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AC induction machine is most used in various area , because of its simple structure,stable running,convenient maintaining,and small inertia,as important power plant it is irreplaceable in economic construction. But when starts AC machine without any set, directly connects it into the power grid voltage, the instant of current and the torque bring impact, not only can cause the motor fever, reduce the machine and the dragging system’s life, simultaneously can reduce power grid voltage quality, and affect the other electric equipment normally operating in the power grid, the traditional soft start also has its malpractice. For good start performance,electronic soft starter has been used widely which uses SCR as main circuit. The studies emphasis is that applying speed sensor-less theory to AC motor starter,during the starting of the motor,uses the voltage and current to estimate start speed and torque ,realizes closed-loop control of the torque, so that it makes motor start as preset torque curve. The paper introduces the principle of speed sensor-less and torque slope in detail,adopts torque slope scheme of AC induction start based on speed sensor-less,the simulation results show that this soft starter has advantages such as simple structure,process of start is smooth and stable,good flexibility,better protect of motor and its dragging system,at the same time it reduces the installation of velocity and torque sensor,and then decreases costing of soft starter system, improves reliability of system. At last, uses MC9S12DG128 micro-processor as controller, designs hardware circuit and software of the soft starter control system.
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