Motor protectors are an important component of the power supply system for motors, playing a crucial protective and control role during the starting and operation of the motor. Therefore, this paper analyzes the protection principle and structure of motor protectors, and elaborates on their application and selection principles in the development process.
I. Electric motors are indispensable transmission devices in modern productivity applications and the power source for other electromechanical equipment. The normal output of an electric motor is a prerequisite for the normal operation of the electromechanical equipment it drives. Today, electric motors are widely used in industry, agriculture, national defense, transportation, mining, chemical industry, and aerospace. Electric motors carry a wide variety of loads, often forming critical components in the entire equipment; therefore, ensuring the normal operation of electric motors is extremely important. Motor protectors (motor protection devices) are crucial components in power generation, supply, and consumption systems. They are energy-saving electromechanical products that are widely used across industries and offer significant energy-saving effects.
Motor protectors provide comprehensive protection and control for motors. They provide alarms and prompts when motors encounter common problems such as overload (overcurrent), undercurrent (light load), phase loss, locked rotor, short circuit, leakage, three-phase imbalance, overheating, overvoltage, undervoltage, grounding, power and electrical quantity analysis, winding insulation, bearing wear, rotor eccentricity, and winding aging. These protection mechanisms then take effect and control the motor. Today, motor protectors have permeated almost all electrical applications, playing an irreplaceable and vital role in the national economy and energy conservation efforts.
II. Protection Principle and Structure of Motor Protectors
From a mechanical perspective, motor failures can be categorized into two main types: winding damage and bearing damage. The main causes of motor damage are as follows: 1. Under prolonged exposure to electrical current, heat, mechanical, and chemical influences, the insulation of the windings ages and deteriorates, leading to short circuits between turns or to ground in the stator and rotor windings. 2. Poor power grid quality, including three-phase voltage imbalance, large voltage fluctuations, distorted power grid waveforms, severe high-order harmonics, or single-phase operation of the motor. 3. Insufficient power supply voltage results in insufficient starting torque, preventing the motor from starting smoothly or causing repeated starts within a short period. Prolonged exposure to excessive starting current leads to overheating. 4. Rotor blockage caused by mechanical failure or other reasons. 5. Failure of the cooling system in some large motors or prolonged operation under high temperature and humidity conditions can cause motor failure.
The study of motor protection principles is crucial for ensuring the performance of motor protectors. Based on the theory of the three-phase symmetrical component method, three asymmetric vectors can be uniquely decomposed into three sets of symmetrical vectors: positive-sequence components, negative-sequence components, and zero-sequence components. The formula for calculating the symmetrical components is as follows:
According to equation (1), the three-phase current of the motor will change when a symmetrical or asymmetrical fault occurs. When the current flowing through the windings of a faulty motor is too large, exceeding the motor's rated current, this characteristic can be used for overcurrent protection. Overload, phase loss, and undervoltage will all cause the winding current to exceed the rated current value. When the power supply voltage is undervoltage, the ratio of the increase in operating current will equal the ratio of the decrease in voltage; when the motor is overloaded, it often causes a stall, at which point the operating current will greatly exceed the rated current. In response to these situations, the motor protector can detect the three-phase operating current and determine different protection methods based on the different characteristics of the operating current, thereby providing power-off protection for the motor. Motor fault types include overcurrent protection, negative sequence current protection, zero sequence current protection, voltage protection, and overheat protection.
Analysis of the protection principle of motor protectors reveals that an ideal motor protector should meet factors such as reliability, economy, and convenience, and possess a high performance-price ratio. Through development and updates, modern motor protectors generally consist of a current detection circuit, an insulation detection circuit, a temperature detection circuit, a reference voltage circuit, a logic processing circuit, a timing processing circuit, a start-up, lockout, and reset circuit, a fault memory circuit, a drive circuit, a power supply circuit, an action control circuit, a parameter display circuit, a digital amplifier circuit, and a signal acquisition circuit.
III. Types and Application Analysis of Motor Protectors
Currently, the most commonly used motor protectors in my country are thermal relays, temperature relays, and electronic motor protectors. Thermal relays, developed in the early 1950s using Soviet technology, are mechanical overload protectors with metal plates. They feature inverse-time performance and simple structure in protecting motors from overload. However, they have limited functionality, lack phase loss protection, and are ineffective against faults such as poor ventilation, rotor rubbing, stalling, prolonged overload, and frequent starts. This is mainly because the thermal relay's operating value does not match the actual protection value of the motor, rendering it ineffective. Furthermore, they have poor repeatability, cannot be reused promptly after large current overloads or short circuits, have large setting errors, are easily affected by ambient temperature or fail to operate, have high power consumption, require many materials, and have outdated performance indicators. Temperature relays are disc-type or other types of relays made of bimetallic strips, with a heating element embedded in the motor. They provide protection based on the motor's temperature. However, for larger motors, they need to be used in conjunction with current monitoring relays to prevent rapid temperature rise during motor stalling, which could damage the motor windings due to the lag of the temperature sensing element. Temperature relays have advantages such as simple structure, reliable operation, and wide protection range, but they are slow to operate and have long return times, making them unsuitable for high-current delta-connected motors. Currently, they are widely used in exhaust fans, refrigerators, and other applications. Electronic motor protectors protect the motor by detecting three-phase current values and setting current values, using potentiometer knobs or DIP switches. The circuits generally employ analog inverse-time or definite-time operating characteristics, and cannot display operating data or set parameters, thus failing to support automated management platforms.
Besides the three common types of 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 relays protect against motor faults by embedding a magnetic field detection coil and temperature probe inside the motor, based on changes in the rotating magnetic field and temperature within the motor. Their main functions include overload, overheating, over/under voltage, locked rotor, short circuit, phase loss, and wear monitoring, offering comprehensive protection. The drawback is the need to install the magnetic field detection coil and temperature sensor inside the motor. Intelligent motor protectors provide intelligent integrated protection for motors, integrating data acquisition, protection, measurement, communication, and display into a single electromechanical system. Setting parameters are digitally set and operated via buttons on the control panel. Users can adjust various parameters according to actual site requirements and protection conditions, selecting to enable or disable protection functions. They also feature a large LCD display window with Chinese and English text, LED fault indicators, motor running time lookup, fault memory function, support for multiple communication protocols, and multi-party control of the transmitted current output. Currently, high-voltage motor protection typically uses Geyao explosion-proof intelligent relays.
IV. Selection Principles for Motor Protectors
The purpose of selecting motor protectors is to ensure that the motor can fully utilize its overload capacity while preventing damage, and to improve the reliability of the electric drive system and the continuity of production. The rational selection of motor protectors can maximize the motor's overload capacity while preventing damage, thereby improving the reliability of the electric drive system and the continuity of production. Specific functional selection comprehensively considers factors such as the value of the motor itself, the load, the operating environment, the importance of the main motor equipment, and whether the motor's shutdown would seriously affect the production system, striving for economic rationality. The simplest protection device should be considered first when the protection requirements are met. Only when the simple protection device cannot meet the requirements, or when higher requirements are placed on the protection functions and characteristics, should a more complex protection device be considered, achieving a balance between economy and reliability.
Today, motor protectors have evolved into the era of microelectronic intelligence, and are becoming increasingly diversified. This necessitates that our staff fully consider the actual requirements of motor protection when selecting models, proactively and accurately diagnose motor faults, and rationally choose protection functions and methods to achieve effective motor protection, realize ideal equipment operational reliability, reduce unplanned shutdowns, and minimize accident losses.
Motor protectors provide comprehensive protection for motors, alarming or protecting them when overload, overcurrent, phase loss, locked rotor, short circuit, overvoltage, undervoltage, leakage, three-phase imbalance, overheating, bearing wear, or stator/rotor eccentricity occur. Compressor motor protectors widely used both domestically and internationally fall into two main categories: bimetallic and thermistor + electronic module types. Their structures and functions differ.
Currently, motor protectors have evolved from mechanical types to electronic and intelligent types, offering high sensitivity, high reliability, multiple functions, and convenient debugging. They can directly display parameters such as motor current, voltage, and temperature, and the type of fault after protection action is immediately apparent, greatly facilitating fault diagnosis and improving on-site fault handling and reducing production recovery time. Furthermore, the technology of detecting motor eccentricity based on the motor's air gap magnetic field makes online monitoring of motor wear possible. By displaying the changing trend of the motor's eccentricity value as a curve and recording the change over two years, bearing faults can be detected early, enabling timely intervention and preventing rotor rubbing accidents.
Classification (1) Bimetallic type
A bimetallic circuit breaker is essentially a switch or relay made of a bimetallic strip, widely used due to its low cost. The bimetallic strip consists of two layers of alloys with different coefficients of thermal expansion; the layer with the larger coefficient is called the active layer, and the layer with the smaller coefficient is called the passive layer. Because of the different coefficients of expansion, the bimetallic strip bends and deforms when the temperature rises, and returns to its original shape when the temperature drops. This phenomenon is used to create switches that can close or open at specified temperatures.
For compressor motors, when the winding temperature rises to a certain level (e.g., 110°C), the power supply needs to be disconnected in time to prevent burnout; and when the temperature drops to a certain level (e.g., 60°C), it can automatically reset, and the compressor resumes operation. This is the working principle of a bimetallic strip protector.
Bimetallic protectors can be divided into two types: thermal protectors and overload protectors. Thermal protectors do not generate heat themselves; the heat comes from the heat generated by the protected component. Overload protectors contain a heater (heating wire or heating plate). When the current is too high, the heating of the heater will cause the bimetallic strip to deform.
The thermal protector, resembling a pencil tip, is typically strapped or affixed to a location on the stator windings where the temperature is high. The winding temperature is transferred to the bimetallic strip through the metal casing. When the winding temperature exceeds the set temperature, the thermal protector trips, disconnecting the control circuit connected to it. This triggers the main circuit contactor to trip, stopping the compressor. The thermal response time of the thermal protector is a crucial parameter; it should typically activate within seconds of reaching the set temperature. During installation, ensuring good thermal contact is essential; otherwise, it will not activate promptly and will fail to provide adequate thermal protection.
Unlike thermal protectors, overload protectors contain one or more small electric heaters (heating wires or heating elements) connected in series in a single-phase or three-phase main circuit. When the motor experiences an overload, the current increases, the heater temperature rises rapidly, causing the bimetallic strip to deform, the contacts connecting to the main circuit to separate, and the compressor stops.
Overload protectors can also transfer heat through their casing, therefore they are also thermal protectors. Overload protectors are large and have a relatively slow thermal response. Furthermore, external overload protectors cannot be used as thermal protectors.
In terms of installation, each has its advantages and disadvantages. Thermal protectors are independent of the main circuit, thus having almost no limitation on motor current, but they need to be connected in series in the control circuit, resulting in complex wiring. Overload protectors are directly connected in series in the main circuit, requiring no additional wiring, making them simple and intuitive, but unsuitable for appliances with very high current, to avoid contact arcing or welding. Thermal protectors can effectively handle motor overheating, such as overheating caused by abnormal voltage, phase imbalance, or even phase loss; overheating caused by insufficient motor cooling (e.g., refrigerant leakage and low return gas pressure); overheating caused by high and low pressure gas leakage (damaged scroll, damaged piston rings, open pressure relief valve, etc.); overheating caused by poor lubrication; bearing seizure; or even stalling. Thermal protectors cannot effectively handle high current problems, therefore, overload protectors or current limiters are often required in the main circuit. Overload protectors react quickly to high currents. Common phenomena causing high currents include: power supply phase imbalance, phase loss (including phase loss caused by contactors), excessively high condensing pressure, bearing seizure due to poor lubrication, stalling due to connecting rod breakage or piston seizure, and stalling due to damage to the scroll or cross ring. Commercial compressors with a capacity of 15HP or less generally use overload protectors from brands such as Klixon.
(2) Composite type
Large compressors operate at very high currents, and when the overload protector engages, it can cause an electric arc, rendering the compressor unusable. A commonly used protection method is thermistor + electronic module protection. Several thermistors are arranged in series (or sometimes in parallel) within the three-phase windings and connected to corresponding terminals (such as S1, S2) of the electronic module. When the thermistor temperature reaches a certain critical temperature, its resistance will drastically increase from several hundred ohms at normal temperature to several thousand or even tens of thousands of ohms, triggering the control circuit (such as M1, M2) within the electronic module to disconnect, stopping the compressor. When the temperature drops back to the set value, the control circuit within the module automatically closes, and the motor resumes operation.
Thermistors are small in size and can be installed within the windings, exhibiting rapid thermal response. Furthermore, their low cost allows for the deployment of multiple thermistors, significantly increasing the monitoring range. In addition to monitoring the thermistor resistance, the electronic module also detects phase loss and phase sequence errors in the main circuit. For scroll compressors, screw compressors, and centrifugal compressors, phase sequence errors are a serious issue, and the module will automatically lock in place.
Large scroll compressors, multi-cylinder piston compressors, screw compressors, etc., generally use thermistor + electronic module thermal protection method.
(3) Intelligent type
The intelligent motor protector is designed for the protection and control of motors in 380/660V low-voltage distribution networks in industrial and mining enterprises. It has functions such as real-time measurement, protection, monitoring, display, and communication.
This protector utilizes advanced integrated circuit and microcomputer technology, employing a microprocessor chip as its core processing unit, resulting in fast operation. It boasts high reliability and comprehensive communication functions and analog transmission output capabilities. It provides multi-faceted and comprehensive protection for motors, offering reliable performance, ease of operation, and convenient installation and maintenance. Its protection range and sensitivity are higher than thermal relays, effectively preventing situations where the motor burns out but the thermal relay fails to trip, making it an ideal upgrade from thermal relays.
Operating environment temperature: -20°C to 55°C, relative humidity: <90%, in places free from corrosive gases, severe vibration, and impact.
Intelligent motor protectors are divided into conventional and pure protection types. In the conventional type, the K1 output of the relay device is a normally closed contact; when the protection action is activated, the contact point can be manually or automatically reset. It can directly replace a thermal relay. In the pure protection type, the K1 output is a normally open contact.
contradiction:
1. A protection scheme using normally open contacts of relays is used. Its characteristic is that the contacts are normally open when the main circuit is not working. Therefore, the contacts must be connected in series with the contactor's self-holding circuit. This makes installation inconvenient for users and prevents its use in automatic control circuits.
2. While protection schemes using the normally closed contacts of relays can have their control contacts directly connected in series with the control circuit, similar to thermal relays, their automatic reset mechanism makes them unsuitable for automatic control systems. This significantly limits the application range of electronic motor protectors. Firstly, replacing thermal relays requires modifying the control circuit; secondly, they cannot be used with equipment requiring automatic control, such as water pumps and compressors, which operate unattended for extended periods and have relatively high failure rates.
A motor protector is an electrical device used to protect electric motors. Its main function is to monitor the operating status of the motor and, in the event of abnormal conditions such as overload, short circuit, or grounding, to prevent damage to the motor by cutting off the power supply or issuing a warning signal, thereby ensuring the safe and stable operation of electrical equipment.
Motor protectors typically consist of basic protection functions such as overload protection, short-circuit protection, and grounding protection, as well as additional functions such as electrical isolation, operation control, and signal output. Overload protection monitors the motor's operating current; when the motor's operating current exceeds the rated current, the protector automatically cuts off the power supply to prevent damage to the motor due to overload. Short-circuit protection monitors the short-circuit current of the cable; when a cable is short-circuited, the protector automatically cuts off the power supply to prevent cable burnout and motor damage. Grounding protection monitors whether the electrical equipment is grounded; when the equipment is grounded, the protector also automatically cuts off the power supply to prevent electrical accidents.
Motor protectors are widely used in electrical equipment, especially in large motors and high-load equipment, where they play a crucial role in protecting not only the motor itself but also related equipment and personnel. With technological advancements, new types of motor protectors are constantly emerging, now possessing functions such as time delay adjustment and frequency response, making them an indispensable part of electrical control systems.
Its main working principle is as follows:
1. Overload protection principle: When the motor is running, if the load is too large, causing the motor current to exceed the rated current, the protector will measure the motor current to determine whether the motor is running under overload conditions, and cut off the circuit within a certain period of time to protect the motor from damage caused by overload operation.
2. Short-circuit protection principle: A short circuit in the motor circuit may trigger the short-circuit protection of the motor transformer. When a short circuit occurs in the motor circuit, the current absorbed by the motor will increase instantaneously. The protector will determine that it is a short-circuit fault by measuring the current and time, and then disconnect the circuit to protect the circuit safety.
3. Undercurrent protection principle: When the motor is running, if the motor current is abnormally low, such as when the motor malfunctions or the circuit is broken, the protector will measure the current and determine whether it is operating under the rated current. If the current is low, the protector will cut off the circuit to protect the safe operation of the motor.
4. Over-temperature protection principle: The protector can also determine whether the motor is overheating by measuring the motor temperature. If the motor temperature is too high and exceeds a certain safe range, the protector will cut off the circuit in time to protect the motor from damage.
In summary, motor protectors are important electrical protection devices. We need to understand their working principles in order to better use, debug, and maintain them, so as to ensure the normal operation and safety of the equipment.
Its main characteristics include the following aspects:
1. Overcurrent protection: The motor protector can detect the load current when the motor is running. Once the load current is too large, the motor protector will cut off the circuit in time to protect the motor and the entire electrical system from damage.
2. Overload protection: When the motor is running, if the rated current of the load is too high, the motor protector will cut off the circuit in time to avoid damage to the motor due to overload.
3. Short circuit protection: The motor protector can detect short circuits in the circuit. If a short circuit is detected, the motor protector will immediately disconnect the circuit to avoid danger and damage.
4. Overvoltage protection: The motor protector can detect overvoltage conditions in the circuit. Once the voltage is too high, the motor protector will immediately activate the protection mechanism to ensure the safe operation of the motor.
