Li Fushu
At the company described in the source article, the slip-ring enclosure of the high-voltage wound-rotor asynchronous motor has an IP23 protection rating. Its inspection doors are often left open for cooling and inspection, allowing fine coal dust generated during mill operation to enter. High-speed friction between the slip rings and carbon brushes produces substantial carbon dust, which accumulates between them and reduces insulation. Poor brush-to-ring surface contact causes sparking. If coal dust inside the enclosure reaches an explosive concentration, personnel and equipment safety are endangered.
Directly replacing ordinary motors with explosion-proof motors is the best way to improve the protection level of the slip ring shell, but it is expensive and the coal mill environment is not a closed space. By modifying the slip ring shell to make the protection level reach IP67, the purpose of slip ring heat dissipation and convenient inspection can be achieved at a relatively small cost.
1 Increase exhaust facilities
Remove the slip-ring enclosure's inspection door and install one sealed, removable end cover at each of the top, bottom, left, right, and rear positions. Provide two transparent inspection windows in each left and right cover, using tempered glass of Ф90 mm × 20 mm, to facilitate routine inspections. The left and right covers can be removed for maintenance and carbon-brush replacement. The top and bottom covers carry the inlet and outlet air ducts for slip-ring cooling, while the rear cover provides access for slip-ring maintenance. Make the covers from 10 mm-thick Q235 steel plate, with Ra1.6 roughness on the contact surfaces between the plates and enclosure, and secure them with M8 bolts.
Figure 1 shows the modification of the slip ring housing of the main motor of the coal mill in the first phase.
Figure 1 Coal mill main motor slip ring protective cover to increase exhaust facilities
As shown in Figure 1, the inlet and outlet ducts are DN50. An air filter is fitted at the inlet, installed outside the coal-mill workshop in a coal-dust-free environment to minimize dust and debris entering the duct. Both ducts have valves to adjust airflow. The centrifugal fan has a flow rate of 504m3/h and a pressure of 860Pa; its motor uses 220VAC and is rated at 370W. It is also installed outside the workshop. With the coal-mill main motor running, the duct valves were opened and the fan started. The measured slip-ring enclosure temperature was 60℃, insufficient for the heat-dissipation needs of prolonged main-motor operation.
2 Replace the centrifugal fan
After modification, the slip ring protective cover failed to meet heat dissipation requirements. Insufficient exhaust capacity of the centrifugal fan was suspected, so it was replaced with a centrifugal fan rated at 850 m3/h airflow, 1020 Pa pressure, a 220 VAC motor supply, and 750 W power. Online testing showed a slip ring housing temperature of 55°C. Although lower, the improvement was unsatisfactory. During maintenance, the housing inspection door was opened and the brush holder temperature measured at 70°C, which was still very high. A layer of carbon dust had also accumulated inside, creating a risk of phase-to-phase slip ring short circuits when the dust became airborne during startup.
The reason may be that the resistance of the exhaust duct is too large, resulting in insufficient heat dissipation capacity. Improvements should be made from two aspects: increasing the exhaust capacity and reducing toner accumulation.
3 Replace the housing air duct
To reduce carbon dust accumulation, the centrifugal fan's exhaust direction was changed from the upper side of the slip-ring enclosure to the lower side. To reduce duct resistance, the centrifugal fan was moved from outdoors to an indoor position below the slip-ring enclosure. Because the system extracts air, there is no issue of coal dust entering the slip-ring interior. Measurements after the modification showed an enclosure temperature of 42°C, but a further measurement one day later showed that it had risen to 55°C, still failing to meet the cooling requirement. The inlet duct was therefore enlarged from DN50 to DN75, the outlet duct from DN50 to DN100, and the valve in the outlet duct was removed. Measurements over several days showed a temperature of only around 33°C, meeting the main motor's operating requirements. Figure 2 shows the modification.
Figure 2 Schematic diagram of the modification of the slip ring protective cover of the main motor of the coal mill
The same modification was applied to the slip-ring enclosure of the equally rated motor on the second-phase coal mill. The measured temperature was 63℃ because its main-motor load factor was 78.7%, compared with only 76.8% for the first-phase mill. To improve ventilation, the inlet duct was enlarged from DN50 to DN125 and the outlet from DN50 to DN100. The temperature then measured 46℃, meeting operating requirements. Other coal-mill high-voltage motor slip-ring enclosures were subsequently modified with satisfactory results.
Considering that the slip ring operates in a sealed environment, the maintenance cycle should be shortened accordingly. Therefore, the maintenance cycle is changed from once a month to once a week. The air filter screen is also cleaned once a week. So far, the equipment is operating normally. This time, a total of 3 pieces of equipment were renovated, costing 8250 yuan, which is both economical, practical, safe and reliable.
Source: "Cement"