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Why does the inverter damage the motor bearings?

2020-03-20

Many friends have discovered that the inverter damages the motor。 For example, in a pump factory, in the past two years, his users frequently reported that the pump was damaged during the warranty period。 In the past, the product quality of this pump factory was very reliable。 After investigation, it was found that these damaged pumps were driven by frequency converters。


Inverter damage to the motor includes two aspects, damage to the stator winding and damage to the bearing. This kind of damage generally occurs within a few weeks to ten months. The specific time and the brand of the inverter, the brand of the motor, the power of the motor, the carrier frequency of the inverter, the cable length between the inverter and the motor, the ambient temperature, etc. Many factors are related.


The following introduces the mechanism of inverter damage to motor bearings:


OPE电竞竞猜娱乐The reason that the inverter damages the motor bearing is that there is a current flowing through the bearing, and this current is in a state of intermittent communication。 The intermittently connected circuit will generate an arc, which will burn the bearing。


There are two main reasons for the current flowing in the bearings of AC motors。 First, the induced voltage caused by the internal electromagnetic field imbalance, and second, the high-frequency current path caused by stray capacitance。


The magnetic field inside an ideal AC induction motor is symmetrical. When the currents in the three-phase windings are equal and the phase difference is 120 °, no voltage will be induced on the motor shaft. When the PWM voltage output from the inverter causes the asymmetry of the magnetic field inside the motor, a voltage will be induced on the shaft, and the voltage amplitude is 10 ~ 30V, which is related to the driving voltage. high. When the value of this voltage exceeds the insulation strength of the lubricant in the bearing, a current path is formed. During the rotation of the shaft, at a certain time, the insulation of the lubricating oil blocked the current. This process is similar to the on-off process of a mechanical switch. In this process, an arc is generated, which burns off the surface of the shaft, ball, and bowl, forming a pit. If there is no external vibration, the small pits will not have an excessive effect, but if there is external vibration, a groove will be generated, which has a great impact on the operation of the motor.


In addition, experiments show that the voltage on the shaft is also related to the fundamental frequency of the output voltage of the inverter. The lower the fundamental frequency, the higher the voltage on the shaft, and the more serious the bearing damage.


In the early stage of motor operation, when the temperature of the lubricating oil is low, the current range is 5-200mA, and such a small current will not cause any damage to the bearing。 However, when the motor is running for a period of time, the peak current will reach 5-10A as the temperature of the lubricating oil rises, which will cause flashovers and form small pits on the surface of the bearing components。


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