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Power management

Why a power-factor correction device is better suited to industrial applications

July 26, 2010 | Mohit Arora | 222901251
Why a power-factor correction device is better suited to industrial applications Mohit Arora, Systems Engineer, Freescale Semiconductors, explains the reasons that make power-factor correction devices better suited to industrial applications.
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How often are homeowners approached by a salesperson at the doorstep selling a device that can go in a wall and supposedly save energy or reduce the monthly electricity bill substantially? The so-called "power-saver device" (known by different names) is nothing but power-factor correction (PFC) device that connects to the mains and improves power factor, and thus the apparent power measured by the meter. However, it's important to notice that a residential user's utilities bill is based on real power rather than apparent power, and thus none of these devices really reduce their monthly bills. In this article, I explain the different power types that an electricity meter measures, power factor, and power factors' implications on power measurement. Although a PFC device may be useful for industrial applications, the additional cost does not really justify this device to be used in residential applications, contrary to the claims.

The basics: Watt (W) and Volt-amp (VA)
Watts (W) and Volt-amps (VA) are often considered as interchangeable and equivalent units to measure power. This is far from true in a practical scenario.

Power by definition is the rate of work or energy flow.

Power = Energy/Time

Instantaneous power is defined as (p)t = v(t) (i)t where:

v (t) = instantaneous voltage as function of time t
i (t) = instantaneous current as function of time t

For a simple alternating current (AC) circuit, real (active) power is defined as the average value of the instantaneous power over a complete AC cycle.

Real power (W) = average{v(t) i(t)}

Note that real power is measured in Watts (W) and represents the actual work done by an electric current or actual energy consumed by a load.

When a sinusoidal voltage source is connected to a resistor, current flows and the power is dissipated in the register. Figure 1 shows the instantaneous power as a product of voltage and current with the following values:

VRMS = RMS value of the voltage = 1V
Vpp = Peak to peak voltage = 1.414V
IRMS = RMS value of the current = 1A
Ipp = Peak to peak current = 1.414A
R = Resistor across the voltage source = 1 Ohm

Click on image to enlarge.

Instantaneous power = Vpp Ipp

Since these values are across time, instantaneous power at the positive peak of both voltage and current (Point "X" in Figure 1) = 1.414V x 1.414A = 2W.

Similarly instantaneous power at the negative peak of both voltage and current (Point "Y" in Figure 1) = -1.414V -1.414A = 2W. Note that average power across the sine wave would be 1W

Electrical systems usually have inductors and capacitors, which are referred to as reactive components. For the same Inductive load, current in an ideal inductor would lag exactly 90 degrees behind the applied voltage as shown in Figure 2. Between the two vertical lines shown in the figure, negative current multiplied by positive voltage would give a negative power. For a part of the cycle, negative power would mean energy would actually be transferred from the inductor (load) back to the source.

Click on image to enlarge.

Ideal reactive components (where phase difference between the current and voltage is exactly 90 degrees) do not dissipate any energy, but they actually do draw currents and create voltage drops. This "imaginary power" is called reactive power. Its average value over a complete AC cycle is zero because the phase shift between voltage and current doesn't contribute to net transfer of energy (as shown in Figure 2).

Reactive power is measured in Volt-amps-reactive (VAR). The combination of real (active) power and reactive power makes up apparent (or total) power, measured in Volt-amps (VA).

Understanding power factor:
Power factor (PF) by definition is the ratio of real power to apparent power:

Power Factor (PF) = Real Power (Watts) Apparent Power (VA)

It is found that people often convert Watt to VA (Watts = VA), which is not true unless power factor of a device is 1.

Figure 3 shows power triangle that shows relationship between real (active) power, reactive, and apparent (total) power, all represented in terms of vectors.

Also note that when both voltage and current are sinusoidal, power factor = Cos where is the angle between voltage and current phasors.

Click on image to enlarge.

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