PlantFlow 3D › Guides › Power factor and capacitor banks
Power factor and capacitor banks
What power factor means, why motors lower it, and how an automatic capacitor bank brings it back up and reduces the line current.
Real and reactive power
A motor takes real power (kW), which does the work, and reactive power (kvar), which builds its magnetic field and flows back and forth without doing work. The supply must carry both, so the current is larger than the real power alone would need. Power factor is the ratio of real power to apparent power (kVA): 1.0 is ideal, and a lightly loaded motor may be well below 0.8.
How capacitors help
A capacitor takes reactive power of the opposite sign. Placed near the load, a capacitor bank supplies the motor's reactive power locally, so less of it flows from the supply. The real power stays the same, but the apparent power and the line current fall, which reduces cable losses and can avoid utility penalties for low power factor.
Example: a motor taking 8.3 kW at power factor 0.85 also takes about 5.2 kvar. Adding 4 kvar of capacitors leaves about 1.2 kvar, and the power factor rises to about 0.99.
Automatic switching
Loads change, so banks are split into steps switched by contactors. A power factor relay reads the current and voltage and switches steps in when the power factor is below the target, and out again when the load falls. Too much capacitance makes the power factor leading, which is also undesirable.
Safety
Capacitors store charge. Each step needs time to discharge, normally through built-in resistors, before it may be switched in again, and before anyone touches the terminals. Use the discharge time and ratings from the capacitor data sheet.
Try it yourself
Choose “Power factor correction” in the simulator, start the motor and watch the power meter as the relay switches in the two capacitor steps. Then stop the motor.
Open the PlantFlow 3D simulator →Safety: this guide explains principles for learning. Real installations must follow the applicable codes, the equipment manufacturer's instructions and the approval of a competent, qualified person. Isolate, lock off and prove dead before touching any circuit.