Servo motors enable highly accurate speed and position control, converting voltage signals into torque and speed to drive the controlled object. The rotor speed of a servo motor is controlled by the input signal and can respond quickly. In automatic control systems, they are used as actuators and possess characteristics such as a small electromechanical time constant, high linearity, and low starting voltage. They can convert received electrical signals into angular displacement or angular velocity output on the motor shaft. Servo motors are broadly classified into DC and AC servo motors. Their main characteristic is that they do not rotate when the signal voltage is zero, and their speed decreases uniformly as the torque increases.
Servo motors are divided into AC servo motors and DC servo motors. What are the advantages and disadvantages of each? Let's take a look.
1. Comparison of the basic structures of DC servo motors and AC servo motors
The structure of a DC servo motor is similar to that of a DC motor. The 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. Changing Ua or φ can control the speed of the DC servo motor, but generally the method of controlling the armature voltage is used. In a permanent magnet DC servo motor, the excitation winding is replaced by a permanent magnet, and the magnetic flux φ is constant.
The structure of an AC servo motor is similar to that of an AC asynchronous motor. The stator has two excitation windings, Wf and Wco, with a phase spatial displacement of 90° electrical degrees. Both are connected to a constant AC voltage. The motor's operation is controlled by utilizing the AC voltage applied to Wc or changes in its phase.
Part Two: Comparison of the advantages and disadvantages of DC servo motors and AC servo motors
(1) Advantages and disadvantages of DC servo motors
Advantages: Precise speed control, strong torque-speed characteristics, simple control principle, good linear regulation characteristics, fast time response, easy to use, and inexpensive.
Disadvantages: Brush commutation, speed limitation, additional resistance, generation of abrasive particles (not suitable for dust-free or explosive environments).
(2) Advantages and disadvantages of AC servo motors
Advantages: Excellent speed control characteristics, smooth control across the entire speed range with virtually no oscillation, high efficiency exceeding 90%, low heat generation, high-speed control, high-precision position control (depending on encoder accuracy), stable operation, good controllability, fast response, high sensitivity, and strict nonlinearity indicators for mechanical and adjustment characteristics. Within the rated operating range, it can achieve constant torque, low inertia, low noise, no brush wear, and maintenance-free operation (suitable for dust-free and explosive environments).
Disadvantages: The control is relatively complex, the driver parameters need to be adjusted on-site to determine the PID parameters, and more wiring is required.
The above comparative analysis clearly shows the advantages and disadvantages of DC servo motors and AC servo motors. This analysis can help us to more accurately choose the right motor and achieve the ideal working conditions.
