I hope that my last column (Pumps & Systems, September 2014) provided a clearer definition of power factor and how it can be calculated. Power factor can also be explained using a vector triangle, but comparing the voltage and current wave forms is more intuitive and can lead to a clearer understanding of what is actually going on. In short, power factor is the percentage of the total current in the circuit that does work. From a power perspective, it is the percentage of apparent power in kVA that actually performs work. It is also the ratio of the load power measured in kilowatts (kW) to the apparent power measured in kilovolt-amps (kVA). Why is power factor important? After all, the current required to initiate a magnetic field in the stator is returned to the electrical grid when that field decays. However, power factor is important for several reasons. Even though the current is returned, the utility still has to supply it during each alternating-current (AC) cycle. Because it is not recorded by the kilowatt-hour meter, the utility does not get paid. Also, it is not available for sale to other customers because it must be available to meet the power factor requirement of the motor. Another reason is the increased wire and transformer size that is required to provide the additional current. These factors affect both the utility and the customer.
09/29/2014
Figure 1. A complete AC cycle for a circuit with a motor and capacitor (Graphic courtesy of the author)
The big difference in capacitive current is that it leads voltage by 90 degrees, while magnetizing current lags voltage by 90 degrees. They are completely out of phase throughout the AC cycle. This is a plus because the capacitor discharges current to the circuit as the magnetic field is building. When the field collapses, the capacitor stores it as a charge. In the example shown in Figure 1, the capacitor is sized to store and provide 100 percent of the magnetizing current required by the motor, so the improved power factor is 100 (1). Therefore, the utility would be required to provide the load current only during motor operation. This is an ideal example because power factor correction is usually limited to 95 (0.95).
Utility companies react differently to their customers’ power factor. If the overall load is small, some will often ignore it. Others offer incentives to increase power factor. However, the majority of utilities charge customers an additional fee if the power factor is below 0.9. Some require a minimum of 0.95.
Power factor correction capacitors can be installed and controlled in several ways. One is static or fixed correction, and the other is central or bulk correction.
In the next “Pump Ed 101,” I will discuss how capacitors are installed and the impact of their location in the system. I will also provide references that offer detailed information on capacitor selection and installation.