After analyzing the commonly used soft start and energy saving methods of three-phase asynchronous motor, the logical working principle and PID control method of the soft start controller based on thyristor are analyzed in detail.
Finally, an example is given to verify the good performance of the soft start controller in the motor start-up process. Three-phase asynchronous motor has been widely used in various engineering fields because of its simple structure, reliable operation, convenient maintenance, and low cost. It is the main power carrier for converting electric energy to other energy. As we all know, when the motor is in the direct starting operation, its starting current can generally reach 8 times of the rated current, and some can even reach 15 times.
The powerful starting impact motor will cause great impact to the power supply network, load machinery and the motor itself, not only will affect the comprehensive service life of the motor system and its dragging mechanical equipment. At the same time, it will cause sudden voltage drop in the distribution network of motor power supply, which directly affects the efficient and stable operation of other electrical equipment in the same distribution network.
When the motor operates in the working range near the rated load rate, its efficiency is high, usually around 80%; however, when the motor drag load drops, thermostatic element the operating efficiency of the motor system will also drop significantly. In the selection and design of motor drive system, the motor power is usually selected according to the maximum load and the worst operating conditions of the system.
In the actual operation process, due to load fluctuation and other reasons, the motor system usually operates under light load (no load) or uneven time-varying load conditions, which will inevitably lead to a large amount of power loss and waste in the low-efficiency operating area. Therefore, it is necessary to improve the starting operation conditions of the motor and ensure that the whole motor system has a higher operating efficiency. The traditional series resistance (reactor), shunt self-coupling transformer and star-shaped triangle (Y-Delta) decompression start-up mode not only have high cost, but also the motor will experience a jump process from auxiliary power supply system to power frequency power supply system in the start-up process, which makes some characteristic electrical parameters in the motor system change specially, and it is not stable and smooth to regulate the start-up operation and easy to conduct.
Due to mechanical or electrical faults in the motor system and the increasing complexity of the motor control system, the economic performance of this kind of control method is generally low, so it is gradually replaced by the soft start control system. Because of the direct starting mode of motor caused by power supply distance and motor power, it can not meet the requirements of voltage drop in distribution network of motor system. In order to ensure the efficient and stable start-up operation of motor, corresponding technical means should be adopted.
By reducing the starting mode of motor stator voltage, the purpose of limiting starting current to ensure the efficient and reliable start-up operation of motor is achieved. Based on the main characteristics of motor starting in motor control system and the basic theory and method of energy saving, this paper will discuss the logic structure and operation effect of motor energy saving soft starter in detail.
Fig. 1 shows that when the three-phase asynchronous motor is in the starting process, the voltage, current and speed signals of the motor end will be converted into corresponding data signals by corresponding A/D analog-to-digital conversion circuit and sent to the control system.
After dynamic comparison and analysis with the original set value of the system, the corresponding electric parameter deviation and deviation rate will be calculated, and then the corresponding electric parameter deviation and deviation rate will be obtained. The trigger angle of thyristor is dynamically adjusted by controlling the width of trigger pulse, and the real-time control of motor start-up is completed.
By controlling the trigger angle of the thyristor, the soft start control of the motor can be realized by changing the output voltage of the thyristor. From the starting characteristics of the motor, the trigger angle of the thyristor will be continuously switched on during the starting process of the motor, so as to realize the stepless smooth starting control of the motor from zero speed. When the three-phase asynchronous motor fails, the system will detect the working voltage and current value, the control system will complete the motor protection tripping operation through the protection circuit, prevent the further expansion of the motor fault to bring more serious harm to the motor system, and through the corresponding visual display interface, remind operators to check the relevant abnormal units, greatly improve the electricity. Humanized Service Level of Machine Control System [3].
Because the asynchronous motor control system is a multi-parameter, dynamic time-varying, non-linear, strong coupling multi-order complex large-scale system, it is difficult to use a simple control model to describe and express. In order to analyze the effect of energy-saving soft-start controller on energy-saving and consumption-reducing in three-phase AC asynchronous motor control system, based on the previous theoretical analysis, the corresponding soft-start control system is formed through the combination of corresponding functional units. The basic parameters of the whole system are: rated power of three-phase asynchronous motor is 22KW, rated current is 44A, rated voltage is 380V, and direct power supply mode is adopted.
。 From Table 1, it can be seen that after adding energy-saving soft-start controller, the characteristics of motor start-up conditions have been effectively changed, which not only reduces the starting current of motor (lower than 38.5A under rated voltage, only 34.7A), effectively avoids the impact of strong starting current on power grid and motor, but also improves the power of motor system (from 0.608 to 0.852), and guarantees motor tools. It has high working efficiency and achieves the purpose of saving energy and reducing consumption. The application of energy-saving soft-start controller with thyristor as the core in the control system of three-phase asynchronous motor can effectively solve the problem of low starting performance of traditional asynchronous motor, and achieve the goal of energy-saving, consumption-reducing, stable and economic start-up operation, which can reduce the starting current of motor and improve the operating efficiency.