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1 Causes and hazards of bearing current
In an induction motor, the bearing current of the motor is always present. Driven by a sine wave power supply, due to the deviation of the size of the stator and rotor cogging of the motor, the change of the orientation properties of the magnetic material, or the three-phase imbalance of the power supply, the magnetic flux of the motor will be unbalanced, and the shaft voltage will be generated on the rotor shaft. bearing current. This kind of bearing voltage amplitude is small, and it is not harmful. Under the drive of the frequency converter, the damage of the motor bearing current is greatly increased due to the different generation principles. General-purpose inverters mostly use PWM modulation, and inverter circuits use high-frequency power components to obtain approximately sinusoidal voltage waveforms on the motor. The synthetic vector of the fundamental wave component of the three-phase voltage is zero, but in fact, the sum of the three-phase voltage vector is not zero at every instant, and the three-phase voltage is unbalanced. The amplitude of the synthesized common mode voltage is equal to the DC side voltage of the inverter, and the frequency is equal to the switching frequency of the inverter.
2 Path of common mode current
Due to electrostatic coupling, there are large or small distributed capacitances between various parts of the motor, which constitute the discharge path of the common mode current on the motor stator. Most of the common mode current passes through the path of stator-chassis-ground-inverter shell, and a small part passes through the path of stator-rotor-shaft-bearing-chassis-ground-inverter shell. When the impedance between the grounding of the inverter and the motor casing is higher than the impedance between the inverter casing and the load, a shaft extension current will also be generated. The discharge of the ground-inverter shell circuit will not only damage the bearing at the load end of the motor, but also damage the load bearing and coupling. The latter two common-mode currents flow through the motor bearing, causing harm and even more harm in the second way.
3 Common bearing current suppression measures
There are many ways to suppress the bearing current, such as adding a sine wave filter on the output side of the inverter to improve the output current waveform; insulating the bearing at one end of the motor to cut off the circulating current flowing through the motor shaft and bearing; on the motor output shaft Install grounding carbon brushes, suppress the shaft extension current that is grounded by the load, etc. These various ways can inhibit the corresponding.
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