Abstract: This article introduces the design points of a large capacity 2P explosion-proof high-speed self-lubricating three-phase asynchronous motor, starting from controlling the heating of the motor bearing pads and improving the heat dissipation ability. It focuses on the measures taken to control the stator temperature rise and strengthen the heat dissipation ability of the bearing pads.
Keywords: flameproof motor; High speed; Self lubrication; Bearing shell
0 Introduction
Due to the fact that most of the oil transportation pipelines are located in the wilderness, the power supply conditions and control protection of the motor supporting the oil transportation pump are limited by the environment. Generally, explosion-proof high-speed self-lubricating motors are used, which eliminates the need for oil stations and their control systems, resulting in low investment and fewer fault links. However, due to the fact that the supporting motor of the conveying pump is usually a high-speed motor with a speed of 3000 rpm, when the motor capacity is greater than 2000kW or above, the heat generated by the motor bearing pad lubrication increases the difficulty of cooling by relying on its own heat dissipation. This article analyzes the key points in the development of large self-lubricating explosion-proof motors.
At present, the two largest capacity explosion-proof sliding bearing motors in China all use forced lubrication because the sliding bearing and shaft are in surface contact. When the motor rotates, the friction between the shaft and the bearing pad generates a large amount of heat, most of which is taken away by the flowing lubricating oil (that is, the flowing lubricating oil not only serves as lubrication but also as cooling). Self lubricating motors rely solely on their own heat dissipation, which can easily cause the bearing pad temperature to be too high, Therefore, controlling the temperature of the bearing pads is the primary key technology of this motor.
According to the AC motor design manual, it can be seen that the key to the heating of high-speed self-lubricating motor bearings lies in how to reduce the heat generated by the bearings and how to dissipate the heat as soon as possible. This article starts from these two points and combines the YB800-2 2500kW explosion-proof high-speed self-lubricating three-phase asynchronous motor developed for oil transportation pipelines to analyze and demonstrate.
Determination of electromagnetic design scheme
1.1 Basic parameters of electric motors
a) Power: 2500kW
b) Rated voltage: 6kV
c) Rated current: 288A
d) Rated frequency: 50Hz
e) Rated speed: 2985r/min
f) Protection level: main body IP54, junction box IP55
g) Thermal classification: 155 (F) level
h) Explosion proof grade: Exd Ⅱ BT4
i) Working system: S1
j) Cooling method: IC511
k) Installation method: IMB3
l) Rotation direction: clockwise
m) Efficiency: 94.0%
n) Power factor: 0.89
o) Locked rotor torque multiple: 0.6
p) Z large torque multiple: 1.80
q) Locked rotor current multiple: 6.5
1.2 Basic principles of electromagnetic design
Considering the requirements for self-lubrication of the motor, the electromagnetic scheme of the motor is designed based on two principles:
The temperature rise design of the motor should be as low as possible to avoid the contact temperature between the bearing pads and the internal air circuit of the motor being too high due to the high internal air temperature, which cannot dissipate heat or even heat the bearing pads.
The rotor of the motor should be minimized as much as possible, with a cast aluminum rotor and a high-strength shaft. The rotor core should be shortened as much as possible to reduce the load on the bearing pads and reduce heat generation.
The load borne by the motor bearing mainly comes from the weight of the rotor and the unilateral magnetic pull caused by the non concentricity of the stator and rotor. In order to reduce the load borne by the bearing, the three circles of the motor should be fully considered in the design [4], and the outer diameter of the rotor should be reduced as much as possible within the allowable range of various loads to reduce the weight of the iron core. A lighter weight cast aluminum rotor should be selected in the guide bar material.
1.3 Design of insulation system
In order to reduce temperature rise, advanced epoxy anhydride paint and its matching less adhesive insulation structure (main insulation reduced to 1.0mm, turn to turn insulation 0.4mm, without slot lining) are used. At the same time, vacuum pressure impregnation is used to solidify the insulation system into a whole, which not only improves slot utilization but also facilitates heat dissipation of the winding.
1.4 Determination of Electromagnetic Load and Estimation of Temperature Rise
It is necessary to consider both self-lubricating of the bearing pads and reducing the temperature rise of the motor, as well as appropriately reducing the weight of the rotor and bearing load. Therefore, based on previous tests of similar motors, the stator electrical density is set to 2.66A/mm2 and the thermal load is set to 1750A2/cm · mm2.
Based on electromagnetic calculations and previous experimental data, as well as the temperature rise calculation formula of Siemens in Germany, the estimated temperature rise of the motor for this design is 63.9K.
2. Overall structure and main component structure design
2.1 Overall structure
Motor structure installation form IMB3; The cooling method is IC511; Protection level IP54; The protection level of the junction box is IP55; Cylindrical uniaxial extension; End cover sliding bearing self-lubricating; Cast aluminum rotor structure; The stator adopts a buckle type external pressure structure; The motor adopts a symmetrical radial ventilation structure on both sides internally; The motor is equipped with two axial flow internal fans; The external fan adopts a rear tilting centrifugal fan, and the specific structure is shown in Figure 1.
Figure 1 Motor Structure Diagram
2.2 Design of Flameproof Shell Structure
The motor adopts an explosion-proof structure with an explosion-proof shell. The base shell is rolled from 12 steel plates Q235A, and the two end plates are 50 steel plates Q235A. There are 8 reinforcing ribs welded inside the base, and cooling pipes are distributed around the base. The cooling pipes are fixed to the end plate of the base through tensioning technology.
The end cover and the base are equipped with a flat flameproof surface, while the junction box and the base are equipped with a flat flameproof surface. The shaft connection adopts a push cover type flameproof structure. The flameproof principle is that there is a diameter difference of 0.15~0.2mm between the push cover and the shaft, which not only ensures that the push cover can move freely on the shaft, but also greatly reduces the axial explosion-proof clearance and improves safety. The push cover rotates with the shaft through the keys on the shaft without relative friction, Avoiding the possibility of easily holding the shaft due to small clearance. When an explosion occurs in the inner cavity of the machine base, the explosion pressure pushes? The cover causes the push cover to move outward and collide with the bearing sleeve, blocking the path of explosive flame propagation and playing an explosion-proof role; Meanwhile, due to the free movement of the push cover on the shaft to adjust radial friction, the wear of the push cover is very slight. According to the regulations of explosion-proof standards, anti rust measures are required for the explosion-proof surface. The explosion-proof surface that matches the push cover is made of stainless steel material, including: stainless steel shaft sleeve (interference fit with the shaft), inner sleeve, and baffle. The push cover material is wear-resistant material silicon brass, and both silicon brass and stainless steel are materials that are not prone to sparks, improving the safety of motor operation. The specific structure is shown in Figure 2.
Figure 2 Schematic diagram of push cover explosion-proof structure
2.3 Measures to reduce the heat generation of bearing pads
Due to the constant speed of the motor, in order to reduce the heat generation of the bearing pads, the only way to reduce friction loss is to reduce the linear speed and improve the oil film quality by reducing the journal. Therefore, the shaft material of this motor is selected as high-quality alloy forged steel 42CrMo [9], and subjected to quenching and tempering treatment. After quenching and tempering, the alloy steel is strengthened in both strength and hardness, which can reduce the journal to a maximum extent within the allowable strength range. After strength calculation, the diameter at the bearing pad of the motor has changed from normal φ 140mm can be reduced to φ 110mm, the linear speed is effectively reduced, reducing heat generation.
In order to improve the quality of the oil film, the lubricating oil grade is first determined based on the motor speed and the load borne by the bearing pads. A double oil ring structure is selected for the bearing pad structure. During assembly, the bearing pads are scraped and polished to ensure a convergent wedge shaped gap between the shaft and the bearing pads, making it more conducive to the formation of the oil film during operation.
2.4 Reduce heat transfer to bearing shells
When the motor is running, the temperature rise generated by the rotor will be directly transmitted to the bearing shell through the shaft. In the inner cavity of the base, the high-temperature air generated by the stator temperature rise will directly blow onto the bearing shell, which will also cause the temperature rise of the bearing shell. Therefore, the motor strictly controls the temperature rise of the motor during design. At the same time, a push cover assembly is added in the base to separate the bearing shell from the inner cavity of the base, preventing the high-temperature air from the inner cavity of the base from directly blowing onto the bearing shell.
2.5 Improving the structure of bearing shells
The motor adopts an end cover sliding bearing oil ring self-lubricating method. According to the requirements of the technical agreement, the bearing temperature does not exceed 95 ℃ and the oil temperature does not exceed 65 ℃. After calculating the temperature rise of the bearing bush, Z determines the model of the motor bearing bush as DQZ12-110B/BJ. The specific structural dimensions of the bearing are shown in Figure 3.
This series of bearings adopts a larger heat dissipation area in the design of the bearing seat, and increases the oil storage capacity in the oil chamber, increasing the heat dissipation area of the bearing. After installation, the entire bearing ball is located on the outer side of the motor chamber, which is more conducive to heat dissipation. At the same time, the bearing is equipped with two oil rings, increasing the oil carrying capacity of the oil ring, which can meet the needs of full film lubrication, accelerate the lubricating oil circulation speed, and promote heat exchange.
Through improvement, the temperature rise of the bearing under rated operating conditions was calculated under existing conditions, and the results are as follows.
Motor speed: 2985r/min
Radial load of each bearing: 12205N
Relative clearance of bearings: 1.9 ‰
Z Small oil film thickness: 25 μ M
Lubrication method: self lubrication
Lubricating oil grade: ISOVG32
Bearing temperature rise: 32K
2.6 Strengthen the cooling of bearing shells
Based on our company's previous test results, the temperature of the bearing shell at the tail end of the motor is 8 ℃~13 ℃ lower than that at the extension end due to indirect cooling by the external fan, indicating that the cooling effect of the fan on the bearing shell is very significant. Therefore, in the design of this motor, an axial flow fan is also added at the shaft extension end of the motor, which can effectively carry away the heat generated by the extension end bearing bush in a timely manner and reduce the temperature of the shaft extension end bearing bush.
Table 1 Bearing Temperature Rise
|
motor speed |
2.985r/min |
|
Radial load per bearing |
12 205 |
|
Relative clearance of bearings |
1.9% |
|
minimum oil film thickness |
25um |
|
Lubrication method |
self-lubricating |
|
Lubricating oil grade |
ISO VG32 |
|
Bearing Temperature Rise |
32K |
2.7 Avoid direct sunlight exposure
Due to the use of this motor in unmanned outdoor locations, direct sunlight on the bearing pads can also cause an increase in temperature. Therefore, a protective cover has been added at the motor shaft extension end, which not only makes the air path of the axial fan at the shaft extension end smoother, but also avoids direct sunlight exposure.
3 Conclusion
By taking the above measures, the motor has passed the type test in one go. The stator temperature of the motor has increased by 63.5K, and the temperature of the bearing bush does not exceed 85 ℃ when operating at a high Z ring temperature of 40 ℃. Compared with imported motors used in China, the temperature of the bearing bush is lower. And we conducted online tests with the pump and passed various performance assessments in one go.
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