Guo Miaogang, the first rolling mill of Changzhi Iron and Steel Group Co., Ltd., analyzed the causes of motor burn-out, and proposed overheat protection and over-current protection and phase-failure protection technology for phase asynchronous motors and requirements for motor protectors. Asynchronous motor protection technology Asynchronous phase-phase asynchronous motors have the advantages of simple structure, low price, reliable operation, and easy maintenance. They are the most widely used types of motors. 1 Causes of motor winding burn-out and protection measures For the reasons of phase-to-phase asynchronous motor stator winding burnout, general protection techniques include over-temperature protection, over-current protection and phase-failure protection. Ii Overheat Protection The view of the overheating part is that the fault caused by whatever reason will eventually cause the stator winding of the motor to overheat and burn. Therefore, as long as the motor winding is prevented from overheating, the motor is protected. But in fact, the motor itself has the difference in insulation heat-resistant grades. The maximum allowable temperature upgrade 105 0 120, 8 130, 155, 1 1800. In the same ambient temperature operating conditions of temperature rise, some motors will be damaged, and some will not be damaged. At the same time, the bearing damage caused by overheating of the motor and the poor ventilation of the rotor of the stator and rotor should be considered by the electrician on a regular basis, and patrol inspections must be discovered and solved, which is not the main discussion area of ​​the protection technology. In addition, the heating and cooling of the motor is a slow process of change. The installation of temperature rise monitoring and overheat protection devices for the motors of large and medium-sized important positions is a device that requires over-temperature alarms and then trips. It is not a good idea for a small motor to use an overheat protection device. Our factory's current 05,04, inside the rolling mill main motor windings are installed within the thermal resistance 00 as the motor's overheat protection detection element. 1.2 Overcurrent protection Over current protection is not necessary for motors with almost constant load. However, some motor loads often change. Over-capacity stalls often occur, causing the motor windings to burn. For Guo Miaogang, male. Born in 1964. He completed his career in electrical technology at Shanxi Radio and TV University in 2009 and is currently working as an electrical engineer. Motors affixed to this operating condition must be equipped with overcurrent protection devices. Although the phase asynchronous motor has strong overload capacity, it is necessary to protect the motor overload against inverse time characteristics. Over the years, our plant's power supply system for high-voltage motor transformers has adopted over-current protection with inverse time characteristics, which has played a good protective effect in our work. Low pressure control systems also use overcurrent protection. 1.3 Phase-failure protection The motor is damaged. Most of them are caused by the phase-failure operation. What kind of harm people will have to the motor in phase-failure operation and what kind of protection method should be adopted is appropriate. So far there is no more intention. For a long period of time, the common view is that a phase-failure operation will cause the motor windings to overheat and be damaged. Another point of view is that the phase-cut operation of the motor will result in the occurrence of a back electromotive force that is several times higher than the rated voltage at the ends of the phase-breaking winding at the moment of phase breaking, and it is very easy for the motor windings to break down during the turn to be damaged. Due to the different hazards to the motor caused by the open-phase operation, there are two different meanings when the motor is phase-faulted. 1 The view that the open phase will be the motor damage caused by overheating. To protect the motor from overheating or inverse time limit Characteristic protection, thus adopting the thermal relay scheme thermistor scheme Phase-to-phase over-current delay protection scheme The view that the phase-breaking causes the high-voltage back-EMF breakdown in the motor windings, adopts the instantaneous action protection scheme for the phase-break, so some electronic protection The advent of the device. To sum up, the danger that the back electromotive force generated several times higher than the rated voltage at the moment of the phase breaking moment to the motor is far greater than the damage caused by the overheating to the motor, and the fault of the phase failure cannot be automatically eliminated. The phase-failure protection should adopt instantaneous action protection instead of inverse time characteristic protection and overheat protection. The protector should take action sensitively, and there is no positive meaning for the delay of a few seconds delay after phase failure. 2 Requirements for Motor Protectors 2.1 Current Sampling This method can not only take into account the different characteristics of over-current and phase-failure protection, but also fully reflect the fact that no matter where the phase is broken, it must be reflected in the current of the power supply line, avoiding other The disadvantages and limitations of the sampling plan. 2.2 Select the sensitive electronic protection device scheme to ensure that the phase-failure starts and refuses closing, and the instantaneous tripping occurs when the phase is open. 2.3 Protection by Normally Closed Contact Action of Protector The protector is required to perform relay operation when the main circuit is not working and working normally, and it operates when the motor starts and fails during operation. This kind of protector is suitable for all kinds of automatic control switch interfaces, and has no limitations. 2.4 There is a wide range of current adaptation phase Asynchronous motor no-load current is about the rated current in the integrated protector should be from low to high 20 times the current allowable range. 2.5 Not using the power supply protector The power supply protector can be used for any occasion. At the same time, it can prevent the relays caused by the power failure of the protector from deactivating and will not provide protection. 2.6 In the protector, the phase-off and over-current should be separately controlled. This is to prevent mutual misunderstanding or misoperation. 2.7 Overheat protection should be installed inside the motor. When the motor overheats, it needs to be alarmed for a long time before it trips to protect the motor. 3 Conclusion The good application of motor protection technology can ensure the reliable operation of the power system, reduce the burning of the motor and improve the output and quality of the product. Reference text, edited by 1 Electric World Magazine. Electric world. Electric World Magazine Press 995.2, 3
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