| Power Factor (Part Two): Electricity Behaving Better |
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| Written by Joe Evans, Ph.D. | |
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Page 2 of 2
FixesThere are several ways to improve PF. The most common one will increase the PF of a motor, other inductive devices or the entire system as a whole. Another involves the use of synchronous motors. Still another, the VFD, is limited strictly to single motors. Let's look at the first one and save the others for another column. So far we have seen examples of circuits that are purely resistive, purely inductive, and one that is a combination of the two. There is, however, another type - the purely capacitive load. The capacitor is a very simple, yet interesting and useful electronic device. It is similar to a battery in that it stores a charge but, unlike the battery, it does not produce electrons through a chemical reaction. It simply stores them for use at some later time. Another difference between a capacitor and a battery is the discharge rate. A battery discharges slowly, over a period of hours or days, while the capacitor can release its entire charge in a few milliseconds! Its design consists of two metal plates that are kept from touching one another by some dielectric material (insulator). When a capacitor is placed in a DC circuit, the plate that is connected to the negative terminal accepts electrons while the other plate loses electrons. When it is fully charged, voltage reaches its maximum and the flow of current in the circuit ceases. When a capacitor is placed in an AC circuit, current behaves differently. It flows continuously because the capacitor is always charging and discharging due the changing polarity of the AC wave form. These rapid changes have an effect on the voltage and current waves similar to the one we saw in an inductive circuit. But it is exactly backwards - voltage lags current. Unfortunately, this particular lagging relationship or "capacitive reactance" is more complex than its inductive cousin. Let's just say that it has to do with frequency and the time required for voltage and current to change within the capacitor. But the result is the important point. Figure 2 shows the relationship of voltage and current in a purely capacitive circuit. Here we see the exact opposite of the inductive circuit we saw last month - voltage is lagging current by 90-deg. As with inductive reactance, power is not consumed during charging, but is returned to the circuit when the capacitor discharges.
Figure 2The addition of capacitive reactance to a circuit containing both resistance and inductive reactance changes our equation for impedance in a very beneficial way. Now we have √R2 + (XL2 - XC2), where XC is capacitive reactance. If capacitive reactance and inductive reactance are equal, reactance goes away and the circuit becomes purely resistive! When properly sized capacitors are added to a circuit that contains an inductive load, power factor will increase. This occurs because the current leading effect of the capacitor will cancel all or a part of the current lagging effect of the inductor. There are a couple of different ways of using capacitors to improve power factor. The most simple, "static" correction, will increase the PF of an individual motor. In this application, an appropriately sized capacitor is installed in parallel with the motor windings and is located between the starter (across the line) and the motor. In this location, the capacitor will always be in the circuit when the motor is running and then removed automatically when the starter disengages. Its removal prevents possible overcorrection when the motor is not running. It is important to follow the motor manufacturer's guidelines when sizing a static capacitor in order to avoid under- or over-correction. As a rule of thumb, correction usually should not exceed 80 percent of the motor's magnetizing current. Static correction capacitors should not be installed on the output side of a solid-state soft starter because of the potential damage that they can cause. In these installations, capacitors should be located on the input side and well upstream of the soft starter. Another method is known as "bulk" correction. In this form of PF improvement, a group or bank of capacitors can be installed right after the supply transformer to correct the PF of the entire system that it feeds. A controller monitors the total power factor of the system and switches groups of capacitors on and off depending upon the inductive load at any point in time. Bulk correction can approach unity (PF = 1), but a PF of 90 percent to 95 percent is more typical. Adding capacitance to correct power factor can be a relatively easy and inexpensive fix for a potentially expensive problem. There is, however, a growing PF problem that cannot be fixed by adding capacitors to a circuit: switched mode power supplies which are found in personal computers and other electronic devices use rectifiers and switching transistors to regulate voltage. These nonlinear components produce harmonics that can feed back into the circuit and, if there are a significant number of these devices, overall PF can be reduced. This is becoming a significant concern for utilities because capacitors alone will not fix the problem. Expensive filters, composed of high current inductors, are needed to remove the harmonics and return the current to its linear form. Because of this, we are beginning to see mandated PF correction for power supplies that exceed a certain power requirement. Joe Evans is the western regional manager for Hydromatic Engineered Waste Water Systems, a division of Pentair Water, 740 East 9th Street Ashland, OH 44805. He can be reached via his website at http://www.pumped101.com/. If there are topics that you would like to see discussed in future columns, drop him an email.
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