Structural characteristics and basic working principles of AC servo motors

Update:26 Jan 2021

The structure of the stator of AC servo motor is basically similar to that of capacitor split phase single-phase asynchronous motor. There are two windings on the stator, one is the excitation winding RF, which is always connected to the AC voltage UF; the other is the control winding L, which is connected to the control signal voltage Uc. So AC servo motor is also called two servo motors.
The rotor of AC servo motor is usually made into squirrel cage type, but in order to make the servo motor have a wide speed range, linear mechanical characteristics, no rotation phenomenon and fast response performance, it should have two characteristics of large rotor resistance and small moment of inertia compared with ordinary motor.
At present, there are two types of rotor structure which are widely used
One is a squirrel cage rotor made of high resistivity conductive material with high resistivity bars. In order to reduce the moment of inertia of the rotor, the rotor is made slender;
The other is the hollow cup-shaped rotor made of aluminum alloy. The cup wall 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 hollow cup-shaped rotor has small moment of inertia, rapid reaction and stable operation, so it is widely used.
When there is no control voltage in AC servo motor, there is only pulsating magnetic field produced by excitation winding in stator and rotor is stationary. When there is a control voltage, a rotating magnetic field is generated in the stator, and the rotor rotates along the direction of the rotating magnetic field. Under the condition of constant load, the speed of the motor changes with the size of the control voltage. When the phase of the control voltage is opposite, the servo motor will reverse.
Although the working principle of AC servo motor is similar to that of split phase single-phase asynchronous motor, the rotor resistance of AC servo motor is much larger than that of phase single-phase asynchronous motor
1. Large starting torque
Because of the large rotor resistance, the torque characteristic curve of induction motor is obviously different from that of ordinary motor. It can make the critical slip S0 > 1, which not only makes the torque characteristic more linear, but also has larger starting torque. Therefore, as soon as there is control voltage in the stator, the rotor will rotate immediately, which has the characteristics of fast starting and high sensitivity.

2. Wide range of operation
3. No rotation
The normal operation of the servo motor, as long as the loss of control voltage, motor immediately stop 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 and synthetic torque characteristics produced by the interaction of two opposite rotating magnetic fields in the stator and the rotor, the output power of AC servo motor is generally 0.1-100w.
When the power frequency is 50 Hz, the voltage is 36 V, 110 V, 220 V and 380 V; when the power frequency is 400 Hz, the voltage is 20 V, 26 V, 36 V and 115 v.
AC servo motor runs smoothly with low noise. But the control characteristic is nonlinear, and because the rotor resistance is large, the loss is large, the efficiency is low, so compared with the same capacity DC servo motor, the volume is large, the weight is heavy, so it is only suitable for 0.5-100w small power control system.
The main advantages of permanent magnet AC servo motor are as follows:
1. It has no brush and commutator, so it works reliably and has low requirement for maintenance.
2. The heat dissipation of stator winding is convenient.
3. Small inertia, easy to improve the rapidity of the system.
4. It is suitable for high speed and large torque working state.
5. It has smaller volume and weight under the same power.

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