If you are asking what does a capacitor do for a motor, the direct answer is this: in a single-phase AC induction motor, a capacitor creates a critical phase shift in the auxiliary winding. This shift generates a rotating magnetic field that provides the starting torque needed to get the rotor moving, and in the case of run capacitors, it maintains that field to optimize efficiency and power factor while the motor operates.
Without this component, a single-phase motor would simply sit there, vibrating and humming, because single-phase AC power crosses zero 120 times a second (on a 60Hz grid), creating torque deadbands where the magnetic field collapses. By delaying the current in the start winding, the capacitor essentially tricks the motor into thinking it is running on two-phase power.
The Physics: Start vs. Run Capacitors
Not all motor capacitors do the same job. Bench technicians and HVAC pros divide them into two distinct categories based on their duty cycle and internal construction.
- Start Capacitors: These are designed for high capacitance (typically 50 µF to 1200 µF) but very short duty cycles. They stay in the circuit only until the motor reaches about 75% of its rated speed, at which point a centrifugal switch or a potential relay physically disconnects them. They use an electrolytic dielectric, which allows high energy density but cannot handle continuous AC current without overheating and venting.
- Run Capacitors: These remain in the circuit continuously. They have lower capacitance (typically 1.5 µF to 100 µF) and use a metallized polypropylene film dielectric. This construction handles continuous AC ripple current without degrading. They keep the auxiliary winding energized to smooth out the torque curve and improve the motor's power factor.
Motor Type Comparison: Where Capacitors Fit In
Understanding what a capacitor does requires knowing which motors actually need them. Three-phase motors generate their own rotating field natively and do not use these components. For single-phase and specialized drives, here is how the common types stack up.
| Motor Type | Torque Curve | Control Needs | Relative Cost | Best Load Profile |
|---|---|---|---|---|
| Capacitor-Start (CSIR) | Very high starting torque (250-400% of full load), drops to standard running torque. | Centrifugal switch or potential relay to drop the start winding/capacitor. | Medium | High-inertia, hard-starting loads (air compressors, conveyors, large pumps). |
| Permanent Split Capacitor (PSC) | Low to moderate starting torque (30-150%), very smooth running torque. | Direct-on-line AC connection; no switches or relays required. | Low | Fans, blowers, and low-inertia continuous duty applications. |
| Capacitor Start/Run (CSR) | High starting torque, high running efficiency and power factor. | Switch/relay for the start cap; run cap stays permanently in circuit. | High | Heavy continuous loads requiring high breakaway torque (large woodworking equipment). |
| Three-Phase Induction | High starting torque, highly efficient across the entire curve. | Requires 3-phase power or a Variable Frequency Drive (VFD). | High (System) | Industrial machinery, heavy commercial HVAC, manufacturing lines. |
Terminal Identification and Sizing Rules
When replacing a failed unit, you must correctly identify the motor terminals and size the replacement. Most single-phase hermetic compressors and fractional-horsepower motors use three terminals: C (Common), S (Start), and R (Run).
Identifying C, S, and R with a Multimeter
Set your multimeter to Ohms (Ω). Measure the resistance between all three pairs of terminals. The rule of thumb for identifying them is:
- Highest Resistance (C to S): This is the Start winding. It has many turns of fine wire.
- Medium Resistance (C to R): This is the Run winding. It has fewer turns of thicker wire to handle continuous current.
- Lowest Resistance (R to S): This should equal the sum of the other two readings (C-S + C-R = R-S).
The terminal common to both the highest and medium readings is C. The capacitor wires connect between S and R (in parallel with the run winding, in series with the start winding).
Sizing Rule of Thumb and Worked Example
If the original nameplate is missing, you can estimate the required microfarad (µF) rating based on the motor's horsepower and the specific mechanical load context. Converting HP to kW without load context is useless for sizing; a 1 HP fan needs a vastly different capacitor than a 1 HP compressor.
- Start Capacitors: ~500 to 800 µF per Horsepower.
- Run Capacitors: ~25 to 30 µF per Horsepower.
Worked Load Example: You are replacing the run capacitor on a 1.5 HP (approx. 1.1 kW) single-phase pool pump driving a continuous-duty centrifugal impeller load.
Calculation: 1.5 HP × 30 µF/HP = 45 µF.
Voltage Rating: The motor runs on a 240V AC nominal line (measured 235V-245V). The capacitor voltage rating must be at least 1.25× the line voltage. 240 × 1.25 = 300V. Therefore, you select a 45 µF, 370 VAC (or 440 VAC) metallized polypropylene run capacitor. Never use a DC-rated capacitor here; the AC ripple will destroy it instantly.
Failure Signatures: Diagnosing a Bad Capacitor
Capacitors are often the weakest link in a motor circuit due to heat and dielectric degradation. Recognizing the failure signatures saves you from unnecessarily replacing a perfectly good motor. According to diagnostic guidelines from Fluke, visual and operational symptoms are your first clues.
- The Hum and Stall: You apply power, and the motor emits a loud 60Hz/120Hz hum but the shaft does not turn. If you manually spin the shaft with a non-conductive stick and it suddenly accelerates to speed, your start capacitor is dead (open circuit) or the centrifugal switch is stuck open. The motor lacks the phase shift to break the zero-crossing torque deadband.
- Overheating and Thermal Overload Trips: If a run capacitor degrades and loses capacitance (e.g., drops from 40 µF to 15 µF), the phase shift angle collapses. The motor draws excessive amperage on the main winding to compensate for the lost torque, overheating the stator and tripping the internal thermal overload protector.
- Physical Bulging or Leaking: Electrolytic start capacitors will bulge at the top or vent dielectric fluid when they fail due to heat or over-voltage. Film run capacitors may look physically perfect but fail internally; you must test them with a multimeter that has a dedicated capacitance setting.
- Rapid Clicking: If you hear a rapid click-click-click from the motor housing, the motor is starting, the centrifugal switch drops the start capacitor, the motor loses torque and slows down, the switch re-engages, and the cycle repeats. This usually indicates a weak start capacitor or a mechanical load that is too high for the motor's breakaway torque.
Frequently Asked Questions
What does a start capacitor do for a motor vs a run capacitor?
A start capacitor provides a massive, temporary phase shift to generate the high breakaway torque needed to get a heavy load moving from a dead stop. It is switched out of the circuit once the motor reaches operating speed. A run capacitor provides a smaller, continuous phase shift to keep the auxiliary winding energized, which smooths out the torque delivery, reduces vibration, and improves the electrical power factor during normal operation.
What happens if you use the wrong size capacitor for a motor?
If the capacitance is too low, the motor will struggle to start, draw excessive current, overheat, and likely trip its thermal overload. If the capacitance is too high, the phase shift angle over-shoots the optimal 90 degrees, causing excessive current to flow through the auxiliary winding. This will rapidly overheat the start winding, potentially melting the insulation and burning out the motor. Always stay within ±5% of the original microfarad rating for run capacitors, and within ±20% for start capacitors.
Can a single-phase motor run without a capacitor?
Standard single-phase induction motors cannot start without a capacitor (or a shaded pole/centrifugal mechanism). However, if a PSC or Capacitor-Start/Run motor is already spinning at full speed and the run capacitor fails open, the motor will often continue to run on the main winding alone. It will, however, lose significant torque capability, draw higher amperage, and run much hotter. It will not be able to restart if it is turned off or if the load causes it to stall.
How do I know if my motor capacitor is blown?
Visual inspection is the first step: look for bulging, split seams, or leaked fluid. For a definitive electrical test, safely disconnect power, discharge the capacitor terminals with a 20k-ohm 5W bleeder resistor, and use a digital multimeter with a capacitance function. Measure the microfarads across the terminals. As noted in All About Circuits motor theory guides, if the measured value is more than 5% below the nameplate rating, or if the meter reads an open circuit (OL) or a dead short, the capacitor must be replaced.
