3 system software design
3.1 Monitoring procedures
In the intelligent motor protector, the MCU only performs sampling, calculation and analysis of current and voltage to realize various protection functions, and displays the parameters of the line and records the fault status in real time.
In the system software, the global monitoring software is used to cyclically scan the status of each unit subroutine, including the data sampling of the analog-to-digital converter, calculate the data after the data acquisition is completed, and then combine the protection strategy according to the calculation result and the preset motor parameters. Determine the current state of the motor. Finally, the corresponding status information and control functions are issued through the LCD display and the output control interface.
3.2 ADC configuration and use
Since the motor protector collects the 50Hz power frequency voltage and current, and in order to be able to monitor the higher harmonic components in the power grid (mainly the 3rd harmonic), the sampling frequency of the ADC needs to be set at an integral multiple of the power frequency. This results in the most accurate accuracy when sampling Fast Fourier (FFT) operations. At the same time, when data is sampled, since the calculation power requires simultaneous acquisition of voltage and current values, in the system design, the three-phase parameters are respectively assigned to two analog-to-digital converters, and the temperature is placed in the third analog-to-digital conversion. Device. All analog to digital conversions are triggered by an internal timer interrupt.
For voltage and current acquisition, the system uses synchronous sampling modulo, that is, simultaneously collects the voltage and current values of the same channel; at the same time, in the sampling channel configuration register, three voltage current channels are sequentially arranged, so that under one timer trigger , complete conversion of all channels at one time.
Since the FFT requires a set of data for calculation, the system design uses DMA to complete the transfer of the conversion results for minimal CPU intervention. When programming, it is necessary to set two data storage areas for alternate storage of sampled data; at the same time, the preset value of the byte count of DMA transfer is the length of the Fourier transform array multiplied by the number of channels acquired.
3.3 Remote communication
The motor protector is designed as a MODBUS slave. All motor running states, control states and other parameters are placed in the register of the system's agreed address. At the same time, the MODBUS slave needs to preset a unique slave address in the network for each motor controller, so it is also necessary to use the button and LCD screen to set the slave address.
This paper proposes a new intelligent motor protector based on the STM32 series. This design makes full use of the resources of the STM32 chip and provides the necessary peripheral components to form a complete system. The practical application proves that this system can effectively protect the motor, and has the advantages of simple structure, perfect function and rich interface. It can also develop interfaces such as USB and CAN open according to actual needs, so this system can be more widely used in industrial production. each field.