The difference between AC servo motor and DC servo motor

2021-11-03 09:36:52 jact_elec

The difference between AC servo motor and DC servo motor

The word "servo" is derived from the Greek word "slave". "Servo motor" can be understood as a motor that absolutely obeys the control signal of the inverter: before the control signal is sent, the rotor is stationary; when the control signal is sent, the rotor immediately rotates; when the control signal disappears, the rotor can stop immediately.

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Servo motor is a micro motor used as an actuator in an automatic control device. Its function is to convert an electrical signal into the angular displacement or angular velocity of the rotating shaft.

Servo motors are divided into two categories: AC servo and DC servo. The basic structure of AC servo motors is similar to AC induction motors (asynchronous motors). Connected to the frequency converter, there are two excitation windings Wf and control winding WcoWf with a phase space displacement of 90° electrical angle on the stator, connected to a constant AC voltage, and using the AC voltage applied to Wc or the phase change to achieve the purpose of controlling the operation of the motor . The AC servo motor is connected to the frequency converter, which has stable operation, good controllability, fast response, high sensitivity, and strict non-linearity indicators of mechanical characteristics and adjustment characteristics.

The basic structure of a DC servo motor is similar to that of a general DC motor. Motor speed n=E/K1j=(Ua-IaRa)/K1j, where E is armature back electromotive force, K is constant, j is magnetic flux per pole, Ua and Ia are armature voltage and armature current, Ra is Armature resistance, changing Ua or changing φ, can control the speed of the DC servo motor, but generally the method of controlling the armature voltage is adopted. In the permanent magnet DC servo motor, the field winding is replaced by a permanent magnet, and the magnetic flux φ is constant. . The DC servo motor has good linear regulation characteristics and fast time response.

The advantages of DC servo motors are precise speed control, very hard torque and speed characteristics, and the excitation current and armature current of the DC motor are relatively independent, so the control principle is simple, easy to use, and cheap. The excitation magnetic field of the DC motor is perpendicular to the armature magnetic field, while the magnetic field of the AC motor is induced, which is inconvenient to control. The current vector control and torque control are all trying to convert the AC motor to the control direction of the DC motor. However, DC servo motors also have their shortcomings. When connected to a frequency inverter, such as brush commutation, speed limitation, additional resistance, it is easy to produce wear particles. If it is required to be dust-free and explosive, it is not suitable for work.

The AC servo motor has two-phase AC windings with a space difference of 90 points. One of them is the excitation winding and the other is the control winding. Its control methods include amplitude control, phase control, and amplitude-phase composite control. Mostly adopt compound control. The rotor resistance of the AC servo motor is generally very large, which can prevent rotation. When the control voltage disappears, due to the excitation voltage, there will be pulsating magnetomotive force in the AC servo motor at this time. Due to the large resistance, the TS curve will shift , The T generated by the reversing magnetic field will become larger, so the synthesized T at this time is of a braking nature and will stop rotating.

The advantages of AC servo motors include good speed control characteristics, smooth control in the entire speed zone, almost no oscillation, high efficiency of more than 90%, low heat generation, high-speed control, high-precision position control (depending on the encoder accuracy) , In the rated operating area, it can achieve constant torque, low inertia, low noise, no brush wear, and maintenance-free (suitable for dust-free and explosive environments). The disadvantage of AC servo motors is that the control is very complicated, the drive parameters need to be adjusted on-site to determine the PID parameters, and more connections are required.

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