Comparison Of AC Servo Motor Selection

AC servo motor
The structure of the stator of an AC servo motor is basically similar to that of a capacitor split phase single-phase asynchronous motor. The stator is equipped with two windings with a 90 ° difference in position, one of which is the excitation winding Rf, which is always connected to the AC voltage Uf; The other is to control winding L and connect the control signal voltage Uc. So AC servo motors are also known as two servo motors.
The rotor of an AC servo motor is usually made into a squirrel cage type, but in order to ensure that the servo motor has a wide speed range, linear mechanical characteristics, no "rotation" phenomenon, and fast response performance, it should have two characteristics compared to ordinary motors: high rotor resistance and small moment of inertia. At present, there are two types of rotor structures that are widely used: one is a squirrel cage rotor made of high resistivity conductive materials with high resistivity conductive bars. In order to reduce the rotational inertia of the rotor, the rotor is made slender; Another type is a hollow cup rotor made of aluminum alloy, with a thin cup wall of only 0.2-0.3mm. In order to reduce the magnetic resistance of the magnetic circuit, a fixed inner stator needs to be placed inside the hollow cup rotor. The hollow cup rotor has a small moment of inertia, rapid reaction, and smooth operation, so it is widely used.
When there is no control voltage in the AC servo motor, only the pulsating magnetic field generated by the excitation winding in the stator, and the rotor remains stationary. When there is a control voltage, a rotating magnetic field is generated in the stator, and the rotor rotates in the direction of the rotating magnetic field. Under constant load, the speed of the motor changes with the magnitude of the control voltage. When the phase of the control voltage is opposite, the servo motor will reverse.
Permanent magnet AC servo motor
Since the 1980s, with the development of integrated circuits, power electronics technology, and AC variable speed drive technology, permanent magnet AC servo drive technology has made outstanding progress. Famous electrical manufacturers in various countries have successively launched their own series of AC servo motors and servo drives, which are constantly improved and updated. AC servo systems have become the main development direction of contemporary high-performance servo systems, posing a crisis of elimination for the original DC servo system. After the 1990s, the AC servo systems that have been commercialized in various countries around the world are driven by sine wave motors using fully digital control. The development of AC servo drive devices in the field of transmission is changing rapidly.
The main advantages of permanent magnet AC servo motors compared to DC servo motors are:
⑴ Without brushes and commutators, it works reliably and has low requirements for maintenance and upkeep.
⑵ The stator winding has convenient heat dissipation.
⑶ Small inertia, easy to improve the system's speed.
Suitable for high-speed and high torque working conditions.
Under the same power, it has smaller volume and weight.
Comparison between servo motors and single-phase asynchronous motors
The working principle of AC servo motors is similar to that of split phase single-phase asynchronous motors, but the rotor resistance of the former is much greater than that of the latter. Therefore, compared with single asynchronous motors, servo motors have three significant characteristics:
1. High starting torque
Due to the high rotor resistance, there is a significant difference in the torque characteristic curve compared to ordinary asynchronous motors. It can make the critical slip rate S0>1, which not only makes the torque characteristics (mechanical characteristics) closer to linearity, but also has a larger starting torque. Therefore, as soon as the stator has control voltage, the rotor immediately rotates, which has the characteristics of fast starting and high sensitivity.
2. Wide operating range
3. No autorotation phenomenon
A normally running servo motor will immediately stop running as soon as the control voltage is lost. When the servo motor loses control voltage, it is in a single-phase operation state. Due to the high rotor resistance, the two opposite rotating magnetic fields in the stator interact with the rotor to generate two torque characteristics (T1-S1, T2-S2 curves) and composite torque characteristics (T-S curves)
The output power of AC servo motors is generally 0.1-100W. When the power frequency is 50Hz and the voltage is 36V, 110V, 220V, 380V; When the power frequency is 400Hz, there are various voltages such as 20V, 26V, 36V, 115V, etc.
The AC servo motor runs smoothly and has low noise. However, the control characteristics are nonlinear, and due to the high rotor resistance, high losses, and low efficiency, compared to DC servo motors of the same capacity, the volume is large and the weight is heavy, so it is only suitable for small power control systems of 0.5-100W.