1 The status quo of motor starting Three-phase squirrel cage induction motor is widely used as the main power equipment to convert electrical energy into mechanical energy in agricultural irrigation and drainage due to its advantages of simple structure, reliable operation, easy maintenance, low inertia and low price. . However, due to its large starting current, the impact on the power grid and the impact on the working machinery (such as pumps, flaps, etc.) are very large, and therefore must take some technical measures to start the process of starting current and impact (starting electromagnetic turn Moment) to be reasonable and effective control, to achieve a more stable start, thereby improving the system equipment conditions, effectively extending the life of the system and reduce the incidence of failure. Asynchronous motor starting problems, has been of concern to the industry. Asynchronous motor starting method in principle there are only two: direct start and step-down start. Direct start, that is, the motor is still at a standstill directly with the rated voltage, the motor voltage directly under the action of the rated voltage to complete the starting process. Direct start torque, start time is short, simple start control, equipment investment and small, so small and medium-sized motor start widely used. However, the direct starting method is also subject to many limitations, mainly in the following three aspects: (1) starting current can be large motor rated current of 4 to 7 times, some domestic motor starting current actually measured even up to 8 to 12 times. If you directly start a larger motor, the excessive starting current will cause the grid voltage dropped significantly, affecting the same power grid and other electrical equipment and electronic equipment normal operation, serious part of the equipment will be due to low voltage out of operation, and even power Circuit relay protection device overcurrent protection trip, so that the line power supply interruption. (2) Direct starting will cause mechanical impact on the driven working machine. For the pump load, too high starting torque will cause soft damage to the blades, bearings, and flaps (mechanical deformation, fatigue aging ) And hard damage (cracks, fractures, etc.) are more common and may even cause serious water hammer damage to the equipment due to the excessive impact (and reaction) of the water flow on the pipe. (3) The direct starting requires that the capacity of the power transformer is large, and the capacity of the transformer that supplies farmland irrigation and drainage pumping stations often can not meet the requirements of the grid capacity for direct starting. In the case of direct start is not allowed, it is necessary to start the buck start, that is, lower the motor voltage to start. Step-down start usually star / delta start, the stator circuit in series resistance, reactor start-up, autotransformer buck start and the recommended soft-start and other methods. The star / delta starter is one of the simplest and lowest cost components in a step-down starter, however, its performance is limited by (1) the inability to control the current and torque drops and these values ​​are fixed , 1/3 of the rated value. (2) When the starter is switched from the star connection to the delta connection, large current and torque changes usually occur. This will cause mechanical and electrical stress, resulting in recurring failures. Autotransformer starters provide more control than the star / delta starters by varying the I stage start-up voltage (typically 65% ​​and 80% start-up taps) via transformer taps. However, its voltage is graded, so its performance is limited as follows: (1) The step change of the voltage (generated during the step transition) causes large current and torque variation, which is the same as the performance of the star / delta starter Restrictions "2" will lead to the same mechanical and electrical failures occur. (2) A limited range of output voltages (limited number of starting voltage taps) limits the choice of ideal starting current. Because autotransformer-type starter control uses a lower voltage level than the rated voltage to start the buck, it controls the motor parameters of the voltage rather than current, so when the grid voltage fluctuations and load changes (such as irrigation station water level changes) , The starting current curve will significantly deviate from the ideal curve design, thus deteriorating the starting performance, equipment in poor conditions will greatly reduce the service life, increase maintenance costs. Resistive starters also provide better start control than the star / delta starters. However, it also has some performance and usage limitations, including: (1) Startup characteristics are difficult to optimize. The reason is that when the starter is manufactured, the resistance value is determined and it is very difficult to change during use. Although the taping can be performed by changing the tap, it is inevitable to increase the complexity of the control system when the number of stages is high, and the manufacturing cost and fault The rate will also be substantially increased, so the general resistance starter are in 2 to 5 levels. In this way, added to the motor stator winding voltage, current and other major power parameters in the start level is still a lot of volatility. (2) The starting characteristics under frequent starting conditions are not good. The reason is that during start-up, the resistance changes with the temperature of the resistor, requiring a long period of cooling during the stop to restart. (3) Degradation of operating characteristics under heavy load or long start-up time is due to the resistance value changing with the temperature of the resistor. (4) Resistive starters can not provide the ideal start-up effect in applications where the size of the load often changes (for example, the water level at the irrigation and drainage station varies a lot). In summary, the traditional step-down start-up equipment has many performance limitations and restrictions on use, more and more difficult to adapt to the growing needs of complex motor starting needs. 2 soft-start technology works Soft-start technology is developed on the basis of thyristor chopper technology, the use of thyristor chopper technology for power frequency voltage regulation 50Hz sine wave each half-week fixed time (zero-crossing delay t1) To the thyristor VT1 gate to a trigger pulse, according to the thyristor characteristics, after the trigger pulse, the thyristor will remain conductive for the rest of the half weeks until the voltage zero again, so long as the regulation VT1 trigger pulse time, the output Voltage u0 will be 0 ~ 100% of the input voltage (ui) be adjusted. If the thyristor chopper voltage regulator technology used in three-phase power, then add modern electronic technology such as SCM control technology can be made of soft starter, which in large three-phase squirrel-cage AC induction motor can be used to start. Soft-start motor voltage, current curve shown in Figure 2. It can be seen from the voltage characteristic curve u = f (t) that the soft starter gives an AC induction motor an initial voltage Ust (Ust is generally adjustable between 10% and 60% Ue) from the start and starts at a user- Tst (Tst is generally set freely within the range of 1 ~ 60s) within the load voltage rises uniformly to the motor rated voltage Ue. Due to its own current limiting function of the soft starter, the starting current always does not exceed the starting limit current ILIM during starting (ILIM is generally set freely within 2 ~ 5Ie). In order to compare the characteristics of start-up, here is the most common application of traditional start-up --- autotransformer voltage step-down start voltage and current characteristics. Both stages of two-stage start-up have large starting inrush current, impacting the power grid, and two large step voltages of 0 → Ust and Ust → Ue will cause very large torque abrupt changes, resulting in mechanical shock . The motor soft-start, regardless of the current curve or voltage curve point of view, have been the impact of electrical and mechanical shocks to a minimum extent. 3 soft-start technology application The composition of the soft starter soft-start control system can take two types: (1) online-controlled soft-start system and bypass switching soft-start system. On-line control of soft-start system to take the "one by one" way, that is, each load motor starting by the appropriate soft starter to complete the selection of long-term work of the soft starter system, you can start the motor-run-stop Process control, and the main wiring and control system are very simple. Bypass switching soft-start system is more than one motor share the same soft starter. When a motor starts to complete, the bypass contactor will pull the motor into the grid to power off the soft starter direct operation, so that the soft starter after the completion of the start of a motor can control another motor start. Bypass switching soft-start system in the control of a large number of motors can greatly reduce system costs, and soft-starters are working in short-time work system, can greatly reduce the failure rate of the soft starter, the only increase is the main wiring and The complexity of the entire system. With the development of science and technology, the control mechanism and technical requirements of the motor are getting higher and higher. The traditional step-down starting equipment has been unable to meet the needs of various industries. In recent years, with the gradual localization of soft starter equipment, the application of soft starter technology will become the mainstream of large squirrel cage induction motor starting mode in the future, and will eventually replace the traditional start-up mode and get a comprehensive Promotion.
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