Capacitors — Run and Start

A capacitor in an HVAC context has one job: shift the phase of current in one of a motor’s windings to create the rotating magnetic field that makes the motor start and run. Without the capacitor, a single-phase induction motor has no inherent starting direction — it would sit and hum. The cap tricks it into spinning by offsetting the current peak in the start or auxiliary winding relative to the run winding.

Capacitors are cheap, failure-prone, and often the reason “the motor doesn’t work.” Diagnosing a failed capacitor takes 30 seconds with the right meter, and replacing one takes five minutes. Learning to check caps early in the diagnostic sequence saves a lot of wrongly-replaced motors.

Run vs. start capacitors

Run capacitors stay in the circuit continuously. They’re in circuit during startup and during normal operation, providing the phase shift that keeps the motor running efficiently. Typical residential blower: 5–15 µF. Typical residential compressor: 30–60 µF. Run caps are oil-filled, metal-cased, and rated for continuous duty — capable of dissipating the heat generated by continuous AC current flow.

Start capacitors are higher-capacitance and only in circuit during startup. They provide a strong phase shift that maximizes starting torque. Once the motor reaches running speed, a start relay disconnects the start cap from the circuit. Typical residential compressor start cap: 100–300 µF. Start caps are usually plastic-cased and not rated for continuous duty; they’d overheat and fail if left in circuit continuously.

Dual-section capacitors (often called “dual run caps” or “440V duals”) combine two run caps in one can. The compressor section is larger (30–60 µF) and the fan section is smaller (5–10 µF). Three terminals: C (common), HERM (compressor), FAN. A dual cap that fails on one section can be replaced with a single dual cap, or with two separate run caps if matching dual replacements are unavailable.

Safely discharging a capacitor

A charged capacitor can store enough energy to deliver a painful and sometimes dangerous shock even after power is disconnected. Always discharge before handling.

Procedure:

1. Power off at the disconnect. Verify with a meter.
2. Use an insulated screwdriver with a plastic handle (not metal-shaft). Touch the screwdriver tip across the capacitor’s terminals — common to HERM on a compressor dual cap, common to FAN on the fan section.
3. You’ll often see a small spark and hear a pop on a freshly-disconnected cap.
4. Verify discharge: meter across the terminals; should read ~0 VDC and stable.

Some techs use a discharge resistor (10 kΩ, 5W or similar) clipped across the terminals for a few seconds instead of a screwdriver short. Gentler on the cap and slightly safer. Either way, don’t handle a capacitor that hasn’t been verified-discharged.

Testing a capacitor

The right tool is a capacitance meter. Most modern DMMs have this function (labeled µF or “CAP” on the dial).

Procedure:

1. Power off, lock out, discharge as above.
2. Disconnect cap leads (remove the push-on terminals).
3. Set meter to µF mode.
4. Connect leads across the cap terminals (polarity doesn’t matter for non-polarized AC caps).
5. Read the value. Compare to the cap’s nameplate rating.

Interpretation:

• Within ±6% of rated: healthy.
• Reading low (e.g., rated 40 µF, reads 30 µF): cap is losing capacitance. Motor will run with diminished torque, higher current, reduced efficiency. Replace.
• Reading near zero or OL: cap is open-circuited internally. Motor won’t start or won’t run correctly. Replace.
• Reading high: unusual; sometimes internal shorts between sections produce inflated readings. Replace.

Cross-referencing replacements

When the original cap is unavailable, cross-reference:

• µF value must match. Same number within tolerance. Don’t substitute a 35 µF for a 40 µF; the phase shift will be off and the motor will run inefficiently or overheat.
• Voltage rating must equal or exceed. 370 VAC → 440 VAC is fine; 440 VAC → 370 VAC is not.
• Hz must match. Always 60 for residential.
• Physical size and terminal style matter for fit, not function. Most residential caps use 1/4” blade terminals, but some older units have screw terminals.

Many replacement caps are “universal” dual caps with tapped values — you use the 5 µF or 7.5 µF tap depending on your need for the fan section. Read the label carefully to confirm which terminals correspond to which values.

From the field

Compressor tripping thermal overload every 10–15 minutes of running. Amp draw high but not extreme. Homeowner said it had been “running hot” for weeks. I measured the run capacitor: rated 45 µF, reading 32 µF. Replaced with a new 45 µF cap. Amp draw dropped about 15%, compressor ran cooler, overload stopped tripping.

The cap had been slowly losing capacitance over probably a year. Each decrease in capacitance meant slightly less phase shift, slightly less torque per RPM, slightly more current to make the same work, slightly more heat. The compressor had been stressed for a long time before anyone noticed. Not a dramatic failure — just a slow decline that finally crossed the threshold where symptoms appeared. $8 cap, 10 minutes of work, saved a compressor.

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Check your understanding

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01A run capacitor is rated 40 µF, 370 VAC. After disconnection and discharge, your capacitance meter reads 28 µF. The motor it serves is still running. What should you do?

ANothing — the motor is still workingBReplace the capacitor — it's degraded 30% from rated and is stressing the motor, even though the motor is currently tolerating itCWait until it fails completelyDReplace the motor

02You're replacing a 370 VAC run capacitor. The only replacement at the supply house is rated 440 VAC with the same µF value. Is this acceptable?

ANo — voltage ratings must match exactlyBYes — a higher voltage rating is always acceptable; it just means the cap can handle more stress than requiredCOnly if you cut the voltage in halfDOnly on three-phase systems

03A service tech clips a discharge resistor across a compressor run cap for 2 seconds, then disconnects the wires and picks up the cap. She still gets a moderate shock. What went wrong?

AThe resistor was wrongBTwo seconds isn't enough for a compressor run cap (often 40–60 µF) to discharge fully through a 10kΩ resistor — time constant is about RC = 10,000 × 0.00005 = 0.5s; multiple time constants are needed to bleed fullyCThe cap was defectiveDThe meter was wrong

Capacitors are among the cheapest components in HVAC equipment and among the most commonly failing. Check them early in any motor-won’t-start or motor-runs-hot call, carry common values in your truck, and respect their stored energy before you handle them.