Capacitors on Motors: Start vs Run Selection, Sizing, and Substitution Guide

The Direct Answer: Sizing and Selecting Capacitors on Motors

For standard 120V/240V AC single-phase induction motors, you must use a CBB60 or CBB65 metalized polypropylene film run capacitor rated for at least 370VAC (440VAC preferred) for continuous phase shifting, and a CD60 electrolytic start capacitor rated for 250VAC or higher strictly for the momentary starting circuit. Never swap them. The exact microfarad (uF) rating must match the motor nameplate within 5% for run circuits, while voltage ratings can safely be substituted upward but never downward.

Choosing the right capacitors on motors is not a guessing game; it is a strict exercise in matching dielectric chemistry to duty cycle. A run capacitor stays energized the entire time the motor spins, enduring continuous heat and voltage spikes. A start capacitor only stays in the circuit for a fraction of a second via a centrifugal switch or potential relay, dumping massive current to create starting torque before disconnecting. Mixing these up results in immediate catastrophic failure or a motor that hums, overheats, and trips the breaker.

Start vs. Run Capacitors: The Core Decision Matrix

To understand which type for which job, you have to look at the internal construction. Run capacitors use thin polypropylene film that can self-heal from minor internal arcing. Start capacitors use aluminum electrolytic construction, which provides massive energy density (high uF in a small can) but cannot dissipate continuous heat.

Decoding Motor Capacitor Markings and Codes

When you pull a capacitor off an HVAC compressor or a bench grinder, the label is your primary data source. Unlike small ceramic capacitors that use cryptic three-digit EIA codes, motor capacitors print their values explicitly. However, legacy terminology often trips up modern DIYers.

• MFD vs. uF: Older capacitors use 'MFD' (MicroFarad). This is exactly the same as 'uF' or 'μF'. A 35 MFD capacitor is a 35 uF capacitor. Do not confuse MFD with milli-Farads (mF); in the motor world, MFD always means micro.
• VAC Rating: This is the maximum continuous alternating current voltage the dielectric can withstand. Because single-phase motors generate back-EMF (electromotive force) that can push the voltage across the auxiliary winding up to 1.5x or 2x the line voltage, a 240V motor requires a minimum 370VAC run capacitor, with 440VAC being the modern gold standard for surge margin.
• Hz Rating: Usually listed as '50/60 Hz'. This dictates the reactive impedance ($X_c = \frac{1}{2\pi fC}$). A 60Hz capacitor will function on 50Hz, but its effective current-passing capability will drop by roughly 17%.
• EIA-456-A: This is the crucial reliability standard. If a run capacitor bears the EIA-456-A mark, it has been tested to survive 60,000 hours of continuous operation at rated voltage and temperature. According to EC&M's motor component guidelines, avoiding non-EIA-certified generic caps is the single best way to prevent premature HVAC compressor failures.

Visual and Electrical Failure Modes

Capacitors on motors fail in distinct ways depending on their type and the stress applied. Diagnosing them requires both a visual inspection and a multimeter equipped with a dedicated microfarad (uF) testing function.

Run Capacitor Failures: The Silent Drift

Metalized film run capacitors feature a 'self-clearing' mechanism. When a microscopic short occurs in the film, the localized energy vaporizes the metal coating around the flaw, clearing the short and allowing the capacitor to keep working. However, this process physically removes active surface area from the capacitor plates.

• Visual Symptom: Often, none. The top dome remains perfectly flat. In severe over-pressure events, the internal pressure interrupter pops the top dome up to break the circuit, but it rarely bulges like a start cap.
• Electrical Symptom: Microfarad drift. A 40 uF capacitor might measure 32 uF. Once a run capacitor drops below 90% of its nameplate rating, the motor's auxiliary winding falls out of the optimal 90-degree phase shift. The motor will draw higher amperage, run hotter, and eventually trip thermal overloads.

Start Capacitor Failures: The Violent Vent

Start capacitors fail when the centrifugal switch sticks, a potential relay fails to open, or the motor is subjected to rapid short-cycling (starting again before the capacitor has cooled down).

• Visual Symptom: The top of the can bulges outward. You may see dried, crusty black or brown electrolyte residue around the terminals or the vent plug. In extreme cases, the case splits at the seam.
• Electrical Symptom: Complete open circuit or massive ESR (Equivalent Series Resistance) spike. The motor will refuse to start, emitting a loud 60Hz hum until the thermal overload clicks off. For deeper diagnostics on motor starting circuits, AmRad Engineering's technical bulletins provide excellent bench-testing protocols.

Safe Substitution Rules When the Exact Part is Missing

You are on a jobsite or at the bench, and the motor requires a 45 uF, 370VAC run capacitor. Your truck only has 30 uF and 15 uF caps. Can you make it work? Yes, but you must follow strict electrical substitution rules.

Voltage Substitution: You can always substitute a higher voltage capacitor for a lower one. A 440VAC capacitor can seamlessly replace a 370VAC or 250VAC capacitor. The reverse is strictly forbidden; installing a 370VAC cap in a circuit designed for 440VAC will lead to dielectric puncture and failure.

Capacitance Substitution: For run circuits, you must stay within ±5% to ±10% of the original value. If you go too high, the auxiliary winding will overheat. If you go too low, the motor loses torque. For start circuits, you have more flexibility. A start capacitor's exact value is less critical than its ability to deliver a massive phase-shift burst. You can safely substitute a start capacitor within a range of -20% to +20% of the original uF rating, provided the voltage rating is equal or higher.

The Final Decision Path: Picking Your Exact Part

Stop guessing. Use this decision tree to terminate your selection process and order the exact, correct component family for your motor application.

By adhering to these dielectric boundaries and substitution rules, you ensure the motor's rotating magnetic field remains perfectly phased. The CBB60/CBB65 film families will handle your continuous run requirements for decades, while the CD60 electrolytic family will safely deliver the high-torque starting kick without risking a catastrophic case rupture.