If you are asking which is true about a capacitor start motor, the defining mechanical truth is that it sacrifices continuous electrical efficiency to generate massive starting torque. A capacitor-start induction motor (CSIM) uses a phase-shifting electrolytic start capacitor and a mechanical centrifugal switch to create a rotating magnetic field. This setup generates 200% to 350% of full-load torque (FLT) to break high-inertia loads loose. Once the rotor reaches roughly 75% of synchronous speed, the centrifugal switch physically opens, dropping the start winding and capacitor out of the circuit so the motor runs solely on its main winding.

This architecture makes the CSIM the undisputed choice for hard-starting, high-inertia equipment that runs continuously once up to speed—specifically reciprocating air compressors, positive displacement pumps, and heavy-duty chip conveyors. It is not meant for frequent stop-start cycling or variable speed control.

Single-Phase Motor Comparison: Where Capacitor-Start Wins

To understand where a capacitor-start motor fits in your load profile, you have to look at the starting torque curve and the control hardware it demands. Below is a data-dense comparison of standard single-phase AC motor topologies.

Motor Type Starting Torque (% FLT) Torque Curve Profile Control / Drive Demands Relative Cost & Complexity
Shaded Pole < 50% Very low starting torque; peaks near synchronous speed. Direct-on-line (DOL); simple relay or switch. Lowest cost; no capacitors or switches.
Permanent Split Capacitor (PSC) 30% - 150% Smooth, moderate torque; excellent for variable speed. DOL or single-phase VFD / ECM controller. Medium cost; run capacitor only, no switch.
Capacitor-Start (CSIM) 200% - 350% Massive initial torque spike; drops to standard run torque. DOL starter, contactor, or manual motor starter. No VFDs. Medium-high cost; start capacitor + centrifugal switch.
Cap-Start / Cap-Run (CSCR) 200% - 350% High starting torque with improved running power factor. DOL starter or contactor. No VFDs. Highest cost; both start and run capacitors + switch.
Load Profile Match: Choose a Capacitor-Start motor when your load requires a massive initial push to overcome static friction or fluid compression (like a loaded piston compressor), but does not require speed variation once running. If your application requires soft-starting or speed control via a Variable Frequency Drive (VFD), you must use a PSC or a 3-phase motor instead.

Terminal Identification and Wiring the Start Circuit

Wiring a capacitor-start motor requires understanding the NEMA standard terminal markings for single-phase motors. Unlike 3-phase motors that just take L1, L2, and L3, a CSIM has distinct run and start winding taps.

  • T1 and T2: Main (Run) Winding leads. These connect directly across your line voltage (e.g., 240V).
  • T3 and T4: Start Winding leads. These are wired in series with the start capacitor and the centrifugal switch.
  • T5 and T8: Thermal overload or thermostat leads (if equipped). These are typically wired in series with the contactor coil or the main line to break the circuit if the motor casing overheats.

The centrifugal switch is mechanically linked to the rotor shaft. At zero RPM, spring tension holds the switch contacts closed, completing the start circuit. As the rotor accelerates, calibrated weights on the switch assembly are thrown outward by centrifugal force. This pulls an insulating collar back, allowing the contacts to snap open.

Controller Demands: A capacitor-start motor demands a simple magnetic contactor (like a Definite Purpose Contactor rated for motor starting) or a manual motor starter with a magnetic trip and thermal overload. Never connect a standard VFD to a capacitor-start motor. The VFD's varying frequency will disrupt the RPM-based actuation of the centrifugal switch, and the high-frequency PWM output will rapidly degrade the electrolytic start capacitor, leading to catastrophic failure.

Sizing Rules and a Worked Load Example

The golden rule for sizing a capacitor-start motor branch circuit is to accommodate the massive inrush current without causing nuisance breaker trips, while relying on the motor's internal thermal overload for continuous running protection. A standard CSIM will draw 600% to 800% of its Full Load Amps (FLA) for the first 1 to 3 seconds of startup.

Under NEC-style guidance (specifically NFPA 70 / NEC Article 430.52), you are permitted to size the inverse-time circuit breaker up to 250% of the motor's FLA to handle this inrush.

Worked Example: 2 HP Air Compressor

Let's size the breaker and wire for a 2 HP, 240V single-phase capacitor-start air compressor.

  1. Identify FLA: A typical 2 HP, 240V motor has an FLA of roughly 12A (check the specific nameplate, as this varies by efficiency and service factor).
  2. Calculate Inrush: 12A × 7 (average multiplier) = 84A inrush current for ~2 seconds.
  3. Size the Breaker: 12A FLA × 2.5 (NEC max for inverse-time breaker) = 30A. We select a 30A 2-pole breaker. A standard 15A or 20A breaker would trip instantly every time the compressor tried to start under pressure.
  4. Size the Wire: The wire must be sized at 125% of the FLA. 12A × 1.25 = 15A. While 14 AWG is technically rated for 15A, the 30A breaker requires a minimum of 10 AWG copper (THHN or NM-B) to protect the wire from the breaker's trip curve.

Failure Signatures: Diagnosing Hum, Overheat, and Stall

Because the capacitor-start topology relies on a mechanical switch and an electrolytic capacitor, it has highly specific failure signatures. When troubleshooting, refer to these exact symptoms and measurements, drawing on standard motor troubleshooting practices.

Symptom 1: Loud Hum, Breaker Trips, Motor Stalls

  • The Cause: The start capacitor has failed open, or the centrifugal switch contacts are pitted and failing to make contact at zero RPM.
  • The Physics: Without the phase-shifted current from the start winding, the motor only produces a pulsating magnetic field, not a rotating one. It cannot generate starting torque.
  • The Fix: Disconnect power, lock out the breaker, and discharge the capacitor with a 20k-ohm 5W resistor. Test the capacitor with a multimeter in capacitance mode. If it reads open or more than 10% below its printed microfarad (µF) rating, replace it. If the capacitor is good, manually actuate the centrifugal switch collar to verify the contacts snap shut with < 1 ohm of resistance.

Symptom 2: Motor Starts, but Overheats and Trips Overload

  • The Cause: The centrifugal switch has failed to open (often due to welded contacts, a broken return spring, or mechanical binding from dust).
  • The Physics: The start winding is wound with thinner wire and is only rated for a few seconds of duty. If it remains energized at running speed, it will rapidly overheat, melt its insulation, and short out the main winding.
  • The Fix: This requires a teardown. Remove the rear bell housing of the motor. Inspect the switch contacts for arcing pits. If they are welded shut, the entire centrifugal switch assembly must be replaced. Do not attempt to file the contacts, as this alters the mechanical gap and switch actuation speed.

Symptom 3: Slow Acceleration and Voltage Sag

  • The Cause: Undersized feeder wire, loose terminal connections, or a failing run winding.
  • The Fix: Measure the voltage at the motor terminals (T1 and T2) during startup using a multimeter with a min/max hold function. If the voltage drops below 90% of nominal (e.g., below 216V on a 240V system) while the motor is accelerating, the start torque drops exponentially (torque is proportional to voltage squared). You must upsize the feeder wire or shorten the run to reduce voltage drop.
Safety Warning: Always de-energize and verify dead with a tested CAT III or CAT IV multimeter before opening a motor terminal box. Start capacitors can retain a lethal charge even after power is removed. Always bridge the terminals with an insulated discharge tool before handling.