One, 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 position difference of 90°, one is the field winding Rf, which is always connected to the AC voltage Uf; the other It is the control winding L, which is connected to the control signal voltage Uc. Therefore, AC servo motors are also called two servo motors.

The rotor of an AC servo motor is usually made of a squirrel cage, but in order to make the servo motor have a wider speed range, linear mechanical characteristics, no "rotation" phenomenon and fast response performance, it should have The rotor resistance is large and the moment of inertia is small. At present, there are two types of rotor structures that are widely used: one is a squirrel cage rotor with high-resistivity guide bars made of high-resistivity conductive materials. In order to reduce the rotational inertia of the rotor, the rotor is made slender; One is a hollow cup-shaped rotor made of aluminum alloy. The wall of the cup is very thin, only 0.2-0.3mm. In order to reduce the magnetic resistance of the magnetic circuit, a fixed inner stator should be placed in the hollow cup-shaped rotor. The rotor has a small moment of inertia, quick response, and stable operation, so it is widely used.

　　 When the AC servo motor has no control voltage, there is only the pulsating magnetic field generated by the excitation winding in the stator, and the rotor is 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. When the load is constant, the speed of the motor changes with the control voltage. When the phase of the control voltage is opposite, the servo motor Will be reversed.

　　 Although the working principle of AC servo motor is similar to that of split-phase single-phase asynchronous motor, the rotor resistance of the former is much larger than that of the latter. Therefore, compared with the single-phase asynchronous motor, the servo motor has three notable characteristics:

1. Large starting torque

　　 Due to the large rotor resistance, its torque characteristic curve is shown as curve 1 in Figure 3. Compared with the torque characteristic curve 2 of an ordinary asynchronous motor, there is a clear difference. It can make the critical slip rate S0>1, which not only makes the torque characteristic (mechanical characteristic) closer to linear, but also has a larger starting torque. Therefore, when the stator has a control voltage, the rotor immediately rotates, which has the characteristics of fast starting and high sensitivity.

2. Wide operating range

3. No rotation

　　A normally running servo motor, as long as the control voltage is lost, the motor immediately stops running. When the servo motor loses the control voltage, it is in a single-phase operation state. Due to the large rotor resistance, the two torque characteristics (T1-S1, T2-S2 curve) generated by the two rotating magnetic fields in the stator and the rotor acting in opposite directions ) And synthetic torque characteristics (T-S curve)

　　 The output power of AC servo motor is generally 0.1-100W. When the power frequency is 50Hz, the voltage is 36V, 110V, 220, 380V; when the power frequency is 400Hz, the voltage is 20V, 26V, 36V, 115V, etc.

　　 AC servo motor runs smoothly and has low noise. But the control characteristic is non-linear, and because the rotor resistance is large, the loss is large, the efficiency is low, so compared with the same capacity DC servo motor, it is large and heavy, so it is only suitable for 0.5-100W small power control system.