A start capacitor and a run capacitor serve different purposes in electric motors. A start capacitor is designed to provide the initial torque to start a motor, particularly one that requires high starting torque, like a compressor motor in refrigeration equipment. It is typically larger in capacitance value compared to a run capacitor and is only in the circuit during the motor startup phase. Once the motor reaches a certain speed, the start capacitor is disconnected from the circuit by a centrifugal switch or a relay.
To distinguish between start and run capacitors, you can check their physical characteristics and capacitance values. Start capacitors are often larger physically and have capacitance values typically ranging from 70 to 800 microfarads (µF). They are designed to handle brief high-current demands during motor startup. Run capacitors, on the other hand, are smaller and have capacitance values ranging from 1 to 100 µF. They remain in the circuit continuously during motor operation to improve efficiency and power factor.
Using a run capacitor as a start capacitor is generally not recommended due to differences in design and capacitance values. Run capacitors are not typically designed to handle the high starting currents that a start capacitor can manage. Attempting to use a run capacitor as a start capacitor could lead to overheating, premature failure, or improper motor operation.
To determine if your start or run capacitor is bad, observe the motor’s behavior during operation. Symptoms of a bad capacitor include difficulty starting, humming noises, or failure to start altogether. Visual inspection can also reveal signs such as bulging, leaking electrolyte, or a burnt smell. Use a multimeter to test the capacitance of the capacitor; if the measured capacitance is significantly lower than the rated value or if the multimeter shows no capacitance reading, the capacitor is likely faulty and should be replaced.
The main difference between a start capacitor and a dual run capacitor lies in their usage and design. A start capacitor is used exclusively for motor starting, providing the initial torque necessary to overcome inertia and start the motor. It is typically larger in capacitance value and is disconnected from the circuit once the motor reaches operational speed. In contrast, a dual run capacitor combines two capacitors in one unit: a run capacitor and a start capacitor. This design allows a single component to provide both continuous operation support (run capacitor function) and initial starting torque (start capacitor function) in certain types of motors or compressors.