VA and watts? Both are expressions of electrical power. In a purely resistive circuit, both are calculated by multiplying volts times amps. That is where the VA name comes from. In fact, there really is no difference between VA and watts in a purely resistive circuit. A 5000 watt electric resistance heater is also a 5000 VA electric heater, but you would probably not see a VA rating on a resistive device.

The term VA is commonly used to describe the total amount of apparent power of an inductive device, such as a transformer or motor. An inductive device is one that uses alternating current to create electromagnetism to operate the device. A funny thing happens inside inductive devices – they actually generate voltage that is opposite the original voltage connected to them. The magnetic field created by the current moving through the wires changes directions when the alternating current changes directions. This has the effect of moving the magnetic field back and forth across the wire – which is pretty much what a generator does. This moving magnetic field generates a voltage in the inductive device that is opposite the original voltage, called counter electromotive force, or back emf for short. Since this voltage is pulling the current in the opposite direction, it dramatically reduces the amount of current traveling through the device. This property of magnetic devices operated on alternating current is called inductive reactance. That is why the physical resistance of inductive devices is so low – most of their opposition to current comes from inductive reactance.

Another effect of inductive reactance is to delay the rising and falling of the current, causing it to lag behind the rising and falling of the voltage. Since the voltage and current are not rising and falling together, they do not work together, so the true power of the device is less than you would get by just multiplying the volts times the amps, or the apparent power. So VA is a measure of the apparent power of an inductive device ( V x A ) while watts is a measure of the true power (V x A x power factor). Power factor is a measure of how out of whack the volts and amps are (note this is probably not the definition your instructors will want) It can be calculated using a bunch of really cool formulas, or you can measure it with a wattmeter, voltmeter, and ammeter. Then use the relatively simple formula of (volts x amps) divided by watts.  

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