Start capacitors and run capacitors differ primarily in their design and function within electrical systems, particularly in motors and compressors. A start capacitor is typically used in single-phase AC motors to provide the initial phase shift needed to start the motor. It has a higher capacitance value compared to run capacitors and is designed to provide a brief boost of power during motor startup.
Start capacitors are often electrolytic capacitors, which can handle high currents for short durations and are then disconnected from the circuit by a centrifugal switch or relay once the motor reaches operational speed.
In contrast, a run capacitor is designed to continuously provide phase shift and improve the efficiency of the motor during its operation. It has a lower capacitance value compared to start capacitors and is usually connected in parallel with the motor windings throughout the motor’s operation.
Run capacitors help to maintain a stable voltage supply to the motor, improve power factor, and ensure smoother operation.
They are often metallized film capacitors, which offer good self-healing properties and can handle continuous operation without overheating.
While run capacitors are not typically used interchangeably with start capacitors due to their lower capacitance value and continuous operation, there are instances where a run capacitor can be temporarily used as a start capacitor.
However, this is not ideal because run capacitors are not designed to handle the high currents and brief duration required during motor startup. Using a run capacitor as a start capacitor can lead to overheating and potential failure of the capacitor or motor over time. It’s recommended to use capacitors specifically designed for their intended application to ensure reliable operation and longevity of the motor or compressor.
In an air compressor, the start capacitor and run capacitor serve distinct purposes to facilitate motor operation.
The start capacitor provides the initial torque needed to overcome inertia and start the compressor motor from a stationary position. It assists in generating a phase shift in the motor windings, allowing the motor to start smoothly and efficiently.
Once the motor reaches operational speed, the start capacitor is typically disconnected from the circuit to prevent overheating and unnecessary energy consumption.
On the other hand, the run capacitor in an air compressor continues to operate in parallel with the motor windings throughout its operation.
It helps to maintain a stable voltage supply, improve motor efficiency, and reduce power consumption by providing additional phase shift and improving power factor. Run capacitors are designed for continuous operation and are essential for ensuring reliable and efficient performance of the compressor motor during its entire operating cycle.
A dual run capacitor combines the functions of both a run capacitor and a start capacitor into a single unit. It is designed to support both the starting and running operations of a motor or compressor.
A dual run capacitor typically has three terminals: one common terminal (C) and two capacitance terminals (HERM for compressor and FAN for fan). It allows for a compact and efficient solution in applications where both capacitors are required, such as in HVAC systems where one capacitor can support the compressor motor (start and run) and another capacitor can support the fan motor.
This configuration simplifies wiring, reduces space requirements, and ensures proper capacitance values for both starting and running operations of the motors.