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The role of starting capacitors

Views: 3     Author: Site Editor     Publish Time: 2022-11-16      Origin: Site

Some household appliances used in our daily life, such as air conditioners, refrigerators, washing machines, electric fans, etc., generally use single-phase asynchronous motors, and these motors all use starting capacitors.

First of all, let's briefly understand the principle of the starting capacitor. From an academic point of view, it may be difficult to understand. If you want to know, you can check this information. I personally understand that the starting capacitor is to give the motor a thrust when the motor is started, so that the motor can change from moving to rotating. Without him, when the single-phase ac motor starts, it will vibrate at the origin instead of rotating.The starting capacitor is the "leading angle" of the two-phase ac motor. Without it, the magnetic field cannot exert force on the rotor, and the rotation is of course impossible. From this aspect, it is easy to understand.

Regarding the structure and principle of the starting capacitor in the single-phase ac motor, it is generally divided into two windings: the starting winding and the running winding. The starting winding is generally called the auxiliary winding, and the running winding is called the main winding.

The role of starting capacitors

After understanding the above content, when encountering some two-phase ac motors that "hum" but do not rotate, it is natural to think of checking whether the starting capacitor is damaged.

But the starting capacitor is also divided into different sizes. It is not enough to install a random capacitor to make the motor run. If it is too large, the motor will run too fast and will generate heat. It will burn out the motor easily if it runs for a long time;If it is too small, it will not be able to give enough force to the rotor. If the thrust is too weak, the motor will not start. Therefore, when changing the starting capacitor, do not change the size of the original capacitor without authorization.

The role of starting capacitors

To put it simply, the capacitor is used to generate a phase difference, so that the motor magnetic field is "asynchronous", and the motor rotates.A single-phase asynchronous motor consists of a main winding (also known as the running winding) and an auxiliary winding (also known as the starting winding), and these two windings are spatially different by 90° in electrical angle.After the capacitor is phase-separated, the two windings are fed with alternating current with different phases, and a rotating magnetomotive force will be generated in the motor.

If only the main winding is fed with single-phase alternating current, the motor generates pulsating magnetomotive force, which can be decomposed into two rotating magnetomotive forces with equal size, same speed and opposite direction.These two opposite magnetomotive force fields act on the rotor together, and the electromagnetic torque generated at rest is equal in magnitude and opposite in direction, so it can’t be started.

If the number of turns of the main winding and the auxiliary winding are equal, the space difference is 90° electrical angle, and the alternating current with 90° phase difference is connected, then a circular rotating magnetic field will be generated in the motor, and the motor rotor will rotate with it under its action.

If the two windings are asymmetrical, the phase difference of the current passing through them is not equal to 90°, and the elliptical rotating magnetic field generated in the motor is decomposed into a larger and a smaller circular rotating magnetic field, and their speeds are equal and the directions are opposite , the motor rotates in the direction of rotation of the magnetic field with a larger magnetomotive force.Changing the first and last ends of the auxiliary winding (or the first and last ends of the main winding) can change the direction of the elliptical rotating magnetic field, and the motor can also be reversed. But changing the first and last motors of both windings at the same time will not change the steering.

A single-phase motor needs to have two sets of windings with a space of 90° electrical angle,and the alternating current with phase difference will generate the rotating magnetic field, and the motor will start by itself and rotate along the direction of the rotating magnetic field.

Strictly speaking, the motor cannot be distinguished by the voltage level. The so-called 220V and 380V are just our daily abbreviations. Here we should say single-phase and three-phase.

The rotation of an ac motor depends on the rotating magnetic field generated by the current.The three-phase motor flows a three-phase current with a phase difference of 120 degrees, which can generate a rotating magnetic field. However, the single-phase current flowing through a single-phase motor cannot generate a rotating magnetic field, and methods need to be adopted to make it generate a rotating magnetic field.The use of capacitors is one of the methods, and it is also a common method. Capacitors are used to separate phases. The purpose is to make the currents in the two groups produce a phase difference close to 90° to generate a rotating magnetic field.In three-phase electricity, the current itself between each two phases has a phase difference, and there is no need to separate the phases.

Capacitive induction motors have two windings, a start winding and a run winding. The two windings are spaced 90 degrees apart.A capacitor with a large capacity is connected in series on the starting winding. When the running winding and the starting winding pass through a single alternating current,due to the effect of the capacitor, the current in the starting winding is 90 degrees ahead of the current in the running winding in time, and reaches the peak value first.Two identical pulsed magnetic fields are formed in time and space, so that a rotating magnetic field is generated in the air gap between the stator and the rotor,under the action of the rotating magnetic field, an induced current is generated in the rotor of the motor, and the interaction between the current and the rotating magnetic field generates an electromagnetic field torque, which makes the motor rotate.


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