I plant three relay pumps for steam turbine public equipment, relay pump model J122--2, for the squirrel-cage motor , rated capacity 185 kw, rated voltage 380v, rated current 331A, rated speed 2960r / min. Its main role is to send chemically treated demineralized water to the condenser to ensure that the condenser water level is within the allowable range. Every year from September to the next year in May and June, especially in winter when the heating task is heavy, it is necessary to start the relay pump frequently to ensure the balance of soft drinks, the most frequent half an hour to start once every half an hour of operation, This day need to start 24 times. When starting frequently, it is easy to produce overheating and rupture in the connecting region between the rotor cage and the end ring, resulting in damage of the rotor, and also posing a great threat to the insulation of the stator end winding. According to incomplete statistics, a total of three relay pump has been burned nine times, in the motor found three Overhaul Taipower machines have different levels of the main stator winding insulation damage, inter-turn short circuit. Each low-voltage motor is worth about 40,000 yuan, only this one of the economic losses amounted to 360,000 yuan. With the deepening of the power system reform, power plants will face more severe competition . How to reduce the rate of power plants, save the cost of power generation and make the units run economically and safely has also become a problem that we attach importance to. 2. There are problems in the operation of relay pump Relay pump start mode is full pressure start. In the full pressure start mode, the general squirrel-cage motor start 5-7 times the starting torque of 1-2 times the rated torque. 2.1 relay pump frequently start the hazards of the motor 2.1.1 There are stator winding insulation damage, wear, turn short circuit. The reasons are: First, the squirrel-cage motor at full pressure start, the smaller starting torque, longer start-up time, combined with a large starting current, the motor frequently starts the process, the stator windings in the heat and electric field, so that insulation gradually Aging, this deterioration of the insulation in a very low over-voltage breakdown easily, resulting in damage to insulation, turn short turns and other failures. Second, the motor will start the process will have a greater impact, due to frequent start, the end of the winding will be lax tied, bar have a greater displacement, triggering bar fatigue wear and tear failure. 2.1.2 rotor cage and the end of the ring connecting the area of ​​fracture. The reason is that when the motor is started, the rotor bar is subjected to a great thermal stress and mechanical centrifugal force, which is the most likely to cause the cages to break, especially for the two-pole high-speed motor (close to 3000 r / min). According to the calculation and measurement, at the start-up, the short-term temperature of the cage can reach 300 ° C., the temperature rising speed is very fast, the mechanical strength is rapidly decreased, and the current distribution along the height direction of the cage is uneven due to the skin effect of the current, So there is a big temperature difference, up to dozens of degrees, so that the cage thermal stress. In addition, due to the role of magnetic leakage, so that cage produce a lot of electromagnetic force, the force is proportional to the square of the current, the cage pulled to the bottom of the tank, and twice the current frequency vibration, cage fatigue rupture. Start frequent or heavy start, this sabotage role. 2.2 relay pump frequently start the switch on the hazard Relay pump switch model DW15-C630-type, rated current of 630A instantaneous current 6300A, the switch arc tube insulated copper plated steel composition, poor arc performance, the relay pump each start and stop will be High temperature arc occurs between the contacts of the switch. Frequent start-up over a long period of time causes the heat generated by the high-temperature arc to aggravate the degree of oxidation of the contact. The contact resistance between the contacts becomes larger and larger and the main contact of the switch burns, damages and even causes Switch explosion, causing the accident to expand. According to statistics, three relay pump operation has been burned eight switches. 2.3 relay pump frequent start economic losses Repeated start relay pump will lead to increased power loss, is not conducive to saving power generation costs. When the relay pump water is too large, the need to adjust the opening of the valve to adjust the pump output, resulting in throttling loss, poor economy. 3. Countermeasures 3.1 Selection of winding motor to improve motor starting performance Winding motor rotor wound on a symmetrical three-phase winding. During normal operation, the three-phase winding is shorted out by the resistance of the slip ring. Start, the rotor winding string into a start-up resistance, to achieve the purpose of reducing the starting current. As the start-up resistor can be removed section by section, appropriate string into the start-up resistor, you can make the starting torque to reach the maximum motor torque, shortening the start-up process. In addition, the rotor series resistance, but also can speed. Compared with squirrel-cage motor, which is characterized by motor complex structure, high cost, convenient operation and maintenance, maintenance care workers as large. Taking into account the replacement of wound motor, although the motor starting performance can be improved, but it can not fundamentally solve the problem of frequent motor starts, but also need to replace the new motor, large investment , energy saving effect is not satisfactory. 3.2 Selection of frequency control asynchronous motor Frequency control motor is by changing the motor stator power frequency to change the motor speed. From the speed of the asynchronous motor formula: n = ns (1-s) = 60f (1-s) / p Where: n - asynchronous motor speed; It can be seen that when changing the power frequency f, synchronous speed and frequency is proportional to the change, so the asynchronous motor speed n also changes. In the same premise of efficiency, the pump flow Q, head H, power P and the relationship between the speed is as follows: Qαn, Hαn2, Pαn3. The advantages of using frequency start with; 3.2.1 relay pump frequency control motor, the motor speed at different speeds to meet the different requirements of the output, reducing the number of motor starts, so that frequent motor start-up motor itself reduces the failure rate. 3.2.2 relay pump frequency control motor, reducing the motor starting current, reducing the impact on the motor to extend the service life of the motor, improve equipment utilization. 3.2.3 Unit In the same power, the use of frequency control of motor running current is small, can greatly save power, reduce the plant power rate. 3.2.4 by changing the speed of the motor to change the pump output, to avoid the throttling loss. 3.2.5 The use of inverter speed control will not produce harmonic interference on the power grid, there is no common-mode voltage no special requirements on the motor, do not replace the original supporting motor, thus reducing equipment maintenance costs and improve energy efficiency. 3.2.6 Frequency Control With good reliability, a wide range of speed and smooth, with a small start-up current, speed control before and after the motor overload capacity, power factor, efficiency is basically unchanged, thus ensuring the speed before, After the motor performance. 3.2.7 using frequency converter speed control can reduce the number of start-up and start-up current, it will avoid the burn out of the switch. 4. Economic Analysis From the above analysis we can see that changing the relay pump speed can change the relay pump shaft power so the use of speed control flow is an effective energy-saving measures. FIG. 1 is a pump speed saving schematic: Curve H1 is the characteristic curve of pressure and flow at constant speed, intersecting with the pipe network characteristic curve R1 at point A, and the corresponding flow rate is QA. At this point, pump shaft power is: P1 = QAHA / 1000ηF. To reduce the flow rate, when adjusting the valve opening method, the new pump characteristic curve R2 and curve H1 intersect at point B, and the corresponding flow rate is QB. At this point, pump shaft power is: P2 = QBHB / 1000ηF. As can be seen from Figure 1, P1 ≈ P2, that is, the two power is almost the same. If the speed control method, that is, the pump speed from n1 to n2, the corresponding pressure and flow characteristics of the curve for the H2, the curve H2 and curve R1 intersect at point C, at this time, the pump shaft power becomes: P3 = QCHC / 1000ηF. As can be seen from Figure 1, P3 <P2 ≈ P1, indicating a lot of decline in shaft power, energy-saving effect is significant. Through the above analysis shows that the relay pump frequency control device, not only can reduce the motor starting current and the number of starts, to avoid frequent start burning motor, the switch situation occurs, and you can use the speed control flow to reduce shaft power , To achieve significant energy savings.
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ns - synchronous rotation
s - slip rate
f - power frequency
p - Motor pole pairs.
1. Overview