The rotor speed of three-phase asynchronous motor is lower than that of the rotating magnetic field. The rotor winding induces electromotive force and current due to the relative movement between the rotor and the magnetic field, and generates electromagnetic torque by interaction with the magnetic field to realize energy conversion. Compared with single-phase asynchronous motor, three-phase asynchronous motor running performance, and save a variety of materials. According to the different rotor structure, three-phase asynchronous motor can be divided into cage and wound two.
Cage rotor asynchronous motor structure is simple, reliable operation, light weight, low price, has been widely used, the main drawback is the speed control difficulties. Winding three-phase asynchronous motor rotor and stator also set up a three-phase winding and through the slip ring, brush and external rheostat connection. Adjusting the varistor resistance can improve the motor starting performance and adjust the motor speed.
Three-phase asynchronous motor characteristics inherent mechanical characteristics and that of the mechanical characteristics of the points.
1, inherent mechanical properties: it has 4 special points above.
(1) The motor runs idly without any load. At this point, the speed is the maximum, which is the ideal no-load point of the motor.
(2) The normal operation of the motor under load, this time for the motor rated operating point.
(3) the motor just started the moment, that is, did not turn up, to overcome the moment when the rotor weight, this point for the motor starting point.
(4) the motor drag the maximum torque load, the speed is relatively modest, this point the motor critical operating point.
At this time if the voltage is too low or a huge impact load, it will cause the motor to shut down.
2, artificial mechanical properties
(1) Voltage drop
When the motor is running, if the voltage drops too much, it will greatly reduce its overload capacity and starting torque, even if the motor with a fixed load or can not start the phenomenon. In addition, the motor will be burned after starting.
(2) stator circuit access resistance, then the maximum torque than the original; rotor circuit resistance or change the stator power supply frequency, then start to increase torque, the maximum torque unchanged.
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