Introduction
Soft start is an innovative motor control device integrating motor soft starting, soft stopping, light-load energy saving and multiple protection functions. Soft start differs from traditional reduced-voltage starting methods in the following aspects:
No Inrush Current
When the soft starter energizes the motor, the motor starting current rises linearly from zero to the preset value. It generates no impact on the motor, improves power supply reliability, achieves smooth start-up, reduces impact torque on the mechanical load, and extends equipment service life.
Soft Stop Function
This function enables smooth deceleration and gradual shutdown. It can eliminate the drawbacks of abrupt shutdown upon power loss, reduce mechanical shock, avoid water hammer effect for pump-type loads and lower the risk of equipment damage；Adjustable Starting Parameters；Parameters can be selected according to load conditions and grid relay protection characteristics. The starting current can be steplessly adjusted freely to the optimal value.
1. Scheme Selection
After comprehensive communication with Shandong Qifa Pharmaceutical Co., Ltd., our GSDQ series high‑voltage water resistance soft starter was finally selected. The main parameters of on-site equipment are shown below: Motor Model: ASZK-LDAO4 (USA) Rated Power: 1100kW Speed: 2965rpm Rated Voltage: 10kV Rated Current: 75.7A Full-voltage starting current multiple: 7.2 times Motor neutral point opened: No Motor Load: Centrifugal compressor (refrigeration unit)
2.Working Principle
After the high‑voltage switchgear closes, the PLC (Programmable Logic Controller) of the water resistance starter receives the signal from its auxiliary normally‑open contact closure (i.e., start command). Following PLC logic judgment, the starter runs in accordance with the preset program. At this moment, the servo motor drives the transmission mechanism and electrode plate to move downwards. The gap between the moving plate and fixed plate gradually narrows, the resistance between the two plates decreases progressively, and the main motor accelerates smoothly. When the moving plate reaches the bottom dead centre, the resistance approximates zero. The vacuum switch is shorted so that the main motor operates at full voltage. Meanwhile, the moving plate travels upward to the top dead centre to get ready for the next start.
3. Design Features
The GSDQ series high‑voltage water resistance soft starter adopted in this project has the following characteristics:
- Good recoverability: The water tank has large heat capacity and supports continuous start‑up without burnout, with simple maintenance. Even if part of the resistance liquid evaporates, its performance can be restored by adding clean water.
- No harmonic pollution: The device generates no high‑order harmonics during operation and causes no pollution to the power grid.
- Controllable starting current (controllable starting process): According to actual load conditions, the resistance value can be changed by adjusting and preparing the concentration of electrolyte solution, or by adjusting the gap between moving and fixed plates. Meanwhile, current negative feedback and other methods are controlled via PLC programs to realize starting current regulation.
- Adjustable starting time: Based on on-site working conditions and user requirements, the travel time of the moving plate can be modified by editing the PLC control program, adjusting the initial starting position of the moving plate and changing the rotating speed of the drive motor, so as to adjust the motor starting time.
- Smooth start process, shock-free switchover between start and operation: The linear movement of the moving plate reduces the resistance of the series water resistor linearly, enabling uniform, smooth and stepless motor start-up. At the instant of switchover upon start completion, the resistance of the water resistor approaches zero. Therefore, no current shock will be imposed on the power grid and no mechanical shock on the mechanical transmission system during switchover.
- Closed-loop current control: The current negative feedback closed-loop control enables constant-current start of the motor and shortens the starting duration.
4. Concluding Remarks
Since the GSDQ series high‑voltage water resistance soft starter was put into service at Qifa Pharmaceutical, the equipment has been operating in good condition and fully achieved the expected results, bringing remarkable economic benefits
