Power factor describes how much of the current flowing into a facility does useful work. This guide explains what the number means, why utilities charge for a low one and how capacitors raise it, drawing on fact sheets from the U.S. Department of Energy (DOE) Motor Challenge program and a technical paper on capacitor resonance. Sizing and installing correction equipment is work for qualified engineers, capacitor suppliers and the equipment manufacturer.

Real, reactive and apparent power

DOE defines power factor as the ratio of real (working) power, measured in kilowatts (kW), to apparent (total) power, measured in kilovolt-amperes (kVA). Reactive power, measured in kilovars (kVAR), is the nonworking power caused by the magnetizing current that inductive equipment needs.

DOE explains it with a horse pulling a railcar from the side of the track: only the forward part of the pull moves the car, and the sideways part is wasted effort. The closer the horse walks to the center of the track, the nearer power factor gets to 1 and reactive power to zero.

In DOE’s example, a load drawing 100 kW of real power and 142 kVA of apparent power has a power factor of 100/142, or 0.70. DOE says that means only 70% of the current supplied by the utility is producing useful work.

Why it is low

Low power factor is caused by inductive loads such as transformers, electric motors and high-intensity discharge lighting, according to DOE. A second DOE fact sheet explains that inductance in motor windings delays current flow, shifting it out of phase with voltage. It stresses that low power factor does not imply lost or wasted power, just excess current: the energy is stored in the windings’ magnetic field and returned to the line each cycle, but the distribution system must be sized to carry it.

Load matters. The same fact sheet says motor power factor tends to drop off sooner, but less steeply, than efficiency as load decreases, and that below about 50% load, reactive magnetizing current causes power factor to degrade.

Why it costs money

  • Utility charges. DOE says many utilities charge an additional fee when power factor is below 0.95.
  • Lost capacity. Extra current reduces the distribution system’s capacity and causes power losses and voltage drops.
  • Equipment wear. Excessive voltage drops can cause overheating and premature failure of motors and other inductive equipment.

How to raise it

  1. Cut idle running. Minimize operation of idling or lightly loaded motors.
  2. Watch voltage. Avoid running equipment above its rated voltage.
  3. Replace with care. Replace standard motors with energy-efficient ones as they burn out, though DOE notes that power factor still depends on load and a motor must run near rated capacity to gain the benefit of a high power factor design.
  4. Add capacitors. Install capacitors in the AC circuit to reduce reactive power.

How capacitors do it

DOE says reactive power from inductance acts at 90 degrees to real power, while capacitance acts in the opposite direction, 180 degrees from inductance. With both in a circuit, energy passes back and forth between capacitor and inductor, reducing the current drawn from the supply.

DOE’s worked example starts with the 100-kW load at 0.70 power factor and 100 kVAR of reactive power. Adding 67 kVAR of capacitance leaves 33 kVAR, cuts apparent power from 142 to 105 kVA, reduces current by 26% and raises power factor to 0.95. DOE says capacitor suppliers and engineering firms can help determine the optimum correction and locate and install capacitors correctly.

Common problems

A white paper by Cowles Andrus III of Power Monitors, Inc., dated November 1, 2014, describes how correction capacitors can interact with the rest of the system.

  • Harmonic resonance. Resonance occurs at the frequency where the capacitors’ reactance equals the inductive reactance of the substation transformer and loads. The paper says it typically falls between 200 and 700 Hz, that systems with several capacitors can have several resonances, and that resonance is sharper when capacitors sit near large motors.
  • Amplified harmonics. Near resonance, harmonic voltage or current can be multiplied by up to a factor of two for a very sharp resonance. In the paper’s example, a 120-kVAR bank on a 10-MVA source resonates at about 547 Hz, close to the ninth harmonic at 540 Hz.
  • Moving targets. Resonant frequency shifts with capacitor kVAR and can move with load changes, producing intermittent problems. Energizing a capacitor can itself excite a resonance.
  • Symptoms. The paper lists overvoltage and overcurrent stress, tripped breakers and blown fuses, damaged capacitors and, in serious cases, transformer failure or fire. A blown fuse can remove the capacitor and leave power factor uncorrected.
  • Mitigation. The paper suggests sizing capacitors so resonance avoids known harmonic trouble spots, or adding a harmonic filter designed to resonate below any expected harmonics. It recommends recording total harmonic distortion and lower-order odd harmonics, and points to IEEE 519 for harmonic control practice.
  • Capacitor ratings. Citing IEEE Std 18-2002, the paper says shunt capacitors must withstand continuous overvoltage of 110% and overcurrent of 180%.