Power factor penalties are the utility’s polite invoice for your magnetizing current. Correcting them is old, solved engineering — right up until a plant full of drives meets a capacitor bank designed for 1985.
Sizing from evidence
The utility bill is the sizing document: reactive energy charged, target cos φ per tariff, and the load profile’s shape. Steps sized so the controller can follow real load swings (many small steps beat few large ones), and the controller’s C/k and switching-delay settings actually commissioned rather than left at defaults — a hunting bank that cycles contactors all day is a maintenance bill on a timer.
The harmonics collision
Capacitors and supply inductance form a resonant circuit; drive-rich plants excite it. When the resonance lands near the 5th or 7th harmonic — exactly where six-pulse rectifiers shout — currents amplify, capacitors cook, and fuses tell confusing stories. The modern default in any plant with meaningful drive load is therefore detuned banks: series reactors shifting resonance safely below the excitation, at modest extra cost. The diagnostic tell for existing plants: capacitor fuses blowing “randomly”, bank running hot, THD measurements worsening with the bank switched in.
Maintenance reality: capacitors age (capacitance drops, a stage quietly weakens), contactors wear from the switching duty, and a yearly measurement visit — currents per stage, temperatures, THD with and without — keeps the bank an asset instead of an incident.
FAQ
Fixed compensation at the motor or central bank? Both have homes: fixed at large steady motors (sized below magnetizing current, never on drive-fed motors), automatic central for the plant’s swing.
Active filters instead? Where harmonics and reactive power both need fixing, an active solution can close both — price it against detuned banks per case.
Zone Otomasyon surveys, sizes and maintains compensation systems with measurements, not folklore. Power quality services.