A three-phase induction motor is an AC motor in which the rotating magnetic field formed by the stator winding interacts with the magnetic field of the induced current in the rotor winding to generate electromagnetic torque to drive the rotor to rotate. It is a kind of induction motor.
Working principle of three-phase induction motor Three-phase induction motors have three-phase symmetrical windings embedded in the stator slots, and guide bars are placed in the rotor slots. The two ends of all the guide bars are respectively short-circuited with two short-circuit rings to form a closed rotor winding. If the rotor speed is zero at the beginning, the rotor bar cutting the rotating magnetic field will generate induced electromotive force e2. Since the rotor bar is short-circuited, a short-circuit current i2 will appear in the rotor conductor. According to the right-hand rule: the direction of the induced electromotive force in the rotor conductor under the N pole and the direction of the active component of the current enter the paper; under the S pole, it flows out of the paper. The rotor current i2 interacts with the air gap magnetic field to generate electromagnetic force, the direction of which is determined by the left-hand rule. The torque produced by electromagnetic force is called electromagnetic torque. Under the action of electromagnetic torque, the rotor will rotate in the direction of the rotating magnetic field. Since the electromotive force and current on the rotor side are generated by electromagnetic induction, this kind of motor is called an induction motor. In addition, if the rotor speed is the same as the synchronous speed of the rotating magnetic field, there will be no induced electromotive force in the rotor bar, and there will be no electromagnetic force and electromagnetic torque. Therefore, when the induction motor is running normally, between the rotor speed and the rotating magnetic field speed There are always differences, so induction motors are also called asynchronous motors.
Common sense of motor protection:
1. Motors are easier to burn out than in the past: Due to the continuous development of insulation technology, the design of motors requires both increased output and reduced size, so that the heat capacity of the new motor is getting smaller and the overload capacity is getting weaker; Due to the increase in the degree of production automation, the motors are required to run frequently in various ways such as frequent starting, braking, forward and reverse rotation, and variable load, which puts forward higher requirements for motor protection devices. In addition, the motor has a wider range of applications, often working in extremely harsh environments, such as humidity, high temperature, dust, and corrosion. All of these make the motor more prone to damage, especially the highest frequency of faults such as overload, short circuit, phase loss, and bore sweeping.
2. The protection effect of the traditional protection device is not ideal: the traditional motor protection device is mainly thermal relay, but the thermal relay has low sensitivity, large error, poor stability and unreliable protection. The fact is also true. Although many devices are equipped with thermal relays, the phenomenon of damage to the motor that affects normal production is still widespread.
The principle of protector selection: reasonable selection of motor protection devices to realize the full use of the motor's overload capacity and avoid damage, thereby improving the reliability of the electric drive system and the continuity of production. The specific function selection should comprehensively consider the value of the motor itself, the load type, the use environment, the importance of the main equipment of the motor, whether the motor exits operation will cause a serious impact on the production system and other factors, and strive to be economically reasonable.
4. The ideal motor protector: The ideal motor protector is not the most functional, nor the so-called most advanced, but should meet the actual needs of the site, achieve the unity of economy and reliability, and have a higher cost performance. Choose the type and function of the protector reasonably according to the actual situation of the site, and consider the simple and convenient installation, adjustment and use of the protector, and more importantly, choose the high-quality protector.
Selection of protector
Basic principles of selection:
There is no unified standard for motor protection products on the market, and there are various models and specifications. In order to meet the different needs of users, manufacturers derive many series of products, which have a wide variety of products, which brings a lot of inconvenience to the selection of users; users should fully consider the actual needs of motor protection when selecting models, and choose protection functions and methods reasonably. In order to achieve a good protection effect, achieve the purpose of improving the reliability of equipment operation, reducing unplanned shutdowns, and reducing accident losses.
The basic method of selection:
1. Conditions related to selection
1) Motor parameters: You must first understand the motor specifications, functional characteristics, protection type, rated voltage, rated current, rated power, power frequency, insulation class, etc. These contents can basically provide a reference basis for the user to correctly choose the protector.
2) Environmental conditions: Mainly refer to room temperature, high temperature, high cold, corrosion, vibration, wind and sand, altitude, electromagnetic pollution, etc.
3) Motor use: Mainly refers to the required characteristics of the driving mechanical equipment, such as fans, water pumps, air compressors, lathes, oil field pumping units and other mechanical characteristics of different loads.
4) Control mode: The control modes include manual, automatic, local control, remote control, stand-alone independent operation, and centralized control of production lines. Start-up methods include direct, step-down, star angle, frequency sensitive rheostat, frequency converter, soft start, etc.
5) Other aspects: the user's monitoring and management of on-site production, the severity of the impact of abnormal shutdowns on production, etc.
There are many factors related to the selection of the protector, such as installation location, power supply situation, power distribution system situation, etc.; also consider whether to configure protection for newly purchased motors, upgrade motor protection, or improve protection for accidental motors, etc. ; Also consider the difficulty of changing the motor protection mode and the degree of impact on production; it is necessary to comprehensively consider the selection and adjustment of the protector according to the actual working conditions on site.
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