详情描述

Motor protector's protection principle and application analysis

Abstract: Motor protectors are an essential component in the power supply system of motors, playing a critical protective control role in the motor's start-up and operation. Therefore, an analysis of the protection principle and composition of motor protectors is provided, along with an explanation of their applications and selection principles throughout their development.

One. Electric motors are an indispensable transmission equipment in the current application of productivity, serving as the power source for other electromechanical equipment. The normal output of electric motors is a prerequisite for the normal operation of the driven electromechanical equipment. Nowadays, electric motors are widely used in industries such as agriculture, transportation, mining, chemical, aerospace, and aviation. The loads carried by electric motors are diverse and often a critical part of the entire equipment, thus ensuring the normal operation of electric motors is of great importance. Motor protectors (motor protection devices) are a key component in power generation, supply, and consumption systems, and are energy-saving electromechanical products with significant energy-saving effects across various industries, with large quantities and widespread application.

Motor protectors provide comprehensive protection and control for motors, alerting to and protecting against overloads (overcurrent), underloads (light loads), phase loss, stalling, short circuits, leakage, unbalanced three phases, overheating, overvoltage, undervoltage, grounding, power, energy consumption analysis, winding insulation, bearing wear, rotor eccentricity, and winding aging. With these alarm prompts, protective actions are controlled. Today, motor protectors are almost ubiquitous in all electrical usage fields, holding an indispensable and crucial position and role in the national economy and energy-saving endeavors.

TwoPrinciples and Composition of Motor Protectors

For electric motors, their failure forms can be divided into two aspects from a mechanical perspective: winding damage and bearing damage. The main causes of motor damage are as follows:1. Extended exposure to electrical, thermal, mechanical, and chemical stresses causes insulation aging and damage in the motor's windings, resulting in short circuits between turns or to ground in the stator and rotor windings. 2. Poor quality of power supply from the grid, with unbalanced three-phase voltage, significant voltage fluctuations, distorted waveform due to grid voltage sag, severe harmonics, or single-phase operation of the motor. 3. Low supply voltage results in insufficient starting torque for the motor, preventing smooth startup or repeated startups within a short period, leading to overheating due to prolonged exposure to high startup currents. 4. Obstruction of the motor's rotor due to mechanical failure or other reasons. 5. Faults in the cooling system of certain large motors or failure caused by prolonged operation under high temperature and humidity conditions.

The study of motor protection principles is crucial for determining the performance of motor protectors. According to the theory of three-phase symmetrical component method, three unsymmetrical vectors can be decomposed into three groups of symmetrical vectors, respectively being the positive sequence components, negative sequence components, and zero sequence components. The calculation formula for symmetrical components is as follows:

Based on1) In the event of both symmetrical and asymmetrical faults, the three-phase current of the motor will change. The current flowing through the winding of the motor during a fault exceeds the motor's rated current, thus enabling overcurrent protection for the motor. Overload, phase loss, and under-voltage can all cause the winding current to exceed the rated value. When the supply voltage is under-voltage, the increase in operating current will be proportional to the decrease in voltage; during motor overload, it often results in stalling, with the operating current significantly exceeding the rated current. In response to these conditions, motor protectors can detect the three-phase operating current, determine different protection methods based on the nature of the current, and thereby provide power-off protection for the motor. The types of motor faults include overcurrent protection, negative sequence current protection, zero sequence current protection, voltage protection, and overheat protection.

By analyzing the protection principle of motor protectors, it can be seen that an ideal motor protector should meet elements such as reliability, economy, and convenience, and have a high performance-to-price ratio. Through development and updates, modern motor protectors typically consist of current detection circuits, insulation detection circuits, temperature detection circuits, reference voltage circuits, logic processing circuits, timing processing circuits, start block and reset circuits, fault memory circuits, driving circuits, working power supply circuits, action control circuits, parameter display circuits, digital amplification circuits, and signal collection circuits.

III.Motor protector types and application analysis

 The motor protectors commonly used in our country include thermal overload relays, temperature relays, and electronic motor protectors. The thermal overload relay, developed based on Soviet technology in the early 1950s, is a mechanical motor overload protector using metal strips. It features reverse-time performance and a simple structure for overload protection. However, it lacks functionality, offers no phase loss protection, and doesn't activate in case of inadequate ventilation, scraping, stalling, prolonged overloading, or frequent startups. This is primarily due to the mismatch between the thermal relay's operating value and the motor's actual protective value, rendering it ineffective. It also has poor repeatability, cannot be reused promptly after a large current overload or short circuit, and has significant setting errors. It is sensitive to environmental temperatures and can malfunction, consuming a lot of power, requiring much consumable material, and having outdated performance indicators. The temperature relay, made of bimetallic strips and available in disk or other forms, incorporates heat elements in the motor for temperature-based protection. However, when the motor capacity is high, it must be used with current monitoring relays to prevent rapid temperature rise during stalling. Due to the lag in temperature sensing elements, this can cause motor winding damage. The temperature relay offers advantages such as a simple structure, reliable operation, and wide protection range, but it has slow action, a long return time, and is not suitable for motors with a large current delta connection. It is widely used in exhaust fans, refrigerators, and other applications. The electronic motor protector, through the detection of three-phase current values and a fixed current value, protects the motor by using potentiometer knobs or toggle switches for operation. The circuit typically employs analog reverse-time or time-limit characteristics, cannot display operating data or set parameters, and cannot be integrated into an automated management platform.

In addition to the three common motor protectors mentioned above, magnetic field temperature detection relays and intelligent motor protectors are also widely used in motor fault protection. Magnetic field temperature detection protectors work by embedding magnetic field detection coils and temperature probes within the motor, protecting it based on changes in the internal rotating magnetic field and temperature. Their main functions include overload, overheating, over/under voltage, stall, short circuit, phase loss, and wear monitoring. They offer comprehensive protection features, but the drawback is that they require the installation of magnetic field detection coils and temperature sensors inside the motor. Intelligent motor protectors can achieve comprehensive intelligent protection for motors, integrating protection, measurement, communication, and display. The set parameters are set digitally, and operations are performed through a control panel. Users can independently adjust various parameters based on actual site requirements and protection conditions, choosing to enable or disable protection functions, and also incorporate...LCD display with large Chinese and English characters, LED indicator for faults, query motor operating time and fault memory function, supports multiple communication protocols, current transmitter output for actual multi-party control, currently, all high-voltage motor protection adopts Gere防爆 intelligent models.

Section 4: Selection Principles for Motor Protectors  

The purpose of using motor protectors is to ensure that motors fully utilize their overload capacity without sustaining damage, while also enhancing the reliability of the power drive system and the continuity of production. Proper selection of motor protector devices not only maximizes the motor's overload capability but also prevents damage, thereby improving the reliability of the power drive system and the continuity of production. Specific functions include:Considering the value of the motor itself, its load, operating environment, the importance of the main motor equipment, and whether the motor's shutdown will severely impact the production system, we strive to achieve economic rationality. We prioritize simple protective devices when they meet the protection requirements. Only when simple devices are insufficient or higher requirements for protection functions and characteristics are needed do we consider using complex protective devices, ensuring a balance between economy and reliability.

V. Conclusion

Motor protectors have now entered the microelectronic intelligent era and are evolving towards diversification. This requires our staff to carefully consider the actual requirements of motor protection when selecting, to proactively and accurately identify motor faults in a timely manner, and to rationally choose protection functions and methods to achieve effective motor protection, ideal equipment reliability, reduce unplanned downtime, and minimize accident losses.