The characteristic of the capacitor is to prevent the voltage abrupt change, that is, to make the voltage lag by 90°. Since the alternating current is a sine wave, the voltage increases from 0° to 90°, the capacitor is charging, and the starting current is large. When the capacitor voltage = alternating current , the current = 0, and the phase difference is exactly 90°. The role of the inductance is to prevent the current from abrupt changes, the alternating current voltage increases, and the inductance suppresses the increase of the current. When the voltage begins to decrease, the inductance compensates the voltage and the current increases, which is also 90°. Therefore, no matter the capacitor or the Inductor, the phase can be changed, which is the so-called phase shift.
Capacitor is the most common component in the circuit. In the DC circuit, the capacitor is equivalent to an open circuit, but in the AC circuit, it can provide phase shifting, coupling, filtering, compensation, resonance, energy storage, rectification and other requirements for the circuit, so how does the capacitor work? What about phase-shifted?
Ignoring the resistance value of the resistor, calculate the relationship between the current and the voltage on the pure capacitor circuit separately. When a voltage is applied between the positive and negative plates of the capacitor, the plate begins to store the charge Q=CU, and the charge flows through the circuit to form current.
When the voltage at both ends of the gear plate changes, the charge stored on the plate will also change, that is to say, the capacitor starts to charge when the voltage increases, and starts to discharge when the voltage decreases.
First assume that the power supply u=Umsinwt is brought into the above formula to get the current:
So it can be concluded that the phase of the capacitor current in the circuit leads the voltage by 90 degrees.
The circuit diagram of the three-phase asynchronous motor after adding a capacitor between the two windings is as follows:
When a single-phase swirl current passes through the stator winding, the motor will generate an alternating magnetic field. The strength and direction of this magnetic field change with the regular swirl law, so this magnetic field is called an alternating pulsating magnetic field. This alternating pulsating magnetic field can It is decomposed into two rotating magnetic fields with the same speed and opposite directions. When the rotor is stationary, the two rotating magnetic fields generate two torques of equal magnitude and opposite directions on the rotor, so that the combined torque is zero, so the motor cannot rotate. At this time, we use external force to rotate the motor clockwise, the relative speed of the motor rotor and the rotating magnetic field in the clockwise rotation direction becomes smaller, and the relative speed between the rotor and the rotating magnetic field in the counterclockwise rotation direction becomes larger, so the balance is broken, and the rotor Rotate in the direction of push.
Adding a starting capacitor to the other winding of the three-phase motor, the current passing through the other winding is 90 degrees ahead of the current in the main winding column, which is equivalent to giving a starting torque to rotate the rotor when the motor starts.