Using Power Factor to Check Capacitors Under Load

The power triangle of an inductive load contains three sides: reactive power on the side, real power on the bottom, and apparent power on the hypotenuse. Apparent power (volt-amps/VA) is a function of real power (true watts) and reactive power (VAr), and the power factor (theta or cos theta) is the angle between apparent power and real power. An inductive load generates a magnetic field, which will result in some reactive power that doesn’t do any work.

As we get more reactive power, the gap between apparent power and real power increases. When that happens, the inductive load uses more utility power to work and becomes inefficient. Capacitors provide reactive power, thus decreasing the reactive power required from the utility and closing the gap between real and apparent power. The best power factor is unity or 1; anything less than that indicates that there is reactive power.

If you set the meter to kilowatts, the meter will display kilowatts, amps, and voltage to see the real power. You can also see the lag or lead, which indicates reactive power. Pressing “function” will show you the apparent power. Pressing “function” again will show you cos theta, which is your power factor. A power factor of unity (1) is ideal but not always realistic; a low power factor may indicate a weak capacitor.

When measuring the amperage, you want to make sure that the clamp is as far away from other wires as possible as not to pick up readings from nearby wires. The wire should also be at the center of the clip. Meanwhile, you use the meter leads to check the voltage (usually at the contactor or across the capacitor). You can use the measurements taken with the Supco Redfish iDVM550 and enter them into the Under Load Capacitor Test tool on the HVAC School app.

Alternatively, you could use the Supco TechLink app. The meter’s readings link directly to the app, which allows you to see the power factor automatically.

Supco Redfish iDVM550 Power Quality Meter and resources: https://www.trutechtools.com/Redfish-iDVM-550-Wireless-Power-Clamp-Meter