Difference Between Start Capacitor and Run Capacitor

When a single phase induction motor is working, it will involve the problems of start capacitor and run capacitor. Because single phase is different from three phase, a single phase motor cannot form a rotating magnetic field when it is started, and uses the characteristic of the current in the capacitor to generate a rotating magnetic field to start the motor. Therefore, the start capacitor in a single phase motor is essential. But the run capacitor is not necessary.

Start Capacitor

Single phase induction motors are different from three phase induction motors. Three phase induction motors have three windings. After power-on, a rotating magnetic field can be generated, and the rotor can be rotated by the action of the magnetic field. The single phase induction motor has only two windings, one is the starting winding and the other is the running winding. It also has only one live wire and one neutral wire for power supply. The running winding is the main winding of the single-phase AC motor and is responsible for the normal operation of the motor. However, to make the motor rotor rotate, more than two alternating magnetic fields are needed to generate a rotating magnetic field. The two magnetic fields have to be staggered by an angle.

The starting winding can also be called a secondary winding, which can provide a current of a different phase (90 degrees) from the main winding when starting, and generate a rotating magnetic field for the motor to start smoothly. Therefore, we need to let the start winding pass current to generate the magnetic field, and then run the winding and then energize to generate the magnetic field, which produces a rotating magnetic field that can make the motor rotate in time and space.

Run Capacitor

After the single phase induction motor is running normally, its internal centrifugal switch will cut off the connection of the start capacitor, and only the running capacitor will accompany the rotation of the motor. In fact, the start capacitor and the run capacitor have the same function, except that the starting capacitor has a larger capacity and can get a larger starting current, thereby generating a larger starting torque. During long-term operation, a large capacitor is not needed. If the capacitor is too large, it will cause the startup winding to heat up during long-term operation. Therefore, when the start-up is completed, the centrifugal switch inside the single-phase induction motor will switch to a smaller-capacity operating capacitor to maintain the rotating magnetic field and avoid heating of the starter winding. The running capacitor allows the motor to perform capacitance compensation during operation. Therefore, the starting capacitor is necessary, but the running capacitor is not necessary. Some single phase induction motors do not have a running capacitor.

ATO single phase induction motors with a start capacitor and run capacitor to provide zero starting torque at rest, used for drills, air conditioners, compressors, pumps, cranes, garage door opening and closing systems.