A capacitor on motor circuits serves a singular, critical purpose in single-phase AC induction motors: it creates a phase shift in the auxiliary winding to generate a rotating magnetic field. Without this phase shift, a single-phase motor produces only a pulsating magnetic field, resulting in zero starting torque. The capacitor shifts the current in the start winding by roughly 90 electrical degrees relative to the run winding, effectively simulating a two-phase power supply to get the rotor spinning and maintain running efficiency.
Why Single-Phase Motors Need a Capacitor
Three-phase motors naturally produce a rotating magnetic field because their three power legs are already offset by 120 degrees. Single-phase power, however, pulses along a single axis. When you apply 120V or 240V single-phase power to a stator, the magnetic field expands and collapses but does not rotate. The rotor will just vibrate and hum.
By placing a capacitor in series with an auxiliary (start) winding, we delay the current flow in that specific winding. This time delay creates the spatial and temporal offset needed to establish a rotating magnetic flux. According to the Engineering ToolBox's guide on single-phase motors, the exact microfarad (µF) rating and the voltage rating of the capacitor dictate the strength of this starting torque and the motor's running power factor.
Motor Type Comparison: Which Fits Your Load Profile?
Selecting the right motor architecture depends entirely on the mechanical load profile. A fan requires very little torque to start but runs continuously, while a loaded air compressor requires massive starting torque to overcome initial cylinder pressure. The NEMA MG 1 Motors and Generators standard categorizes these designs based on their torque curves and control requirements.
| Motor Type | Torque Curve | Control / Driver Needs | Typical Cost | Best Load Profile |
|---|---|---|---|---|
| PSC (Permanent Split Capacitor) | Low starting torque (30-50% of full load), smooth acceleration. | Simple contactor or toggle switch. No centrifugal switch or relay required. | $80 - $150 | HVAC blowers, exhaust fans, pool pumps (low-inertia loads). |
| CS (Capacitor-Start) | High starting torque (200-300% of full load), abrupt transition. | Requires a centrifugal switch or potential relay to drop out the start capacitor at ~75% RPM. | $150 - $250 | Compressors, deep well pumps, conveyors (high-breakaway torque). |
| CSR (Capacitor-Start / Capacitor-Run) | High starting torque, high running efficiency, excellent power factor. | Requires both start and run capacitors, plus a potential relay and/or centrifugal switch. | $250 - $400+ | Commercial HVAC compressors, heavy industrial machinery, large chipper/shredders. |
Sizing Rules and Wiring Terminal Identification
Capacitor sizing is not arbitrary; it is mathematically tied to the motor's horsepower, voltage, and the specific phase angle the designer targeted (usually between 40 and 80 degrees, as a true 90-degree shift is electrically inefficient).
The Sizing Rule of Thumb
For 120V/240V, 60Hz single-phase motors, jobsite rules of thumb provide a reliable baseline when replacing a missing nameplate:
- Run Capacitors: 10 to 20 µF per horsepower.
- Start Capacitors: 50 to 100 µF per horsepower (often 5x to 10x the run capacitor value).
Worked Load Example: 1/2 HP, 120V Air Compressor (CS Motor)
Load Profile: High breakaway torque required to start against tank pressure.
Run Cap Sizing: 0.5 HP × 15 µF/HP = 7.5 µF. (Select standard 7.5 µF or 10 µF, 370VAC rated).
Start Cap Sizing: 0.5 HP × 150 µF/HP = 75 µF. (Select standard 108-130 µF or **189-226 µF**, 125VAC rated).
Note: Start capacitors are always rated for lower AC voltages (125V or 250V) because they are only in the circuit for milliseconds and experience lower continuous RMS stress, but they must handle massive inrush current.
Wiring and Terminal Identification
Single-phase hermetic compressors and many industrial motors use a three-terminal block. Identifying these correctly prevents immediate winding burnout.
| Terminal Label | Name | Wiring Destination & Function |
|---|---|---|
| C | Common | Connects to the main power Line (L1). This is the shared return path for both the start and run windings. |
| S | Start | Connects to the start winding. In a CS motor, this routes through the potential relay and start capacitor. In a PSC, it routes directly through the run capacitor. |
| R | Run | Connects to the run winding. Routes to the other side of the run capacitor (and eventually to L2/Neutral). |
Verification Step: If terminals are unlabeled, use a multimeter in resistance (Ohms) mode. Measure across all three pairs. The highest resistance reading is between S and R. The lowest resistance is between C and R. The intermediate resistance is between C and S.
Failure Signatures: Diagnosing Hum, Overheat, and Stall
When a capacitor degrades, the motor's electrical symmetry collapses. The physical symptoms on the bench or jobsite map directly to the internal failure mode of the capacitor.
- Symptom: Loud Hum, Will Not Spin (Rotor locked)
Cause: Open start capacitor or failed centrifugal switch/potential relay. The start winding is receiving zero current, meaning no rotating field is generated. The motor draws locked-rotor amperage (LRA) and will trip the thermal overload in 5-15 seconds.
Fix: Discharge and test the start capacitor. If it reads "OL" (open) on the capacitance meter, replace it. If the capacitor tests good, manually spin the shaft. If it runs, the centrifugal switch contacts are burned or the potential relay coil is open. - Symptom: Runs, but Overheats and Draws High Amps
Cause: Degraded or shorted run capacitor. As the dielectric film inside a metallized polypropylene run capacitor breaks down, its microfarad value drops. A 45 µF capacitor that has degraded to 20 µF fails to provide the correct phase shift for the running magnetic field. The motor's power factor plummets, and it pulls excessive current to maintain mechanical output.
Fix: Replace the run capacitor. Always use a capacitor with the exact µF rating (±5% tolerance) and a voltage rating equal to or higher than the original. - Symptom: Motor Stalls Under Load
Cause: Weak run capacitor or incorrect sizing. If a CSR motor stalls when the compressor unloader engages, the run capacitor cannot sustain the magnetic flux density required for breakdown torque.
Fix: Verify the run capacitor µF rating against the motor nameplate. Do not "upsized" the run capacitor to fix a stall; a higher µF will over-excite the start winding, causing it to overheat and melt the insulation.
Frequently Asked Questions
Can I use a run capacitor in place of a start capacitor on a motor?
No, and doing so will likely result in a fire or explosion. Start capacitors use an electrolytic dielectric designed to handle massive inrush currents for a maximum of 3 seconds (intermittent duty). Run capacitors use an oil-filled metallized polypropylene film designed for continuous duty but cannot handle the thermal stress of starting inrush. If you wire a start capacitor into a continuous run circuit, the electrolyte will boil, build pressure, and violently vent the capacitor casing. Always use the correct duty-rated component.
Why does my motor hum but not spin when the capacitor is wired?
Humming without rotation indicates that the main run winding is energized, but the auxiliary start winding is not contributing to the magnetic field. This is almost always caused by an open (dead) start capacitor, a broken wire in the start winding, or a failed switching mechanism (centrifugal switch or potential relay) that is failing to close the circuit to the start winding at zero RPM. Give the shaft a manual spin with a non-conductive stick; if the motor accelerates to full speed, the start circuit is definitively open.
How do I safely discharge a capacitor on a motor before testing?
Never short a motor capacitor directly with a screwdriver. The instantaneous current spike can weld the screwdriver to the terminals, destroy the capacitor's internal foil, and shower you with molten metal. Instead, use a dedicated capacitor discharge tool or a 20,000-ohm, 5-watt wirewound resistor attached to insulated alligator clips. Clip the resistor across the capacitor terminals for 10 to 15 seconds, then verify the voltage is below 1V DC using your multimeter before touching the terminals.
What happens if I wire a capacitor with a higher microfarad rating than specified?
Installing a run capacitor with a higher µF rating than the motor design calls for will increase the current flow through the auxiliary start winding. While this might slightly increase starting torque, it will cause the start winding to overheat during continuous operation because it is not wound with heavy enough gauge wire to handle the excess current. Over time, this degrades the winding insulation, leading to an inter-turn short and total motor burnout. Always match the nameplate µF rating within a ±5% tolerance.
