A single-phase AC induction motor cannot generate a rotating magnetic field on its own. Without intervention, applying single-phase power simply creates a pulsating field that hums and overheats. A motor capacitor works by shifting the phase of the alternating current in the auxiliary (start) winding relative to the main winding. This phase shift—ideally approaching 90 electrical degrees—creates the artificial two-phase condition necessary to generate starting torque or improve running power factor. If you are selecting a motor for a specific mechanical load, understanding how single-phase motors utilize capacitors is the difference between a system that runs for a decade and one that trips the breaker on day one. This guide breaks down the physics, wiring terminals, sizing math, and failure diagnostics to help you make a definitive motor and drive selection.

The Core Physics: Start vs. Run Capacitors

Motor capacitors are not interchangeable. They are strictly divided into two functional categories based on their dielectric construction and duty cycle.

Start Capacitors (Electrolytic)

Start capacitors are designed to maximize starting torque for high-inertia loads. They use a non-polarized electrolytic dielectric to achieve high microfarad (µF) ratings (typically 70 µF to 800 µF) in a compact physical size. Because electrolytic capacitors have high internal equivalent series resistance (ESR), they generate significant heat. They are strictly rated for intermittent duty—usually less than 3 seconds per start and no more than 20 starts per hour. A centrifugal switch or potential relay must disconnect them once the motor reaches roughly 75% of synchronous speed.

Run Capacitors (Metallized Polypropylene)

Run capacitors remain in the circuit continuously while the motor operates. They use a metallized polypropylene film dielectric, which offers low ESR, high stability, and continuous duty capability. Their capacitance values are much lower (typically 1.5 µF to 80 µF). Their primary job is to optimize the magnetic field angle during operation, reducing slip, lowering operating temperature, and improving the power factor. According to the Department of Energy's motor selection guidelines, properly sized run capacitors are critical for minimizing I²R losses in continuous-duty HVAC and pump applications.

Bench Rule: Never substitute a start capacitor for a run capacitor. The start cap will overheat, vent its electrolyte, and fail catastrophically within minutes of continuous 60Hz cycling. You can, however, temporarily parallel multiple run capacitors to achieve a higher µF value for emergency starting, provided they are switched out of the circuit.

Motor Type Comparison: Matching the Load Profile

Selecting the right capacitor requires first selecting the right motor architecture for your mechanical load. Stepper and servo motors handle positioning via digital pulse trains; single-phase AC induction motors handle continuous rotational work. Here is how the capacitor-dependent AC motor types compare.
Motor Type Torque Curve & Starting Profile Control / Switching Needs Relative Cost Best Load Profile
Shaded Pole Very low starting torque. Slips heavily under load. None (No capacitor). Simple on/off. Lowest Small fans, blowers, dampers.
PSC (Permanent Split Capacitor) Low-to-medium starting torque. Smooth, quiet operation. Run capacitor only. No centrifugal switch. Low-Medium HVAC blowers, garage door openers, belt-driven fans.
Capacitor-Start (CS) High starting torque (200-300% of full load). Drops to medium running torque. Start capacitor + centrifugal switch or potential relay. Medium Air compressors, piston pumps, conveyors.
Cap-Start / Cap-Run (CSR) Very high starting torque. High running torque and efficiency. Both start and run capacitors. Switching mechanism required. Highest Heavy-duty agricultural pumps, large woodworking machinery.

Wiring, Terminals, and Sizing Rules of Thumb

When wiring a single-phase motor, you will encounter standard NEMA terminal markings. Miswiring the main and auxiliary windings will result in the motor running in reverse or failing to start.

Terminal Identification (NEMA Standard)

  • T1, T2, T3, T4: Main (Run) Winding terminals. T1 and T4 are typically the ends of the main winding. T2 and T3 are often internal thermal overload connections or taps for multi-voltage (115V/230V) wiring.
  • T5, T8: Auxiliary (Start) Winding terminals. The start capacitor and centrifugal switch are wired in series with this winding.
  • P1, P2: Thermal protector terminals (if externally accessible).

For a standard 115V connection, line voltage (L1) goes to T1 and the neutral (L2) goes to T4. The start circuit (T5 to T8) is wired in parallel with the main winding, with the capacitor and switch interrupting the T5 leg.

Capacitor Sizing Math

While exact sizing requires motor design software, field replacements and custom builds rely on empirical rules of thumb based on the motor's horsepower (HP) rating and the mechanical load's inertia.

  • Start Capacitor Sizing: 70 µF to 80 µF per Horsepower.
  • Run Capacitor Sizing: 2 µF to 5 µF per Horsepower.

Worked Load Example: 3/4 HP Well Pump

Load Context: A 3/4 HP (approx. 560W mechanical output) submersible well pump pushing water up a 150-foot column. This is a high-inertia, high-breakaway-torque load. A PSC motor will stall and overheat. You must select a Capacitor-Start or Cap-Start/Cap-Run motor.

Start Cap Calculation: 0.75 HP × 75 µF/HP = 56.25 µF. Selection: Choose a standard 53-64 µF, 250VAC electrolytic start capacitor.

Run Cap Calculation (if CSR): 0.75 HP × 4 µF/HP = 3.0 µF. Selection: Choose a standard 3 µF, 370VAC metallized polypropylene run capacitor.

Failure Signatures: Hum, Overheat, and Stall

Capacitors are often the first component to fail in a motor circuit due to thermal stress and dielectric degradation. When testing a suspected bad capacitor, always discharge it first with a 20k-ohm, 5W bleeder resistor, and verify with a multimeter equipped with a capacitance setting. A reading more than 5% outside the printed µF tolerance indicates replacement is mandatory.
Symptom Probable Cause Diagnostic Measurement / Fix
Loud Hum, Motor Stalls Open start capacitor, or failed centrifugal switch. The auxiliary winding is not energizing. Measure capacitance. If reading is 0 µF or OL, replace start cap. If cap tests good, inspect the centrifugal switch contacts for pitting or mechanical binding.
Motor Overheats, High Amp Draw Shorted run capacitor, or incorrect µF value installed. The phase angle is wrong, causing high slip. Clamp the main winding current. If it exceeds the nameplate FLA by >10% and the cap reads shorted or >10% over rated µF, replace with exact OEM µF spec.
Rapid Clicking at Startup Potential relay failure or start cap not disconnecting. The relay is bouncing, or the centrifugal switch is chattering. Check the potential relay's pick-up and drop-out voltage ratings against the motor's back-EMF. Replace relay if contacts are welded.
Motor Starts, Then Dies Start capacitor failing to disconnect (switch stuck closed). The start cap is overheating and opening its internal safety interrupter. Inspect the centrifugal switch mechanism. Ensure the spring tension is correct and the flyweights move freely.

The Decision Tree: Picking Your Motor and Capacitor

Do not default to the cheapest motor on the shelf. Match the drive to the mechanical reality of the load. Use this decision path to terminate your selection process with a concrete bill of materials.
  • IF the load is a low-inertia fan or blower (starts easily, runs continuously) THEN select a PSC (Permanent Split Capacitor) Motor. It requires no maintenance-prone centrifugal switch and runs quietly.
    • Concrete Pick: Dayton 1/3 HP PSC Motor (NEMA 48Y frame) paired with a Genteq 5 µF, 370V run capacitor.
  • IF the load is high-inertia, high-breakaway torque (piston compressor, positive displacement pump, loaded conveyor) THEN select a Capacitor-Start (CS) Motor. The high µF start cap will provide the 250% breakaway torque required to overcome static friction and fluid pressure.
    • Concrete Pick: Dayton 1/2 HP Cap-Start Motor (NEMA 56C frame) paired with a Genteq 108-130 µF, 250V start capacitor.
  • IF the load requires both massive starting torque AND continuous high-efficiency running under heavy load (large agricultural augers, industrial lathes) THEN select a Cap-Start / Cap-Run (CSR) Motor.
    • Concrete Pick: Baldor-Reliance 2 HP CSR Motor, utilizing a 216-259 µF start cap and a 15 µF run cap.
Final Default Recommendation: If you are replacing a generic 1/2 HP motor on a standard shop air compressor and the original nameplate is illegible, do not guess with a PSC motor. Default to a NEMA 56-frame Capacitor-Start induction motor (such as the Dayton 119-104 or equivalent) and wire it with a fresh 108-130 µF electrolytic start capacitor. This combination guarantees the breakaway torque needed to clear the compressor's unloader valve and ensures reliable 120V/240V operation without nuisance breaker trips.