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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.

It should be noted that power factor, like all ratio measurements, is a unitless quantity.

For the purely resistive circuit for example incandescent light bulb, the power factor is 1 because the reactive power equals zero. Here, the power triangle would look like a horizontal line, because the opposite (reactive power) side would have zero length.

For the purely inductive circuit, the power factor is zero, because true power equals zero. Here, the power triangle would look like a vertical line, because the adjacent (true power) side would have zero length.

The same could be said for a purely capacitive circuit. If there are no dissipative (resistive) components in the circuit, then the true power must be equal to zero, making any power in the circuit purely reactive. The power triangle for a purely capacitive circuit would again be a vertical line (pointing opposite direction than it was for a purely inductive circuit).

Power factor can be an important aspect to consider in an AC circuit; because any power factor less than 1 means that the circuit's wiring has to carry more current than what would be necessary with zero reactance in the circuit to deliver the same amount of (true) power to the resistive load.

Let's consider two motors with the following characteristics.

Motor 1: 2KW, 230V, PF = 0.7
Motor 2: 2KW, 230V, PF = 1 (Though having a PF =1 for a motor may not be practical but lets assume it for now).

Current dissipated in both the motors would be as follows:

From above, it is clear that Motor 1 with poor power factor makes for an inefficient power delivery system.

Table 1 shows power factor (PF) for various type of electrical equipment.


Click on image to enlarge.

Note that any electrical equipment with non-resistive load has a power factor of less than 1. As an example, we know that CFL (compact fluorescent lamp) light bulbs have a Power factor of between 0.5 and 0.7 depending on maker and wattage, while incandescent light bulbs have a power factor of 1. Note that what a residential consumer is billed is for real power and not for reactive power that is a result of power factor so a CFL usage is purely from an electrical consumption point of view is good for consumer but bad for the power company. In the home, the amount of power consumed for the same level of lighting is highly reduced. The utility must transport the power to the home and if the reactive power gets higher, it means more losses in the lines and transformers, etc., along the way.

Electric motors, fluorescent lamps, refrigerator, air conditioning, and consumer electronics (such as televisions and computers) are examples of appliances that have power factors of less than one. This is because they include some type of storage element such as a capacitance or inductance.

Power factor correction
Poor power factor can be corrected by adding another load to the circuit drawing an equal and opposite amount of reactive power, to cancel out the effects of the load's inductive reactance. Inductive reactance can only be canceled by capacitive reactance. So power factor correction device typically includes nothing but a capacitor that is to be connected in parallel as the additional load. The effect of these two opposing reactances in parallel is to bring the circuit's total impedance equal to its total resistance (to make the impedance phase angle equal or at least closer, to zero).

Figure 4 shows a Wattmeter and ammeter connected to a typical load. Note that Wattmeter would measure real power while ammeter will measure apparent power.


Click on image to enlarge.

Here:


Click on image to enlarge.

If this load is inductive (i.e., electric motor), power factor (PF) would be lagging, which means a capacitor of appropriate size is required to be wired in parallel to be able to cancel out reactive power. Capacitor value can be calculated as follows:


Click on image to enlarge.

Modified circuit is shown in Figure 5.


Click on image to enlarge.

Since the capacitor's current is 180 degrees out of phase from the load's inductive contribution to current draw, the capacitor's reactive power will directly subtract from the load's reactive power resulting in zero or close to zero reactive power.

This correction, of course, will not change the amount of true power consumed by the load, but it will result in a substantial reduction of apparent power, and of the total current drawn from the 230-Volt source.

The correction unit includes a big bank of high-voltage capacitors and a controller that switches appropriate caps across the mains supply. This configuration is the principle behind most of the power-saver devices available in open market. As explained earlier, this type of device does not have an impact on the real power, and since residential consumers are billed on real power, this device has no impact on the monthly bill.

PFC is more suited for industrial applications since the load is mainly reactive, running huge motors; thus, industrial customers are billed based on apparent power. It's also typical that large industrial users are charged a penalty for a net power factor of less than cut of number because it directly impacts distribution losses for a utility company.

Even though PFC device may not reduce the electricity bill, it does not change the fact that inductive loads run more efficiently with corrected or at least improved power factors. By furnishing the necessary magnetizing current for induction motors and transformers, PFC capacitors reduce the current drawn from the power supply. Less current means less load on transformers and feeder circuits. Since the electrical current in the lines is reduced, I2R losses decrease, thereby improving efficiency and any unnecessary blackouts. Other benefits include less wear and tear on the appliance (in other words, the motor, since current drawn by the motor is reduced), thus improving life and better immunity to any current harmonics.

Since residential load is mostly resistive, these benefits bring little to a residential user; however, this can be necessary in an industrial application.

One situation where PFC can be extremely useful for residential is in designing a back-up energy system like home inverter or UPS (uninterruptible power supply). Correcting the power factor from 0.65 to 1 results in a 35% reduction in the size (VA rating), thus a less expensive inverter can be chosen though power consumption remains the same.

In homes today, and particularly since Energy Star rated appliances have hit the scene, motor driven appliances already have an appropriate sized capacitor attached to them.

Energy star products not only define maximum real power an appliance can draw in various modes but also defines the minimum power factor appliance must satisfy to get energy star certification. For example, Energy Star version 5.1 for game console/computer requires power factor to be greater than 0.9 at 100% rated output or maximum rated output less than 75W to be able to meet the requirements. By purchasing another PFC attach to your electric meter, you are basically adding a redundant device into your home than your appliance can use, so there is really no monetary savings by doing so and is certainly a marketing gimmick.

Mohit Arora is a systems engineer in Freescale Semiconductors. He is focused on energy/utility metering market. He has been involved in product definition and specification for ColdFire based products for mid-high end industrial market space. He earned a bachelor's degree in electronics and communication engineering from Netaji Subhas Institute of Technology (NSIT), India. He can be reached at mohit.arora@freescale.com.

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