Sep 09, 2020
Talking about the influence of soft starter on asynchronous motor
As an important power device, electric motors have been widely used in industry, agriculture, transportation, national defense and military facilities and daily life. The speed regulation of DC motors has always dominated in the past, but due to its own structural reasons, such as the low mechanical strength of the commutator and the easy wear of the brushes, it is far from adapting to the requirements of modern production for the development of high-speed and large-capacity. AC motors, especially three-phase squirrel-cage asynchronous motors, have been widely used in industrial production due to their simple structure, convenient manufacturing, low price, sturdiness and durability, small inertia, and reliable operation. Playing an increasingly important role.
1 Comparison of the status quo of soft start and various start methods
AC motors have many advantages compared with DC motors, but there are many disadvantages when asynchronous motors are started. The so-called starting process is in the AC drive system, when the asynchronous motor is put into the grid, its speed starts to rise from zero, and the speed rises to the whole process of stable speed. If no starting device is used, when the rated voltage is directly added to the stator winding to start the motor, the starting current of the motor can reach 4 to 8 times the rated current, and its speed will also rise from zero to the rated speed in a short time. . At the same time, the torque impact of a three-phase induction motor when starting is relatively large, generally up to more than twice the rated torque. On the one hand, the excessively high current during starting will cause serious grid impact, cause excessive voltage drop to the grid, reduce the power quality of the grid and affect the normal operation of other equipment. The excessive torque impact will cause mechanical stress impact and affect the service life of the motor itself and its driving equipment. Therefore, always strive to get a large enough starting torque under a small starting current, for this reason it is necessary to choose a suitable starting method. When choosing the starting method, you can choose according to the specific requirements of the specific situation.
The starting current limit of the three-phase squirrel-cage asynchronous motor usually includes stator series reactor starting, Y-△ starting, self-coupling transformer step-down starting, and Yanbian delta starting. For wound AC motors, the rotor is connected in series with a frequency-sensitive rheostat to start, and the stator series resistance is started in stages. These traditional starting methods have some problems.
(1) Stator series resistance starting: due to the external series resistance, there is a large active power loss in the resistance, especially for medium-sized and large-scale asynchronous motors, which is less economical, so while reducing the starting current, it pays a lot The cost of this, that is, the starting torque is reduced more, generally only used for no-load and light-load.
(2) Y-△ starting: Although the Y-△ starting method is simple, only one Y-△ conversion switch is needed. However, the six outlet ends of the stator winding of the Y-△ starting motor must be drawn out, which has certain difficulties for high-voltage motors, and is generally only used for 380 V motors.
(3) Autotransformer step-down starting: Compared with the starting of the stator series reactor, when the limited starting current is the same, the starting torque loss is less; compared with Y-△ starting, There are several types of taps for selection, which are more flexible and can be started by dragging a larger load. However, the autotransformer is large in size and expensive, and it cannot be started with a heavy load.
(4) Yanbian delta start: The Yanbian delta start squirrel cage asynchronous motor is used. In addition to a simple winding connection switching device, no other special starting equipment is required. However, the stator windings of the motor are not only delta-connected and have taps, but also need to be specially designed. The taps cannot be changed at will after they are made.
With the continuous development of power technology (especially integrated circuits, microprocessors and new generation power electronic devices), the problems of too high starting current and too small starting torque in the starting process of asynchronous motors have been well solved.
Since the 1970s, the soft starter made by using thyristor AC voltage regulation technology has been popularized, and the use of microcontroller instead of analog control circuit has developed into a modern electronic soft starter.
2 Features of soft start
Compared with the traditional starting method, the outstanding advantages of electronic soft starter are reflected in:
(1) The power semiconductor switch is a non-arc switch and continuous current adjustment, so the electronic soft starter is stepless adjustment, which can continuously and stably adjust the starting of the motor, while the adjustment of the traditional starting is divided into stages, that is, it is a stepped adjustment range.
(2) Impact torque and impact current are small. When the soft starter starts the motor, it gradually increases the conduction angle of the thyristor to limit the motor starting current within the set value, so the impulse current is small, and the torque can also be controlled to rise smoothly to protect the transmission machinery, equipment and personnel. .
(3) The soft starter can introduce current closed-loop control, so that the motor maintains a constant current during the starting process and ensures the smooth start of the motor.
(4) According to the load conditions and the characteristics of the power grid relay protection, it can be steplessly adjusted to the best starting current to save energy.
(5) Due to the use of microcomputer control, the main circuit can be diagnosed before starting, and the digital control has relatively stable static characteristics, and is not easily affected by factors such as temperature, power supply voltage and time changes, thus improving the reliability of the system It is helpful for system maintenance. At the same time, the soft starter can also realize direct computer communication control, laying a good foundation for automatic control.
From a short-term point of view, soft start will still take various forms of reduced voltage (current limiting) soft start as its main form. In theory, products with high performance-price ratio will occupy a larger market share. However, in various applications, people have different emphasis on various performances, so that various starting products (including traditional star-delta starting) may win their own markets. But in the long run, variable frequency soft start will become the mainstream of soft start. Various forms of reduced voltage soft start will be combined with star-delta starting and other technologies into traditional starting technology. With the gradual decline in the price of frequency converters and the further improvement of reliability, the soft starting device that will become mainstream products in the future will be a cheap frequency converter with soft switching function.
The SAFESAVE soft starter SJR3 series adopts a general soft start structure, with universal applicability and good quality, and can adapt to the use environment under various conditions. Welcome to consult.