Lesson AC Capacitance · Field integration
AC capacitance integrated
This lesson ties Units 20–23 together for field work: what a capacitor is, how banks combine, RC timing, types and tests, AC leading current, XC, and series/parallel RC with power factor. Leave with a practical sequence: identify the job of the capacitor → verify type and ratings → discharge → measure → decide replace or redesign.
The story in four beats
- Storage (Unit 20): Plates + dielectric store energy. C = Q/V. Parallel adds C; series reduces C. τ = RC; five time constants ≈ done. Polarized vs nonpolarized, markings, tests.
- AC behavior (Unit 21): Charge/discharge every half-cycle. Current leads voltage by 90° in pure C. XC = 1/(2πfC). Frequency and µF set current.
- RC series (Unit 22): One current; voltage triangle; Z = √(R²+XC²); leading PF = R/Z.
- RC parallel (Unit 23): One voltage; current triangle; IT = √(IR²+IC²); leading PF = IR/IT.
If you can sketch those four beats on a whiteboard, you own this course.
Field decision tree
What is this capacitor doing?
| Role | Clues | Key checks |
|---|---|---|
| Motor run | Dual/single can near compressor | µF, Vac, Herm/Fan/C |
| Motor start | Often in series with start winding / relay | Drop out after start; Vac; duty |
| PF correction | Panel or MCC bank | Current, fuses, stages, harmonics |
| Filter / bypass | Across DC or signal | Polarity if electrolytic; ESR/µF |
| Timing RC | Near logic/control | R and C both correct |
Safety that never gets optional
- Assume charge until proven otherwise
- Discharge with a rated resistor, not a screwdriver
- LOTO before rebuilding banks
- Respect Vac/Vdc and polarity
Capacitors can deliver very high current for a short time. Treat large cans like stored energy devices.
Formulas worth muscle memory
C = Q/V τ = RC (≈5τ full) XC = 1/(2πfC) Z_series = √(R² + XC²) IT_parallel = √(IR² + IC²) PF = P/VA (leading in RC)
Link forward to RLC
Real machines combine R, L, and C. XL rises with frequency; XC falls. When XL = XC you get resonance — high currents or high voltages if you are not careful. The next course (RLC and filters) starts from that collision of reactances.
Integration checks before you leave a job
- Nameplate µF and voltage match the replacement.
- Measured C within allowed tolerance.
- Terminals correct on dual cans.
- No bulge, oil, or burned terminals left in service.
- For PF banks: current reasonable at operating voltage; stages work.
- Document what you changed for the next tech.
Numbers you should be able to work cold
Carry these without notes:
- Parallel CT = sum; series 1/CT = sum of reciprocals
- τ = RC; ~5τ full
- XC = 1/(2πfC); I = E/XC (pure C)
- Series RC: Z = √(R²+XC²); PF = R/Z leading
- Parallel RC: IT = √(IR²+IC²); PF = IR/IT leading
Replacement script (say it aloud)
“Discharge. Read µF and Vac. Match type — run vs start, polarized vs not. Measure the new can before install. Land terminals correctly. Retest under load.”
If any step is skipped, call-backs climb.
Bridge to RLC
Net plant behavior is rarely pure RC. Motors add XL. When XL and XC meet, resonance and filters appear. Keep capacitive tools; add inductive ones next.
Field case
Situation. Rooftop unit: compressor hums, fan runs. Dual run capacitor: fan section OK, herm section 40% low. Tech has a single 35 µF / 370 V in the truck; nameplate wants 35/5 µF dual.
What you do. Replace with a proper 35/5 µF dual (or two cans wired correctly). Discharge, pull old can, verify new µF, reconnect Herm/Fan/C, test start current and pressures. Do not “make do” by paralleling random electrolytics from a DC drawer.
Applied lesson. Integration means matching role + ratings + topology, not only a µF number.
In the field
Symptom
Hard start, high amp draw, PF penalty, control noise, timing faults
Where to look
Entire capacitive path: type, wiring series/parallel, R companions, frequency
Likely causes
- Open/low C, wrong Vac, reversed electrolytic, harmonic overload, wrong RC
What to measure
- Discharge → µF → Vac/Vdc → E and I → infer XC → PF if needed
What not to do
- Swap parts by case size
- mix AC motor cans with DC electrolytics
Checklist
- I can explain DC storage and AC leading current in one short story
- I choose series vs parallel rules correctly
- I calculate XC and expected current
- I handle series and parallel RC triangles
- I apply PF leading/lagging language correctly
- I follow discharge → test → replace discipline
- I know when to escalate to RLC/resonance thinking