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AC motor soft starter based on single chip microcomputer and its application in electric drive of machine tool

Nov 03, 2020

AC motor soft starter based on single chip microcomputer and its application in electric drive of machine tool

0 Preface

  As we all know, three-phase asynchronous motors are widely used in various industries due to their low cost, high reliability and easy maintenance. Especially in machine tool equipment, it is used as a main power equipment, often used to drive spindle, workbench, cooling pump, oil pump and other devices. However, when it is started directly, since the starting current is as high as 5-7 times the rated current, it will cause a great impact on the power grid and load, affect the work of peripheral electrical appliances, increase the wear of mechanical transmission parts, and reduce the equipment life. The three-phase asynchronous motor soft starter based on the single-chip AT89C5l introduced here can not only solve the above problems, and essentially improve the starting characteristics of AC motors, but also has functions such as power-saving operation, overcurrent protection, overload protection, and lack of phase protection.

1 Working principle and hardware composition


   When starting (receiving a start command), a phase-shifting 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 motor operation, the power factor of the motor is detected in real time, and the conduction angle α is changed accordingly to realize power saving operation. Overcurrent and overload detection adopt conventional current transformer circuits, which are sent to the single-chip microcomputer after rectification, filtering, amplification, A/D conversion and isolation, and data processing and judgment are completed by software. The lack of phase detection adopts the method of simultaneously detecting the three-phase power supply to judge the lack of phase fault. 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 microcontroller for processing and judgment.


The A-phase current and voltage signals are converted into square waves by the optocoupler and then sent to the XOR gate. Channel 0 of the timing/counter 8253 (working in mode 2) is used to detect the phase difference between the A-phase voltage and current, and the gate when the voltage crosses zero. The control terminal GATE0 gets a high level and starts counting. When the current crosses zero, GATEo changes to a low level, and the counting stops. The microcontroller is in GATE. The voltage and current phase difference can be obtained by reading the count value during the low level period. By the 8253 pass [industrial electrical appliances network-cnelc] channel 1, 2 (working in mode 5) delay to realize the phase shift control of the trigger pulse, the A phase voltage zero crossing is the synchronization signal start timing. Channel 1 cooperates with the single-chip timer/counter T0 to control the turn-on moments of No. 1, 2 and 3 thyristors. Channel 2 cooperates with the single-chip timer/counter T to control the turn-on moments of No. 4, 5, and 6 thyristors, T0, T1 The timing time 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 machining process, when the motor is required to start (stop), the main controller will issue a start (stop) command, and the soft starter controls the motor to start (stop). In machine tool equipment, interlocking control is often required, so when the motor fails and shutdown protection, a stop signal needs to be fed back to the main controller, and the main controller can then perform interlocking control processing

2 Control software

   The task of the software is to control the hardware system to automatically collect and detect the input signal, judge and process the input data, and output the required control signals as required. After receiving the start command, start the motor first, and then enter the cycle work process until receiving the stop command or fault shutdown. Due to strong functions


With the cooperation of the hardware circuit, the main program can perform fault detection, current value sampling and display, power factor angle measurement, trigger pulse phase shifting and output control in each cycle, so it achieves rapid control, sampling and fault diagnosis. error. The INTo and To interrupt programs realize the phase shift and positioning control of the trigger pulse of No. 1, 2 and 3 thyristors. The INT1 and T1 interrupt programs realize the phase shift and positioning control of the trigger pulse of No. 4, 5, and 6 thyristors.



3 Conclusion

The three-phase asynchronous motor soft starter controlled by the single-chip microcomputer has the characteristics of perfect function, reliable operation, convenient use, low cost, etc., which meets the requirements of the enterprise for equipment reliability, production efficiency and resource optimization, and has certain application value .