Introduction

A company’s micro powder production line uses the main motor of 10kV and 4500kW vertical mill. After winter maintenance, it is found that the temperature of the bearing bush after the main motor increases significantly. When the ambient temperature is 8 ~ 10℃, the temperature of the bearing bush is measured to be 70℃ and the vibration is 3 ~ 4mm/s, which seriously affects the output of the station and the normal operation of the equipment. For this purpose, several demolition inspections were carried out.

01 Disassembly and inspection analysis for three times

(1) The first dismantling inspection. After the first fault was found, because the specific reasons were not sure, the first consideration was that the lubricating oil had a problem, and then the sample inspection was carried out. It was found that the oil pollution level reached 12, and there were metal particles in it. Therefore, all the lubricating oil was replaced, and a series of optimization and treatment were carried out on the filter screen of the oil station. After disassembling and inspecting the bearing bush, it is found that the shaft damage is striped, as shown in Figure 1. Contact the technician of the manufacturer to scrape the tile.

Fig. 1 Striped axial damage

(2) The second demolition and inspection. After the second failure, contact the manufacturer’s technical personnel to check, found that the problem is serious, the shaft and the shaft bush damage significantly increased, the technical personnel suspected that the base or foundation is uneven, so the main motor alignment. In order to judge whether the fault was caused by uneven foundation and because the construction period was tight and difficult, the main motor (#2) of the vertical mill in another production line was temporarily replaced with this main motor (#1). It was found that the main motor (#2) was normal while the main motor (#1) still had problems, and the basic problems were eliminated. So I decided to return the motor to the factory for maintenance.

(3) The third demolition and inspection. After the main motor was returned to the factory for maintenance, it was operated with load for about 3 days, and the problem appeared again. The data measured by the point inspection instrument are shown in Table 1.

Table 1 point inspection vibration data

Analyze the failure of electrical cause rather than mechanical cause. With this idea, I began to collect technical materials and asked relevant experts. The reason was gradually established as the breakdown of oil film caused by axial voltage or axial current, and I decided to measure it first.

Troubleshooting by shaft voltage and shaft current
Shaft voltage refers to the magnetic circuit imbalance around the motor shaft caused by the imbalance of the magnetic flux cutting shaft, shaft voltage induced at both ends of the shaft (the voltage between the two ends of the shaft or between the shaft and the bearing seat). Shaft voltage is harmful, it can form a circuit through the generator shaft, journal, bearing bracket, unit base, breakdown of the oil film discharge between the shafts and bearing. Although the shaft voltage is not high, but because the circuit resistance is very small, the shaft current generated is very large, which will make the lubricating and cooling oil gradually deteriorate. In serious cases, the bearing bush will burn out, causing forced shutdown.

The multimeter was used to measure the voltage at the shaft end of the motor, and the voltage difference between the two ends of the rotor of the main motor was about 0.8V, which belonged to the normal range, indicating that there was no obvious abnormal discharge phenomenon between the stator and rotor of the main motor. Winner motor back axle measurement with the earth, the voltage of about 0.8 V between the main motor bearing shell after insulation board with the earth voltage of 0.09 V, repeated measurement confirmation, think back axle bearing surface insulation to ground voltage is greater than after voltage, this means that after the rear axle and bearing insulation board between voltage of about 0.8 ~ 0.09 V material 0.71 V, the voltage is the voltage on the oil film bearing, this voltage is sufficient to produce a large current, lead to the oil film breakdown. According to the test results of the oil, it is indeed the shaft current that causes the deterioration of the oil, which fully shows that the insulation of the main electric machine against the shaft current is very low.

According to the access to the data, a detailed inspection, including the rear axle housings of insulation, insulation oil conveying pipeline insulation, bearing shell temperature measuring instrument, found that the main motor front axle warm directly grounding resistance measuring, the original main motor overhaul in winter, once after the replacement bearing shell temperature resistance measurement, fixed with screw thread USES is the original resistance insulation rubber thread, and the replacement, because of the lack of related knowledge, change with thermal resistance of the shell of the same material metal thread (see figure 2).

Fig. 2 Resistance thread before and after replacement

When the fault point is found, replace it immediately, and re-scrape the bearing bush and filter the lubricating oil. After reinstallation, no-load test of the motor was conducted, and U1≈0.8V, U2≈0.8V, and U3≈0.76V were measured, which met the operating conditions.

It has been running for more than half a month since the last fault treatment, and the overall operating indicators are normal, as shown in Table 2. The temperature of the front and rear axles is at 55℃, and the vibration is about 1.5mm/s, and the fault has been completely eliminated.

Table 2 Spot inspection vibration data

03 Conclusions

The normal operation of the equipment is the result of the joint action of electromechanical and process. It is necessary to maintain a strict and careful attitude for the maintenance of the equipment. This fault is due to the careless maintenance work, which has caused repeated disassembly and inspection of the bearing bush and the main motor, resulting in a huge waste of manpower and material resources. Equipment failure, especially electrical failure, cannot be seen or touched, there may be a lot of non-standard places behind the normal surface, and these irregularities will gradually become hidden dangers, until the failure. I hope the process of this case can provide reference for the personnel in the industry.

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