1. Introduction In modern industry, electric motors, as a type of drive machinery, form the foundation of all power machinery. With the continuous advancement of science and technology and the constant improvement of process control, especially the requirements of automated production, there is an urgent need to develop and improve electric motor control and protection equipment to achieve functions such as remote control, telemetry, remote adjustment, fault diagnosis, and centralized control of production processes and large machinery. Meanwhile, the petrochemical industry itself is characterized by high-throughput, full-load continuous operation, thus placing extremely high demands on the reliability and stability of motor equipment. In the event of an accident, the cause of the fault must be identified and eliminated immediately to ensure timely resumption of production. Jinhua Chemical Group has previously used thermal relays as overload protection and control components for electric motors, but due to limitations in component quality and manufacturing processes, they can no longer meet the increasingly demanding requirements of process automation. Therefore, after years of trials and repeated practice, we have finally selected a new type of intelligent electric motor protector (referred to as a motor protector) as the protection component for electric motors and have gained some experience in its use. In particular, during the construction of several key national projects at Jinhua, including the 80,000-ton ion-exchange membrane caustic soda renovation project, the 120,000-ton ion-exchange membrane caustic soda project, the 80,000-ton polyvinyl chloride project, the 80,000-ton propylene oxide project, and the 50,000-ton granular caustic soda project, we used over 1,600 motor protectors in the low-voltage power distribution system. This not only effectively ensured the safe operation of the motors and completely replaced thermal relays, but also, by utilizing advanced communication technology, achieved partial automation of motor control, resulting in significant economic benefits. Therefore, the following is a brief introduction to the engineering applications of motor protectors. 2. Basic Working Principle: Motor protectors are a new type of electronic multi-functional integrated motor protection device developed in the last ten years or so. They integrate overload (light) protection, phase loss, over (under) voltage, locked rotor, leakage current, grounding, and three-phase imbalance protection into one low-voltage protection system. They have advantages such as high setting accuracy, energy saving, sensitive operation, and reliable operation, making them an ideal replacement for traditional thermal relays. They typically consist of a current sensor, a comparison circuit, a microcontroller, or an output relay. Its basic principle and working process are shown in Figure 1: The sensor linearly reflects the change in motor current to the sampling port of the protector. After rectification, filtering and other links, it is converted into a DC voltage signal proportional to the motor current. It is sent to the corresponding part for comparison with the given protection parameters. Then, it is processed by the single-chip microcomputer circuit to drive the power circuit and make the relay act. When the motor current increases due to overload of the drive part, the voltage signal obtained from the current sensor will increase. When this voltage value is greater than the setting value of the protector, the overload circuit works. After a certain (adjustable) delay, the RC delay circuit drives the output relay to act, so that the contactor cuts off the main circuit. The working principle of the undervoltage and phase loss protection functions is basically the same. 3 System functions (1) Protection function. The motor protector can realize comprehensive protection of the motor. Its specific functions include grounding protection, phase loss protection, short circuit protection, over (light) load protection, unbalanced overload protection, locked rotor protection, low voltage and overvoltage protection. When the motor is working normally, the protector displays the motor's working current and voltage in real time through the LCD screen on the panel. The motor protector with communication function transmits the collected signals to the background microcomputer through the CAN bus at regular intervals when the microprocessor is idle. When a fault occurs, the protector acts quickly, saves the fault type, time and motor operating parameters on site, and then reports to the background monitoring computer through the CANBUS network. (2) Online monitoring function. The control room operator can complete the retrieval, modification and setting of various protection setting data of all protectors on the background microcomputer. At the same time, the online monitoring of the motor's operating load and the quality of the power supply creates conditions for early detection of overload and grounding faults. In addition, the powerful information management function of the background microcomputer can effectively record the model, operating mode and historical maintenance records of each motor, providing a reliable basis for ensuring the safe production of the process system and reducing accident shutdowns. 4 Selection principle (1) According to the structural form, motor protectors are generally divided into two types: integrated type and split type. The former is an independent whole, so it is suitable for fixed cabinet installation. For example, in the renovation of old switch cabinets or GGD cabinets, the protector can be directly fixed on the mounting panel and the feeder cable can be threaded through the core. The latter, because the transformer part and the display part are separate, can be placed arbitrarily within a certain range, so it is very suitable for the installation of new drawer-type switch cabinets. In our engineering practice, since we have widely adopted the new Domino type or GCK series low-voltage drawer-type switch cabinets, we have creatively embedded the display head in the movable panel of the switch drawer. This simplifies the wiring inside the cabinet and facilitates the system to adjust, set parameters and display and monitor at any time. At the same time, the digital display panel also adds to the uniformity and aesthetics of the cabinet surface; making the equipment operation status and fault status in the power distribution room clear at a glance, which greatly facilitates the system inspection and maintenance. (2) Motor protectors are divided into two types according to the rated working voltage and current range: AC380V and AC220V; the corresponding ranges are 6A and 60A, respectively. When using them, the appropriate range range can be selected according to the working voltage of the motor control circuit and the rated current of the load. In engineering applications, due to the large number of large motors in chemical plants, in order to facilitate interchangeability and reduce the types of spare parts, for motors of 110KW and above, we adopt the method of installing (□/5) current transformers in the main circuit to convert the primary current into a secondary current of less than 5A, and then connect it to a motor protector with a range of 6A, so that large equipment can also be effectively controlled and protected.