Differences Between Motor Start Capacitors and Run Capacitors

Both start capacitors and run capacitors of electric motors are used in single-phase AC induction motors. Their core function is to cooperate with the start winding to generate a rotating magnetic field. However, there are essential differences between the two in terms of operating principles, parameter specifications, and working modes. The detailed comparison and explanation are as follows:

1. Differences in Core Functions and Operating Principles

2. Comparison Item	Start Capacitor	Run Capacitor
   Core Function	Provides starting torque only at the moment of motor startup to help the rotor overcome static friction and start rotating.	Operates continuously throughout the entire motor running process, improves the circuit power factor, maintains the stability of the rotating magnetic field, and ensures the motor outputs rated torque.
   Working Phase	Automatically disconnected by a centrifugal switch/relay and separated from the circuit after the startup is completed (when the speed reaches 70%-80% of the rated speed).	Connected in series with the start winding during the entire process from startup to operation and continuously participates in the work.
   Effect on Magnetic Field	Temporarily increases the current phase difference of the start winding to form a strong rotating magnetic field, which assists in startup.	Maintains the current phase difference (approximately 90掳) between the start winding and the run winding to ensure the stability of the magnetic field.

3. Differences in Key Parameters and Structures
4. Capacitor Type

Start Capacitor: Mostly uses non-polar electrolytic capacitors, which can withstand instantaneous high-current impacts. They have large capacitance but poor durability and are not suitable for long-term operation.

Run Capacitor: Mostly uses metallized polypropylene film capacitors, which are resistant to AC voltage, have good insulation performance, low power loss, and high stability, enabling long-term continuous operation.

2. Capacitance Range

Start Capacitor: Has a large capacitance, usually ranging from tens to hundreds of microfarads (μF). Examples include 100μF, 200μF, and 300μF. The specific capacitance is matched according to the motor power.

Run Capacitor: Has a small capacitance, usually ranging from several to tens of microfarads (μF). Examples include 2μF, 10μF, and 30μF. The capacitance is positively correlated with the motor power.

3. Voltage Rating Requirements

Start Capacitor: The voltage rating is generally 250VAC/330VAC, which only needs to meet the instantaneous operating voltage.

Run Capacitor: Has a higher voltage rating, usually 450VAC/630VAC. It must withstand the rated operating voltage for a long time to prevent breakdown.

4. Appearance and Installation

Start Capacitor: Has a large volume. Due to its large capacitance, its pins are thick, and it is often installed together with a centrifugal switch.

Run Capacitor: Has a small volume and a compact structure. It is directly fixed on the motor housing, and the wiring is simpler.

III. Applicable Motor Types and Fault Symptoms

1. Applicable Motors

Only Start Capacitor Required: Capacitor-Start Induction-Run (CSIR) motors, such as high-power air compressors and water pump motors. They have heavy starting loads and rely on start capacitors to provide strong torque.

Only Run Capacitor Required: Permanent Split Capacitor (PSC) motors, such as fan motors and air conditioner indoor unit motors. They have light loads, and the run capacitor serves both startup and operation functions.

Both Capacitors Required: Capacitor-Start Capacitor-Run (CSR) motors, such as washing machine motors and compressor motors. Both capacitors work during startup, and the start capacitor is disconnected after the startup is completed.

2. Fault Symptoms

Damaged Start Capacitor (Capacitance Attenuation or Breakdown): The motor fails to start, emits a buzzing sound when powered on, and the rotor gets stuck and cannot rotate. Or, the motor has difficulty starting and requires external force to assist in rotation.

Damaged Run Capacitor (Capacitance Reduction or Open Circuit): The motor can start but has insufficient torque, the speed is lower than the rated value, the motor body heats up severely, and it is prone to shutdown when under load.

1. CoreSummary

A start capacitor is a “temporary booster” for the motor, characterized by large capacitance and short-term operation. A run capacitor is a “long-term stabilizer” for the motor, characterized by small capacitance and long-term operation. The two cannot be mixed. When replacing, the capacitor specifications (capacitance+voltage rating) marked on the motor nameplate must be strictly matched.

Start capacitors and run capacitors of electric motors must not be directly interchanged for use. Their design principles, parameter characteristics, and working modes are completely different. Forced interchange will lead to motor failures and even potential safety hazards. The specific reasons and hazards are as follows:

1. Core Reasons: Mismatched Parameters and Structures
2. Differences in Capacitor Type and Durability

A start capacitor is a non-polar electrolytic capacitor, which can only withstand instantaneous high-current impacts. It has large capacitance but poor durability and is not suitable for long-term operation.

A run capacitor is a metallized polypropylene film capacitor, which is resistant to AC voltage, has good insulation performance, low power loss, and high stability, enabling long-term continuous operation in the circuit.

If a start capacitor is used as a run capacitor, the electrolyte will volatilize and the capacitor will bulge due to long-term power-on heating, eventually leading to breakdown and short circuit, and even fire.

2. Mismatch Between Capacitance and Torque Requirements

A start capacitor has a large capacitance (tens to hundreds of microfarads), aiming to provide a strong phase difference at the moment of startup to generate sufficient starting torque to overcome the static friction of the rotor.

A run capacitor has a small capacitance (several to tens of microfarads), which only needs to maintain a stable rotating magnetic field and does not require large torque output.

If a run capacitor is used as a start capacitor, insufficient capacitance will result in failure to generate enough starting torque. The motor will emit a buzzing sound but fail to start when powered on. A stuck rotor will cause the winding current to rise sharply, burning the motor windings in a short time.

3. Differences in Voltage Rating Compatibility

The voltage rating of a run capacitor (450VAC/630VAC) is much higher than that of a start capacitor (250VAC/330VAC). If a start capacitor is used as a run capacitor, it will be broken down by the continuous AC voltage in the circuit due to insufficient voltage resistance. If a run capacitor is used as a start capacitor, although the voltage resistance is sufficient, the capacitance disadvantage cannot be compensated, and the motor still cannot complete the startup.

1. Special Case: Strict Restrictions for Temporary Emergency

Only in emergency situations can a run capacitor with the same voltage rating and capacitance be temporarily used as a start capacitor, and two conditions must be met:

1.  The motor starts under light load (such as fans and small water pumps) without heavy load conditions.
2.  The single startup time does not exceed 5 seconds, and the motor must be shut down immediately after startup, with no continuous operation allowed.

This replacement is only an expedient measure. Long-term use will still damage the motor, and the start capacitor of the original factory specification should be replaced as soon as possible.

Conversely, a start capacitor must never be used as a run capacitor, and there is no feasible emergency scenario for such replacement.

III. Summary

The functions, structures, and parameters of start capacitors and run capacitors are tailored to different working phases of the motor. They have a relationship of “short-term booster” and “long-term stabilizer”. Direct interchange will cause serious consequences such as motor burnout and capacitor explosion.

When replacing a capacitor, the capacitor type, capacitance, and voltage rating must be selected in strict accordance with the specifications marked on the motor nameplate.