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Why can't the starting capacitor replace the running capacitor?

Views: 4     Author: Site Editor     Publish Time: 2022-08-31      Origin: Site

The two capacitors of the single-phase dual-value capacitor motor, one of which is responsible for starting and the other is responsible for running, with clear responsibilities. The single-phase asynchronous motor has large starting torque, high force and energy index, little change in speed and low noise during operation.

Why do single-phase motors use capacitors?

This has to start with the working principle of the ac motor.Because the AC motor rotates with a rotating magnetic field, and the single-phase motor is used, it only has two-phase windings, and the power supply used is also single-phase electricity. When the single-phase electricity is connected, although the magnetic field is rotating, there is no starting torque, so after power-on, the motor can't start, and an external force is needed to start the rotor, so I thought of the working characteristics of the capacitor, changed the original alternating current by a phase difference, and connected the capacitor to the starting winding of the motor to generate the starting torque, so that the motor achieves the purpose of turning.Therefore, this capacitor is called the startup capacitor. Therefore, as long as the capacitor is normal, the phase difference between the main winding current and the secondary winding current of the motor can be made 90° out of phase, and a rotating magnetic field is formed in the air gap, so that the rotor can obtain the starting torque.

For the single-phase dual-value capacitor motor, the starting capacitor and the running capacitor in parallel with each other are connected in series in the secondary winding of the motor. When the motor starts to energize, the parallel start and run capacitors work together, allowing the motor to obtain a high starting torque and a short start time.For the single-phase dual-value capacitor motor, the starting capacitor and the running capacitor in parallel with each other are connected in series in the secondary winding of the motor. When the motor starts to energize, the parallel start and run capacitors work together, allowing the motor to obtain a high starting torque and a short start time.The running capacitors, on the other hand, continue to be connected in series with the secondary windings of the motor and work together with the primary windings of the motor.

capacitor1 

Why can't the starting capacitor replace the running capacitor?

 

The above picture is a schematic diagram of a single-phase dual-value capacitor motor, where C1 is the running capacitor, C2 is the starting capacitor, and K is the centrifugal switch. The selection of these two capacitors, first determine the capacitance value of the running capacitor, and then determine the capacitance value of the starting capacitor. The capacitance capacity formula of the dual-value capacitor running capacitor; C1=1200×I/U×cosφ, the dual-value capacitor starting capacitor capacity formula: C2=(2~3)×C1.

It can be seen from the above formula and the parallel connection of capacitors that if the starting capacitor is used instead of the running capacitor, it is equivalent to two starting capacitors, and 2C2 is greater than C1+C2, which means that the starting capacitor capacitance increases on the original basis when the motor starts. Although the starting torque of the motor is increased at this time, the motor will increase the starting current by a large ratio, thereby burning the motor.

The reason for the motor burnout; theoretically speaking, the starting capacity of the motor is increased by 100%, while the starting torque can only be increased by half, but the starting current is increased by 200%. When the starting capacity of the motor is increased by 200%, its starting torque can be increased by nearly 200%, but the efficiency of the motor will be reduced by half. Therefore, the motor cannot drive the original load, so the motor is in an overload state. Once the time is prolonged, the coil windings of the motor will be burned.

To sum up, it is not advisable to replace the running capacitor with the starting capacitor in the single-phase dual-value capacitor motor, which will cause the motor to burn out. Simply put, when the motor starts, the starting capacitor and the running capacitor work together. Only when the speed of the motor reaches 75% of the synchronous speed, the centrifugal switch on the motor shaft is used to cut off the starting capacitor. The connection to the secondary winding, followed by the running capacitor is working. Therefore, in the process of starting the motor, increasing the capacitance on the original basis is likely to burn out the motor.

Can the fan start capacitor be replaced with another capacitor?

Most of the electric fans on the market have only one starting capacitor. This capacitor is connected in series to the motor starting winding to shift the phase. If the starting capacitor fails or the capacitance value decreases, it will cause the motor to fail to start.We generally need to replace the capacitor of the same type.

capacitor2 

Why can't the starting capacitor replace the running capacitor?

If there is no capacitor of the same value on hand, you can use two capacitors in parallel, but pay attention to the following two points:

一、Two non-polar capacitors of the same type should be selected for the parallel capacitor. Generally, PP film capacitors are used in fans.

二、The capacitance values of the capacitors are added in parallel, but the withstand voltage value depends on the smaller one. Therefore, two capacitors with the same withstand voltage value should be selected in parallel, and the withstand voltage value should not be less than 400V.


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