Introduction
AC asynchronous motors have been widely applied in China’s coal mining, petroleum, chemical, steel, power generation, water supply, cement and building materials industries, owing to their simple construction, high operational reliability, strong environmental adaptability and excellent drive performance. Nevertheless, AC asynchronous motors have a critical drawback: severe starting impact, which exerts adverse effects on the system. The main impacts fall into two categories:
(1)Electrical issues：
The starting current can reach 5–7 times the rated current. This causes excessive temperature rise in motor windings, accelerating insulation ageing, triggering voltage fluctuations in the power supply network and impairing the normal operation of other equipment connected to the same grid.
(2)Mechanical issues ：
Excessive starting torque generates mechanical shock, imposing heavy impact on driven equipment, shortening service life and compromising precision. Typical failures include coupling damage and belt tearing. It results in abnormal wear and shock of mechanical transmission components, accelerates ageing, shortens service life and increases maintenance workload.
Jiangsu Kangjie Environmental Engineering Co., Ltd. is an environmental protection enterprise integrating R&D and design of air pollution control projects, equipment manufacturing, installation and commissioning, as well as operation management. This case covers two dust removal fans supplied by Kangjie Company to Xuzhou Southeast Iron and Steel Co., Ltd. The motors feature relatively high power ratings (10kV / 1250kW and 500kW). First, substantial mechanical shock occurs to the fans during start-up and increases system maintenance workload; the starting current equals six times the rated current. Second, severe grid voltage drop arises during direct-on-line motor start-up. When the grid voltage drops to 85% of the rated value, other equipment fails to operate normally.
1. Scheme Selection
After extensive communication with Jiangsu Kangjie Environmental Engineering Co., Ltd., our GSDQ series high-voltage water resistance soft starter was finally selected. The main parameters of on-site equipment are listed below:
Dust removal fan: YKK630-6
Rated power: 1250kW
Rated voltage: 10kV
Dust removal fan: YKK500-6
Rated power: 500kW
Rated voltage: 10kV
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.
2. Technical Characteristics
(1) Low starting current, no more than 2–3.5 times the motor rated current. It reduces heat generation during motor start-up and effectively extends motor service life.
(2) Smooth start-up that mitigates shock to mechanical equipment. The service life of mechanical equipment and motors can be prolonged by approximately 20%.
(3) Simple structure and convenient maintenance.
3. Operating Performance
The following compares the performance of the GSDQ soft starter against full-voltage direct start:
(1) Inducing grid voltage fluctuations and interfering with operation of other equipment on the same grid When an AC motor starts directly at full voltage, the starting current can reach 5–7 times the rated current. For motors with relatively large capacity, such starting current will lead to sharp grid voltage drop and disrupt normal operation of other grid-connected equipment. When the GSDQ soft starter is adopted, the starting current is generally 2–3.5 times the rated current, and the grid voltage fluctuation rate is kept within 10%, resulting in minimal impact on other equipment.
- Impacts on the power grid Impacts on the power grid are mainly reflected in two aspects:
- The heavy current from direct motor start imposes impact on the grid nearly equivalent to that of a three-phase short circuit, which often causes power oscillation and grid instability.
- The starting current contains abundant high-order harmonics, which may trigger high-frequency resonance with grid circuit parameters and lead to faults such as maloperation of relay protection and failure of automatic control systems. When the GSDQ soft starter is used, the starting current is greatly reduced and all the above adverse effects can be completely eliminated.
(3) Damage to motor insulation and shortened motor service life
a. Joule heat generated by heavy current repeatedly acts on the external insulation of conductors, accelerating insulation ageing and reducing service life.
b. Mechanical force produced by heavy current causes friction between conductors and shortens insulation life.
c. Contact bounce during high-voltage switch closing generates switching overvoltage on motor stator windings, which may exceed 5 times the applied voltage in some cases. Such high overvoltage causes severe damage to motor insulation. With the GSDQ soft starter, the maximum current is reduced by roughly half and instantaneous heat output is only about one quarter of that in direct-on-line start, greatly extending insulation life. The motor terminal voltage can be adjusted from zero during soft start, which completely avoids overvoltage damage.
(4) Electro-dynamic damage to motors Heavy current produces strong impact force on motor stator coils and rotor squirrel-cage bars, which may result in clamping loosening, coil deformation, squirrel-cage bar fracture and other failures. When the GSDQ soft starter is applied, the impact force is significantly lowered thanks to the reduced maximum current.
(5) Damage to mechanical equipment The starting torque during full-voltage direct start is approximately twice the rated torque. Applying such large torque abruptly to stationary mechanical equipment accelerates fan blade fatigue and may even cause blade fracture. The torque of the GSDQ soft starter will not exceed the rated torque, so the above drawbacks can be fully eliminated. Reduced-voltage start only mitigates the above hazards to a certain extent; GSDQ soft start almost removes all such hazards completely. Direct start with an independent transformer power supply can merely ease grid voltage fluctuations, while hazards in other aspects still persist.
4. Concluding Remarks
The two dust removal fans achieve smooth start-up and fully meet the expected objectives. The solution not only improves equipment efficiency and satisfies production process requirements, but also substantially cuts equipment maintenance and repair costs, delivering remarkable direct and indirect economic benefits.
