Power factor correction

Induction motors draw magnetizing VARs. Transformers do too. The utility (and your generators) must supply those VARs, which increases current and may trigger PF penalties.

PF = P / VA Low PF → higher VA for the same P → higher I.

Capacitors draw leading current (supply leading VARs, in power-accounting language). Paralleled on the bus, they cancel lagging VARs:

VARs_net = VARs_L − VARs_C

As net VARs fall, VA falls toward P, PF rises, and ILine drops.

• At the service (global correction)
• At MCCs or large motors (local correction)
• Automatic staged banks that switch with load

Local caps reduce current in the feeders upstream of the bank — a real conductor and transformer loading benefit.

You need enough capacitive kVAR to move from the existing PF to the target PF at the operating kW. Engineering tables and formulas convert “kW today at PF1 → desired PF2” into kVAR. Field techs often install OEM-specified banks; changing stages still requires understanding lead/lag.

At night or weekends, motors stop but fixed caps may stay online → leading PF. Some generators and UPS systems dislike strong leading PF. Automatic controllers or contactors tied to load prevent that.

Harmonics plus capacitors can resonate — coordinate with the RLC course lessons when VFDs dominate.

Capacitor banks store charge. Discharge resistors should bleed them down; still verify with a meter. Use rated fuses; caps have high inrush when switched.

Rough idea: to move kW load P from PF1 to PF2,

kVAR_needed ≈ P × (tan θ1 − tan θ2) where θ = cos⁻¹(PF).

Example: 100 kW from 0.70 → 0.95 lagging needs substantial capacitive kVAR — compute exactly before buying. Field techs confirm with engineering sheets; still understand the direction of the math.

Energizing caps causes inrush. Use rated contactors and fuses. Never substitute general-purpose breakers without checking capacitor duty.

VFDs present? → harmonic study may be required. On-site generators? → watch leading PF. Existing banks? → measure before adding parallel µF.

Situation. A shop installs a fixed PF bank sized for daytime machine load. After hours, a small generator feeding a few lights and a freezer shows voltage rise and hunting.

What happened. Leading VARs from the bank dominated the light lagging load.

Applied lesson. Stage or switch caps with load. Measure PF lead/lag, not only “PF number.”

### Teaching pause — say this out loud

Before you leave this lesson, explain the main idea to an imaginary first-month helper in under one minute. If you need the book open to do it, reread How it works once more. Field diagnosis only helps after the concept is yours.

Also sketch the key diagram from memory (triangle, wye/delta, filter shape, or charge curve — whichever this lesson used). Labels beat artistic skill.

### Why this lesson matters on Monday morning

Power factor correction is not trivia. You will meet it when a meter reading looks “impossible,” when a replacement part is almost right, or when a helper asks why the book uses √3 or lead/lag. Master the model here so the next call is pattern recognition, not panic.

Common Monday uses: verify a nameplate against clamps, explain a PF or capacitor change to a customer, or catch a miswired series/parallel or wye/delta assumption before energizing.