Power Factor Correction

Published:
By Cole Ferguson



One of the great things to do after you get a power system study completed, is implement power factor correction. With power factor correction you will be able to properly support the voltage at your site, but more importantly you will likely be able to save some money. Large loads on a utility are charged on power factor, maximum demand and energy use. At home you will only be charged on energy (kWhr).

If you can increase the power factor to unity, you will save on energy, and any reactive power charges. The maximum demand will be slightly helped, but you will more likely need to change procedures to affect this aspect of the bill.

What is Power Factor Correction?

Power factor correction is any action on an electrical system that brings the power factor of that system closer to unity.

How does it work?

Power consists of both real and reactive components. The real component of power is the part that does all of the actual work. The reactive component of power is the power generated by the magnetizing current in a motor on transformer. This magnetizing current is necessary to operate machines with alternating current, but does no usable work. Power factor correction works by reducing the reactive power component while maintaining the real power component. This reduces the overall current consumption of your system. The simplest and most common way to improve your power factor is static power factor correction (adding capacitors to your system).

Capacitors

In many electrical systems the current and voltage waveforms are out of sync by a certain amount. In an induction motor, the current lags the voltage. A capacitor however is a purely reactive load, and as a result current drawn by a capacitor leads voltage. The effect this has is that as the alternating current cycles back and forth, it looks like the reactive component of the capacitor current and the reactive component of the motor current "cancel" each other out. You can see this is the phasor diagram below.

stock photo

Fig. 1 is the phasor diagram of the induction motor. The current lags the voltage. Fig. 2 is the phasor diagram of the capacitor: the current leads the voltage. Fig. 3 is the result of adding a capacitor to your motor circuit: the capacitor current and motor current add together to reduce the phase angle and bring it closer in line with the voltage angle.| Three phasor diagrams of voltage (V) and current (I). Fig. 1 is the phasor diagram of the induction motor: the current lags the voltage. Fig. 2 is the phasor diagram of the capacitor: the current leads the voltage. Fig. 3 is the result of adding a capacitor to your motor circuit: the capacitor current and motor current add together to reduce the phase angle and bring it closer in line with the voltage angle.

The great thing about capacitors is that they are cheap, reliable, and come in many different sizes. If you can't find a capacitor with the specific size you need, you can combine multiple capacitors in a capacitor bank.

Where to place the Power Factor Correction Equipment?

Since you want your capacitors to draw reactive current away from your motor load, you want to place the capacitors in parallel with your motor. "You can place the capacitors at the equipment, distribution board, or the origin of the installation. Static power factor correction must not be applied at the output of a variable speed drive, solid state soft starter or inverter as the capacitors can cause serious damage to the electronic components. " (John Ware, IEEE Wiring Matters, Spring 2006).

So there you have it! If you want to perform power factor correction on your system to save yourself some money and possibly increase the capacity of your systems, you have to add capacitors in parallel with your loads. Thanks for reading!

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