A Brief Discussion on the Application of Nidec KDS Traction Machines in Subway Projects

Abstract

With the accelerated global urbanization process, the rail transit industry is developing rapidly. As a key urban infrastructure, subways are facing the trends of increasing passenger flow and diversified travel demands of passengers, which bring new challenges to the traction machine—the core component of subway elevators. These challenges include how to balance efficiency and safety in scenarios with large passenger flow, how to meet passengers' growing needs for comfort and convenience, and how to balance costs and design life. This article will briefly discuss how Nidec KDS, as an expert in traction machine design and manufacturing, provides professional solutions for customers.

Keywords

Subway Elevators, Duty Cycle, Energy Conservation and Environmental Protection, Reliability Analysis, Overload Capacity, Service Life Design

According to the classification method in the 2021 Review of World Urban Rail Transit Operation Statistics and Analysis, urban rail transit can be divided into three categories: subways, light rails, and trams. Generally, rail transit has advantages such as large transportation capacity, high speed, frequent departures, safety and comfort, high on-time rate, low fares, energy conservation and environmental protection. However, it also requires high technical standards and maintenance costs.[1]

As a crucial part of urban rail transit, subways play an undoubtedly important role in the development of urban transportation. They help alleviate urban traffic pressure, expand the urban radius and residents' living radius by building an integrated transportation network, improve residents' quality of life, enhance the overall image of the city, and promote urban development and progress as well as social communication and prosperity.

Passengers can efficiently and conveniently enter stations, transfer, or exit stations by taking vertical elevators. The technical level of elevator motors plays a key role in ensuring passengers' riding experience. Nidec KDS has been deeply engaged in the motor industry for more than 60 years. Its products integrate internationally advanced technology design, excellent production, and quality management. With high-quality products and a comprehensive service system, it has stably supplied elevator traction machines for large Indian customers in subway projects for more than a decade.

01 Global Scale and Prospects of the Urban Rail Transit Industry

Statistics show that by the end of 2022, urban rail transit had been put into operation in 545 cities across 78 countries and regions worldwide, with an operating mileage exceeding 41,386.12 km. Compared with 2021, the total global urban rail transit mileage increased by 4,531.92 km, a year-on-year growth of 11.0%.[1]

Figure 1 shows the overall scale of urban rail transit across major continents in the world (Note: All cities in Russia are classified into Europe for calculation). Data indicates that subways and trams are the mainstream types of urban rail transit globally, and global urban rail transit is mainly concentrated in Eurasia, with subways mainly distributed in Asian countries.[1]

Figure 1 Summary of Urban Rail Transit Operating Mileage by Continent in the World in 2022 (km)

Globally, the urban rail transit industry is in a critical development stage. Governments and enterprises around the world are continuously investing in new technologies and facilities to improve transportation efficiency, reduce environmental pollution, and provide better services to a large number of passengers. The construction and application of global rail transit are constantly expanding, and some developing countries are also actively promoting the construction of rail transit.

India has promoted large-scale subway expansion since 2014. According to The Times of India, by April 2022, India's subway network had covered 870 km, serving 18 cities. Currently, approximately 1,000 km of subway tracks are under construction in 27 cities, with nearly 6 km of new tracks put into use each month. The speed and scale of India's subway construction over the past decade have been impressive.

As a professional provider of permanent magnet synchronous motor solutions for elevators, Nidec KDS has supplied more than 1,600 vertical elevator motors to large Indian customers in the past decade. Key projects are shown in Figure 2. With strong technical and manufacturing capabilities, Nidec KDS deeply participates in urban construction and contributes to the building of the international image of local cities.

Figure 2 Indian Subway Projects Won by Nidec KDS

02 Subway Industry Chain and Elevators

Rail transit connects urban industries, promotes the extension of the industrial chain, and drives the rapid development of supporting industries such as equipment manufacturing and technological R&D. The subway industry chain is shown in Figure 3, where all links are interdependent and drive the economic development of urban agglomerations and metropolitan areas.[2]

Figure 3 Subway Industry Chain

As an upstream industry in the subway supply chain, elevators not only provide great convenience and guarantee for urban transportation but also reflect the country's care and attention to the elderly, the disabled, and citizens carrying heavy loads. The future development of cities is closely linked to the construction of subways. Building a rail transit network is an important government project, and its construction level will leave a profound mark on the city's image.

Figure 4 Application of Nidec KDS Motors in Subway Elevators

03 Technical Key Points of Subway Elevator Motors

As the core component of subway vertical elevators, the design of the traction machine must take into account the service environment and application scenarios of subway elevators. The excellent performance of Nidec KDS traction machines stems from the accurate grasp of the application environment and the technical accumulation of permanent magnet synchronous motors, as detailed below:

1. Requirements for High Duty Cycle and High Energy Efficiency

Against the background of global advocacy for green and low-carbon travel, rail transit construction has put forward higher requirements for energy conservation. Therefore, compared with other elevators, subway elevators require traction machines to have higher efficiency to meet urban scenarios with large passenger flow. For elevators with a speed of approximately 1 m/s, their efficiency can reach up to 90%. Moreover, under the requirements of elevator installation, the weight and size of the traction machine must be controlled. Therefore, traction machine design engineers need to have rich experience in electromagnetic design, optimize the electromagnetic design with advanced electromagnetic field analysis software (Figure 5), and select appropriate electromagnetic materials to meet the requirements.

Due to the requirement for transportation capacity, traction machines operate frequently, so they have high requirements for the duty cycle, generally S5-60% or above. At the same time, the requirements for temperature rise (Figure 6) are also very high.

This requires that the design of subway traction machines must comply with the principles of compactness and economy in addition to meeting the performance requirements of energy conservation and low temperature rise.

Figure 5 Electromagnetic Field Analysis

Figure 6 Temperature Rise Simulation

2. Requirements for High Safety and Reliability

High safety and reliability are the basic requirements and attributes of elevators, and people have even higher requirements for the safety and reliability of subway elevators. Therefore, we conduct a detailed analysis of mechanical strength (Figure 7), and put forward special requirements and upgrades for mechanical load-bearing capacity and braking performance. The safety factors for load-bearing of key mechanical components such as the machine base, wheel hub, traction sheave, and shaft have been increased accordingly.

At present, most of the subway traction machines exported by us adopt drum brakes, which have large brake torque margin and stable braking performance. They are matched with power controllers that have been fully verified. The input voltage and current of the brakes are not affected by power grid fluctuations or interference from other electrical equipment, providing a guarantee for the reliable operation of the traction machine.

Figure 7 Mechanical Strength Analysis

3. Strong Overload Stability and High Requirements for Riding Comfort

One of the purposes of equipping subway vertical elevators is to provide convenience for passengers. Citizens carrying heavy loads and groups in need of special care, such as the elderly and the disabled, are the main service objects of vertical elevators. Therefore, subway elevator traction machines need to have strong overload capacity. According to overload analysis, the overload capacity is more than 2 times, and it is required that the torque can be output stably regardless of high speed or low speed, light load or heavy load.

To provide a comfortable riding experience, the design of the traction machine also pays special attention to harmonic suppression, especially the suppression and reduction of low-order harmonics (which have a great impact on comfort) and their force wave amplitude (Figure 8), as well as the reduction of low-speed torque ripple (Figure 9) and cogging torque. This ensures that the elevator runs smoothly throughout the process with low noise and low vibration, providing passengers with a good riding experience.

Figure 8 Force Wave Analysis

Figure 9 Torque Analysis

4. Requirements for Long Design Life and Humanized Maintenance

The design of subway traction machines must also focus on the service life, which is not limited to the above-mentioned aspects such as low temperature rise and high mechanical strength to extend the service life of the traction machine, but also includes other aspects such as motor insulation, bearings, lubricating grease, and permanent magnets. For example, in terms of insulation, insulating paper with high mechanical properties and dielectric strength is selected as slot insulation, and electromagnetic wires with stronger impulse voltage resistance are used. In the process, the VPI (Vacuum Pressure Impregnation) varnishing method is adopted, which greatly improves the ability of the traction machine stator to resist voltage impact and extends the service life of the traction machine.

Most subway elevators are used in machine-room-less applications. The main models of Nidec KDS motors applied in subway projects are shown in Figure 10. In the design, we try our best to adopt integrated design, maintenance-free design, and visual design of vulnerable parts. For example, plug-in wiring, maintenance-free design of rotating parts, high-protection design of electrical components, and visual display of brake friction pad wear. These designs extend the service life of traction machine components and the interval between replacements, reduce the difficulty of maintenance, and make the traction machine run more smoothly, quietly, and durably. The integration of multiple technologies creates high-quality products and saves customers maintenance costs.

Figure 10 Main Models of Nidec KDS Motors Applied in Subway Projects

04 Future Outlook

The world's urban rail transit continues to develop upward. It is expected that by 2024, the operating mileage of the world's urban rail transit will exceed 44,500 km, and the future development prospect of the rail transit industry is very broad. In the era of rapid technological advancement, the technical and quality requirements for elevator motors will be increasingly higher, requiring adaptation to diverse regional cultural and geographical differences around the world.

With high-quality products, professional solutions, and an excellent service team, Nidec KDS has been deeply engaged in important urban infrastructure projects such as subways for more than a decade. It has helped global customers win many major government projects, provided passengers with a comfortable and safe riding experience, and helped customers establish a trustworthy and good reputation.

In the future, Nidec KDS will adhere to the business philosophy of "Quality First, Customer Success", focus on customers, and with solid design and manufacturing capabilities and high-quality services that exceed customer expectations, work with customers to create more and better elevator solutions.