The primary purpose of a capacitor in a single-phase AC motor is to create an artificial second phase. Single-phase grid power alone produces a pulsating magnetic field, which yields zero starting torque. By placing a capacitor in series with the auxiliary (start) winding, the current in that winding is phase-shifted by up to 90 electrical degrees. This phase shift generates a rotating magnetic field, providing the breakaway torque needed to spin the rotor. In run-capacitor designs, it also corrects the power factor and maintains auxiliary winding current to boost efficiency under mechanical load. Without it, the motor simply hums, draws locked-rotor amperage (LRA), and trips the breaker.
Motor Type Comparison: PSC vs. Capacitor-Start vs. Shaded Pole
Not all single-phase motors use capacitors, and those that do use them differently. Choosing the wrong architecture for your load profile will result in stalled shafts or burned windings. Here is how the three dominant single-phase AC motor types stack up on the workbench.
| Motor Type | Torque Curve (Start / Run) | Control & Drive Needs | Typical Cost (1/2 HP) | Best Load Profile |
|---|---|---|---|---|
| CSIR (Capacitor Start, Induction Run) | High Starting (250-300% FLT) / Medium Run | Direct-on-line (DOL); requires centrifugal switch or potential relay | $120 - $180 | Hard-starting, high-inertia loads (air compressors, well pumps) |
| PSC (Permanent Split Capacitor) | Low Starting (100-150% FLT) / High Run Efficiency | DOL; easily speed-controlled via TRIAC or variable frequency drives (VFD) | $90 - $140 | Variable torque, continuous duty (HVAC blowers, exhaust fans) |
| Shaded Pole | Very Low Starting (50-75% FLT) / Low Run | Simple DOL; no external capacitors or switches required | $20 - $40 | Fractional HP, low-inertia (desk fans, small blowers, display motors) |
Wiring and Terminal Identification for Capacitor Motors
When wiring a CSIR or PSC motor, you must correctly identify the main (run) and auxiliary (start) windings. Reversing the line voltage across the start winding without the capacitor in series will instantly burn out the thinner gauge start wire. According to NEMA MG 1 standards, single-phase motor leads are typically marked with numbers:
- T1 and T4 (or 1 and 4): Main (Run) Winding leads.
- T5 and T8 (or 5 and 8): Auxiliary (Start) Winding leads.
- Line 1 (L1): Connects to T1 and T5 (through the centrifugal switch/capacitor branch).
- Line 2 (L2): Connects to T4 and T8.
PSC Wiring: The run capacitor (typically an oval CBB60 metallized polypropylene can) is wired permanently in series with T5 and T8. L1 and L2 connect directly across the main circuit.
CSIR Wiring: The start capacitor (a cylindrical CD60 electrolytic can) is wired in series with T5, passing through a centrifugal switch. When the rotor hits ~75% of synchronous speed, the switch opens, physically disconnecting the start capacitor and the start winding from the circuit.
Sizing Rule of Thumb and Worked Load Example
Capacitor sizing is dictated by the motor's horsepower, line voltage, and the mechanical breakaway torque of the load. Using an undersized start capacitor results in a sluggish start and tripped breakers; an oversized one causes excessive winding current and insulation breakdown.
The Bench Rules of Thumb:
- Start Capacitors (CD60): 50–100 µF per HP for easy-starting loads; 100–150 µF per HP for hard-starting loads (at 120V). Halve the µF value for 240V systems. Voltage rating must be at least 1.5x line voltage (e.g., 250VAC for a 120V circuit).
- Run Capacitors (CBB60): 2–3 µF per 100 Watts of motor output. Voltage rating must be at least 1.25x line voltage (e.g., 370VAC or 440VAC for a 240V circuit).
Worked Example: 1/2 HP, 120V Air Compressor
Air compressors are high-inertia, hard-starting loads because the piston must overcome residual tank pressure.
- Identify HP and Voltage: 0.5 HP, 120VAC.
- Calculate Start Capacitor: Using the hard-start rule (120 µF/HP average), 0.5 HP × 120 µF = 60 µF. To ensure reliable breakaway torque against 100 PSI tank pressure, we bump this to the next standard commercial value: 108–130 µF at 250VAC.
- Select the Part: A standard Dayton 2MDV6 (108-130 µF, 250V) start capacitor. Note: This motor will be a CSIR type, so no run capacitor is needed.
Failure Signatures: Hum, Overheat, and Stall
Capacitor degradation is the number one cause of single-phase motor failure. Because capacitors dry out or short internally over time, recognizing the acoustic and thermal signatures will save you from replacing a perfectly good motor. For deep diagnostic procedures, reference Fluke's motor capacitor testing guidelines using a multimeter with capacitance mode.
| Symptom | Most Likely Cause | Bench Verification & Fix |
|---|---|---|
| Motor hums loudly, shaft refuses to turn, casing gets hot within 10 seconds. | Open Start Capacitor or stuck centrifugal switch. The main winding is energized, but no rotating field is generated. | Disconnect power. Discharge cap with a 20kΩ resistor. Measure capacitance. If reading is < 80% of nameplate µF or reads open (OL), replace the CD60 start cap. |
| Motor starts fine, but bogs down, loses torque, and overheats under mechanical load. | Failed Run Capacitor (in PSC/CSCR motors). The auxiliary winding drops out, killing the power factor and running torque. | Measure the CBB60 run cap. A bulging case or a µF reading below 90% of the rated value means the dielectric film has degraded. Replace with exact µF and ≥370VAC rating. |
| Motor starts, reaches speed, but emits a loud buzzing and the start capacitor explodes/vents. | Welded centrifugal switch contacts. The switch failed to open at 75% RPM, keeping the start winding and electrolytic cap in the circuit continuously. | Inspect the centrifugal switch mechanism on the rear shaft. Clean pitted contacts with a fine file or replace the switch assembly. Never just replace the blown capacitor without checking the switch. |
Decision Path: Picking the Right Capacitor Motor for Your Load
Stop guessing which motor architecture to buy for your DIY build or replacement job. Use this decision matrix to lock in the correct motor type, drive requirement, and a concrete default part number.
| IF your load profile is... | THEN select this motor type... | Required Drive / Controller | Concrete Bench Pick (Default Part) |
|---|---|---|---|
| High breakaway torque, intermittent duty (e.g., Air compressor, shallow well jet pump, chipper) | CSIR (Capacitor Start, Induction Run) | Direct-on-line contactor or heavy-duty toggle switch. No VFD. | Dayton 11932 (1/2 HP, 1725 RPM, 115V CSIR). Built-in centrifugal switch handles high-inertia starts reliably. |
| Continuous duty, variable airflow (e.g., HVAC blower, attic exhaust fan, duct booster) | PSC (Permanent Split Capacitor) | DOL for single speed, or a standard TRIAC-based fan speed controller for multi-speed. | Baldor-Reliance FDL3710M (1/3 HP, 1075 RPM, 230V PSC). Industry standard for continuous blower duty with high efficiency. |
| Fractional HP, low inertia, cost-sensitive (e.g., Project box cooling fan, small appliance) | Shaded Pole | Simple SPST switch. No capacitors, no relays. | Dayton 4 shaded pole motors (e.g., 1/100 HP). Cheap, stall-proof, but highly inefficient. |
The Default Recommendation: If you are building a motorized DIY fixture in the 1/4 HP to 1 HP range and the load involves any mechanical resistance at startup (gears, belts, fluid pressure), default to a CSIR motor with a properly sized CD60 start capacitor. The upfront cost is roughly 20% higher than a PSC motor, but the 250% starting torque margin prevents the nuisance breaker trips and stalled rotors that plague under-spec'd PSC installations on the workbench.
