 Control Methods of AC Servo Motor

The adjustment of the rotational speed of an AC servo motor relies on the automatic adjustment of the ellipticity of the elliptical rotating magnetic field formed by the combination of two-phase windings.

When the ellipticity is large, the forward rotating magnetic field will correspondingly weaken, the corresponding forward torque will decrease, the reverse rotating magnetic field will strengthen, and the corresponding reverse torque will increase. As a result, the synthetic torque will decrease, and the rotational speed will reduce; conversely, the rotational speed will increase.

The change in the direction of rotation of the AC servo motor relies on the control of the power source's phase reversal, causing the synthetic magnetic field to reverse and the rotor to follow the reversal. The adjustment of the ellipticity relies on changing the voltage magnitude and phase applied to the control winding. Therefore, the AC servo motor can use the following three methods to control the servo motor's rotational speed and direction.

AC servo motor: Amplitude Control

The adjustment of the ellipticity relies on changing the voltage magnitude and phase applied to the control winding. Therefore, the AC servo motor can use the following three methods to control the servo motor's rotational speed and direction.

The two-phase windings are connected to the same single-phase power source, with an appropriately selected capacitor C ensuring a 90° phase difference between U and U. By changing the resistance R, thereby altering the control voltage U., different mechanical characteristic curves can be obtained under various control voltages.

Under a certain load torque, the higher the control voltage, the smaller the slip rate, and the higher the motor's speed. Therefore, changing the voltage can alter the motor's speed.

AC servo motor: Phase Control

Phase control circuitry alters the motor's speed by adjusting the phase of the control voltage (i.e., adjusting the phase angle between the control voltage and the excitation voltage) while keeping the amplitude of the control voltage constant.

When the phase angle is zero, the motor stops. As the phase angle increases, the electromagnetic torque also increases, raising the motor's speed. This method is rarely used.

AC servo motor: Amplitude-Phase Control

Amplitude-phase control circuitry, in this method, the excitation winding is connected to the stabilized power source through capacitor C. By adjusting the amplitude of the control voltage U., the motor's speed is altered, and simultaneously, the phase angle between the excitation voltage and the control voltage also changes. Hence, it's called amplitude-phase control. This method is simple to set up and relatively low-cost, making it the most commonly used control method.