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1 The status quo of motor starting Three-phase squirrel cage induction motor is widely used as the main power equipment to convert electrical energy into mechanical energy in agricultural irrigation and drainage due to its advantages of simple structure, reliable operation, easy maintenance, low inertia and low price. . However, due to its large starting current, the impact on the power grid and the impact on the working machinery (such as pumps, flaps, etc.) are very large, and therefore must take some technical measures to start the process of starting current and impact (starting electromagnetic turn Moment) to be reasonable and effective control, to achieve a more stable start, thereby improving the system equipment conditions, effectively extending the life of the system and reduce the incidence of failure. Asynchronous motor starting problems, has been of concern to the industry. Asynchronous motor starting method in principle there are only two: direct start and step-down start. Direct start, that is, the motor is still at a standstill directly with the rated voltage, the motor voltage directly under the action of the rated voltage to complete the starting process. Direct start torque, start time is short, simple start control, equipment investment and small, so small and medium-sized motor start widely used. However, the direct starting method is also subject to many limitations, mainly in the following three aspects: (1) starting current can be large motor rated current of 4 to 7 times, some domestic motor starting current actually measured even up to 8 to 12 times. If you directly start a larger motor, the starting current will result in a large grid voltage dropped significantly, affecting the same grid and other electrical equipment and electronic equipment normal operation, serious part of the equipment will be due to low voltage out of operation, or even Power line relay protection device overcurrent protection trips and trip, so that the power supply line interrupted. (2) Direct starting will cause mechanical impact on the driven working machine. For the pump load, too high starting torque will cause soft damage to the blades, bearings, and flaps (mechanical deformation, fatigue aging ) And hard damage (cracks, fractures, etc.) are more common and may even cause serious water hammer damage to the equipment due to the excessive impact (and reaction) of the water flow on the pipe. (3) The direct starting requires that the capacity of the power transformer is large, and the capacity of the transformer that supplies farmland irrigation and drainage pumping stations often can not meet the requirements of the grid capacity for direct starting. In the case of direct start is not allowed, it is necessary to start the buck start, that is, lower the motor voltage to start. Step-down start usually star / delta start, the stator circuit in series resistance, reactor start-up, autotransformer buck start and the recommended soft-start and other methods. The star / delta starter is one of the simplest and lowest cost components in a step-down starter, however, its performance is limited by (1) the inability to control the current and torque drops and these values ​​are fixed , 1/3 of the rated value. (2) When the starter is switched from the star connection to the delta connection, large current and torque changes usually occur. This will cause mechanical and electrical stress, resulting in recurring failures. Autotransformer starters provide more control than the star / delta starters by varying the I stage start-up voltage (typically 65% ​​and 80% start-up taps) via transformer taps. However, its voltage is graded, so its performance is limited as follows: (1) The step change of the voltage (generated during the step transition) causes large current and torque variation, which is the same as the performance of the star / delta starter Restrictions "2" will lead to the same mechanical and electrical failures occur.