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1. Causes and hazards of motor shaft voltage and bearing current
During the operation of the motor, any imbalance in the magnetic circuit of the stator and rotor or the phase current around the shaft will generate the flux linkage of the rotating system. As the shaft turns, these flux linkages create a potential difference across the shaft, known as the shaft voltage. The shaft voltage can excite the circulation in the loop (closed circuit) formed by the shaft and the casing through the bearings at both ends. This current is called the shaft current. like
In addition, there is a large amount of residual magnetism in the rotor core. For wound rotor motors, if two or more windings are shorted to the rotor core or shaft, shaft voltage and shaft current will also be generated.
The size of the bearing current is related to factors such as motor structure, motor power, driving voltage amplitude, pulse rise time, and cable length. The greater the power of the motor, the higher the driving voltage, the steeper the rising edge of the driving voltage, the shorter the cable, and the greater the carrying current.
2. Measures to block shaft current - use insulated bearings
In order to avoid the shaft current from burning the bearing, effective measures should be taken to isolate the shaft current.
For large motors with separate bearing housings at both ends, spacers made of insulating material can be placed between the bearing housing and the metal base. For ordinary motors in which the bearing and the casing are integrated, an insulating bearing is generally used at one end (often arranged at the end of the non-main shaft extension). For occasions with high requirements, insulating bearings are installed at both ends. The insulated bearings used are generally the method of adding (generally coating) an insulating layer to the outer ring, and in some occasions, the method of adding an insulating layer to both the inner and outer rings
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