A single phase motor capacitor is the critical component that creates the phase shift necessary to generate a rotating magnetic field in an AC induction motor. Without it, the motor simply draws locked-rotor current, hums loudly, and trips the thermal overload. If you are replacing a failed unit or sizing one for a custom build, the direct answer is this: Run capacitors (typically 2–80 µF, 370V/440V AC) remain in the circuit continuously to optimize efficiency and torque, while Start capacitors (typically 70–800 µF, 125V/250V/330V AC) provide high initial starting torque and must be disconnected from the circuit within 3 seconds via a centrifugal switch or potential relay.

Run vs. Start: Which Single Phase Motor Capacitor Fits Your Load?

Not all single-phase motors are built the same, and the capacitor configuration dictates the motor's torque curve and ideal application. Selecting the wrong motor type for your load profile will result in stalled compressors or burnt windings. Below is a comparison of the four primary single-phase motor architectures you will encounter on the bench or jobsite.

Single-Phase Motor Type Comparison
Motor Type Torque Curve Profile Control / Switching Needs Relative Cost Typical Applications
Shaded Pole Very Low (Starts at ~50% speed) None (Direct-on-line) Lowest Small fans, blowers, microwave turntables
Split-Phase Low to Medium Centrifugal switch for start winding Low Furnace blowers, washing machines, light belts
Capacitor-Start High Starting Torque (200-300%) Start capacitor + Centrifugal switch/relay Medium Air compressors, deep well pumps, conveyors
Cap-Start / Cap-Run Highest (High start, high running) Both start and run caps + switching mechanism Highest Heavy machinery, large HVAC compressors, woodshop tools

Once you identify the motor architecture, you must select the correct physical capacitor. The internal dielectric construction is entirely different between start and run variants. Never swap them.

Capacitor Specifications and Construction Data
Capacitor Type Capacitance Range (µF) Voltage Rating (VAC) Duty Cycle Dielectric / Construction ESR Characteristics
Motor Start 70 µF to 1000+ µF 125V, 250V, 330V AC Intermittent (< 3 sec) Non-polarized Electrolytic High ESR (generates heat rapidly)
Motor Run 2 µF to 80 µF 370V, 440V AC Continuous Metallized Polypropylene Film Very Low ESR (runs cool)
Dual Run (HVAC) 15-60 µF (HERM) / 3-10 µF (FAN) 370V, 440V AC Continuous Metallized Polypropylene Film Very Low ESR (two caps in one can)
Hard Start Kit Matches OEM start cap + 20% 250V, 330V AC Intermittent (< 3 sec) Electrolytic + PTC Thermistor High ESR (PTC handles switching)
Bench Tip: If you are testing a suspected run capacitor with a multimeter that only measures resistance (Ohms), you are wasting your time. An open electrolytic start cap and a degraded polypropylene run cap can both show 'infinite' resistance on a basic DMM. You must use a meter with a dedicated Microfarad (µF) setting to measure actual capacitance against the nameplate tolerance (usually ±5% for run, ±20% for start). See Fluke's guide on capacitor testing for proper discharge and measurement procedures.

Sizing Rules and a Worked 1.5 HP Compressor Example

The golden rule of single phase motor capacitor replacement is to match the original nameplate microfarad (µF) and voltage rating exactly. Upsizing a run capacitor by more than 10% will push excessive current through the auxiliary winding, causing it to overheat and fail prematurely. Downsizing it will result in low running torque, high slip, and elevated main winding current.

But what if the nameplate is missing, painted over, or you are engineering a custom drive? You can use standard sizing heuristics based on the motor's horsepower and voltage.

Worked Load Example: 1.5 HP, 120V Air Compressor

Let's size the capacitors for a 1.5 HP (1119 Watts), 120V, 3450 RPM capacitor-start/capacitor-run air compressor. This is a high-inertia load that requires massive starting torque to overcome initial cylinder compression, followed by efficient continuous running.

  • Start Capacitor Sizing: The rule of thumb for start capacitors is roughly 70 to 120 µF per horsepower for 120V systems. For a 1.5 HP motor, we need between 105 µF and 180 µF. However, compressors demand high breakaway torque. We will select a standard off-the-shelf 270-324 µF, 250VAC start capacitor to ensure it overcomes the Locked Rotor Amps (LRA) spike without stalling.
  • Run Capacitor Sizing: Run capacitors are sized to create an optimal 90-degree phase shift in the auxiliary winding at full load. The approximation formula is C (µF) = (120,000 × I_aux) / V_line. Assuming an auxiliary winding current of roughly 1.8A at full load: C = (120,000 × 1.8) / 120 = 1800 µF? No, that formula applies to raw reactive power compensation. The practical bench standard for a 1.5 HP 120V motor is a 15 µF or 20 µF, 370VAC run capacitor. We will select 15 µF, 370VAC.
Voltage Rating Warning: Always use a run capacitor rated for at least 370VAC on a 240V circuit, and preferably 440VAC if the supply is prone to spikes. A 240V nominal line can easily see 252V+ during low-load utility conditions, which will cause a 250V-rated capacitor's dielectric to break down and vent violently.

Terminal Identification and Wiring the Replacement

Wiring a single phase motor capacitor correctly requires understanding the terminal markings on both the capacitor and the switching mechanism. Unlike DC circuits, AC motor capacitors are non-polarized; it does not matter which spade terminal gets which wire on a standard single run or start capacitor.

Dual Run Capacitor Terminals (HVAC Applications)

If you are working on a dual run capacitor (common in central air condenser units), the top will feature three distinct terminal clusters:

  • C (Common): This is the shared line connection. It receives the hot leg (usually from the contactor) that feeds both the compressor and fan motor auxiliary windings.
  • HERM (Hermetic Compressor): Connects directly to the auxiliary/start winding terminal of the compressor.
  • FAN: Connects directly to the auxiliary/start winding terminal of the condenser fan motor.

Start Capacitor and Potential Relay Wiring

For high-torque loads like our 1.5 HP compressor example, the start capacitor must be removed from the circuit once the motor reaches roughly 75% of synchronous speed. While small motors use a mechanical centrifugal switch inside the motor housing, larger or sealed compressors use an external potential relay (e.g., Supco SUPR SPU410 or Steveco 180).

The potential relay has three terminals:

  • Terminal 1: Connects to the motor's start winding.
  • Terminal 2: Connects to the motor's main winding (run) and one side of the start capacitor.
  • Terminal 5: Connects to the other side of the start capacitor and the incoming line voltage.

How it works: As the motor spins up, the back-EMF (voltage generated by the spinning rotor) across the start winding increases. When this induced voltage hits the relay's pickup threshold (usually 300V-400V on a 240V system), the electromagnetic coil pulls the contacts open, physically disconnecting the start capacitor from Terminal 1.

Failure Signatures: Diagnosing Hums, Overheats, and Stalls

Capacitors are the most common failure point in single-phase motor systems. Diagnosing the exact failure signature saves you from replacing a perfectly good motor. It is also critical to understand what driver or controller these motors demand—and more importantly, what they cannot use.

Controller Demands: Unlike 3-phase AC motors, standard single-phase induction motors cannot be driven by a standard Variable Frequency Drive (VFD). Altering the frequency on a single-phase supply without a phase-shifted auxiliary supply causes severe torque pulsation, magnetic saturation, and rapid winding destruction. Instead, single-phase motors demand electromechanical switching (centrifugal switches, potential relays) for starting. If you need to reduce the massive inrush current (LRA) on a weak generator or solar inverter, you must use a specialized single-phase soft-starter (like the Micro-Air EasyStart or ESI SoftStart), which uses triacs to phase-angle chop the voltage during the first 2 seconds of startup.

Symptom 1: Motor Hums Loudly, Shaft Will Not Turn

  • Most Likely Cause: Open start capacitor, or a failed centrifugal switch/potential relay that is stuck open.
  • The Physics: Without the start capacitor's phase shift, the motor produces a pulsating magnetic field rather than a rotating one. It has zero starting torque and simply vibrates in place while drawing 500%+ of full load current.
  • The Fix: Disconnect power, safely discharge the capacitor with a 20k-ohm 5W resistor, and test the start capacitor for continuity and µF. If the cap tests good, manually spin the shaft (with power off) to check for a seized mechanical centrifugal switch.

Symptom 2: Motor Runs, but Overheats and Trips Thermal Overload

  • Most Likely Cause: Degraded run capacitor (capacitance has dropped below 90% of nameplate value).
  • The Physics: Metallized polypropylene film capacitors suffer from 'capacitance loss' over time due to microscopic dielectric clearing events. As the µF drops, the phase angle in the auxiliary winding shifts away from the optimal 90 degrees. The motor loses running torque, slip increases, and the main winding draws excessive current to maintain the load, leading to a thermal trip.
  • The Fix: Measure the run capacitor with a µF meter. If a 15 µF cap reads 11 µF, replace it. Do not attempt to 'compensate' by adding a second cap in parallel unless you are permanently out of space; just buy the correct OEM spec.

Symptom 3: Start Capacitor is Bulging, Vented, or Exploded

  • Most Likely Cause: The centrifugal switch welded itself closed, or the potential relay coil failed, keeping the start capacitor in the circuit past its 3-second duty cycle.
  • The Physics: Start capacitors use an electrolytic dielectric with high Equivalent Series Resistance (ESR). They are designed to handle high current for less than 3 seconds. If left in the circuit during continuous running, the high ESR generates massive internal heat, boiling the electrolyte and causing the pressure relief vent to rupture.
  • The Fix: Replacing the start capacitor is not enough. You must diagnose why it didn't drop out. Test the potential relay coil for an open circuit, or disassemble the motor bell housing to clean carbon buildup off the centrifugal switch contacts. For a comprehensive breakdown of single-phase motor winding and switching theory, refer to the All About Circuits AC textbook chapter on induction motors.
Pro-Tip for Hard-Starting Compressors: If your compressor consistently struggles to start on a long extension cord or a generator (causing the breaker to trip before the motor reaches speed), install a Hard Start Kit (like the Supco SPP6). This combines a high-µF start capacitor with a Positive Temperature Coefficient (PTC) thermistor. The PTC acts as a solid-state switch: it passes current when cold, but as it heats up from the starting current, its resistance spikes exponentially, effectively removing the start cap from the circuit without the need for mechanical contacts that can pit and weld over time.