At AMETEK Programmable Power, we are committed to helping you select the best AC power source for your test system, even if you're not currently an expert on AC power. The first thing that you need to know are the terms used by AC power engineers. Below, you'll find definitions for the three most basic AC power terms you'll need to know: true power, apparent power, and power factor.
- True Power. We're all taught that power consumed by a load equals the voltage across the load times the current flowing through the load. While this is certainly true for DC loads, the situation is a little more complicated for loads that are reactive. To calculate the true power being consumed by a load, you need to take into account the non-sinusoidal waveform shapes that may be present, and the current lead or lag angles caused by reactive elements in the load. The true power drawn by a load will be less than the simple product of the voltage across the load and the current through the load as a result of these factors.
- Apparent Power (or Volt-Amps). When a reactive load is connected to an AC power source, it appears to consume more power than it actually does, hence the term “apparent power.” The reason a reactive load appears to consume more power than it actually does is is because the reactive load will actually return some of the power back to the source. For this reason, we do not measure apparent power in watts, but rather volt-amps. Volt-Amps, or VA, is the product of true rms current multiplied by the true rms voltage.
Knowing about volt-amps is very important when selecting AC power sources and designing the wiring and circuit protection of a test system that uses an AC power source. The reason for this is that while the apparent power may be greater than the true power being consumed, the current flowing through the load is very real. For example, a reactive load across a 120 VAC source may have a true power rating of 2,400 W, but an apparent power rating of 3,600 VA. The load current in this case would be 30 A, and not only would the AC source need to provide 30 A, wire sizes and circuit protection devices would need to be chosen to handle this current.
- Power Factor. Power factor is the ratio (unitless) of true power (measured in watts) to apparent power (measured in volt-amperes). Power factor can range from 0 for a purely reactive load to 1 for a purely resistive load. When the load is purely resistive, the power factor is 1, and the true power equals apparent power. When a load is reactive, the power factor will be less than 1, and the true power will be less than the apparent power. Let's calculate the power factor for the example we used in the definition of apparent power:Power Factor (PF) = true power / apparent power = 2400 / 3600 = 0.667