Motor protector functions and common applications
The motor protector serves to provide comprehensive protection and control for the motor. It alerts for conditions such as overload (current), underload (current), phase loss, stalling, short circuit, leakage, unbalanced three phases, overheating, overvoltage, undervoltage, grounding, power analysis, winding insulation degradation, bearing wear, rotor eccentricity, and winding aging, thereby triggering protective control actions.
OneWhy are motor windings now more prone to burning out compared to the past? Owing to the continuous advancement in insulation technology, there's a demand for increased power in motor design while reducing size, leading to smaller heat dissipation capacity and weaker overload capacity in new motors. Moreover, the acceleration of automation in production management causes motors to frequently operate, start, reverse, brake, and change speeds under overload, thereby raising the bar for motor protector functions. Additionally, the application of motors has expanded to broader fields, often in harsh working environments with poor ventilation. Coupled with irregularities in motor manufacturing, issues like power quality problems, equipment management shortcomings, and more contribute. All these factors lead to a shorter lifespan for motors now, with an increased frequency of failures, especially overloading, which causes phase loss, short circuits, leakage, unbalanced three-phase, overheating, locked rotor, and other sweeping faults.
Please provide the Chinese content to be translated.Why do the protective devices in the traditional motor protection series not offer ideal protection? Traditional motor protection products mainly rely on electronic protectors, thermal overload relays, fuses, and time relays as primary protection. For instance, electronic protectors primarily use transistor analog circuits as the main control element, with significant potentiometer adjustment errors; fuses are primarily intended for protecting against line shorts, and the current rating of fuses must consider the motor's starting current, so relying solely on fuses for motor protection is not sufficient. Thermal overload relays are widely used for motor overload protection, but they lack sensitivity, have a broad range, are prone to errors, and are unstable, varying with environmental temperature changes, making them not very reliable for motor protection. According to real-world conditions, many installations with thermal overload relays still experience motor failures affecting normal production. Many traditional protection devices also exist, but they all suffer from some degree of range deviation, simple functions, low automation levels, lack of systematization, complex operation, and low accuracy.
Motor protector protection principle and application analysis
Summary: Motor protectors are an essential component in the power supply system of electric motors, playing a critical role in protecting and controlling the motor during startup and operation. Therefore, an analysis of the protection principles and composition of motor protectors is provided, along with an explanation of their applications and selection criteria 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 smooth operation of the driven electromechanical equipment. Nowadays, motors are widely used in industrial and agricultural, transportation, mining, chemical, aerospace, and aviation fields. The types of loads carried by electric motors are diverse and often a critical part of the entire equipment, making the normal operation of electric motors of great importance. Motor protectors (motor protection devices) are a crucial component in power generation, supply, and consumption systems. They are energy-saving electromechanical products with significant energy-saving effects, spanning across various industries, with large quantities and widespread applications.
Motor protectors provide comprehensive protection and control for motors, addressing issues such as overload (overcurrent), underload, phase loss, stalling, short circuit, leakage, unbalanced three phases, overheating, overvoltage, undervoltage, grounding, power, energy consumption analysis, coil insulation, bearing wear, rotor eccentricity, and aging of windings, triggering alarm alerts and controlling protective actions. Today, motor protectors are almost ubiquitous across all electrical usage sectors, 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 modes can be categorized from a mechanical perspective into two aspects: winding damage and bearing damage. The main causes of motor damage are as follows:1. Long-term exposure to electrical, thermal, mechanical, and chemical stresses on the motor causes insulation aging and damage to the winding, resulting in short circuits between turns of the stator and rotor windings or to ground. 2. Poor quality of power supply from the grid, with unbalanced three-phase voltage, significant voltage fluctuations, distorted waveform of the grid electricity, severe higher harmonics, or the motor operating with a missing phase. 3. Low power supply voltage results in insufficient starting torque for the motor, making it unable to start smoothly or repeatedly start within a short period, leading to excessive starting current over time and motor overheating. 4. Mechanical failure or other reasons cause the motor rotor to be blocked. 5. Failure of the cooling system in certain large motors or operation under high temperature and humidity conditions for extended periods leads to motor failure.
The research on the protection principle of electric motors is crucial for determining the performance level of motor protectors. According to the theory of three-phase symmetrical component method, three asymmetrical vectors can be decomposed into three groups of symmetrical vectors, respectively: 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. When the motor's fault current exceeds its rated current due to excessive current flowing through the winding, overcurrent protection for the motor can be activated. 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 rise in operating current will be proportional to the drop in voltage; during motor overload, it often leads to stalling, with the operating current significantly exceeding the rated current. For these situations, the motor protector can detect the three-phase operating current, determine different protection methods based on the characteristics of the operating current, and thereby provide electrical disconnection 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, with 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 disablement and reset circuits, fault memory circuits, drive circuits, working power supply circuits, action control circuits, parameter display circuits, digital amplification circuits, and signal acquisition circuits.
Three.Types and Applications Analysis of Motor Protectors
The motor protectors commonly used in our country include thermal relays, temperature relays, and electronic motor protectors. The thermal relay, introduced and developed in the early 1950s based on Soviet technology, is a mechanical motor overload protector using metal strips. It features a reverse-time characteristic and a simple structure for overload protection of motors. However, it lacks functionality, offers no phase failure protection, and does not protect against issues like inadequate ventilation, bearing damage, stalling, prolonged overload, and frequent startups. This is primarily due to the mismatch between the thermal relay's operating value and the actual motor protection value, rendering it ineffective. Additionally, it has poor repeat performance, cannot be used immediately after a high current overload or short circuit, has large setting errors, is sensitive to environmental temperatures, and may fail to operate, consuming high power, using excessive materials, and having outdated performance indicators. The temperature relay is a disk-type or other form of relay made of bimetallic strips, with a heating element embedded in the motor to protect it based on the motor's temperature. However, when the motor capacity is large, it needs to be used with current monitoring to prevent rapid temperature rise during stalling. Due to the lag in temperature measurement, the motor windings may be damaged. The temperature relay has the advantages of a simple structure, reliable operation, and a wide protection range, but it operates slowly, has a long return time, and is not suitable for motors with a large current delta connection. It is currently widely used in exhaust fans, refrigerators, and other applications. The electronic motor protector, by detecting the three-phase current value and the set current value, protects the motor through potentiometer dials or plug switches. The circuit typically uses an analog reverse-time or timing characteristic, cannot display operating data or set parameters, and cannot achieve automated management platform functionality.
In addition to the three common types of motor protectors mentioned above, magnetic field temperature detecting relays and intelligent motor protectors are also widely applied in motor fault protection. Magnetic field temperature detecting protectors achieve protection by embedding magnetic field detection coils and temperature probes within the motor, based on changes in the internal rotating magnetic field and temperature. Their main functions include overload, overheating, over/under voltage, stalling, short circuit, phase loss, and wear monitoring. The protection functions are comprehensive, but the drawback is the need to install magnetic field detection coils and temperature sensors internally. Intelligent motor protectors can realize comprehensive intelligent protection for motors, integrating protection, measurement, communication, and display. The set parameters are digitally set, and operations are performed through the control panel buttons. Users can independently adjust various parameters according to actual usage requirements and protection conditions, choose to activate or deactivate protection functions, and more.LCD display window for large Chinese and English characters, LED fault indication, query motor running time and fault memory function, supports multiple communication protocols, transmitter current output for actual multi-party control, currently, all high-voltage motor protections use Gerey explosion-proof intelligent models.
Four: Principles for Selection of Motor Protectors
The purpose of selecting a motor protector is to fully utilize the motor's overload capacity while ensuring it is never damaged, thereby enhancing the reliability of the power drive system and the continuity of production. Properly selecting the motor protector device can fully exploit the motor's overload capacity and prevent damage, thus improving the reliability of the power drive system and ensuring production continuity. Specific functions include: We strive to achieve economic rationality by considering various factors, such as the inherent value of the motor, its load, operating environment, the significance of the main motor equipment, and the potential impact on the production system if the motor is removed from operation. We prioritize simple protection devices when they meet the protection requirements. Only when simple devices are insufficient or when higher requirements are placed on protection functions and characteristics, do we consider using complex protection devices, ensuring a balance between economy and reliability.
V. Conclusion
Motor protectors have now entered the microelectronic intelligent era and are diversifying in development. This requires our staff to carefully consider the actual requirements of motor protection during selection, to accurately and timely judge motor faults proactively, and to rationally choose protection functions and methods. The aim is to achieve excellent motor protection, ideal equipment reliability, reduce unplanned downtime, and minimize accident losses.
