Research on AC motor soft starter based on single chip microcomputer and its application

    The motor is one of the important traction equipment in industrial production. AC asynchronous motors are widely used in industry, agriculture and other production due to their simple structure and easy maintenance. However, the current when the motor is directly started is too large, and the mechanical shock caused by the torque at the moment of starting will also affect the service life of the motor itself and the traction equipment. Excessive starting current also accelerates the aging of the motor insulation. When the motor is started, it will cause a large grid voltage drop, affecting the power supply of the grid and the operation of other equipment. In the case of frequent motor starting, the operation of the equipment will also be affected by motor short circuit, phase loss, overcurrent, undervoltage, stall and other faults. Therefore, soft start control technology should be used to improve the poor starting performance of the motor, extend the life of the motor, and reduce the impact of the grid. The three-phase asynchronous motor soft starter based on the single-chip microcomputer AT89C5l introduced in this article essentially improves the starting characteristics of the AC motor, and has functions such as power saving operation, overcurrent protection, overload protection, and phase loss protection.

　　1 Working principle and hardware composition

　　The hardware circuit structure block diagram of the soft starter is shown in Figure 1.

　　When starting (after receiving the start command), a phase-shift trigger pulse is generated from the output port of the single-chip microcomputer, and the conduction angle α of the bidirectional thyristor connected in series in the winding of the three-phase asynchronous motor is controlled to realize the ramp mode decompression start. During the operation of the motor, the motor power factor is detected in real time, and the conduction angle α is changed accordingly to achieve power-saving operation. Overcurrent and overload detection uses a conventional current transformer circuit, which is sent to the single-chip microcomputer after rectification, filtering, amplification, A/D conversion and isolation, and the software completes data processing and judgment. Phase loss detection uses the method of simultaneously detecting the three-phase power supply to judge the phase loss fault. The detection circuit of each phase is shown in Figure 2. During the conduction period of the thyristor, the circuit Vo should output a high-level signal, otherwise the phase is missing (the current is zero). The detection signal of each phase is also sent to the single-chip microcomputer for processing and judgment.

　　The phase detection and synchronization signal generation circuit is shown in Figure 3. The current and voltage signals of phase A are converted into square waves by the optocoupler and then sent to the XOR gate. Channel 0 of the timer/counter 8253 (working in mode 2) is used to detect the phase difference between the voltage and current of phase A. When the voltage passes through zero, the gate terminal GATE0 gets a high level and starts counting. When the current passes through zero, GATEo becomes a low level and the counting stops. The microcontroller reads the count value during the low level of GATE to obtain the voltage and current phase difference. The phase shift control of the trigger pulse is realized by the delay of channels l and 2 of 8253 (working in mode 5). The zero crossing of the voltage of phase A is the start timing of the synchronization signal. Channel 1 cooperates with the timer/counter T0 of the single-chip microcomputer to control the conduction moment of thyristors 1, 2, and 3. Channel 2 cooperates with the timer/counter T of the single-chip microcomputer to control the conduction moment of thyristors 4, 5, and 6. The timing time of T0 and T1 is 3.3ms. It uses the previous timing interrupt to start the next timing, and generates three trigger control pulses with a difference of 3.3ms in one cycle.

　　The start (stop) command comes from the main controller of the machine tool electrical control system. According to the processing flow, when the motor is required to start (stop), the main controller will issue a start (stop) command, and the soft starter will control the motor to start (stop). Interlocking control is often required in machine tool equipment. In this way, when the motor fails and stops for protection, a stop signal needs to be fed back to the main controller, and the main controller can then perform interlocking control processing. [page]

　　2 Control Software

　　The task of the software is to control the hardware system to automatically collect and detect input signals, judge and process the input data, and output the required control signals as required. The main program flow chart is shown in Figure 4. After receiving the start command, the motor is started first, and then enters the cyclic working process until receiving the stop command or the fault stops. Due to the cooperation of the hardware circuit with strong functions, the main program can perform fault detection, current value sampling and display, power factor angle measurement, trigger pulse phase shift and output control in each cycle, so that the control is fast, sampling and fault diagnosis are not wrong. The INTo and To interrupt programs realize the trigger pulse phase shift and positioning control of thyristors 1, 2, and 3. The flow chart is shown in Figures 5 and 6. The INT1 and T1 interrupt programs realize the trigger pulse phase shift and positioning control of thyristors 4, 5, and 6. The flow chart is similar to Figures 5 and 6.

   

　　3 Conclusion

　　The three-phase asynchronous motor soft starter controlled by a single-chip microcomputer has the characteristics of complete functions, reliable operation, easy use and low cost. It meets the requirements of enterprises on equipment reliability, production efficiency and resource optimization, and has certain application value.