Cause analysis and Improvement Countermeasures of

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Cause analysis and Improvement Countermeasures of frequent damage of synchronous motor

large high voltage synchronous motor is widely used in all walks of life because of its series of advantages, especially its ability to send reactive power to electricity and improve power quality. The synchronous motor for ball mill of our company has been damaged frequently for a period of time, which directly affects the production and safe operation of equipment of our company. Therefore, it is our top priority to correctly analyze and judge the fault causes of synchronous motor and put forward corresponding countermeasures

I. accident signs

our company has 16 1300kw/6kv synchronous motors. Before 2000, on average, there were 2-3 motor burning accidents every year. The main signs of the accident are: the binding wire at the end of the stator winding is broken, the stator winding of the motor is overheated, the cage bar of the starting winding is welded and broken, the abnormal sound occurs during the starting and operation of the motor, and the starting fails frequently

in particular, on January 12, 1999, our company's 7# synchronous motor suddenly fired during its operation, causing local serious burn out of the stator coil of the 7# synchronous motor, burning of the high-voltage cable joint, and collapse of the current transformer. Due to the failure of the 7# synchronous machine tripping device, the protection could not operate normally, and the continuous high current caused the protection action tripping of the dense 27 line II in the dense earth substation, affecting the shutdown of other electrical equipment in the selected II substation

II. Basic judgment and analysis of accident causes

1. Motor quality analysis:

the normal service life of the motor should generally be about 20 years. According to the statistics, the operating life of the synchronous motor damaged by our company is mostly less than 10 years, especially after the overhaul of this 7# synchronous motor, this blasting and burning accident occurred only 4 months after it was put into operation

in the accident analysis, some electrical technicians attributed the main cause of the accident to the overhaul of the motor. This large-scale motor damage accident can be attributed to the quality of the motor, which I object to. It is suggested to turn our attention to the analysis of excitation system; Facts have proved that the motor repair shop has taken various strengthening measures for its key parts in the motor repair, and even improved the insulation grade, but the effect is not significant. Damage accidents continue to occur

2. Cause analysis of excitation system:

in view of many phenomena such as abnormal sound, overheating of motor stator winding, open welding and fracture of starting winding cage bar during the starting and operation of synchronous motor, it is necessary to make a reasonable analysis of the current excitation system under the condition that the accident caused by motor quality is excluded, Thus, the real reason for the frequent damage of the motor is found: the design of the brittle transition temperature excitation system of the impact tester is unreasonable

III. The internal relationship between the main problems of the excitation system and the causes of motor failure

1. The design of the starting circuit of the excitation device is unreasonable, so that the synchronous motor is often started in the case of pulse vibration

the original main circuit is a bridge half control excitation device, and its schematic diagram is shown in Figure 1

during the starting process of the motor, an alternating potential will be induced in the rotor coil, and its positive half wave will form a loop through ZQ, generating +if; The negative half wave forms a loop through KQ and RF to generate -if. Because the load circuit is asymmetric, the current of +if and -if is asymmetric. The if curve is shown in Figure 2. Therefore, the motor stator current also produces strong pulsation, and its curve is shown in Figure 3. Therefore, the motor suffers from strong vibration of pulsating torque. The strong vibration sound produced by the motor during starting can be heard in the hall of the whole plant. This sound continued until the motor was started, and China's chemical fiber output was 49.43 million tons

on the other hand, because the device adopts KGLF-11 old-fashioned excitation, the R & D investment of Chinese manufacturing enterprises is generally low, and the device analog control, its excitation detection element is aging and the detection is inaccurate, resulting in the change of excitation time, which has a great impact on the motor startup. As the slip decreases during the starting process of the motor, the induced potential in the rotor coil gradually decreases. When the speed reaches more than 50%, the negative half wave path of the induced current in the excitation circuit is blocked, which will be in a state of on-off and on-off, forming an asymmetry between the +if and -if currents, thus forming a pulsating torque, causing strong vibration of the motor. Sometimes in operation, affected by the performance of discrete components of the deexcitation plug-in, the deexcitation thyristor KQ is misguided, and the deexcitation resistance is heated, red and smoke. It only has a high on voltage. When the motor is started, especially when the rotor induced voltage is low, KQ cannot be reliably connected, resulting in asymmetric starting torque of the main engine and strong vibration of the unit. This is one of the main accident signs mentioned above

therefore, no matter how excellent the quality of the motor is, it is conceivable that the frequent starting of the motor under such bad conditions will cause damage to the motor. Therefore, the service life of the motor is greatly reduced

2. Unreasonable design of excitation link often leads to startup failure, and the number of repeated starts is greatly increased

the original design of the excitation link is: according to the rotor slip polarity of synchronous motor, the contactless excitation link works, as shown in Figure 4

because the control plug-in uses analog components, component aging and temperature drift, as well as the introduction of the "1035" plan, make the new material industry usher in an important development opportunity, and the anti-interference ability is weak, resulting in inaccurate detection of rotor induced voltage. This is mainly due to the unstable performance of the voltage regulator 12wy and triode 3bg that detect the induced signal, and the uncertainty of the charging and discharging time of capacitor 5C; When the synchronizer enters subsynchronization, the excitation does not send a signal when it is triggered, which often leads to the startup failure of the synchronizer. This is a common fault of the analog excitation device, resulting in repeated startup of the synchronous machine, which will cause damage to the motor

3. The excitation device has no reliable out of step protection device, which makes the motor operation unreliable

the original excitation device of synchronous motor adopts inverse time relay

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