If your air conditioner is humming but the fan isn’t spinning, or the compressor is tripping the breaker, you are likely looking at a failed motor capacitor. To answer the core question—what does the capacitor do on an AC unit?—it acts as a temporary energy reservoir that shifts the electrical phase of the current feeding the motor’s auxiliary winding. Single-phase AC motors inherently lack starting torque; the capacitor creates an artificial second phase, generating the rotating magnetic field required to spin the compressor and condenser fan.
Without this phase shift, the motor just sits there vibrating and drawing locked-rotor amps (LRA) until the thermal overload switch trips or the windings melt. Below, we break down the physics, decode the physical markings on the metal can, and outline the strict rules for substituting parts when you are stuck on a hot jobsite without an exact match.
The Physics: Start vs. Run Capacitors in HVAC Systems
In a pure capacitive circuit, current leads voltage by exactly 90 degrees. In the real world of copper windings and steel laminations, an HVAC motor capacitor shifts the phase by roughly 80 degrees. This offset creates the torque needed to overcome the static friction of the compressor and the aerodynamic load of the fan blades.
HVAC systems primarily use three types of capacitors, each engineered for a specific duty cycle and thermal profile. The most critical distinction is between start and run topologies. A start capacitor is designed for massive, short-duration torque, while a run capacitor is designed for continuous, efficient magnetic field maintenance.
| Type | Dielectric / Construction | Capacitance Range (µF) | Voltage Rating | Tolerance | Duty Cycle & Tempco |
|---|---|---|---|---|---|
| Start Capacitor | Electrolytic (Non-polarized), plastic case | 70 µF to 200+ µF | 125VAC, 250VAC, 330VAC | +20% / -0% | Intermittent (max 3 sec on, 20+ cycles/hr) |
| Run Capacitor | Metallized Polypropylene Film (CBB65), metal case, oil-filled | 2 µF to 80 µF | 370VAC, 440VAC | ±5% or ±6% | Continuous duty, rated to 70°C or 85°C ambient |
| Dual Run Capacitor | Metallized Polypropylene (CBB65), dual-section metal can | 15+3 µF to 70+10 µF | 370VAC, 440VAC | ±6% (typically) | Continuous duty, shares one common terminal |
| Hard Start Kit | Start cap + potential relay (or solid-state PTC thermistor) | Matched to compressor LRA | 250VAC to 330VAC | +20% / -0% | Milliseconds; relay drops it out at ~75% RPM |
A Worked Numeric Example: Run Capacitor Impedance
Let’s look at the actual current flowing through a standard 45 µF run capacitor on a 240V, 60Hz residential condenser unit. We calculate the capacitive reactance ($X_c$) using the formula $X_c = 1 / (2 \pi f C)$.
- $X_c = 1 / (2 \times 3.14159 \times 60 \text{ Hz} \times 0.000045 \text{ F})$
- $X_c = 1 / 0.01696 = 58.9 \Omega$
Using Ohm’s Law ($I = V / X_c$), the continuous current is $240\text{V} / 58.9\Omega = \mathbf{4.07\text{ Amps}}$. This means the run capacitor and the start winding are continuously passing over 4 amps of reactive current while the AC runs. This is why run capacitors must be oil-filled metal cans (specifically the CBB65 standard) to dissipate heat, rather than the cheaper, plastic-cased CBB60 types found in ceiling fans.
Decoding the Markings: MFD, VAC, and Terminal Codes
When you pull the access panel off the condenser, the capacitor label is your blueprint. Here is how to read the spec sheet printed on the metal can:
- MFD vs. µF: In the HVAC industry, MFD (Microfarad) and µF mean the exact same thing. Older schematics and US manufacturers prefer 'MFD', while international and electronics contexts use 'µF'. A label reading '45 MFD' is a 45 µF capacitor.
- VAC (Voltage Alternating Current):strong> You will see 370 VAC or 440 VAC. This is the maximum continuous AC voltage the dielectric film can withstand. Never substitute a DC-rated capacitor (like those in power supplies) here; the dielectric breakdown characteristics are entirely different.
- Hz Rating: Usually printed as '50/60 Hz'. A capacitor rated for 60 Hz will have a lower reactance (and pass more current) on a 50 Hz grid, which can overheat the auxiliary winding if not accounted for.
- Terminal Identifiers (Dual Caps): A dual run capacitor has three terminals on top: C (Common), HERM (Hermetic Compressor), and FAN. The C terminal is the shared internal connection point for both the compressor and fan capacitor sections.
Visual Failure Modes and Diagnostic Symptoms
Capacitors do not usually fail without a trace. The metallized polypropylene film inside degrades over time due to thermal stress, voltage spikes, and dielectric absorption. Here is what to look for on the bench:
- The Domed Top (Pressure Interrupter): Modern CBB65 run capacitors feature a scored internal membrane. When the dielectric breaks down and generates gas, the top of the can bulges upward. This physically snaps the internal wiring, disconnecting the capacitor to prevent an explosion. If the top is convex rather than flat, it is dead.
- Dielectric Oil Leaks: Run capacitors are filled with mineral or polybutylene oil to displace oxygen and cool the foil. If you see a greasy residue around the base or terminals, the seal has failed. Moisture ingress will rapidly destroy the capacitance value.
- Humming and High LRA: If the capacitor has lost its microfarad rating (dropping below the ±6% tolerance) but hasn't bulged, the motor will hum, spin slowly, or fail to start. The compressor will draw massive Locked Rotor Amps, heating the windings until the internal thermal overload clicks off.
- ESR Spike: Using a dedicated ESR (Equivalent Series Resistance) meter, a healthy 45µF HVAC cap should read well under 1 ohm. If ESR climbs above 3 or 4 ohms, the internal connections are corroding, and the cap will fail under load even if a standard multimeter reads the correct capacitance.
Safe Substitution Rules When the Exact Part is Missing
You are on a roof in July, the 40/5 MFD 440V dual capacitor is dead, and your truck only has a 45/5 MFD 370V and a separate 40 MFD run cap. Can you make it work? Yes, if you follow the strict substitution hierarchy used by HVAC-R technicians.
Rule 1: The Capacitance Tolerance Limit
Run capacitors must stay within ±6% of the original equipment manufacturer (OEM) spec. Replacing a 40 µF cap with a 45 µF cap is a 12.5% increase, which will over-amp the start winding and burn out the compressor motor. Never round up on microfarads for run capacitors. Start capacitors, however, are rated +20% / -0%. You can safely use a slightly larger start cap to boost torque on an aging compressor, provided the physical size fits.
Rule 2: Voltage Up-Rating (The Golden Rule)
You can always substitute a higher voltage capacitor for a lower one, but never the reverse. A 440 VAC capacitor can safely replace a 370 VAC capacitor. The 440V version simply uses a thicker dielectric film, which makes it run cooler and last longer. Substituting a 370V cap in a 440V circuit will result in rapid dielectric breakdown and a blown capacitor within weeks. For deeper theory on how dielectric thickness affects voltage breakdown, All About Circuits provides an excellent breakdown of capacitive reactance.
Rule 3: Combining Caps for Dual Replacements
If you need a 35/5 MFD dual cap but only have single run caps, you can wire two separate capacitors together. The Wiring Trick: Connect the 'C' (Common) terminal of the 35µF cap to the 'C' terminal of the 5µF cap using a jumper wire. Connect your incoming common line to this jumper. Wire the HERM compressor wire to the HERM terminal on the 35µF cap, and the FAN wire to the FAN terminal on the 5µF cap. Electrically, this is identical to the internal construction of a dual can.
Sourcing and Bench Verification
Not all replacement capacitors are created equal. The market is flooded with cheap, import-grade CBB60 capacitors disguised as heavy-duty HVAC run caps. When sourcing replacements, look for the CBB65 designation printed on the label. CBB65 specifies a metal case, a specific pressure-interrupter safety vent, and metallized polypropylene film rated for the harsh thermal environment of an outdoor condenser unit.
Premium brands like AMRad, Genteq, and Titan Pro use high-purity aluminum cases and superior edge-winding techniques that resist the micro-fractures caused by the constant vibration of the compressor. While a generic import cap might cost $8 and last two summers, a $25 AMRad 45/5 440V capacitor will routinely outlast the compressor itself.
Before installing any new capacitor, bench-test it with a multimeter that features a dedicated capacitance setting. Verify that a 45µF cap reads between 42.3µF and 47.7µF (the ±6% window). If a brand-new capacitor reads outside this tolerance straight out of the box, return it. For more information on maintaining HVAC efficiency and component longevity, review the EPA's Energy Star heating and cooling guidelines.
