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| The main pumps of large and medium-sized drainage pumping stations in our country generally use axial flow pumps or mixed flow pumps in the 1950s and 1980s, due to the limitations of grid capacity and power supply quality. Synchronous motors are often used as the power machine of the water pump. Although synchronous motors have the characteristics of improving load power factor, higher efficiency and flat efficiency curves. However, there are also shortcomings such as small starting torque, frequent restarting when out of step, and the need for DC excitation equipment. At present, my country's power grid is strong enough, and the impact of the power load of the pumping station on the power grid is getting smaller and smaller. Therefore, it is necessary to re-discuss the selection of the matching motors for large and medium-sized axial flow pumps. 1 Axial flow pump's requirements for electric motors. With electric motors as the prime mover, the basic requirement for electric motors is to provide the required mechanical power and torque to the load from time to time. Since the power of the axial flow pump increases with the decrease in the flow rate, when the flow rate is equal to 0, the power reaches the full load value. Therefore, for the motor driving a large axial flow pump, in addition to providing sufficient power and torque for the axial flow pump In addition, the pump must also meet the requirements of the starting characteristics of the motor. In addition, because the load of the pump load is variable, it is required that the motor can adjust its working state by itself when the input voltage, frequency and other electrical power remain unchanged to meet the load of the axial flow pump. The load of axial flow pump is generally low speed and high torque load. When starting or water flow changes drastically, it may be overloaded instantaneously. Therefore, the motor is also required to have a strong overload capacity. Comparison of the operating principles of the two motors After the three-phase windingsof the asynchronous motor stator are connected to the three-phase power supply, a rotating magnetic field rotating at a synchronous speed is established in the air gap between the stator and the rotor. Since the short-circuited rotor winding is cut by the magnetic field lines of this rotating magnetic field, induced electromotive force will be generated in the rotor winding, and then electric current will be generated. Since the coil is in the rotating magnetic field and carries induced current, the rotor coil will inevitably be subjected to electromagnetic force. This produces an electromagnetic torque in the same direction as the rotating magnetic field. If this torque is large enough to overcome the braking torque, the rotor will rotate along the direction of the rotating magnetic field at a speed of n. After the three-phase winding of the synchronous motor stator is connected to the three-phase power supply, the field winding is connected to the DC power supply and the synchronous motor has started , Two rotating magnetic fields are generated in the air gap of the stator and rotor, one is the stator rotating magnetic field (the speed is also the other is the rotor rotating magnetic field (the speed is n). Due to the interaction of the two magnetic fields, the stator magnetic field drags the rotor When the motor rotates, the motor is in the state of the motor. At this time, the stator magnetic field leads the rotor magnetic field. If the rotor is dragged by other prime movers at the speed, the rotor magnetic field leads the stator magnetic field at this time, then the stator end will generate an output current to the machine and synchronize The electric motor can drive the axial flow pump. Comparison and comparison of the working characteristics of asynchronous motors Projects Synchronous motor, asynchronous motor speed n does not change with the increase of the load slightly decreases electromagnetic torque M increases with the increase of the load increases with the increase of the load power factor cosh decreases with the increase of the load rises first with the increase of the load The rear drop efficiency Z dropped slightly. 1994-2015 ChinaAcademic is precisely due to the different operating principles of the two motors, which leads to the difference in their structure. The stator part of the two motors is similar, but the rotor part is quite different. The rotor winding of the synchronous motor is a non-enclosed winding, and the excitation current needs to be input; the rotor winding of the asynchronous motor is a short-connected closed winding. In addition, due to the need of starting the synchronous motor, a cage is also installed on the pole shoe of the rotor pole Because the rotor of the synchronous motor is more complicated than the asynchronous motor, the price of the synchronous motor is higher than that of the asynchronous motor under the same power and number of pole pairs. In addition, synchronous motors require DC excitation current, so synchronous motors have an extra set of excitation and corresponding protection and control equipment than asynchronous motors, which also increases the cost of the pump unit. In general, the structure of the synchronous motor is more complicated than that of the asynchronous motor, the supporting equipment is also more, and the cost of the pump unit is also higher. Comparison of the working characteristics of the two motors The working characteristics described in this article refer to the relationship between the motor speed, electromagnetic torque, power factor, efficiency and output power when the grid voltage Ui and the excitation current If the grid frequency is a constant value. They are the operating characteristics of synchronous motors and asynchronous motors. When the input voltage, frequency, and other electric quantities are unchanged, the two motors can adjust their working status according to the change of load to meet the requirements of axial flow pumps. At the same time, when the output power is in the heavy load area, there are the following comparison results: As can be seen from Table 1, there is no big difference between the other two types of motors except that the synchronous motor is slightly better than the speed. And the slip rate s of the asynchronous motor is only 0~0.04 in stable operation, especially for the high-power, low-speed asynchronous motor, the slip rate is about 0.01, and the motor speed decreases very little. The effect of the water pump performance is not large. Therefore, only from the operating characteristics, the start and stop of the asynchronous electric motor for the water pump unit driven by the motor, the torque balance equation during the starting process can be expressed by the following formula: total resistance torque; Mp is The static friction resistance torque when the unit starts; Mf is the torque during the starting process of the water pump rotor; (GD2/4g) dk/dt is the acceleration inertia torque of the unit rotor. The above formula can be expressed. In the figure, M is the starting torque of the motor. The unit is stationary before starting. At the moment of starting, the frictional resistance torque of the rotor of the unit must be overcome to make the water pump rotate and gradually accelerate. Therefore, it is required that Mi>Ma starts as the electromagnetic torque of the motor. Mm is greater than the total resistance torque M of the unit, and the remaining torque Mm-M can be transmitted to the rotor to accelerate the rotation, so (GD2/4g) dk/dt is also called acceleration inertia torque. Various torques in the starting process Schematic diagram The electromagnetic torque of an asynchronous motor can be expressed by the following formula: This electromagnetic torque is only a function of the slip rate s, as shown in the Mm curve. It can be seen from this that as long as the asynchronous motor is not locked at the moment of starting, it can be guaranteed to start and accelerate the rotor. When the speed reaches a certain value, the total resistance torque curve of the unit and the electromagnetic torque curve intersect at point A. Then Mm-M=0, that is, dk/dt=0, at this time, n is a constant, and the unit enters a stable operation state. The power factor of asynchronous motor is lagging when it is running and starting. Therefore, for pump stations that use asynchronous motors to drive water pumps, when starting the second and later motors, it will cause a large voltage drop. Synchronous motors generally start Divided into 2 stages. The stator winding of the i-stage is connected to the AC grid, so that the synchronous motor is started as an asynchronous motor (asynchronous start). In the second stage, when the motor speed reaches 95% of the synchronous speed, the excitation winding of the rotor is connected to the DC power supply, so that the motor is brought into synchronization and transferred to stable operation. In the asynchronous starting phase, in addition to the torque MD produced by the start winding, the induced current in the excitation winding (which is equivalent to a single-phase winding at this time) and the air gap magnetic field phase *Mf. The characteristic of the uniaxial torque is that A negative torque loss occurs near the slip rate s = 0.5 (<0.5). In this way, due to the influence of the single-axis torque, the motor's synthetic torque curve will be clearly concave near 0.5, forming a " Therefore, if the starting load is too heavy to exceed the loadable torque of the motor, the motor may be "stuck" near half the synchronous speed A and cannot continue to increase speed to reduce the single-axis torque. A current-limiting resistor can be connected in series in the excitation winding, and after the speed exceeds 0.5 ns, the resistor is gradually removed. When the rotation speed is close to the synchronous rotation speed (that is, the slip rate s=0.05, and the corresponding electromagnetic torque is called the pull-in torque, such as M2 in), if M2>M, the excitation can be turned on, and the pull-in torque will be The rotor is pulled into synchronization. The M=f(s) curve when the synchronous motor starts asynchronously must point out that the excitation winding cannot be opened when the synchronous motor starts asynchronously. Otherwise, dangerous high voltage will be induced in the excitation winding during starting, and the winding insulation will be broken down and cause accidents. Therefore, we must pay attention The excitation link of the excitation device. If the excitation is switched on too early, the rotor speed and the synchronous speed will differ greatly, and sufficient pull-in torque will not be generated to make the synchronous motor pull in synchronization. The synchronous motor is an inductive load during asynchronous starting, but it is a capacitive load to the power grid during operation. Because inductive and capacitive compensate each other, for pump stations that use synchronous motors to drive pumps, start the second and subsequent ones. When the motor is started, the voltage drop is better than the asynchronous motor. When the synchronous motor stops, the magnetic field energy stored in the field winding is large. If the field winding is open, a high induced voltage will be generated at both ends of the field winding, which may break down the rotor. Insulation Therefore, when the synchronous motor stops, it is necessary to short-circuit a de-excitation resistor with the excitation winding to dissipate the energy stored in the excitation winding on the resistor. It can be seen that the start and stop of a synchronous motor is much more complicated than that of an asynchronous motor, and it is prone to problems. |
浙公网安备 33022602000421号
Mobile浙公网安备 33022602000421号