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Analyze the difference between servo motor and single asynchronous motor

Date: 2020-08-13
Clicks: 306
Date: 2020-08-13
Clicks: 306


Comparative Analysis of AC servo Motor and Single Asynchronous Motor:

      Theoretically, the stator of ac servo motor is basically similar to that of capacitor phase-separated single-phase asynchronous motor. The stator is equipped with two windings 90° apart from each other, one is excitation winding Rf, which is always connected to AC voltage Uf. The other is the control winding L, which connects the control signal voltage Uc. So ac servo motor is also called two servo motors.

      The rotor of ac servo motor is usually made of squirrel cage type, but in order to make the servo motor has a wide range of speed, linear mechanical characteristics, no 'rotation' phenomenon and fast response performance, compared with the ordinary motor, it should have large rotor resistance and small moment of inertia these two characteristics. At present, there are two types of rotor structures that are widely used: one is a squirrel-cage rotor with a high resistivity guide bar made of conductive material. In order to reduce the rotor's rotational inertia, the rotor is made slender; The other is a hollow cup rotor made of aluminum alloy, the cup wall is very thin, only 0.2-0.3mm, in order to reduce the magnetic circuit reluctance, the fixed inner stator should be placed in the hollow cup rotor. The hollow cup rotor has a small moment of inertia, rapid response, and smooth operation, so it is widely used.

      When ac servo motor has no control voltage, the stator only has pulsating magnetic field generated by excitation winding, and the rotor is stationary. When there is a control voltage, a rotating magnetic field will be generated in the stator and the rotor will rotate along the direction of the rotating magnetic field. Under the condition of constant load, the speed of the motor will change with the size of the control voltage. When the phase of the control voltage is opposite, the servo motor will reverse.

  The working principle of AC servo motor is similar to that of single-phase asynchronous motor, but the rotor resistance of the former is much greater than that of the latter. Therefore, compared with the single-phase asynchronous motor, the servo motor has three significant characteristics:

  1、Wide range of operation

  2、High starting torque

 As the rotor resistance is large, its torque characteristic curve is shown in FIG. 3, curve 1, which is significantly different from the torque characteristic curve 2 of ordinary asynchronous motors. It can make the critical slip S0 > 1, which not only makes the torque characteristic (mechanical characteristic) more linear, but also has a larger starting torque. Therefore, when the stator has a control voltage, the rotor rotates immediately, that is, it has the characteristics of fast starting and high sensitivity.

  3、No rotation

  Servo motor in normal operation, as long as the control voltage is lost, the motor will stop running immediately. When the servo motor loses its control voltage, it is in a single-phase operation state. Due to the large rotor resistance, two torque characteristics (T1-S1, T2-S2 curves) and synthetic torque characteristics (T-S curves) are generated by the interaction between the rotating magnetic fields rotating in opposite directions in the stator and the rotor.

  The output power of ac servo motor is generally 0.1-100W. When the power supply frequency is 50Hz, the voltage is 36V, 110V, 220, 380V; When the power frequency is 400Hz, the voltage is 20V, 26V, 36V, 115V and so on.

  Ac servo motor runs smoothly and has low noise. However, the control characteristics are nonlinear, and due to the large rotor resistance, large loss and low efficiency, compared with the dc servo motor with the same capacity, it is large in size and heavy in weight, so it is only suitable for the small power control system of 0.5-100W.

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