What is the difference between a running and a starting capacitor?

The running capacitor and the starting capacitor mainly have the following differences:

I. In terms of usage

1. Starting capacitor

• Mainly used to provide a momentary high current when the motor starts, helping the motor overcome the inertia of the stationary state and start smoothly. For example, in a single-phase asynchronous motor, the starting capacitor is connected in series with the starting winding. At the moment the motor starts, a rotating magnetic field with a large phase difference is generated, enabling the motor to start quickly.

• Once the motor reaches a certain speed, the starting capacitor is usually automatically disconnected through a centrifugal switch or other devices and no longer participates in the operation of the motor.

2. Running capacitor

• Continuously plays a role during the operation of the motor and is used to improve the power factor of the motor and improve the operating performance of the motor. For example, in some motors that need to run continuously, such as air conditioning compressors and fan motors, the running capacitor is connected in parallel with the main winding of the motor. By compensating the reactive power of the motor, the efficiency and power factor of the motor are improved.

• The running capacitor will always be connected in the circuit and work as the motor runs.

II. In terms of capacity

1. Starting capacitor

• Generally has a large capacity. This is because a large current and torque need to be provided at the moment of motor startup, so a capacitor with a large capacity is needed to generate a sufficient phase difference. For example, for some small single-phase asynchronous motors, the capacity of the starting capacitor may be between several tens of microfarads and several hundred microfarads.

• Since the starting capacitor only works at the moment of startup, its capacity can be relatively large without adversely affecting the long-term operation of the motor.

2. Running capacitor

• The capacity is usually smaller than that of the starting capacitor. Because only a certain amount of reactive power needs to be compensated during the motor's operation, there is no need to provide a huge current like at startup. For example, the capacity of a running capacitor may be between several microfarads and several tens of microfarads.

• If the capacity of the running capacitor is too large, it may cause overcompensation of the motor and reduce the efficiency and performance of the motor instead.

III. In terms of voltage withstand requirements

1. Starting capacitor

• Due to the large current impact at the moment of startup, the voltage withstand requirement is relatively high. For example, the starting capacitor usually needs to be able to withstand the high voltage and large current impact at the moment the motor starts. Its voltage withstand value is generally above 400 volts AC.

• In order to ensure that the starting capacitor can work reliably under harsh starting conditions, a capacitor with good quality and high voltage withstand performance is usually selected.

2. Running capacitor

• Although it also withstands a certain voltage during operation, compared with the starting capacitor, it withstands less current impact. Therefore, the voltage withstand requirement of the running capacitor is relatively lower, generally between 250 volts AC and 450 volts AC.

• The running capacitor needs to have good stability and reliability to ensure the long-term stable operation of the motor.

IV. In terms of working time

1. Starting capacitor

• The working time is short and only works at the moment the motor starts. Once the motor starts up, the starting capacitor will be disconnected and no longer participate in the operation of the motor. For example, in a single-phase asynchronous motor, the starting capacitor may only work for a few seconds to several tens of seconds.

• Due to the short working time, the starting capacitor generates relatively little heat and has a lower requirement for heat dissipation.

2. Running capacitor

• The working time is long and the same as the running time of the motor. As long as the motor is running, the running capacitor will always work and continuously compensate the reactive power of the motor. For example, in some continuously operating equipment, the running capacitor may need to work continuously for several hours or even longer.

• Due to the long working time, the running capacitor will generate a certain amount of heat, so heat dissipation needs to be considered to ensure its long-term stable work.