Design of soft starter for three-phase asynchronous motor
In the past three decades, with the development of power electronics technology, it has become possible to switch without arc and continuously adjust the current. Power semiconductor switching devices have the characteristics of no wear, long life and low power consumption. Combined with modern control theory and microcomputer control technology, it provides a new idea for realizing soft start of motor. To break through the traditional way of starting, it is inseparable from the development of power electronics technology and computer control technology. The soft starters currently produced on the market are mainly mechanical and three opposite parallel thyristors. The mechanical starter is a widely used starting method, but it is a stepped start, which will generate secondary inrush current. The starting current is still 3~4 times of the nominal current, and it has large volume, high noise and maintenance cost. High, unable to adapt to harsh environments and many other drawbacks.
At present, in foreign countries, motor soft start products in developed countries are mainly solid-state soft start devices - soft starters for thyristors and inverters that also serve as soft starters. When the production process has the requirements of speed regulation, the frequency conversion device is adopted. In the case where there is no speed regulation requirement, the thyristor soft start is generally used when the starting load is light. The variable frequency soft start is only used when the load is heavy or the load power is particularly large. The thyristor soft starter is the mainstream product of soft start in developed countries. All well-known electric companies have their own brand of thyristor soft start, which has their own characteristics. For example, GE's ASTAT intelligent motor soft starter; ABB's PST, PSTB series motor soft starter; Schneider's ATS46 soft starter; Germany's SIEMENS company's 3RW22SIKOSTART soft starter and so on. At present, foreign research on thyristor three-phase AC voltage regulation circuit has been developed from the open-loop and closed-loop mode of control voltage and control motor current to the establishment of a relatively accurate and practical mathematical model to find a suitable three-phase AC voltage regulator circuit. The motor load control method makes the motor load performance of the three-phase AC voltage regulation circuit better [3]. On the other hand, with the development of power electronics technology, asynchronous motors are becoming more reliable, convenient, and miniaturized.
The soft starter is essentially a DC voltage regulator for soft start, soft stop, real-time monitoring and various protection functions. In order to ensure the safe and reliable operation of the system, the powerful control function of the single-chip microcomputer can be fully utilized. The main control circuit monitors the key components of the system and key parameters such as overvoltage, undervoltage, overcurrent, overload, etc. in real time. With the application of digital DC PWM voltage regulation technology and the use of high-performance single-chip microcomputer as the control core of the system, the soft starter can have the advantages of fast and accurate control, fast response, stable operation and reliability. When the three-phase asynchronous motor is not suitable for direct start, it can be considered to use stator string resistance or series reactor start, Y-△ start, autotransformer step-down start, rotor string resistance start, thyristor electronic soft start, step frequency conversion soft Start, two-phase inverter voltage regulation and soft start.
1. Analysis of the starting process of three-phase asynchronous motor
In order to study the relationship between voltage, current, torque and other variables during the starting of three-phase asynchronous motor, and then analyze the relationship between the current, starting torque and applied voltage of the asynchronous motor, it is necessary to study the mathematical model of the motor. For the soft start of the motor, a mathematical model based on the lumped parameter equivalent circuit is often used. Under the premise of not changing the physical quantity in the stator winding of the asynchronous motor and the electromagnetic performance of the asynchronous motor, the frequency, the number of phases, and the effective series connection number of each phase of the asynchronous motor are calculated into the stator and the stator through the calculation of the frequency and the winding. Like the windings, the equivalent circuit of the asynchronous motor can be derived from the basic equations that have been reduced.
2, the main circuit design
2.1 main loop circuit
2.2 Thyristor protection circuit
(1) Overcurrent protection
(2) Overvoltage protection
3, voltage detection loop design
3.1 Synchronous signal detection
3.2 voltage feedback loop
4, current detection circuit design
4.1 current feedback loop
4.2 over current protection circuit
5 Conclusion
Based on the research of soft starters at home and abroad, the main research on the hardware design of thyristor phase shift AC voltage regulating induction motor soft start system is carried out. Due to the traditional step-down start, there are disadvantages such as ineffective bucking effect, high energy consumption, mechanical contact or inability to smoothly adjust the voltage. In this paper, the soft starter produced by the close combination of high-performance control chip technology and power electronics technology can realize more flexible implementation of step-down starting. Under the premise of meeting the asynchronous motor starting torque requirement and reducing the starting current, the motor can be started smoothly and reliably.
