1 Introduction
In the 1980s, soft starters became the ideal product for motor starting control. However, they all operated via online thyristor control, resulting in very high costs and power consumption, leading to a severe imbalance between cost and technology and making them difficult for users to accept, resulting in very few applications. But due to the alternative of bypass-operation soft starters, the cost of soft starting systems was significantly reduced, and their cost-effectiveness became superior to other starting methods, reaching a level that was easily accepted by users. Furthermore, my technical lectures at various design institutes and industrial and mining enterprises promoted the widespread adoption of soft starters in engineering applications.
2. Soft Starter Type Analysis
2.1 Thyristor Online Soft Starter
A thyristor-controlled online soft starter (or motor controller for short), as the name suggests, refers to a soft starter whose main switching element, the thyristor, operates continuously online. It not only controls the voltage rise and fall of the motor during starting and stopping but also supplies the full voltage to the motor during normal operation, as shown in Figure 1. Its advantages include simple circuitry, flexible control, and strong protection for the motor. Its disadvantages include the high power consumption of the soft starter itself due to the online operation of the thyristor. To address the heat generated by this power consumption, the soft starter body is designed to be large and requires mechanical cooling. If a distribution room has ten 200kW motor soft starters, firstly, these ten soft starters alone would require ten switch cabinets, significantly increasing the floor space, and their combined heat output would reach approximately 15kW. Under such conditions, regardless of winter or summer, the room temperature would exceed the standard, making it difficult for the power distribution system to operate normally. Therefore, when using thyristor-controlled online soft starters, a bypass contactor should be added to supply the motor with full voltage. While adding a bypass contactor solves the power consumption problem, it increases the overall size of the system. Therefore, the system cost is very high. Furthermore, the online operation of the thyristors introduces high-order harmonic pollution into the power grid, making harmonic mitigation more difficult.
2.2 Bypass type soft starter
Thyristor-based online soft starters present significant manufacturing challenges due to their high power consumption and heat generation. This is especially true considering the large size and mechanical ventilation required for the thyristor's heat sink. This leads to a substantial increase in both technical complexity and material costs. To address these heat dissipation and power consumption issues during actual use, the author developed a bypass-type soft starter in 1998, as shown in Figure 2.
Because the thyristor only operates briefly for a few seconds to tens of seconds during startup, its heat dissipation is minimal. Therefore, the soft starter itself does not require a large heatsink, only about 1/10 the size of an inline soft starter, making it several times smaller and eliminating the need for mechanical cooling. This approach is far superior to the previous one, leading to more rational engineering designs. Consequently, bypass soft starters are commonly used in current engineering applications. However, nothing is perfect. The disadvantages of bypass operation include the inability to integrate the starting device, complex circuitry, and the inability of a powerful intelligent soft starter controller to fully utilize its capabilities, while also causing inconvenience for maintenance and repair.
2.3 Built-in bypass soft starter
An internal bypass soft starter (or motor controller for short) is, as the name suggests, an internal contactor connected in parallel with a thyristor. During soft starting and soft stopping of the motor, the thyristor operates, and the mechanical contacts open. When the motor is running normally, the thyristor closes, and the mechanical contacts close. This entire process is automatically completed by the internal controller; from the perspective of external wiring, it is a single device, hence the term "online operation." It can also be called a bypass soft starter, integrating the external contactor into the soft starter while maintaining a compact size. Its advantages are that it possesses all the advantages of the two types mentioned above while avoiding their respective disadvantages: First, the circuit is simple; second, it is naturally air-cooled; third, the thyristor is only responsible for starting and stopping, avoiding the power consumption and heat dissipation caused by the online operation of the thyristor; fourth, the powerful intelligent controller can play a full role, and can start, stop, protect and control the motor; fifth, it saves space; sixth, due to the integrated design of the thyristor and mechanical contact, the intelligent controller realizes that the mechanical contact is arc-free, so that the electrical life of the mechanical contact is equal to the mechanical life, which solves the problem that contactors have long been difficult to solve, and greatly improves the reliability of the contactor compared with the bypass type soft starter, as shown in Figure 3.
It retains the advantages of both online and bypass soft starters while avoiding their disadvantages, making it a leading international technology for soft starters.
2.4 Comparison of Soft Starter Types
Currently, there are three main types of soft starters on the market: online, bypass, and built-in bypass. Online soft starters are primarily foreign brands, accounting for about a quarter of the market share, but these are often image-building projects or projects driven by ignorance. About half of the market uses bypass soft starters. Since the introduction of built-in bypass soft starters in 2003, they have become more widespread. Through the above introduction, readers should have a rational understanding of the technological advantages of built-in bypass soft starters. Below, we will use data comparison for quantitative analysis, as shown in the attached table. The costs in the attached table refer to the manufacturer's publicly quoted prices; the user's purchase price is generally lower than this.
As the table shows, bypass soft starters are superior to online soft starters, while built-in bypass soft starters are superior to both. However, the more advanced the technology, the lower the cost—a general rule in high-tech applications. Comparing the brands listed in the appendix, online soft starters are 1.5 times more expensive than bypass soft starters, and bypass soft starters are 1.4 times more expensive than built-in bypass soft starters (excluding contactor costs). In terms of energy consumption, online soft starters are approximately 10 times more energy-efficient than bypass soft starters, and bypass soft starters are approximately 7 times more energy-efficient than built-in bypass soft starters.
There are approximately 300 million kW of electric motors in operation nationwide. Assuming 40% of these require soft starters, this equates to about 120 million kW. In terms of investment, using online soft starters would require over 25.2 billion yuan, using bypass soft starters would require over 16 billion yuan, and using built-in bypass soft starters would require over 10 billion yuan. Regarding total energy consumption, using only online soft starters would result in an annual power consumption of 6.3 billion kWh, equivalent to the annual output of an 800,000 kW power plant. Using only bypass soft starters would result in an annual power consumption of 630 million kWh, equivalent to the annual output of an 80,000 kW generator. Using only built-in bypass soft starters would result in an annual power consumption of 80 million kWh, equivalent to the annual output of an 8,000 kW generator. Using all bypass-type soft starters can save 5.6 billion kWh of energy per year compared to using all online soft starters. If all soft starters are built-in bypass-type, the energy saving will be 550 million kWh per year compared to using all bypass-type soft starters.
The costs listed in the appendix refer to the manufacturers' publicly quoted prices; users' purchase prices are generally lower than these. (For reference only)
3. Conclusion
The above analysis clearly shows that online soft starters should be replaced by bypass soft starters, and bypass soft starters should be replaced by built-in bypass soft starters. However, in practical engineering applications, some projects prioritize brand image or reliability, knowingly choosing online soft starters when they are deemed unreasonable. Since imported brands do not offer bypass soft starters, they reluctantly opt for imported online soft starters, often due to a lack of understanding of the underlying principles. This article provides an analysis for reference. Currently, there are 5 or 6 manufacturers producing built-in bypass soft starters, meeting users' needs. Examples include: SJR3 (15~400KW) from Shuen, TJNR1 (15~400kw) from Tianjin Nuoerhaton, PST (200~560kw) and Flex (15~250kw) from ABB, 3RW44 (15~250kw) from Siemens, Danfoss (15~110kw) from Danfoss, and Xi'an Xipu (15~75kw). Furthermore, all soft starter manufacturers are developing built-in bypass soft starters, and it is believed that it will soon become a market dominated by built-in bypass soft starters. If users are still choosing bypass or online soft starters, their spare parts and maintenance will be constrained in the near future.
