Abstract : To improve the accuracy and reliability of intelligent motor protectors, an automatic testing system was developed based on digital electronics technology, analog electronic circuits, a single-chip microcomputer, and a host computer language. The system's hardware and software design is introduced, and multiple tests were conducted. Test results show that the system can automatically apply signals to the intelligent motor protector to verify its accuracy, protection function, and additional performance, automatically determine whether the test results meet the technical specifications, and automatically save and print the test data after testing. Compared with conventional manual testing systems, it features high testing efficiency, standardized testing process, high automation, high accuracy, strong versatility, and convenient test data management. This automatic testing system is running normally on the company's production line and has proven to be feasible.
Keywords: intelligent motor protector; automatic detection; microcontroller; host computer software;
0 Introduction
The intelligent motor protector has a remote communication interface, analog input and output, and switch input and output. It can form a control system with sensors, PLC, PC, etc., and can realize remote monitoring of motor operation [1] . It is suitable for automated control in coal mines, petrochemicals, smelting, power, ships, sewage treatment and other fields, as well as intelligent management of civil buildings. Since users need diverse functions and complex protection functions, its reliability involves many aspects. In order to ensure the quality of the product, all indicators of the intelligent motor protector need to be strictly tested before leaving the factory. At present, its testing is done manually, which has problems such as complicated testing procedures, low work efficiency, low reliability, non-standard testing process, troublesome data storage and inability to print automatically. This increases the production cost of the product, restricts the improvement of the enterprise's production efficiency and benefits, and cannot meet the needs of modern large-scale high-efficiency production and testing. In addition, customers have diverse customization of product functions, so realizing automatic testing of intelligent motor protectors is of great significance.
The development of high-precision electronic components, modern microcontroller technology, and database technology has made it possible to automatically detect intelligent motor protectors. The entire detection process can be controlled by a host computer. Microcontroller technology is used to generate signals and control their on/off state. Serial port server technology is used to control multi-tablet communication. Remote server technology is used to store test data, forming an automatic detection system. This reduces production costs, improves detection efficiency, and eliminates the drawbacks of manual recording, such as errors [2,3,4] .
1. System Basic Requirements
The system needs to be able to automatically perform comprehensive testing on all settings, digital display accuracy, protection functions, and additional functions of the protector, and automatically determine whether the test results meet the product's technical specifications. It should also have functions for storing, statistically analyzing, querying, and outputting test reports [5,6] . The main requirements and technical specifications of the system are as follows:
(1) Product range. Suitable for various specifications of intelligent motor protectors.
(2) Working modes. Fully automatic and single-item automatic detection coexist.
(3) Working environment. It is robust and can cope with emergencies such as sudden power outages on the production line and human error.
(4) Testing efficiency. It can test multiple motor protectors of the same specifications at the same time, and the testing time is required to be short.
(5) Technical specifications of the power source. Output voltage range: AC 3×30V~ 3×450V; current range: AC 0.1A~800A; phase shift range: 0~360°; voltage and current output frequency range: 45Hz~65Hz; waveform distortion <0.5% when outputting voltage and current as sinusoidal waves; voltage, current, and power stability of the power source <0.1% when outputting as sinusoidal waves; can drive resistive, inductive, and capacitive (<4uF) loads; has built-in automatic detection and protection functions for output voltage short circuit and output current open circuit.
(6) Sub-unit control unit. It can communicate with the protected device under test and the host computer, collect the digital input and analog input of the protected device, and has switch output control and timing functions.
2. System Hardware Structure and Principle
The basic principle block diagram of the system is shown in Figure 1. The main modules of the system include: PC, RS-485 extended communication module, waveform generation module, current and voltage power amplifier and switching device, extension control module, printer, etc.
1) PC. The PC acts as the host computer, mainly realizing human-computer interaction functions. It is responsible for setting detection parameters, controlling the start and stop of detection, real-time monitoring of the detection process, saving and printing detection results, etc. It is an indispensable device for completing the entire detection process.
(2) RS-485 extended communication module. This module mainly completes the communication between the PC, waveform generation module, sub-control module and the protected device under test.
(3) Waveform generation module. The waveform generation module (precision digital signal source) generates various signals required for detection. The current and voltage outputs are controlled by the current and voltage switching device and finally flow into the intelligent motor protector for detection of accuracy, protection, switching quantity and other functions.
(4) Current and voltage power amplifiers. The system power amplifier adopts PWM power amplification technology and is designed with voltage short-circuit overload, current open-circuit overload and fast current limiting protection measures to ensure long-term stable and reliable operation of the power amplifier.
(5) Current and voltage switching device. A schematic diagram of a single current switching device is shown in Figure 2. This device mainly consists of a short-circuit relay and an output relay, effectively preventing current open circuits. By switching between the short-circuit relay and the output relay, the presence or absence of input current to the protector can be controlled; by selecting one or more short-circuit relays to close, single-phase or multi-phase current can be obtained. A schematic diagram of a single voltage switching device is shown in Figure 3. Whether a voltage signal is input is determined by controlling the opening and closing of the relays.
(6) Sub-unit control module. It has 9 digital outputs and 5 digital inputs, which can control and detect the digital input and output status of the protector; it has a DC analog measurement signal with a range of 4-20mA, which can detect the analog output of the protector; it uses a hardware clock for timing with an accuracy of ms, which can count the tripping time of the fault; it has an RS485 communication interface, which can communicate with the protector and PC.
(7) Printer. Used to print test results and generate product factory inspection reports.
3. Testing process and software
The software for the intelligent motor protector automatic detection system mainly consists of two parts: lower-level software and upper-level software. The lower-level software primarily receives instructions from the upper-level software, performs real-time control of various hardware components, and collects system parameters and variables in real time. The upper-level software mainly implements functions such as human-computer interaction, setting detection parameters, and controlling the detection process.
3.1 Lower-level software
The system's control module uses an STM32 microcontroller with a Cortex-M3 core for real-time control of various hardware modules. The lower-level software, i.e., the microcontroller program, has its main program flow shown in Figure 4. After power-on, the system first performs initialization, including microcontroller configuration and I/O port initialization. Then, different control commands are input via the keyboard or the host computer to control various external modules.
3.2 Host Computer Software
The flowchart of the host computer program is shown in Figure 5. After running the software, first enter the serial number of the protector connected to the corresponding sub-unit, and the system will automatically set the detection parameters; then, start the detection process until the detection is completed.
(1) Detection of set parameters. The value of the protector register to be tested is read and saved, and then compared with the correct configuration parameters. If it passes, proceed to the next test; otherwise, it is returned to the debugging department. Other tests can only be carried out after the debugging is correct.
(2) Digital display accuracy test. The signal source is raised to the level required for the corresponding test. After the signal source stabilizes, the displayed value of the corresponding protector is read, the error is calculated and judged, and this accuracy test ends. A message confirmation mechanism is activated during the test. The flowchart of the accuracy test is shown in Figure 6.
(3) Protection function testing. This includes overload, phase loss, grounding, residual current, locked rotor, blockage, and imbalance testing. The main testing process is shown in Figure 7. After the test begins, to prevent interference from previous tests, the current input signal is first cut off. Then, according to the testing requirements, the corresponding fault signal is raised, the trip enable bit of the corresponding test item is opened, and other trip bits are shielded. In order to prevent the protector from being in a tripped state and affecting the test results, a remote reset operation is required for the protector. Then, the relay is operated to input a signal to the protector, and a timing command (a broadcast packet command) is sent to the sub-unit. After that, the operating status of the protector is continuously read. If the protector is read to be in a tripped state, the sub-unit sends a reading timing command to obtain the tripping time to determine whether it is qualified; otherwise, if it still does not trip after exceeding the specified maximum set non-tripping threshold, the function of the protector is judged to be unqualified.
(4) Additional function testing. Based on the customer's order, additional function testing is performed, mainly including: underload, start-up timeout, short circuit, undervoltage, overvoltage, underpower, overpower, phase sequence protection, as well as fault recording and switching quantity testing. The basic testing process is similar to the protection function testing, but the switching quantity testing requires individual switching quantities to be tested sequentially to prevent short circuits between switching quantities.
(5) Restore factory settings. For any modifications made to the protector parameters during the testing process, the factory settings must be restored before the testing is completed to ensure that the protector meets all technical specifications when it leaves the factory.
(6) Saving Results. The system stores the test data and results in a remote server database, using stored procedures to perform database operations. After the data is saved, inspection personnel can use the report management module in the host computer to output the factory inspection report. The system's reporting function is implemented through an enterprise-level report development tool.
4. Test Results and Analysis
The automated testing system underwent multiple tests on intelligent motor protectors of different specifications and models. The results showed that the system can complete the testing process accurately, systematically, and efficiently. Several full-function protectors were tested in approximately 5 minutes per test, significantly improving production efficiency compared to conventional manual testing. The accuracy and functional test results for a certain full-function protector are shown in Tables 1 and 2, respectively.
5 Conclusion
The intelligent motor protector automatic testing system boasts advantages such as a user-friendly human-machine interface, simple operation, and a high degree of automation. Test results show that the system met all expected technical specifications. Throughout the testing process, minimal human intervention is required, ensuring a high degree of automation and maximizing the standardization of the testing process and the objectivity of the test results. Testing revealed that in automatic testing mode, the time required for multiple full-function protectors to complete the entire testing process was significantly reduced. Furthermore, with minor modifications, the system can be used for the automatic testing of other instruments, demonstrating its wide applicability.
This article is from the 22nd issue of Low Voltage Electrical Appliances in 2013.
References
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