The soft starter is mainly composed of a voltage regulating circuit composed of three pairs of anti-parallel thyristors connected in series between the power supply and the controlled motor, with a microprocessor as the control core, and the entire starting process is automatically carried out under the control of digital program software.
The soft starter utilizes the electronic switching characteristics of the three pairs of thyristors, and controls the timing of the trigger pulse through the microprocessor in the starter to change the size of the trigger delay angle, and the size of the trigger delay angle of the thyristor can also change the conduction of the thyristor. time, thereby ultimately changing the magnitude of the three-phase voltage applied to the stat winding. The characteristic of the soft starter is that the motor torque is approximately proportional to the quadratic of the stat voltage, and the current of the motor is proportional to the stat voltage. Therefore, the limit of the starting torque and initial current of the motor can be realized by controlling the stat voltage. . The soft starter motor stat voltage is controlled by the conduction angle of the thyristor, so different initial phase angles can achieve different terminal voltages to meet different load starting characteristics.
During the starting process of the soft starter motor, the conduction angle of the thyristor gradually increases, the output voltage of the thyristor also gradually increases, and the motor accelerates from zero until the thyristor is fully turned on and the start is completed, thereby realizing the stepless smooth start of the motor. The maximum starting torque and starting current of the soft starter motor can be set according to the load.
The main composition of the soft starter is a three-phase parallel thyristor and its electronic control circuit connected in series between the power supply and the controlled motor. Different methods can be used to control the conduction angle of the three-phase parallel thyristor, so that the input voltage of the controlled motor changes according to different requirements, and different functions can be realized.
Soft starters and frequency converters are two completely different products. The frequency converter is used where speed regulation is required, and its output not only changes the voltage but also the frequency; the soft starter is actually a voltage regulator. When the motor is started, the output only changes the voltage and does not change the frequency. The frequency converter has all the functions of the soft starter, but it is much more expensive and more complicated than the soft starter.
A soft starter just changes the supply voltage and is equivalent to a step-down starter. Frequency converters are much more complex and more expensive than soft starters. The inverter also has a soft starter function, which is achieved by changing the power frequency.
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