A capacitor motor uses a discrete capacitor in series with its auxiliary (start) winding to create a phase shift, generating the rotating magnetic field required to start and run a single-phase induction motor. Without this phase shift, a single-phase motor would merely vibrate and overheat. When selecting a motor for pumps, compressors, or HVAC blowers, the choice between a Permanent Split Capacitor (PSC), Capacitor-Start/Capacitor-Run (CSCR), and an Electronically Commutated Motor (ECM) dictates your starting torque, efficiency, and control complexity.

Single-Phase Motor Selection Matrix

Choosing the right motor requires matching the load's breakaway torque and duty cycle to the motor's torque curve. Below is a data-dense comparison of common single-phase motor types available in 2026, focusing on fractional to small integral horsepower (1/4 HP to 5 HP) ranges.

Motor Type Starting Torque (% FL) Running Efficiency Controller Required 2026 Avg Cost (1 HP) Ideal Load Profile
Shaded Pole 25% - 75% 35% - 50% None (Direct-on-line) $40 - $70 Small desk fans, dampers, low-inertia blowers
PSC (Permanent Split Capacitor) 30% - 90% 65% - 75% None / Simple Relay $130 - $160 HVAC blowers, pool pumps, multi-speed fans
CSIR (Capacitor-Start Induction Run) 200% - 300% 65% - 72% Centrifugal Switch $160 - $190 Conveyors, high-inertia fans, easy-start compressors
CSCR (Capacitor-Start/Capacitor-Run) 200% - 300% 70% - 80% Centrifugal Switch + Potential Relay $190 - $240 Well pumps, hard-start air compressors, wood lathes
ECM (Electronically Commutated) 0% - 150% (Programmable) 85% - 92% Integrated VFD / Microcontroller $350 - $550 Variable-air-volume HVAC, smart circulator pumps
Selection Rule: If your load requires high starting torque (like a positive displacement pump) but runs continuously, you must use a CSCR. A CSIR will overheat under continuous high load because it lacks a run capacitor to optimize the magnetic field during operation, while a PSC will stall and trip the breaker during startup.

Terminal Identification and the Capacitor Circuit

Wiring a capacitor motor correctly requires understanding the internal winding topology. Most industrial single-phase motors follow NEMA MG-1 standards for terminal markings. A standard dual-voltage (115/230V) single-phase motor will have leads marked T1 through T8.

Inside the motor, there are two distinct circuits:

  • The Run (Main) Winding: Designed for continuous duty. It has low resistance and high inductance.
  • The Start (Auxiliary) Winding: Designed for short-term duty (typically under 3 seconds). It has high resistance and low inductance, wound with thinner wire.

Wiring the Capacitors

In a CSCR motor, you will find two distinct capacitors in the terminal box or mounted on the motor belly band:

  1. Start Capacitor: Usually black, cylindrical, and rated between 50 µF and 1000 µF at 125VAC or 250VAC. It is wired in series with the start winding and the centrifugal switch (or potential relay). It provides the massive phase shift needed for breakaway torque.
  2. Run Capacitor: Usually silver or metallic, oval/round, and rated between 2 µF and 80 µF at 370VAC or 440VAC. It remains in the circuit continuously, fine-tuning the phase angle to maximize running efficiency and power factor.
Safety Warning: Never wire a start capacitor in place of a run capacitor. Start capacitors use a dry electrolyte dielectric that cannot dissipate the heat of continuous AC ripple current. If left in the circuit past 3 seconds, a start capacitor will vent, rupture, or catch fire. Always de-energize the panel, lock out the breaker, and verify zero voltage with a CAT III multimeter before opening a motor terminal box.

Sizing Rules and a Worked Compressor Load Example

The golden rule of capacitor motor sizing is that the load's breakaway torque must not exceed 80% of the motor's rated starting torque. If the motor takes longer than 3 seconds to reach 75% of synchronous speed, the centrifugal switch will not open, and the start winding will cook. Furthermore, you cannot simply convert HP to kW and assume a motor is sized correctly without accounting for the mechanical load's inertia.

Worked Example: Sizing a Motor for a 2 HP Stationary Air Compressor

The Load: A 2 HP, single-phase reciprocating air compressor operating at 230V. The pump head has high rotational inertia, and if the unloader valve fails, it must start against partial line pressure.

  1. Select Motor Type: A PSC motor (max 90% starting torque) will fail to break the inertia. A CSIR motor has the starting torque, but the continuous run efficiency will drop, causing thermal overload trips on hot days. We select a CSCR motor (e.g., Baldor-Reliance L1500T series or equivalent) for 250% starting torque and high running efficiency.
  2. Determine FLA (Full Load Amps): Per the NEC Table 430.248, the standard FLA for a 2 HP single-phase motor at 230V is 12A.
  3. Capacitor Sizing: The manufacturer specifies a 30 µF / 370V run capacitor and a 250 µF / 250V start capacitor. Do not upsize the start capacitor to 'get more torque'; excessive capacitance will push the start winding current past its thermal limit, burning out the auxiliary winding.
  4. Breaker Sizing: NEC 430.52 allows a maximum inverse-time breaker size of 250% of FLA for single-phase motors. 12A × 2.5 = 30A. We install a 30A standard breaker, or a 20A dual-element time-delay fuse (which allows 175% sizing, 12A × 1.75 = 21A, rounded down to 20A) to provide better overload protection.

Diagnosing Failure Signatures: Hum, Overheat, and Stall

Capacitor motors fail in highly predictable ways based on which component degrades. According to Fluke's motor troubleshooting guidelines, testing capacitors and switches with a multimeter is the fastest way to isolate the fault.

Symptom 1: Motor Hums Loudly and Trips the Breaker

The Cause: The start circuit is open. The motor is receiving single-phase power but has no phase shift to create a rotating field. It sits at locked rotor, drawing 500% to 700% of FLA (Locked Rotor Amps) until the thermal overload or branch breaker trips.
The Fix: 1. Test the start capacitor with a multimeter in capacitance mode. If it reads open (OL) or significantly below its µF rating (e.g., a 200 µF cap reading 40 µF), replace it. 2. If the capacitor is good, the centrifugal switch contacts are likely stuck open or burnt. You must disassemble the motor bell end to clean the switch contacts or replace the switch assembly.

Symptom 2: Motor Runs but Overheats and Stalls Under Load

The Cause: Run capacitor degradation. Over years of thermal cycling, the dielectric fluid in a run capacitor degrades, increasing its Equivalent Series Resistance (ESR) and dropping its capacitance. The phase shift collapses, power factor drops, and the motor draws excess current to maintain torque, eventually stalling when the pump hits cut-in pressure.
The Fix: Measure the run capacitor. A 30 µF run capacitor should read within ±5% (28.5 µF to 31.5 µF). If it reads 22 µF, the dielectric is failing. Replace it with an identical µF and voltage rating. Never use a lower voltage rating (e.g., replacing a 440V cap with a 370V cap) as the thinner dielectric will fail prematurely under voltage spikes.

Symptom 3: Smells Like Burning Varnish and Won't Restart

The Cause: Centrifugal switch welded closed. The switch is designed to snap open at 75% synchronous speed to disconnect the start winding. If the contacts arc and weld together, the start winding remains energized during continuous operation. Because the start winding uses thin wire not rated for continuous current, it rapidly overheats, melting the insulation varnish and shorting out.
The Fix: This is a fatal motor failure. The start winding is burned open. You must rewind the motor (rarely cost-effective for sub-5HP units) or replace the motor entirely. To prevent this, ensure the motor is not subjected to rapid start-stop cycling (jogging), which causes severe arcing at the centrifugal switch contacts.