The main pumps of large and medium-sized drainage pumping stations in China generally use axial or mixed flow pumps from the 1950s to the 1980s. Due to limitations in the power grid capacity and power supply quality, synchronous motors are often used as the power pumps for the pumps. Although the synchronous motors can improve the load power. Factors, high efficiency and flatness of the efficiency curve, but there are also disadvantages such as small starting torque, frequent restart when stepping out, and the need for DC excitation equipment. At present, China's power grid is strong enough, and the electricity load of the pump station is on the grid. The impact is also getting smaller and smaller. Therefore, it is necessary to re-evaluate the selection of motors for large and medium-sized axial flow pumps.
1 Axial flow pump requirements for the motor The motor is the drag type load of the prime mover. The basic requirement for the motor is to be able to provide the required mechanical power and torque to the load from time to time. Since the power of the axial pump increases as the flow decreases, the power reaches its maximum value when the flow equals 0. Therefore, in addition to providing sufficient power and torque for the axial flow pump for the motor driving the large axial flow pump. Must also meet the pump requirements for motor starting characteristics In addition, due to the load of the pump load changes, it is required that the motor can adjust the working state automatically when the input voltage, frequency, etc., and the load requirements of the axial pump are met. Axial pump loads are typically low speed, high torque loads. When the starting or flow conditions change drastically, they may be overloaded instantaneously. Therefore, the motor is required to have a strong overload capacity.
Comparison of two kinds of motor operation principle After the stator three-phase winding of the asynchronous motor is connected to the three-phase power supply, a rotating magnetic field rotating at the synchronous rotation speed is established in the air gap of the stator and the rotor. Since the short-circuited rotor winding is cut by the magnetic field lines of this rotating magnetic field, an induced electromotive force is generated in the rotor winding, and electric turbulence is generated. Since the coil is in a rotating magnetic field and carries an induced current, the rotor coil is bound to receive electromagnetic force. This produces an electromagnetic torque in the same direction as the rotating magnetic field. If this torque has enough magnitude to overcome the braking torque, the rotor will rotate in the direction of the rotating magnetic field, and the speed is n. The three-phase winding of the synchronous motor stator is connected to the three-phase power, the excitation winding is connected to the DC power supply and the synchronous motor is started. Two rotating magnetic fields are generated in the air gap of the stator and the rotor. One is the rotating magnetic field of the stator (the other is the rotating magnetic field of the rotor (rotational speed is n). Because of 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 and the magnetic field of the stator is ahead of the magnetic field of the rotor. If the rotor is dragged by the speed of the other prime mover, then the magnetic field of the rotor is ahead of the magnetic field of the stator, the output current of the stator will be generated. The motor can pull the axial pump.
Table i is synchronized. The comparison of the operating characteristics of asynchronous motors The comparison project The synchronous motor asynchronous motor speed n does not change slightly with the increase of the load. The electromagnetic torque M increases with the increase of the load. The power factor increases with the load. The cosh decreases with the load and decreases with the increase of the load. After-drop efficiency Z slightly decreases slightly. 1994-2015 China Academy is precisely due to the difference in the operating principle of the two types of motors, resulting in a difference in the structure of the two. Two types of motors are similar in the stator part, and there are large differences in the rotor part. The rotor winding of the synchronous motor is an unclosed winding and requires the input of excitation current; the rotor winding of the asynchronous motor is a short-circuited closed winding. In addition, due to the need of starting the synchronous motor, a pole-shaped pole is arranged on the pole shoes of the rotor pole. Start-up windings Since the rotors of synchronous motors are more complex than asynchronous motors, synchronous motors are more expensive than asynchronous motors at the same power and pole pair count. In addition, the synchronous motor requires a DC excitation current, so the synchronous motor needs one more excitation and its corresponding protection and control equipment than the asynchronous motor, which also increases the cost of the pump unit. In general, the structure of the synchronous motor is more complex than that of the asynchronous motor, and there are also more supporting facilities, and the cost of the pump unit is also higher.
Comparison of the two types of motor operating characteristics The operating characteristics described in this paper refer to the relationship between motor speed, electromagnetic torque, power factor, efficiency, and output power when the grid voltage Ui field current If the grid frequency is constant. They are the operating characteristics of synchronous and asynchronous motors, respectively.
It can be seen from the figure that the two types of motors can automatically adjust the working state according to the change of the load when the input voltage, frequency, and other power are constant, and meet the requirements of the axial flow pump. At the same time, when the output power is in the heavy-load zone, the following comparison results are obtained. From Table 1, it can be seen that except for the synchronous motor with a certain speed, the other two types of motors are not much different. And the asynchronous motor slip s, in the stable operation of the value range is only 0 ~ 0.04, especially for high-power, low-speed asynchronous motor, the slip rate is about 0.01, motor speed is reduced very little, The impact of the performance of the pump is not significant. Therefore, from the standpoint of the operating characteristics, the starting and stopping motor-driven pump units of the asynchronous electric motor can be represented by the following equation: The total resistance torque; Mp is The static friction resistance moment when the unit starts; Mf is the torque of the pump rotor starting process; (GD2/4g) dk/dt is the available inertia acceleration torque of the unit rotor. In the figure, M is the starting torque of the motor. The unit is stationary before starting. The starting moment must overcome the frictional resistance moment of the rotor of the unit to make the water pump rotate and gradually accelerate. Therefore, the motor electromagnetic torque after starting Mi>Ma is required. Mm is greater than the unit total resistance torque M, the residual torque Mm-M can be transmitted to the rotor to accelerate the rotation, so (GD2 / 4g) dk / dt also known as the acceleration torque torque during starting various torque Illustration The electromagnetic torque of an asynchronous motor can be expressed by the following formula: This electromagnetic torque is only a function of the slip s, as shown in the Mm curve. It can be seen that as long as the asynchronous motor is not blocked at the instant of starting, then it can be guaranteed to start and the rotor is accelerated. When the speed reaches a certain value, the total resistance torque curve of the unit intersects with the electromagnetic torque curve 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 the asynchronous motor during operation and start-up is lagging, so the pump station using asynchronous motors to drive the pump will cause a large voltage drop when the second and subsequent motors are started. Divided into 2 phases. The i-th stage stator windings are 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 rotor's excitation winding is connected to the DC power supply, so that the motor pulls in synchronization and goes into stable operation. In the asynchronous starting stage, in addition to the torque MD generated by the start winding, the induced current in the field winding (which is equivalent to a single-phase winding) and the magnetic field in the air gap phase *Mf. In the vicinity of the slip s = 0.5 (M can energize, and the pull-in torque synchronizes the rotor.
When the synchronous motor starts asynchronously, the M=f(s) curve must indicate that the excitation winding cannot be opened when the synchronous motor is started asynchronously. Otherwise, a dangerous high voltage in the excitation winding will be induced to break down the winding insulation and cause an accident. Therefore, attention must be paid. Excitation device investment incentives. If the excitation is premature, the difference between the rotor speed and the synchronous speed is large, and sufficient pull-in torque cannot be generated to bring the synchronous motor into synchronization.
Synchronous motor is an inductive load when starting asynchronously, but it is a capacitive load for the power grid during operation. Since the inductive and capacitive mutual compensation, the pump station using a synchronous motor to drive the water pump starts the second and later. When the motor is started, the magnetic field energy stored in the field winding is much higher than the asynchronous motor when the induction motor is stopped. If the field winding is open, a very high induced voltage will be generated across the field winding and may break through the rotor. Insulation Therefore, when the synchronous motor is stopped, it is necessary to short-circuit a field-eliminating resistor and the field winding to dissipate the energy stored in the field winding on the resistor. It can be seen that the start-stop of synchronous motors is much more complicated than the start-stop of asynchronous motors, and it is prone to problems.
5 Other aspects of the synchronous motor When the synchronous motor is running, DC power is needed. At present, most of the DC power supply adopts a thyristor excitation device. When the motor is started and stopped, the excitation circuit requires a large impedance, and the excitation circuit is in normal operation of the motor. Need less resistance.
Therefore, the excitation device requires higher stability. If there is a slight abnormality in the excitation part, the entire excitation device may be damaged, and even if it is serious, the rotor of the motor may be damaged. Excitation device must meet the needs of normal operation (excitation, demagnetization), and at the same time to avoid the positive and negative half-wave current asymmetry, parasitic circuit, thyristor and power diode components to withstand too high positive and negative Voltage, rectifier bridge subversion and so on. There is no doubt that the circuit of the excitation device will be very complicated and there will be many components. In this way, on the one hand, the cost will be increased, and on the other hand, the reliability will be reduced. Therefore, the excitation device is prone to problems, and the maintenance is also very difficult. Due to the complex structure of the synchronous motor, the requirement for maintenance is high, and the operation requires 2 This kind of power supply, especially the demand of DC power supply, all of which will directly affect the investment and maintenance cost of the pump unit. The analysis of the above, the comparison of the starting and running characteristics of the synchronous and asynchronous motors can be illustrated in Table 2 Table 2 Synchronous motor and Features of Asynchronous Motor Comparison and Comparison Item Starting Current of Asynchronous Motor for Synchronous Motor and Approximate Starting Torque of Squirrel Cage Asynchronous Motor Mouse Cage-type Small, Winding Large Slip No Lead When Power Factor Is Running, Lag Asynchronous Start, High Lag, The low excitation device should not be expensive or complicated. Maintenance can be seen from Table 2. Synchronous and asynchronous motors can meet the requirements of large and medium-sized irrigation and drainage pump stations on the main pump. Compared with asynchronous motors, synchronous motors have power factor High, not only does not reduce the power factor of the power grid when running, on the contrary However, it can improve the power factor of the power grid and its advantages such as high efficiency and flatness of the efficiency curve. However, it also has a complicated structure, a small starting torque, a high technical requirement in the start-up process, and often requires a restart in case of out-of-step, control and protection equipment, and investment. And maintenance costs high disadvantages.
In summary, at present, China’s power grid is strong enough, the proportion of the power load of the pump station in the regional power grid is reduced, and the long-distance transmission voltage level is improved. The quality of the power supply of the power grid is better, and in the better and better conditions, the pump It is no longer necessary for the station to use a synchronous motor to perform reactive power compensation for the regional power grid. That is, the advantage of using a synchronous motor is not obvious. Under this circumstance, it is more appropriate to choose an asynchronous motor as a matching motor for large and medium-sized axial flow pumps. □
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