How to Select the Right Capacitor for a Motor

The core of selecting a matching capacitor for a motor is to match the capacitor type, capacitance, and voltage withstand value. It is necessary to consider the motor type (mainly single-phase asynchronous motors; three-phase motors generally do not require capacitors), fuerza, and nameplate parameters to avoid motor startup difficulties, calentamiento excesivo, or even burnout caused by excessively large or small capacitance. Below is a systematic selection method:

1. First Clarify the Two Core Types of Motor Capacitors

Capacitors for single-phase asynchronous motors are divided into start capacitors and run capacitors, which have completely different functions and selection logic and must be distinguished first:

> Three-phase asynchronous motors usually do not require capacitors. Additional capacitors are only needed in emergency phase loss situations or when converting to single-phase operation.

1. Three Key Steps for Selection
2. Prioritize Checking the Motor Nameplate (Most Accurate)

The nameplate of a standard motor will directly mark the capacitor specifications, Por ejemplo:

Run Capacitor: `Run Capacitor: 20μF 450V AC`

Start Capacitor: `Start Capacitor: 100μF 450V AC`

Directly matching the parameters on the nameplate is the most reliable method.

2. Estimate Capacitance Based on Motor Power When the Nameplate Is Missing

If the nameplate is lost, the capacitance can be estimated based on the motor power (W.). The formula is only applicable to 220V single-phase asynchronous motors:

Run Capacitor Capacitance Estimation Formula

$$do(\mu F) \approx P(W.) \veces 0.06$$

Ejemplo: For a 1.5kW (1500W.) motor, the run capacitor capacitance ≈ 1500 × 0.06 = 90μF

Start Capacitor Capacitance Estimation Formula

Start Capacitor Capacitance = Run Capacitor Capacitance × 2–3 times

Ejemplo: The start capacitor corresponding to the above 90μF run capacitor = 90×2 = 180μF

> Nota: The smaller the power, the larger the estimation deviation. It is recommended to appropriately reduce the capacitance for low-power motors (≤500W) to avoid excessive current.

3. Determine the Capacitor’s Voltage Withstand Value (Key Safety Indicator)

The voltage withstand value of the capacitor must be more than 1.5 times the rated voltage of the motor, and an AC voltage withstand (C.A.) capacitor must be selected. DC capacitors (corriente continua) are strictly prohibited as substitutes.

220V single-phase motor: Capacitor voltage withstand ≥ 450V AC (preferably 450V or 500V)

380V motor converted to single-phase operation: Capacitor voltage withstand ≥ 630V AC

Insufficient voltage withstand capability will cause capacitor breakdown, bulging, or even explosion, which is the core safety red line for selection.

III. Four Important Precautions for Selection

1. Do Not Mix Capacitor Types

Start capacitors (CD60) cannot be used as run capacitors for long-term operation, de lo contrario, they will be damaged due to overheating; run capacitors (CBB60) cannot replace start capacitors either, de lo contrario, the motor will have insufficient starting torque and fail to start under load.

2. Control Capacitance Deviation Within ±10%

Excessively large capacitance → Increased motor current and excessive temperature rise; Excessively small capacitance → Difficult startup and insufficient torque. Especially for motors starting under heavy load (such as compressors and water pumps), the capacitance deviation should be smaller.

3. Prioritize Non-Polar Capacitors

Run capacitors must be non-polar CBB series; although start capacitors are electrolytic capacitors, they are also AC non-polar (Type CD60) and cannot use ordinary DC electrolytic capacitors.

4. Adjustments for Special Scenarios

Motors starting under heavy load (such as crushers and air compressors): The start capacitor capacitance can be appropriately increased (take 3 times the run capacitor capacitance).

High-altitude/high-temperature environments: Select capacitors with higher temperature resistance grades (p.ej., -40℃~105℃), and the voltage withstand value can be increased by one level.

1. Supplementary Selection Method for Capacitors When Converting Three-Phase Motors to Single-Phase Operation

When a three-phase motor is in phase loss or converted to 220V single-phase operation, a run capacitor needs to be configured. The formula is as follows:

Ejemplo: For a 3kW three-phase motor converted to 220V single-phase operation, the capacitor capacitance ≈ (3000×50)/220 ≈ 682μF. In practice, select a CBB capacitor with 600~700μF and 630V AC.

Failures of motor capacitors (start capacitors and run capacitors) will directly cause the motor to fail to work normally and even damage the motor windings. Common faults can be classified according to appearance characteristics and functional failure modes, as detailed below:

1. Visible Appearance Faults

Such faults can be directly judged by visual inspection, which is also the first step in on-site troubleshooting.

1. Case Bulging and Leakage

Fenómeno: The top/side of the capacitor bulges and deforms, the plastic case cracks, and yellowish-brown electrolyte leaks out (common in Type CD60 start capacitors).

Main Causes: Insufficient voltage withstand capability (actual voltage exceeds the rated voltage of the capacitor), long-term high-temperature operation (poor motor heat dissipation or insufficient capacitor temperature resistance grade), and capacitance overload (over-sized capacitor selection).

Hazards: The capacitor completely fails, and the motor cannot start; leakage will corrode motor terminals and metal components of the distribution box.

2. Pin Corrosion and Fracture

Fenómeno: Capacitor pins are rusted, oxidado, or broken at the root, resulting in poor contact at the wiring point.

Main Causes: Humid and dusty installation environment, or excessive tightening torque during wiring causing pin damage due to stress.

Hazards: The motor “starts and stops intermittently” during operation or powers off instantly at startup. Frequent startup and shutdown will burn out the motor windings.

3. Case Carbonization and Smoldering Marks

Fenómeno: The capacitor surface turns black with burning marks, and even accompanied by a burnt smell.

Main Causes: Internal short circuit of the capacitor, which generates large current instantaneously and causes a sharp temperature rise.

Hazards: May cause short circuit of the distribution box circuit, burn out the fuse, and even lead to fire hazards in severe cases.

1. Functional Faults Requiring Instrument Detection

Such faults have no obvious abnormal appearance and can only be confirmed by detection tools such as a multimeter.

1. Capacitance Attenuation or Loss

Fenómeno: Measured with the capacitance range of a multimeter, the actual capacitance is more than 10% lower than the nominal capacitance; the motor has difficulty starting, torque drops, can run no-load but stops under load, and makes abnormal noise during operation.

Main Causes: Capacitor dielectric aging (natural loss due to long-term use), frequent startup and shutdown causing repeated charging and discharging of the capacitor to accelerate aging (common in Type CBB60 run capacitors).

Hazards: The motor operating current increases, and the winding temperature rise is too high. Long-term operation will burn out the motor.

2. Internal Open Circuit

Fenómeno: The capacitance measured by a multimeter is 0; when the start capacitor is open-circuited, the motor hums but does not rotate after being powered on; when the run capacitor is open-circuited, the motor can start but runs at low speed and with insufficient power.

Main Causes: The welding point of the internal lead of the capacitor falls off, or the dielectric becomes brittle and breaks.

Hazards: If the start capacitor is open-circuited and power is not cut off in time, the motor locked-rotor current will rise rapidly, burning out the windings in a short time.

3. Internal Short Circuit

Fenómeno: The capacitance displayed by a multimeter is abnormally large, or directly shows a short circuit; the fuse blows instantly after power-on, and the capacitor heats up rapidly.

Main Causes: The capacitor is broken down due to insufficient voltage withstand capability, or internal impurities cause plate short circuit.

Hazards: Directly cause circuit short circuit, may damage the motor drive circuit, and even pose an electric shock risk.

III. Key Points for Fault Troubleshooting

1. Prioritize visual inspection: Capacitors with bulging, leakage, or carbonization must be replaced immediately and are prohibited from continued use.
2. Confirm with instrument detection: Measure with the capacitance range of a multimeter, and it is recommended to replace the capacitor if the capacitance deviation exceeds ±10%.
3. Strictly match parameters during replacement: The type, capacitance, and voltage withstand value of the capacitor must be consistent with the motor nameplate. It is strictly prohibited to mix start capacitors and run capacitors.