Capacitor Bank (PFC)
PFC
Banks of AC capacitors that supply the reactive current inductive loads (motors, transformers) would otherwise draw from the grid, correcting power factor and cutting utility penalties. Stored charge and resonance make them serious equipment.
Schematic symbol
Capacitor (non-polarised)
How capacitor bank (pfc) appears in a circuit diagram.
Types & variants
Key specs
Reactive power
kvarCorrection capacity, stepped in stages (e.g. 5+10+25 kvar).
Voltage rating
V440–525 V units on 415 V systems for harmonic headroom.
Detuning factor
%7% reactors shift resonance away from 5th/7th harmonics.
Discharge time
sInternal resistors must bleed to <50 V within a minute — verify before touching.
Markings
kvar, voltage, frequency and capacitance on each can; APFC controller shows target cos φ.
Standard values
Cans of 1–25 kvar; APFC panels of 50–300 kvar in stages.
How to choose
From a metered power-factor study: kvar to lift cos φ to ~0.95+, detuned if VFDs/harmonic loads are present (plain caps + harmonics = resonance fires), automatic if load varies.
Pinout & package
Three-phase delta-connected cans with discharge resistors, switched by capacitor-duty contactors in an APFC panel.
Example circuits
- APFC panel on a factory incomer
- Fixed 2 kvar correcting a big compressor motor
- Detuned bank in a VFD-heavy plant
Common failures
Bulged/vented cans (harmonics + heat), failed discharge resistors leaving lethal charge, contactor wear from capacitive inrush, resonance blowing everything when detuning was skipped.
How to test
Qualified personnel: isolate, wait, VERIFY discharged, then capacitance-check each phase against the nameplate (>10% low = replace) and inspect for bulging. Compare phase currents in service — imbalance flags a dying can.
Substitutes
Same kvar/voltage capacitor-duty units only. Never substitute DC or motor-start capacitors into PFC service.