The motor is the most important driving source in the industrial production field. How to effectively monitor the running state of the motor, protect the motor circuit, improve the running time of the motor, reduce the motor fault, and is critical to the operation of the overall grid of the plant.
There are many kinds of motor protection devices. At present, it is more commonly used based on metal plate mechanical thermal relays. It has a simple structure and has inverse time characteristics in protecting motor overload. However, it has fewer protection functions, no phase failure protection, and can not protect the motor from poor ventilation, brooming, stalling, long-term overload, frequent start-up, etc. In addition, the thermal relay also has defects such as poor repeatability, large current overload or short-circuit fault, can not be used again, large adjustment error, easy to be affected by environmental temperature, mis-movement or refusal, large power consumption, consumables, and poor performance indicators.
In response to national energy conservation and emission reduction requirements, the use of microcontroller-based electronic motor protectors to replace existing thermal relays has a broad market. The STM32 series ARM chip with integrated rich peripherals is designed as the core intelligent motor protector, which has the advantages of fast response, less additional chips, simple production debugging, high production and social benefits.
1 intelligent protector function and hardware architecture
The main faults in motor operation include: start timeout, overload, stall, phase loss, unbalance, overheat, underload, overvoltage, undervoltage, etc. Therefore, the smart protector needs to monitor the operating voltage, operating current and chassis temperature of the motor.
At the same time, due to the different types, capacities and load types of the motor, the parameters of the motor protection are also different, so it is necessary to be able to set the protection parameters for different motors.
Furthermore, in order to enable intelligent protection relays to meet the needs of the currently popular Intelligent Motor Control Center (IMCC), intelligent motor protectors also need to have network communication functions.
Figure 1 is a block diagram of the hardware structure of the intelligent motor protector.
2 system hardware design
The MCU is the core part of the motor protector and is responsible for data acquisition, data processing, output control and parameter setting. Here is ST's latest STM32F103xD series ARM chip.
This series of chips uses ARM's 32-bit C0rtex M3 as the core, and the highest frequency is 72MHz. The Cortex core has a single-cycle hardware multiplication and division unit, so it is suitable for high-speed data processing.
The chip has three independent conversion cycles, a minimum of 1s high-speed analog-to-digital converter, and three independent digital-to-analog converters with separate sample-and-hold circuits, so it is especially suitable for three-phase motor control, grid monitoring and multi-parameter instruments. Use of equipment.
The chip also comes with a rich communication unit, including up to five asynchronous serial interfaces, one USB slave device, one CAN device, I2C and SPI modules.
2.2 Analog acquisition unit
The motor protector mainly needs to collect three analog quantities of current, voltage and temperature to monitor and protect the running state of the motor.
There are many types of current sensors, including core current transformers, Hall sensors, and shunt resistors. The motor connected to the motor protector mainly has a motor of several kilowatts to several tens of kilowatts, so the phase current of the motor is mainly from several amperes to several tens of amperes. Therefore, the current transformer is used as the current collecting unit, which has the advantages of wide measuring range, small heat generation and high isolation voltage. At the same time, without changing the parameters of the processing circuit, the current sensor with different ratios can easily change the current detection range of the motor protector, so that it can be conveniently used for larger capacity motor protection.
The voltage is obtained directly by resistor divider, so the entire motor controller is a common system. The resistor uses a high-impedance and high-voltage type resistor. In order to improve the over-voltage capability of the voltage acquisition circuit, the voltage divider circuit uses a multi-resistor series to reduce the rated voltage drop across each resistor and improve the entire branch. The highest withstand voltage.
The temperature sensor uses a common platinum resistance sensor or NTC thermistor, and the corresponding thermal resistance signal conditioning circuit is designed on the protector hardware. Since the thermal resistance is a nonlinear device, the temperature acquisition processing channel needs to be nonlinearly processed. In order to reduce the complexity of the hardware circuit, the actual RTD conditioning unit is only designed to use an instrumentation amplifier, and the nonlinear processing of the RTD is performed by the MCU. carry out. There is also a semiconductor temperature sensor built into the MCU chip to detect the temperature inside the protector to prevent control errors due to overheating of the system.
2.3 LCD display
For a stand-alone motor protector, it is necessary to be able to set the protection parameters, display the current operating status, and also display the fault type when a fault occurs. Therefore, the motor protector requires a display unit.
The system design adopts the dot matrix STN black and white liquid crystal display (LCD) module. Compared with the TFT color LCD module, it has the advantages of wide temperature range, long life and readable under strong light.
The built-in controller of the LCD module uses a parallel data communication interface, including a data bus, read and write control lines, device strobes, and reset pins. In the system design, the multi-function static memory controller (FSMC) using the STM32F103xD chip is connected to the LCD module.
The FSMC module of the STM32F chip is a multi-function static memory controller that supports static memory (SRAM), NOR F1ash and PSRAM. It can support 8-bit or 16-bit wide memory.
The access timing of the LCD module is the same as that of the SRAM, and the interface timing of the 8080 or 6800 type can be selected by the configuration pin. Figure 2 shows the electrical connection between the FSMC interface of the STM32 chip and the LCD. The LCD here is the 8080 interface timing.
2.4 Communication circuit
The control structure of the Intelligent Motor Control Center (IMCC) is mostly a bus-type distributed network structure in which a central controller is responsible for scheduling and monitoring the operation of all motors. Depending on the central controller used (mostly PLC), the system's communication protocols are MODBUS, Fieldbus and Ethernet. The most common of these is the MODBUS protocol. The physical layer of the MODBUS protocol is a half-duplex communication network based on RS485, in which the motor protector is in a slave state.
Since the motor protector is internally heated, RS485 remote communication needs to be isolated from the main circuit of the controller. For the isolation of the RS485 transceiver, the communication signal and the power supply of the transceiver need to be isolated. The communication interface design of the motor protector requires a communication baud rate of up to 57.6 kbps. Therefore, high-speed optocouplers or digital isolation chips are needed to isolate the communication signals.
The digital isolation chip is a new type of device. Companies such as TI, ADI and Silicon Lab have introduced their own patented digital isolation devices, but the pin packages and pin functions of each chip are mostly compatible and can be directly replace. Compared to traditional high-speed optocouplers, digital isolation devices have the advantages of low power consumption, high transmission rate, compatibility with 3V/5V systems, and simple peripherals. The actual connection circuit is shown in Figure 3.