A capacitor in a single-phase AC electric motor creates an artificial second phase. By shifting the current in the auxiliary winding out of phase with the main winding (typically by 70° to 90°), it generates the rotating magnetic field required to produce starting torque and maintain running efficiency. Without it, a single-phase motor connected to a standard wall outlet would simply hum, vibrate, and overheat without turning the rotor.
The Physics of the Phase Shift
Single-phase AC power pulses at 120 Hz (in a 60 Hz system) but does not naturally rotate. To make an induction motor rotor turn, the stator must produce a rotating magnetic field. The main winding creates a pulsating field on a single axis. The auxiliary (start) winding is physically offset in the stator by 90 electrical degrees.
By placing a capacitor in series with the auxiliary winding, the current in that winding leads the supply voltage, while the current in the highly inductive main winding lags. This electrical phase separation, combined with the physical spatial offset, creates the magnetic "push" needed to start the rotor. As All About Circuits details in their AC theory textbook, this principle is what allows single-phase motors to mimic the naturally rotating field of three-phase power.
- Start Capacitors (e.g., CD60 series): High capacitance (70–800 µF), electrolytic dielectric. Designed for short-duty cycles (under 3 seconds). They provide massive starting torque but will overheat and vent if left in the circuit continuously.
- Run Capacitors (e.g., CBB60/CBB65 series): Lower capacitance (2–80 µF), metallized polypropylene film. Designed for continuous duty. They remain in the circuit while the motor runs to optimize the power factor and running torque.
Motor Type Comparison & Load Profiling
Selecting the right motor and capacitor configuration depends entirely on the mechanical load. A capacitor-start motor is useless for a continuous-duty conveyor, and a shaded-pole motor will instantly stall on an air compressor. Below is a data-dense comparison to help you match the motor to the load profile.
| Motor Type | Torque Curve | Control / Switching Needs | Typical Cost (1 HP Baseline) | Best Load Profile |
|---|---|---|---|---|
| Shaded Pole | Very low starting torque, low running torque. | Direct-on-line (DOL) contactor or simple relay. No capacitor. | $40 - $60 | Small fans, record players, low-inertia blowers (< 1/20 HP). |
| PSC (Permanent Split Capacitor) | Low starting torque, medium running torque. | DOL contactor. Run capacitor permanently wired in series with aux winding. | $120 - $180 | HVAC blowers, pool pumps, garage door openers (1/4 to 1 HP). |
| CSCR (Capacitor-Start / Capacitor-Run) | Very high starting torque, high running torque. | Centrifugal switch or potential relay to drop the start cap. Run cap stays. | $200 - $350 | Air compressors, shallow well pumps, heavy conveyors (1/2 to 5 HP). |
| 3-Phase Induction | High starting torque, excellent speed regulation. | VFD (Variable Frequency Drive) or DOL starter. No capacitors required. | $250 - $400 | Industrial machinery, large HVAC chillers, continuous conveyors (> 1 HP). |
Which motor fits your load? If your load requires high breakaway torque (like compressing air against a loaded tank), you must use a CSCR motor. If the load is easy to start but runs continuously with variable speed requirements (like a furnace blower), a PSC motor paired with an ECM (Electronically Commutated Motor) upgrade is the modern standard.
What driver/controller does it demand? Single-phase CSCR motors demand a reliable potential relay (like the Supco SUPR4 or Mars 19 series) to disconnect the start capacitor precisely when the back-EMF peaks. PSC motors only require a simple heavy-duty contactor or smart relay to switch the main line voltage.
Terminal Wiring, Sizing Rules, and Worked Examples
Correctly identifying terminals and sizing the replacement capacitor is where most DIY repairs fail. According to Engineering Toolbox motor standards, mismatching capacitance by more than 10% will cause severe magnetic imbalance, leading to rapid winding insulation breakdown.
Terminal Identification
On a standard single-phase motor terminal board, you will typically see:
- T1 / T2 (or L1 / L2): Main line voltage inputs.
- T3 / T4 (or U1 / U2 / Z1 / Z2): Internal winding taps. Z1/Z2 usually denote the auxiliary (start) winding.
On a dual run capacitor (common in HVAC), the terminals are clearly marked:
- C (Common): Connects to the incoming hot line (L1).
- HERM (Hermetic): Connects to the compressor's start winding.
- FAN: Connects to the condenser fan motor's start winding.
Sizing Rule of Thumb & Worked Load Example
The Rule: For run capacitors, estimate roughly 15 to 20 µF per Horsepower (HP) at 230V/60Hz. For start capacitors, estimate 80 to 100 µF per HP. Always verify against the motor nameplate, as pole count and efficiency class alter these baselines.
Scenario: You have a 1.5 HP (1119W) single-phase PSC pool pump motor operating on 230V nominal (measured 234V). The motor hums and trips the 20A breaker.
Sizing: Using the 15-20 µF/HP rule: 1.5 HP × 18 µF = 27 µF.
Selection: You check the nameplate and it specifies 25 µF, 370VAC. You must buy a 25 µF run capacitor.
Voltage Upgrade: You can safely substitute a 440VAC rated capacitor for the 370VAC original (the higher voltage rating just means thicker dielectric film and longer life), but you must never use a 370VAC cap in a circuit designed for 440VAC. Never substitute a start capacitor (CD60) here; the electrolytic fluid will boil and rupture within seconds of continuous operation.
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. Recognizing the acoustic and thermal signatures of failure will save you from replacing a perfectly good motor.
| Symptom | Most Likely Cause | Diagnostic Measurement & Fix |
|---|---|---|
| Loud Hum, Rotor Locked | Failed start capacitor, or centrifugal switch stuck open. | Disconnect power. Discharge cap with a 20kΩ 5W resistor. Measure with DMM in capacitance mode. If reading is >10% below nameplate µF, replace. If cap tests good, inspect the centrifugal switch contacts for pitting. |
| Overheating Under Load | Degraded run capacitor (motor running on main winding only, causing high slip and high current). | Measure running current with a clamp meter. If current is 15-20% above nameplate FLA (Full Load Amps) and the run capacitor reads low on the DMM, replace the run capacitor. Check for bulging or leaked dielectric fluid. |
| Motor Stalls Mid-Cycle | Severe voltage drop at the panel, or mechanical bind in the driven load. | Measure voltage at the motor terminals under load. If it drops below 197V (for a 230V nominal system), the issue is wire gauge/voltage drop, not the capacitor. Check the driven load for seized bearings. |
Pro-Tip for Bench Testing: When testing a run capacitor with a digital multimeter, ensure the capacitor is fully isolated from the circuit. Modern CBB65 capacitors contain an internal pressure-sensitive interrupter. If the capacitor casing swells due to gas buildup from internal arcing, this interrupter physically snaps open to prevent explosion. If your DMM reads "OL" (Open Loop) on a physically swollen capacitor, the internal fuse has blown—do not attempt to reset it; discard and replace immediately.
