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Why do many motors need to install capacitors when starting?

Date:10-30  Hits:402  Belong to:NEWS

Simply put, the capacitor is used to generate phase difference, while the magnetic field of the motor is "asynchronous", and the motor rotates.


Single phase asynchronous motor is composed of main winding (also known as running winding) and auxiliary winding (also known as starting winding). The two windings have a space difference of 90 ° electrical angle. When the two windings of the capacitor are connected with alternating current with different phases after phase separation, the rotating magnetomotive force will be generated in the motor.


If only the main winding is connected with single-phase alternating current, the pulsating magnetomotive force generated in the motor can be divided into two rotating magnetomotive forces with equal size, same speed and opposite direction of rotation. These two opposite turning magnetomotive force magnetic fields act on the rotor together, and the electromagnetic torque generated at rest is equal in size and opposite in direction, so it cannot be started.


If the turns of the main winding and the secondary winding are the same, the space difference is 90 ° electrical angle, and the AC with 90 ° phase difference is applied, 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 asymmetric and the phase difference of the current flowing into them is not equal to 90 °, the elliptical rotating magnetic field generated in the motor is decomposed into a larger and a smaller circular rotating magnetic field. Their rotational speeds are equal and their directions are opposite, and the motor rotates in the direction of magnetic field rotation with larger magnetomotive force. Changing the head and tail ends of the secondary winding (or the head and tail ends of the main winding) can change the direction of the elliptical rotating magnetic field, and the motor can also reverse. However, the motor at both ends of the two windings will not change the direction of rotation.


A single-phase motor must have two groups of windings with a space of 90 ° electrical angle, and only when AC with a certain phase difference is connected can a rotating magnetic field be generated, and the motor can start up automatically and rotate along the direction of rotation of the rotating magnetic field.


Strictly speaking, the motor cannot be distinguished by the voltage. 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 through 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 the single-phase motor cannot produce the rotating magnetic field, so it needs to take certain methods to make it produce the rotating magnetic field. One of the methods is to use capacitance, which is also the most common method. Capacitance is used for phase separation, so as to make the current in the two Rao groups produce a phase difference of nearly 90 degrees, so as to produce the rotating magnetic field. In three-phase electricity, the current between each two phases has its own phase difference, so phase separation is not necessary.


Capacitive induction motor has two windings, namely starting winding and running winding. The two windings differ by 90 degrees in space. A capacitor with large capacity is connected in series on the starting winding. When the operating winding and the starting winding pass through a single AC power supply, the current in the starting winding is 90 degrees ahead of the current in the operating winding in time due to the effect of the capacitor, and reaches the maximum 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, the induction current is generated in the motor rotor, and the interaction between the current and the rotating magnetic field generates electromagnetic torque, which makes the motor rotate.

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