The difference between AC servo motor and DC servo motor

Servo motors can control speed and position with high accuracy, and can convert voltage signals into torque and speed to drive the controlled object. The rotor speed of the servo motor is controlled by the input signal and can respond quickly. In the automatic control system, it is used as an actuator, and has the characteristics of small electromechanical time constant, high linearity, and starting voltage. It can convert the received electrical signal into angular displacement or angular velocity output on the motor shaft. Servo motors are mainly divided into two categories: DC and AC servo motors. Their main characteristics are that there is no self-rotation when the signal voltage is zero, and the speed decreases uniformly with the increase of torque.

Servo motors are divided into AC servo motors and DC servo motors. They are widely used in the electromechanical industry, but many people cannot tell the difference between them. Let me introduce them to you below.

1: Comparison of basic structures between AC servo motor and DC servo motor

DC servo motor

The structure of AC servo motor is similar to that of AC asynchronous motor. There are two excitation windings Wf and control windings WcoWf on the stator with a phase space displacement of 90° electrical angle, which are connected to a constant AC voltage. The purpose of controlling the operation of the motor is achieved by using the AC voltage or phase change applied to Wc.

AC servo motor

The structure of a DC servo motor is similar to that of a DC motor.

Motor speed n=E/K1j=(Ua-IaRa)/K1j, where E is the armature back electromotive force, K is a constant, j is the magnetic flux per pole, Ua and Ia are the armature voltage and armature current, and Ra is the armature resistance. The speed of the DC servo motor can be controlled by changing Ua or changing φ, but the method of controlling the armature voltage is generally adopted. In permanent magnet DC servo motors, the excitation winding is replaced by permanent magnets and the magnetic flux φ is constant.

2: Comparison of the advantages and disadvantages of DC servo motors and AC servo motors

(1) Advantages and disadvantages of AC servo motors

Advantages: good speed control characteristics, smooth control in the entire speed range, almost no oscillation, high efficiency of more than 90%, less heat, high-speed control, high-precision position control (depending on the accuracy of the encoder), stable operation, good controllability, fast response, high sensitivity, and strict nonlinearity indicators of mechanical characteristics and adjustment characteristics. In the rated operating area, constant torque, low inertia, low noise, no brush wear, and maintenance-free (suitable for dust-free and explosive environments) can be achieved.

Disadvantages: The control is more complicated, the drive parameters need to be adjusted on-site to determine the PID parameters, and more connections are required.

(2) Advantages and disadvantages of DC servo motors

Advantages: precise speed control, very rigid torque-speed characteristics, simple control principle, good linear regulation characteristics, fast time response, easy to use and low price.

Disadvantages: brush commutation, speed limitation, additional resistance, generation of wear particles (not suitable for dust-free and explosive environments)

Through the above comparative analysis, we can clearly know the advantages and disadvantages of DC servo motors and AC servo motors. Such analysis can help us know more accurately how to choose the motor and make our work reach the ideal state.