Lesson AC Capacitance · Capacitance and reactance in AC
Capacitance in AC
In AC, voltage polarity keeps reversing, so a capacitor charges and discharges every half-cycle. Current appears to flow through the capacitor even though the dielectric is still an open circuit. In a pure capacitive circuit, current leads voltage by 90°, and the average true power is zero — energy is stored and returned, not consumed as heat in an ideal C.
Why current “appears” to flow through an open
A capacitor still has no electron path through the dielectric. In AC, the source constantly changes direction. One half-cycle charges the plates one way; the next half-cycle discharges and recharges the opposite way. An ammeter in the leads shows continuous alternating current.
Hydraulic analogy: two tanks connected to a reversible pump. Water never flows through a pipe joining the tanks directly, but water keeps moving as the pump reverses. The capacitor plates are the tanks; the dielectric is the wall between them.
Voltage and current in a pure capacitive circuit
In a purely capacitive AC circuit:
- Current leads voltage by 90 electrical degrees
- When voltage is at maximum, current is at zero (plates fully charged that half-cycle)
- When voltage is crossing zero, current is maximum (fastest rate of change of charge)
Compare with a pure inductor (current lags voltage by 90°) and a pure resistor (in phase). Capacitors are the opposite of inductors in phase behavior.
Power in a pure capacitive circuit
Instantaneous power goes positive and negative. Over a full cycle, true power (watts) averages to zero in an ideal capacitor. The product is reactive power measured in VARs (volt-amperes reactive).
Field meaning: a capacitor does not “burn watts” like a heater, but it still causes current to flow in conductors and transformers. That current heats wiring and counts on the utility meter’s ampacity even when watts are low.
Voltage rating on AC capacitors
Nonpolarized capacitors often list Vac ratings. Peak voltage is higher than RMS:
Vpeak ≈ 1.414 × VRMS (sine wave)
A capacitor on a 240 V RMS line sees about 340 V peak. Manufacturers rate motor-run capacitors for the AC service they expect (for example 370 Vac or 440 Vac). Using a part rated only for the RMS number without understanding peak is how dielectrics get overstressed.
Also respect DC working voltage on electrolytics in filtered supplies: ripple plus DC must stay under the rating.
Frequency effects
Capacitive behavior depends strongly on frequency. At higher frequency the capacitor charges and discharges faster, so more current flows for the same C and voltage — reactance falls (next lesson). At DC (0 Hz), after the transient, current is zero.
That is why a capacitor can block DC and pass AC: steady DC eventually stops; AC never stops reversing.
Series and parallel capacitors in AC
The same C formulas from Unit 20 still set total capacitance. In AC you then convert CT into reactance and current. Series strings still share voltage (watch peaks); parallel strings share current.
Numbers you should be able to work cold
For a sine-wave RMS voltage E on a pure capacitor:
I = E / XC with XC = 1/(2πfC)
If C = 40 µF at 60 Hz on 240 V:
XC ≈ 1/(377 × 40e-6) ≈ 66.3 Ω I ≈ 240/66.3 ≈ 3.62 A
That current is real on the clamp even though ideal true power is zero. Conductors and fuses must be sized for it.
Comparing capacitor and inductor phase at a glance
| Device | Current vs voltage | Energy storage |
|---|---|---|
| Pure R | In phase | None (dissipates) |
| Pure L | Current lags 90° | Magnetic field |
| Pure C | Current leads 90° | Electric field |
When a helper says “reactance,” ask: which kind? The lead/lag answer tells you instantly.
AC voltage rating margin
Motor-run cans often use 370 Vac or 440 Vac even on 208–240 V systems because of peaks, spikes, and manufacturer margins. Copy the OEM Vac class when replacing. A “250 V” film cap from an electronics drawer is not a motor-run substitute.
Field case
Situation. A tech measures 0 VDC across a coupling capacitor on a signal board and concludes “the capacitor is shorted because AC is getting through.”
What happened. The capacitor is doing its job: blocking DC while passing the AC signal. Continuity-style thinking from resistive circuits misled the diagnosis.
Applied lesson. Expect AC current in capacitor leads. Confirm with µF test out of circuit and look at DC bias separately. Leading current and DC blocking are normal capacitive behavior.
In the field
Symptom
Unexpected AC current with “open” looking capacitor; overheating conductors feeding capacitor banks
Where to look
Pure capacitive branches, PF banks, motor run circuits, coupling caps
Likely causes
- Normal leading current
- undersized conductors
- failed shorted dielectric (true fault)
What to measure
- Current in capacitor feed
- voltage rating vs peak
- waveform phase if scope available
What not to do
- Assume any AC current through a capacitor means a short
- ignore Vac vs peak
Checklist
- I explain AC charge/discharge without a path through the dielectric
- I state current leads voltage by 90° in a pure C circuit
- I distinguish watts vs VARs for capacitors
- I respect Vac ratings and peak voltage
- I know capacitors block DC after the transient
- I expect frequency to change capacitor current