1. Causes and harm of motor shaft voltage and bearing current formation
In the operation of the motor, any imbalance in the phase currents in or around the rotor’s magnetic circuit can produce a rotating system flux linkage.
When the shaft rotates, these flux chains can generate a potential difference across the shaft. This potential difference is called the shaft voltage. The shaft voltage can excite a circulating current through a loop formed by the two ends of the bearing in the shaft and the casing (closed circuit), which is called the shaft current.
In addition, the rotor core has more remanence, For a wound rotor motor, shaft voltage and shaft current are generated if two or more windings are short-circuited to the rotor core or the shaft. the bearing current is related to the structure of the motor, the power of the motor, the magnitude of the drive voltage, the rise time of the pulse, the length of the cable, etc. The larger the power of the motor, the higher the driving voltage, the steeper the rising edge of the driving voltage, and the shorter the cable, the larger the bearing current.
2. How to block shaft current – use electrically insulated bearings
In order to avoid damage to the bearing caused by the shaft current, effective measures should be taken to isolate the shaft current. For large motors with independent bearing housings at both ends, spacers made of insulating material can be placed between the bearing housing and the metal base. For a motor in which an ordinary bearing and a casing are integrated, an Electrically insulated bearing is generally used at one end thereof (often arranged at the non-spindle end).
For high requirements, insulated bearings are installed at both ends. The insulating bearing used is generally a method of attaching (generally coating) an insulating layer to the outer ring. In some cases, the inner and outer rings are also provided with an insulating layer.
Electrically insulated bearings avoid damage caused by electrical corrosion, so they are more reliable in operation than in conventional bearings. And compared to other insulation methods, such as shaft or housing insulation, etc. More cost effective and reliable. Insulated bearings have the same dimensions and basic technical features as non-insulated bearings Suitable for motors, generators, especially variable frequency motors.
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