Challenges in Starting Screw Compressors
At the moment of startup, a screw compressor’s motor must overcome substantial static friction and the system’s pressure differential. Conventional direct‑on‑line starting generates an inrush current that can reach several times the rated current, which not only induces severe voltage fluctuations in the power grid but also induces intense mechanical shocks within the compressor, accelerating wear on critical components such as gears, bearings, and rotors.
The basic operating principle of soft start
The core of soft starting lies in the smooth control of the motor’s input voltage using power electronic devices. The primary component is a pair of anti‑parallel thyristors connected in series with the motor stator circuit. During the initial stages of startup, the control system adjusts the firing angles of the thyristors, causing the voltage at the motor terminals to rise gradually from a low level. As the voltage increases smoothly, both the motor’s starting current and torque increase in a controlled manner, thereby preventing abrupt changes in current and mechanical torque.
The core operating phase of soft start
- Startup phase:The controller gradually increases the firing angle of the thyristors according to the preset start-up curve, causing the motor speed to rise smoothly until it reaches its rated speed.
- Bypass Operation Phase:Once the motor has reached full-speed operation, the control system closes the bypass contactor, shunting the thyristors. At this point, the motor is directly connected to the power grid, and the soft starter is disconnected from the main circuit, thereby reducing its own energy losses and heat generation.
- Soft stop phase:During shutdown, the system re‑engages the thyristors to gradually reduce the motor terminal voltage, enabling the compressor to decelerate and stop smoothly and effectively preventing the hazards caused by a sudden drop in pipeline pressure.
Advantages of Soft-Start Technology
The adoption of soft‑start technology delivers significant comprehensive benefits for screw compressors. First, it substantially limits inrush current, reducing the impact on the power supply network and ensuring the stable operation of other equipment connected to the same grid. Second, the smooth mechanical start‑up and shutdown processes markedly decrease stress‑induced fatigue in the drive components, thereby extending the compressor’s overall service life. Moreover, soft‑start units typically incorporate robust protection features against overload, phase loss, and overheating, enhancing the safety and reliability of equipment operation.