Q: What measures can be taken to prevent shaft current from damaging the motor bearing system?

A: To solve the shaft current problem, comprehensive measures need to be taken from the design and manufacturing stages.

For large motors using fan-shaped pieces, it is difficult to solve the shaft current problem from the symmetry of the magnetic circuit. Necessary measures need to be taken in the manufacturing and use stages of the motor. For example, reduce the shaft voltage, cut off the circuit, or create another circuit by diversion to prevent the shaft current from passing through the bearing.

The specific measures can be summarized as follows:

(1) Methods to reduce shaft voltage. This method is more reflected in the design and manufacturing process control stages, such as preventing the eccentricity of the motor stator and rotor, reducing the degree of magnetic circuit asymmetry, ensuring the symmetry of the stator core laminations, and strengthening the monitoring of short-circuit faults between the turns of the excitation winding.

(2) we can take necessary insulation measures for the components of the bearing system to cut off the current circuit. The more mature solutions include the use of insulating bearings, insulating end covers, or adding insulating coatings to the shaft bearings, which can achieve the purpose of cutting off the circuit. Insulated bearings are mostly used in small-sized motors, while large-sized motors mostly use insulating end covers. Some motor manufacturers are applying the method of adding insulating coatings to the bearings, but when the bearings are replaced, the insulating layer may be damaged, which is an issue that must be paid attention to.

(3) Create another way to drain the current. This is a common method for large-sized motors, that is, to drain the current through a grounding carbon brush, but the carbon brush is a consumable part, and maintenance during use is particularly important; there is currently a kind of annular soft carbon brush that can be installed at the shaft extension position, which can also solve the drainage problem well, but the price of this device is relatively expensive, which will cause a large increase in the cost of the motor.

(4) Choose a better quality inverter. For variable frequency motors, the probability of shaft current is higher. In addition to the measures taken by the motor itself, it is recommended to choose a better quality and more stable inverter to control from the power input link.

(5) Add a capacitor filter. This method is suitable for large generators, that is, to install a capacitor filter on the excitation winding of the static excitation generator to absorb some high-order harmonics of the static excitation circuit, so that the capacitive coupling effect between the excitation winding and the rotor body is weakened.