Principle and characteristics of thyristor soft start

Thyristor soft starter is a novel motor control equipment that integrates soft start, soft stop, light load energy-saving, and multifunctional protection, also known as SoftStarter. It not only achieves smooth starting of the motor without impact throughout the entire starting process, but also can adjust the parameters during the starting process according to the characteristics of the motor load, such as current limit value, starting time, etc.

In addition, it also has multiple motor protection functions, which fundamentally solves many drawbacks of traditional voltage reduction starting equipment.

The principle of thyristor soft start

The soft start of thyristors is controlled by the control unit emitting PWM waves to control the triggering pulse of thyristors, thereby controlling the conduction of thyristors and achieving the control of motor starting. Before analyzing the principle of soft start, the following terms should be emphasized:

(1) Trigger angle α: refers to the electrical angle corresponding to the period from the forward voltage of the thyristor to the application of the trigger pulse.

(2) Conduction angle θ: refers to the angle at which a thyristor conducts for a period of time.

(3) Continuation angle φ: The phase angle at which the inductive load current lags behind its corresponding phase voltage.

(4) Turn off angle δ: refers to the angle corresponding to the period from the moment when the current reaches zero until the thyristor of that phase is turned on again. The working principle of thyristor soft start is to increase the terminal voltage of the motor from a pre-set value to the rated voltage by controlling the conduction angle of the three-phase anti parallel thyristor connected in series between the power supply and the controlled motor. The starting torque of a three-phase AC asynchronous motor is directly related to the square of the applied voltage, which means that reducing the voltage on the motor terminal will affect these values.

Principle and characteristics of thyristor soft start

The voltage regulating circuit is composed of six thyristors connected in reverse parallel in pairs, which are connected in series in the three-phase power supply line of the motor. During the starting process, the triggering angle of the thyristor is controlled by software. When the microcomputer control system of the starter receives the starting command, it performs relevant calculations and outputs the signal to trigger the thyristor. By controlling the conduction angle θ of the thyristor, the starter adjusts the output voltage according to the designed mode, so that the voltage applied to the three-phase stator winding of the AC motor gradually and smoothly rises from zero to full voltage. At the same time, the current detection device detects the three-phase stator current and sends it to the microprocessor for calculation and judgment. When the starting current exceeds the set value, the software controls the boosting to stop until the starting current drops below the set value, and then continues to boost and start the motor. If the three-phase starting current is unbalanced and exceeds the specified range, stop starting. After the starting process is completed, the soft starter will close the bypass contactor, short-circuit all thyristors, and directly put the motor into operation on the grid to avoid unnecessary energy loss. Furthermore, according to the principles of electrical engineering, when the input power frequency of the motor remains constant, the output torque of the motor is proportional to the square of the input voltage. Therefore, soft starting not only achieves boost current limiting starting by continuously and smoothly increasing the stator voltage, but also reduces the impact of motor starting torque.

Advantages of thyristor high-voltage soft starter

1. Widely applicable voltage and power range, can be used in conjunction with motors ranging from 6kV-10kV/600kW-22MW;

2. Equipped with soft start and soft stop functions, fully ensuring the safe start and stop of the system;

3. When the motor starts, the current smoothly rises from zero to the set value, and the start is smooth;

4. Mechanical aspects: reduce mechanical impact, prevent overvoltage and water hammer effects, and extend the service life of transmission mechanisms;

5. In terms of electrical aspects: reducing 50% of the starting surge current, extending the service life of the motor, reducing voltage fluctuations in the power grid, and minimizing the impact on other electricity consuming departments;

6. It can achieve soft parking and eliminate the impact and damage of sudden shutdown on certain equipment;

7. Comprehensive protection functions have prevented the occurrence of personal safety accidents;

8. High level of intelligence, simple setup and operation;

9. Small size, saving infrastructure investment;

10. Reduced the capacity of power supply equipment and saved investment costs for the project.

Technical features

1. Using thyristor valves as the main circuit components, it has the characteristics of advanced technology, reliable operation, modular structure, and easy maintenance;

2. Using fiber optic transmission technology to isolate high and low voltage parts;

3. Adopting modern advanced digital signal processing and system integration technology, it has excellent performance, high reliability and stability;

4. Adopting a series of anti-interference measures, it has strong electromagnetic compatibility;

5. The controller adopts a large screen LCD display and a membrane switch setting, with simple operation and a good human-machine interface.