1. The formation of shaft voltage
It refers to the voltage generated between the two ends of the motor shaft or between the rotating shaft and the bearing seat. The main reason for the shaft voltage is caused by the imbalance of the magnetic circuit around the motor shaft. This unbalanced magnetic flux cuts the shaft and induces shaft voltage at both ends of the shaft.
There are many factors that cause the unbalance of the magnetic circuit, such as the non-concentric rotor operation, the asymmetry of the stacking seams of the stator core fan-type punching, the different magnetic permeability of the silicon steel sheet due to the different orientation of the magnetic permeability, and the existence of the iron core slot and the ventilation hole. Unbalance and electrostatic charging caused by oil films with insulating properties in bearings, etc.
2. Formation and harm of shaft current
Conditions for the formation of shaft current: one is the shaft voltage, and the other is the formation of a loop. For motors with normal design and operating conditions, there is only a small difference in voltage across the shaft, and due to the treatment of oil film or bearing insulation, it is not enough to cause harm.
If there is a problem in a certain link, and the shaft voltage exceeds the limit, the original insulation may be broken down, and a circuit will be formed in the rotating shaft, the inner ring of the bearing, the outer ring of the bearing, and the bearing chamber, so the bearing position of the rotating shaft and the surface of the inner ring of the bearing will be damaged. Small and deep circular pits or strip arc scars are produced due to arc discharge, which are serious enough to burn out the journal and bearing bush.
Three, shaft current prevention measures
According to the principle of shaft current generation, there are generally the following measures to solve it:
1. Diversion: Install a grounding carbon brush at the shaft extension end, so that the grounding carbon brush can be reliably grounded and contacted with the rotating shaft to ensure that the potential of the rotating shaft is zero, and the electrostatic charge on the motor shaft can be directed to the ground at any time, so as to eliminate the shaft current. Such measures are generally used in generators, and are occasionally used in large AC motors.
2. Blocking: One of the important conditions for the formation of shaft current is to form a loop. It is a measure adopted by most motor manufacturers to install an insulating pad or use an insulating bearing between the bearing seat and the bearing at the non-shaft extension end to cut off the circuit of the shaft current.
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