What measures can be taken to avoid shaft current damage to motor bearing system?

As for the mechanism of shaft current generation, we have mentioned many times in the previous article that to solve the shaft current problem, it is necessary to conduct comprehensive disposal from the design and manufacturing links.

It is difficult to solve the shaft current problem from the symmetry of magnetic circuit for the large motor using the sector, and necessary measures need to be taken in the manufacturing and use of the motor. One way to solve the problem is to reduce the shaft voltage, the other is to cut off the loop, and the other is to create another loop through the drainage to avoid the shaft current from the bearing. Specific measures can be boiled down to content.

(1) Method of reducing shaft voltage. This method is more reflected in the design and manufacturing process control links, such as preventing the motor rotor eccentricity, reducing the degree of magnetic asymmetry, ensuring the symmetry of the stator core lamination, strengthening the excitation winding turn to turn short circuit fault monitoring measures.

(2) The method of cutting off the loop. We mentioned in the previous article that the attack object of shaft current is the bearing system, so we can take the necessary insulation measures for the components of the bearing system to cut off the current loop. More mature solutions include the use of insulated bearings, insulated end caps, or the addition of insulating coating on the bearing position of the shaft, which can achieve the purpose of cutting off the loop. Insulated bearings are mostly used in small size motors, while large size motors use more ways of insulating end caps. For the method of increasing the insulation coating on the bearing position, some motor manufacturers are in application, but when the bearing is replaced, it may cause damage to the insulation layer, which is a problem that must be paid attention to.

(3) The way to create another primary drainage. This is a commonly used method for large motor, that is, drainage by grounding carbon brush, but the carbon brush is a wearing part, and the maintenance of the use process is particularly important; At present, there is a kind of annular soft carbon brush that can be assembled at the shaft extension position, which can also solve the drainage problem better, but the price of this device is relatively expensive, which will cause a large increase in the cost of the motor.

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

(5) Add capacitive filter. This method is suitable for large generators, that is, capacitive filters are installed on the excitation winding of the static excitation generator to absorb some higher harmonics of the static excitation loop, so that the capacitive coupling effect between the excitation winding and the rotor body is weakened.