Russia: Smooth movement of a train without sharp starting and braking depends on a key component of the electric drive system, a frequency converter. Read about principles of its operation and technical evolution from Sergey Zverev, Chief Designer at KSC Drive Systems.
When it comes to public transport, the word “comfortable” conjures up images of spacious, bright interiors, soft, ergonomic seats with intuitive intelligent control systems, air conditioning, personalised service, etc. Few of us think of a component that makes the ride smooth and prevents sudden starts and stops. This component is called a frequency converter, and it is the key component of the electric drive system. Why is it considered key, what are the principles of its operation and what are the advantages for a rolling stock manufacturer using modern solutions in the design of electric trains?
Sergey Zverev, Chief Designer at KSC Drive Systems. Source: KSC Drive Systems
How does it work
A frequency converter is an electronic device that controls the speed of electric motors by changing the frequency of an alternating voltage using semiconductor elements such as transistors, diodes and microcontrollers.
А frequency converter consists a rectifier and a traction inverter. А rectifier first converts the input AC to DC, а traction inverter does the reverse, converting DC to AC of a different frequency and voltage. Frequency converters change the voltage, frequency and amplitude of the current fed to an electric motor, allowing an electric train to start, gain speed and brake smoothly.
Nowadays, 99% of the world’s rail transport uses electric traction. The first Soviet prototype of the OR22 AC electric freight locomotive with mercury rectifiers was built in 1938. Its advantages over the VL22 DC locomotives included smooth acceleration and quicker entry into the operating mode.
(enlarge) Source: Vektor TMH
The further evolution of converters is characterised by two main factors. The first is a significant positive impact of converters on the performance of electric trains equipped with asynchronous traction drives themselves and their maintenance costs. This trend is growing worldwide.
The second factor is the constant improvement in the parameters and design characteristics of the power semiconductor devices that form the core, the ‘’heart’’ of a converter. The latest sixth generation of these devices has been improving for 30 years, in parallel with the development of high performance insulated-gate bipolar transistors (IGBTs). These are assembled into converter modules, which are then placed in containers located in the undercarriage.
The IGBT module is a device that acts like a transistor and is a cross between a bipolar transistor and a MOS field-effect transistor (MOSFET). IGBTs outperform other semiconductors in terms of high current conduction efficiency, conducting large currents in a small space and being easily controlled by an external voltage. This makes IGBTs the perfect solution for applications where precise current control is required. These modules are small in size and weight, extremely reliable and can be easily replaced if required. They are used in AC to DC converters and inverters, industrial equipment and in modern vehicles such as electric trains, electric and hybrid automobiles, etc.
Different conditions and different applications
The type of converter depends on the type of rail transport in which it is used. For example, they are different for metro and railway. The voltage in the metro network is 750 V DC, while in the railway network it reaches 3,000 V DC. The choice of converter also depends on the motor power and the parameters of the electric train itself, i.e. its mass, design speed, etc. The motor power of the Ivolga 4.0 EMU is 380 kW, while that of the Moskva 2024 metro train is only 170 kW.
As with traction inverters, the design of the converter depends on the technical parameters of the locomotives and cars. Vehicles produced in TMH plants are no exception. One of Russia’s largest manufacturer of converter equipment is KSC Drive Systems, subsidiary of TMH and supplier of the holding’s production and service companies.
The company’s wide range of converters and electrical equipment, includes products such as a traction inverter container. Shown here is the KTI-4U1 used to equip the cars of the Moskva 2024 metro train. Other products include a static traction converter, such as the PST-1200 for the Ivolga EMUs.
The KTI-4U1 in the undercarriage space of the Moskva 2024 metro train. Source: KSC Drive Systems
The upgrade of the traction inverters over the past 18 months ensures continuity of component supply and import substitution process. Replacement of transistors, fast switches, line contactors, current and voltage sensors, capacitors, and coolers, minimising production and operational risks and ensuring uninterrupted component supply to the TMH plants.
It is no secret that any engineering solution must be tested under real conditions to ensure a high level of technical excellence and product quality. With this in mind, KSC Drive Systems specialists are improving their capabilities and skills in a variety of tests. In addition to the modern test facility, the company is implementing a large-scale programme to equip its test centre in St. Petersburg.
At the test centre of the power inverter for the Ivolga 4.0 train. Source: KSC Drive Systems
New solutions
Today, leading rolling stock manufacturers are striving to improve the technical characteristics of their products, with a focus on increasing energy efficiency and reliability. To achieve this, they are developing innovative solutions in the field of electric drives and power converters. KSC Drive Systems is one of these companies: it is implementing a new project to create an advanced three-level converter for new series of EMUs, which would represent a significant step forward in energy conversion.
Three- and two-level inverters compared. Source: KSC Drive Systems
The oscillogram shows that the waveform of a three-level converter is more like a sinusoidal signal. This has a number of benefits, such as reducing acoustic noise and losses in the electric motor, reactor equipment and IGBT modules, thus increasing the efficiency of the electric drive. The three-level converter makes it possible to almost halve the overvoltage slow down the ageing of the insulation of all the motor and converter equipment and reduce the weight and dimensions of reactor equipment. It can be said that the three-level converter is an important component of the equipment reliability when safety and comfort requirements for public transport are growing as they are doing today, and the TMH companies are able to meet these new requirements.
