How does a run capacitor works?

Running capacitors are used to continuously adjust the current or phase shift of the motor winding to optimize the torque and efficiency of the motor. Because it is designed for continuous operation, the failure rate is much lower than that of the starting capacitor.

Start capacitors give a large capacitance value necessary for motor starting for a very short (seconds long) period of time. They are only intermittent duty and will fail catastrophically if energized too long. Run capacitors are used for continuous voltage and current control to a motor's windings and are therefore continuous duty. They are generally of a much lower capacitance value.

In unusual circumstances, a run capacitor could be used as a start capacitor, but the values available for them are much lower than the values usually available for dedicated start capacitors. The capacitance and voltage ratings would have to match the original start capacitor specification. A start capacitor can never be used as a run capacitor, because it could not handle current continuously (only a couple of seconds).

A run capacitor uses the charge in the dielectric to boost the current which provides power to the motor. It is used to maintain a charge. In AC units, there are dual run capacitors. One capacitor provides power to the fan motor. The other sends power to the compressor. Run capacitors measure in at approximately 7-9 micro-Farads. The value or rating of the run capacitor must be accurate. If the value is too high, the phase shift will be less than perfect and the winding current will be too high. If the capacitor value/rating is too low, the phase shift will be higher and the winding current will be too low. If run capacitors are not ideal, then the motor could overheat and the true torque will not be enough to drive current.

As we all know, a single-phase AC motor is not like a three-phase motor. It can be turned on when it is powered. It needs a starting torque to rotate, and the timing of this torque determines the steering of the motor, and there are many ways to start it. Among them, the capacitor start is a kind, it is customary to call the starting capacitor, and the single-phase motor needs it to turn it smoothly.

However, some single-phase motors not only have one capacitor, but some have two capacitors. Why? Because some motors are equipped with a starting capacitor and a running capacitor, what is going on?

Running capacitor: It is to shift the alternating current into the secondary winding to form an alternating magnetic field, and form an approximately circular elliptical rotating magnetic field with the alternating magnetic field of the main winding. So he can be the same capacitor, but its role is different.

Regardless of the type of capacitor, it has a starting action at the beginning of the motor start. However, when the motor reaches about 75 [%] of the rated speed, the starting capacitor is automatically disconnected by the centrifugal switch, and the running capacitor continues to work with the motor. The process of starting the motor is actually the process of "column phase". Because the single-phase motor is different from the three-phase motor, there is no phase difference, and the rotating magnetic field cannot be generated. The function of the capacitor is to make the starting winding current of the motor in time and space, leading the 90 electrical angles of the running winding to form a phase difference. Among them, the operating capacitor also plays a role in balancing the current between the primary and secondary windings. The starting capacitor is short-time operation, and the withstand voltage requirement is above 250V. The running capacitor has to work for a long time, and the withstand voltage is required to be above 450V.

The starting capacitor is to make the starting coil of the single motor energize at startup, and cut off after starting. The running capacitor is to make the motor compensate for the capacitor during operation, so the starting capacitor can't be less, and the running capacitor can be used.

The running capacitor is the starting capacitor used when the press is working normally. When the press starts and the running capacitor is used, the press starts up. After the press is turned up, the starting capacitor is disconnected. The running and starting capacitors are together but the starting capacitor is turned on, and the motor turns the starting capacitor out of use.

Finally, what is the difference between the starting capacitor and the running capacitor? That is, the capacity of the starting capacitor is large, generally 2-5 times of the running capacitor, and the capacity of the running capacitor is small, and the capacity difference between the two is huge, and it is easy to distinguish.