An AC capacitor works by storing and releasing electrical energy in its dielectric field in sync with the alternating cycles of the AC waveform. Because the voltage polarity constantly reverses (typically 50 or 60 times a second), AC capacitors must be strictly non-polarized. Unlike DC electrolytic capacitors that rely on a chemical oxide layer sensitive to reverse voltage, AC capacitors use symmetrical dielectric materials like polypropylene film, ceramic, or non-polarized electrolytic constructions. Their primary jobs in a circuit are phase-shifting (creating starting torque in single-phase motors), AC coupling (blocking DC while passing AC signals), and reactive power compensation.

The Physics of AC Capacitance and Phase Shift

To understand how an AC capacitor works on the bench, you have to look at the phase relationship between voltage and current. In a purely capacitive AC circuit, current leads voltage by 90 degrees. When the AC sine wave crosses zero volts, the capacitor is fully discharged and draws maximum current as it begins to charge. As the voltage peaks, the capacitor becomes fully charged, and current drops to zero.

This phase shift is the exact mechanism that allows single-phase AC induction motors to start. A single-phase power supply creates a pulsating magnetic field, not a rotating one. By placing a capacitor in series with a motor's start winding, the current in that winding is shifted out of phase with the main winding. This creates a rotating magnetic field that gives the rotor the physical 'push' it needs to overcome inertia.

The opposition a capacitor presents to alternating current is called capacitive reactance ($X_C$), measured in ohms. It is inversely proportional to both the frequency ($f$) and the capacitance ($C$):

$X_C = \frac{1}{2 \pi f C}$

Worked Example: If you are testing a 10µF (0.000010 F) motor run capacitor on a standard 60Hz North American bench supply, the reactance is:
$X_C = 1 / (2 \times 3.14159 \times 60 \times 0.000010) = 265.25 \Omega$.
If you connect this to 120VAC, it will draw roughly 0.45A of continuous alternating current ($I = V / X_C$), despite having no physical resistance.

The Water Analogy: Imagine a water pipe with a flexible rubber membrane stretched completely across the inside. If you push water in one direction (DC), the membrane stretches and stops the flow entirely. But if you rapidly push and pull the water back and forth (AC), the membrane flexes in both directions. No water actually passes through the membrane, but the alternating kinetic energy transfers perfectly to the other side. That membrane is the dielectric.

AC Capacitor Types and Selection Matrix

Not all non-polarized capacitors are built for the same AC environment. Selecting the wrong dielectric for an AC application will result in rapid thermal failure or catastrophic venting. Use this matrix to determine which type belongs in your circuit.

AC Capacitor Selection and Specification Matrix
Type / Dielectric Construction & Form Factor Tolerance Tempco / Stability Typical AC Application
Metallized Polypropylene (CBB60/CBB61) Film wrap, epoxy fill, metalized electrodes that 'self-heal' minor shorts. ±3% to ±5% Highly stable; low dielectric loss at high temps. Continuous Motor Run, HVAC compressors, ceiling fans.
Non-Polar Aluminum Electrolytic Two back-to-back anodes in liquid electrolyte, radial or axial can. -10% / +30% Poor; capacitance drops heavily with age and heat. Intermittent Motor Start (duty cycle < 3 seconds).
Class 2 Ceramic (X7R / Y5V) Multi-layer ceramic chip or epoxy-dipped radial disc. ±10% to +80%/-20% Moderate to Poor; highly voltage-dependent (capacitance drops at high V). AC line filtering, snubber circuits, EMI suppression.
Metallized Polyester (CL21) Stacked film or wound, epoxy coated. ±5% to ±10% Good; stable up to 85°C. Audio crossovers, AC signal coupling, power supplies.

Which Type for Which Job?

If the capacitor stays energized while the motor or circuit runs (like a blower motor or a compressor), you must use a Metallized Polypropylene Film capacitor (Motor Run). They have extremely low Equivalent Series Resistance (ESR) and won't overheat under continuous AC load. If the capacitor is only in the circuit for 2 seconds while a heavy load spins up (like a table saw or well pump), a Non-Polar Electrolytic (Motor Start) is acceptable and much cheaper for high µF values, but it will overheat and vent if left in the circuit.

Decoding Physical Markings and Substitution Rules

Reading capacitor markings is where most bench mistakes happen. Manufacturers use different coding systems depending on the physical size and chemistry of the part.

How to Read the Markings

  • Motor Capacitors: Usually printed plainly. A label reading 45+5 µF 370VAC 50/60Hz indicates a dual-run capacitor. It contains two independent internal sections: one 45µF and one 5µF. The 370VAC is the maximum continuous RMS voltage, and it is rated for both global grid frequencies.
  • Ceramic Disc/Film (3-Digit Code): A marking like 104 means 10 followed by 4 zeros, in picofarads (pF). So, $100,000 \text{ pF} = 100 \text{ nF} = 0.1 \text{ µF}$. A marking of 222 is $2200 \text{ pF}$ or $2.2 \text{ nF}$.
  • Letter Suffixes: On ceramic caps, a letter after the digits indicates tolerance. J = ±5%, K = ±10%, M = ±20%. A cap marked 104K is a 0.1µF cap with a 10% tolerance.

Safe Substitution When the Exact Part is Missing

When you are on a jobsite or at the bench and the exact OEM part isn't in your bin, follow these hard substitution rules to avoid blowing up the board or burning out a motor:

  1. Voltage Rating: You can always substitute a higher voltage rating for a lower one. Replacing a 370VAC motor run cap with a 440VAC cap is perfectly safe and often results in a longer lifespan. Never substitute a lower voltage rating.
  2. Capacitance (Motor Run): Must be within ±5% of the OEM spec. If a compressor calls for 40µF, a 45µF will cause excessive current draw and trip the thermal overload.
  3. Capacitance (Motor Start): Can be within ±20%. A motor needing 200µF to start will run fine on a 220µF start cap.
  4. Frequency Shift: If you use a 50Hz-rated cap on a 60Hz supply, the capacitive reactance ($X_C$) drops, meaning it will draw more current and run hotter. Always match the Hz rating for continuous run applications.

Failure Modes and Visual Bench Diagnostics

AC capacitors degrade over time due to thermal stress, voltage spikes, and dielectric breakdown. According to Fluke's electrical diagnostic guidelines, visual inspection and bench metering are mandatory before condemning a motor or compressor.

⚠️ BENCH SAFETY WARNING: Never short an AC capacitor with a screwdriver to discharge it. This causes a massive current spike that can vaporize the internal metalized layers, weld the screwdriver tip, and blind you with molten metal. Always discharge using a 20kΩ, 5-watt wirewound resistor attached to insulated probes for 5 to 10 seconds before handling.

Visual Symptoms of Failure

Capacitor Type Common Failure Mode Visual / Physical Symptom
Motor Run (CBB60) Loss of capacitance / Dielectric thinning Case is perfectly flat, but multimeter reads 20% below rated µF. Motor hums but won't start.
Motor Run (P2 Type) Internal short / Pressure buildup The top dome is visibly swollen or popped. The internal 'P2' pressure interrupter has physically severed the connection to prevent fire.
Motor Start (Electrolytic) Electrolyte boil-off / Venting Rubber vent plug on the bottom is pushed out, or black/brown crusty electrolyte is leaking from the seam.
Ceramic (X7R) Micro-cracking (Piezoelectric/Thermal shock) Hairline crack in the epoxy coating. Often fails dead-short, which will blow the upstream AC fuse instantly.

Bench Testing Procedure

To verify how an AC capacitor is working on your bench, set your digital multimeter to the capacitance (µF/nF) setting. After safely discharging the part, connect the probes across the terminals. For a dual-run capacitor (three terminals: C, FAN, HERM), measure between C and FAN, then between C and HERM. The sum of these two readings should roughly equal the total capacitance printed on the label. If your meter has an ESR (Equivalent Series Resistance) function, use it; a high ESR reading on a motor start capacitor indicates dried-out electrolyte, even if the µF reading looks normal on a basic meter. For deeper AC theory and reactive power calculations, Electronics Tutorials provides excellent reference material on AC capacitance behavior in complex impedance networks.