Introduction

With the expansion of the application range of three-phase asynchronous motors and the development of insulation technology, the working environment of motors is becoming increasingly harsh, such as humidity, high temperature, dust, corrosion, etc.; at the same time, the design of motors requires an increase in output and a reduction in size and weight, resulting in a smaller heat capacity and weaker overload capacity of the motor. On the other hand, with the improvement of the degree of automation in production, motors are required to run under multiple operating conditions such as frequent starting, braking, reversing, and variable loads. Therefore, higher requirements are placed on the protection, monitoring, and maintenance of motors.

Principles of Selecting Protection Devices for Three-Phase Induction Motors

The protection of motors involves monitoring the temperature of the motor windings or bearings and alerting the equipment and maintenance personnel to take necessary measures before the temperature of the windings or bearings rises to the point of failure, thus preventing accidents. The principles of selecting protection devices are: first consider simple protection devices while meeting the protection requirements; when simple protection devices cannot meet the requirements or when higher requirements are placed on the protection function and characteristics, consider applying complex protection devices to achieve a balance between economy and reliability. The reasonable selection of motor protection devices allows the motor to fully demonstrate its overload capacity while running safely, improving the reliability and continuity of the power traction system.

Classification of Temperature Protection for Motors

Motor temperature protection can be divided into winding temperature protection and bearing temperature protection. Winding temperature protection is to control the system and issue an alarm before the insulation of the rotor windings deteriorates due to overcurrent, undercurrent, phase failure, short circuit, etc. Bearing temperature protection is to directly measure the temperature of the bearings and protect the motor by controlling the bearing temperature.

Indirect Protection of Motor Winding Temperature

Traditional motor indirect protection devices include circuit breakers, thermal relays, current protectors, etc. Circuit breakers are the earliest and simplest protection devices used mainly to protect the power supply line during short circuits and reduce the scope of the fault. However, circuit breakers do not provide protection against overload and single-phase, and they can easily cause damage to the motor due to single-phase operation, so they are often used in conjunction with other overload and single-phase protection devices.

Thermal relays are the most commonly used motor overload protection devices, mainly used in ordinary small capacity AC motors with good working conditions. However, thermal relays have single functions, low sensitivity, large errors, and poor stability, which can easily cause unreliable motor protection and are not recommended for use.

Current protectors that are currently being promoted include electronic current protectors and intelligent current protectors. Electronic current protectors use potentiometer knobs or code switches for operation, and the circuit is generally analog. They detect the three-phase current values and use reverse time-limiting or time-limiting working characteristics to provide protection against overload, phase failure, stalling, etc. Fault types are displayed using indicator lights, and running electrical quantities are displayed using digital displays. Intelligent current protectors use microcontrollers for control, digital current settings, and detection of three-phase current values through operation buttons on the control panel. Users can modify various parameters on site according to their actual usage and protection requirements, and display windows are provided using digital displays or large screen LCD displays. They support multiple communication protocols and are used in important scenarios to integrate protection, measurement, communication, and display, realizing intelligent integrated protection control for motors.

Direct Protection of Motor Winding Temperature

Direct protection involves directly measuring the temperature of the motor windings and protecting the motor by controlling the winding temperature. The structure involves pre-embedding temperature sensing components in the three-phase windings of the motor, which are generally positive temperature coefficient (PTC) thermistors. By utilizing their characteristic of a sudden increase in resistance at a certain temperature, they are used in conjunction with GRB motor overheating protectors. When the temperature of the motor windings approaches the insulation temperature, the resistance of the PTC thermistor changes dramatically, transmitting to the control line, causing the thermal relay to act and disconnect the motor circuit, achieving the purpose of protecting the windings. Thermistors are selected according to the insulation level of the motor.

For larger capacity motors, to avoid damage to the motor windings due to the hysteresis of the temperature sensing components during stalling, direct protection is often used in conjunction with current monitoring type protection devices. Magnetic field temperature detectors involve embedding magnetic field detection coils and temperature probes in the motor, and protection is provided based on changes in the internal rotating magnetic field and temperature of the motor. The main functions include overload, stalling, phase failure, overheating protection, and wear monitoring. However, the disadvantage is that magnetic field detection coils and temperature sensors need to be installed inside the motor.

Protection of Motor Bearing Temperature

Motor bearing temperature protection involves drilling holes in the motor end cover bearing chamber or inside and outside the small cover, and installing bearing temperature sensing components PT100 to directly measure the bearing outer wall temperature, and to monitor the bearing temperature online at all times. When the bearing temperature of the motor reaches the specified value, the protection device issues an alarm or disconnects the main circuit through the control circuit, causing the motor to stop working and achieving protection.

Maintenance and Care of Three-Phase Induction Motors

In addition to taking necessary technical protection measures during the operation of the motor to avoid burning out, daily maintenance and care are also crucial. During normal operation, the following work should be done:

1. Keep the motor clean and ensure that the motor has sufficient insulation resistance and good ventilation and cooling conditions, allowing the motor to operate safely and stably for a long time.
2. Ensure that the starting device is working properly and that the electrical control cabinet is located in a dry, well-ventilated, and easily accessible location. Regularly clean it to prevent dust accumulation. Often check whether the contact points of the contactors, the iron cores of the coils, and various wiring screws are reliable, and whether the mechanical parts are flexible. Keep them in good condition to ensure smooth starting of the motor.
3. Ensure that the motor operates under rated current. The main reasons for overload operation of the motor are that the load being driven is too large, the voltage is too low, or the mechanical part being driven is jammed. During operation, pay attention to frequently checking whether the transmission device is running smoothly and reliably, whether the concentricity of the couplings is standard, and whether the gear transmission is flexible. If there is any jamming, stop the machine immediately, find out the cause, eliminate the fault, and then run it again.
4. Often check whether the three-phase current of the motor is balanced. Under the condition that the three-phase voltage of the three-phase asynchronous motor is balanced, the difference between any one phase current and the average value of the three-phase current should not exceed 5%. If it exceeds this, it indicates that the motor has a fault and the cause must be found and eliminated.
5. Regularly check the temperature of the motor. Often check the temperature of the bearings, stator, and outer casing of the motor. Pay particular attention to the temperature rise of motors without voltage, current, and frequency monitoring and no overload protection.
6. Observe whether there is any vibration, noise, and abnormal smell in the motor. During operation, especially for large capacity motors, it is necessary to frequently check whether the bolts of the motor base, motor cover, and bearing cover are loose, whether the grounding device is reliable, and whether there is any sound and abnormal smell. If there are any abnormalities, stop the machine immediately, find out the cause, eliminate the fault, and then run it again.

Conclusion

With the progress of technology and the improvement of motor traction control technology, motor protectors are developing towards microelectronics, intelligence, and diversification. When choosing, fully consider the actual needs of motor protection, reasonably select protection functions and protection methods, and realize good protection of the motor. Do a good job in the maintenance and maintenance of the equipment, monitor and inspect important equipment, and achieve the goal of improving the reliability of equipment operation, reducing unplanned downtime, and reducing accident losses.