Power Factor Basics: Real, Reactive and Apparent Power

Utility Power

Utility power comes to us as Alternating Current electricity.

•  AC electricity is a sinusoidal waveform. Both the voltage and current go up and down over time, like a roller coaster.

•  The frequency is 60Hz (Hertz) or 60 cycles per second in the USA. In Europe, it’s 50 Hz.

Characteristics of the Electrical Load

There are three types of electrical loads: •  Resistive •  Inductive •  Capacitive

Voltage and Current are “in-phase”

When the voltage and current waveforms both cross the zero-point of the Y axis (or magnitude) at the same time, they are said to be “in-phase.” They are synchronized in their cycles and go positive at the same time and negative at the same time- precisely.

Resistive Load

When the load is purely resistive, the current and voltage will always be “in-phase” with one another. In electrical engineering terms, resistive loads have only frequency-independent components. •  That’s important because if there are frequency-

dependent components, it’s possible that the voltage and current may no longer be “In-phase.

Inductive Load

When the load is “Inductive,” it will draw the voltage in advance of the current, pulling them out of phase with one another. We call this a “Lagging” power factor because the current lags the voltage.

Capacitive Load

When the load is “Capacitive,” the current will be drawn in advance of the voltage, pulling them out of phase with one another. We call this a “Leading” power factor, because the current leads the voltage.

The “Power Factor”

“Power Factor” is a unit-less term, computed as the cosine of the phase angle between the voltage and current waveforms.

Power Factor= cos θ  

•  Power Factor = 1.0 (unity), means the voltage and current are exactly in-phase •  The load is purely resistive

•  Power Factor < 1.0, means the voltage and current are out of phase •  The load is either capacitive or inductive

If the Power Factor is not 1.0, it means we have a “Reactive” load. •  We are using energy that is not doing real work for us •  ‘But it’s almost always the case

Let’s be “CIVIL”

A handy way of remembering which is leading or lagging is the “Power 101” cheat term:

 Voltage, “V” is in the middle. The letter, “I” is for current (yes it’s counter intuitive but that’s life). If the load is Capacitive, “C,” the current, “I” is before the voltage, “V” so it is “Leading.” If the load is Inductive, “L” (“L” is the symbol for inductance. ‘Don’t ask), the current, “I” comes after the voltage, “V” and is therefore “Lagging.” ‘Get it?

In  a  Capacitor  the  I  (current)  leads  Voltage,  Voltage  leads  I  (current)  in  an  inductor  L  

Apparent Power, Reactive Power, Real Power

The full glass including the foamy head represents the “Apparent Power.” This is the total power in the circuit.   The beer at the bottom is the “Real Power,” or “True Power.” This is the power that can do real work.   The foam is the “Reactive Power.” Reactive Power doesn’t do real work. It’s just absorbed and returned by the load, but is a necessary part of the overall “Apparent Power.”   Reactive Power places additional design requirements on the electrical distribution infrastructure in the data center, but is necessary in order to deliver the power capacity to the load.

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