The volt-amp is a unit seldom used outside of electrical engineering in general and power systems design in specific. Increasing dependency on electricity in the developed world has, however, exposed this little-known unit of measurement to the general public. Usually abbreviated VA, this term is most often first encountered as a computer's uninterruptible power supply capacity. In most cases, you can simply consider it being the same as the more commonly understood term Watt. In certain rare cases, however, the differences become important.
Resistive Loads
For most power consumers, Volt-Amp (VA) is simply another term for Watt. The majority of the power draw for the average person is used up in such items as light bulbs, power supplies for electronics, heating elements in cooking appliances, and other such uses. These power draws are basically resistive, that is, they can be modeled as resistors in a circuit.
Inductive Loads
For larger power consumers, however, Volt-Amps are a more complicated matter. Large factories will have a number of machines which are mostly powered by electric motors. Electric motors represent a large inductive load, that is, they must be modeled in a circuit using inductors, because their operation depends on the creation of a rotating magnetic field. (The relatively small motors in ceiling fans, vacuum cleaners, and clothes dryers in a typical household are not a significant enough source of inductance to worry about.)
Inductors are loops of wire which create a magnetic field when electric current is passed through them. They have the ability to store energy in this magnetic field, which has the effect of dampening changes in the flow of current through them. This makes them good filters in power quality applications.
Alternating current is, of course, a continuously changing flow of current, along a sine wave. When alternating current is passed through a large inductor, the resulting dampening creates a phase shift in the current. This means that if you were to compare the sine wave of current entering the inductor to the sine wave of current exiting the inductor, the exiting wave would be retarded by several degrees, or shifted a bit to the left on a graph.
Inductors have no effect on the voltage waveform, however, meaning that there is no corresponding phase shift in the voltage. This means that in large inductive loads, the current sine wave lags behind the voltage sine wave. This is referred to as having a lagging power factor.
Capacitive Loads
Capacitive loads have the opposite effect. Capacitors dampen changes in voltage by storing energy as an electric field, and have no effect on current. This means the current sine wave leads the voltage sine wave, so capacitive circuits are referred to as having a leading power factor. Large capacitive loads are rare compared to inductive loads.
Power Factor
The power factor is a measure of how far the current is leading or lagging behind the voltage, and is expressed as the cosine of the phase difference between the current and voltage. Power factor is modeled using the power triangle:
_,-
VA _,-' |
_,-' | VARs
_,-' |
-'______________|
Watts
The power triangle graphically represents the relationship between Volt-Amps, Watts (the ordinary, resistive portion of the load), and a term called Volt-Amps Reactive (VAR) which describes the effect of the capacitive or inductive load. The cosine of the angle between Volt-Amps and Watts is the power factor, and must be specified as leading or lagging (because the cosine of, for example, +15 degrees or -15 degrees is the same, 0.966). This represents the ratio of Watts to Volt-Amps consumed by the system.
Volt-Amps vs. Watts
The Volt-Amp is a vector quantity, and like all vectors can be expressed in two different ways. Either magnitude and direction (e.g. 10,000 VA at a power factor of 0.966 lagging) or more commonly in complex notation (e.g. 5,000W + j100VAR, where j is the electrical engineering version of the imaginary number i, because I represents current).
We see by the triangle that when dealing with a large inductive load (significant power factor angle), Volt-Amps and Watts are not quite the same thing. This is an important distinction to make, since the Volt-Amps are the apparent power draw from the system. This means that the actual current draw of the system is higher than it really has to be (Power = Volts × Amps). If we could eliminate the VARs from the system, we could bring the Volt-Amp draw back down to being equal to, or nearly equal to, the Watt draw.
Power Factor Correction
Since large power factor angles mean that the Volt-Amps consumed will be significantly larger than the actual Watts consumed (Watts being the actual useful power consumed), power generation facilities want to keep their consumer's demand very close to a power factor of 1 (i.e. cos(0) ). This is done through a process called power factor correction, and will be the responsibility of the consumer. If the power factor is not sufficiently corrected, the power company may impose fines.
The first step in power factor correction is to determine whether the load represents a leading or lagging power factor. This is almost always lagging, since the most common source of VARs are induction motors. Next this power factor must be compensated for by an equally strong power factor in the opposite direction. In the case of factories, this is usually done by adding power factor correction capacitors to the motor control center — a number of capacitors are switched on in the MCC to negate the effect of the induction motors. As far as our triangle is concerned, this has the effect of adding VARs in the opposite direction, bringing the triangle's hypotenuse closer to the Watt side. These capacitors will be as close to ideal as practical (that is, having little internal resistance) so that they do not significantly increase the Watt consumption as they lower the power factor angle, which would of course defeat the purpose.