Low-loss chokes for inverters in rail vehicles – Quiet and resistant against impact and temperature

Chokes for inverters perform the task of limiting the input current on the primary side. To minimise the losses within its chokes, SMP uses cores made from special powder composites developed in-house. SMP’s low-loss chokes are contained, for example, in the inverters for the three-phase asynchronous motors of the current ICE 3 units of German Railways, Deutsche Bahn.

In addition to traction units and locomotives, chokes are also used for inverters in passenger coaches. “Because passenger comfort is a key criterion for this application, the chokes must run very quietly. With the magnetostriction-free material developed specially for this purpose, SMP can offer chokes with an exceptionally low noise emission for use in coaches,“ explains Stefan Schauer, Technical Sales Manager at SMP.

Chokes for rail vehicles must be vibration and impact-resistant in all directions. To ensure that they fulfil this requirement they are subjected to rigorous shake tests after being installed in the inverter. They must also have a high rate of voltage rise of up to 10 kV/µs. Because of the high temperatures that occur in railway applications, they must also have a high thermal stability. This is achieved with a UL-listed Class H insulation system, with which all SMP chokes are equipped and which provides thermal protection up to 180 °C.

A further requirement for rail vehicle components is a protective paint coating against dust and water. “Because of protection class IP66, these chokes can be fitted outside the inverters, which means that the heat generated by the choke is not discharged inside the inverter. This results in a lower internal inverter temperature, without the need for cooling fans, saving both energy and installation space. Placing the choke outside the inverter has the further advantage of reducing the inverter’s overall dimensions, which further cuts space and energy demand,” explains Stefan Schauer.

For the choke itself, external mounting has the benefit of allowing it to be designed for lower ambient temperatures. While inverters in railway applications can have an internal temperature of 70 to 80 °C, the outside temperature to which underfloor-mounted electronics are exposed is unlikely to exceed about 40 °C. To simplify mounting outside the inverters, SMP provides the chokes with special mounting fixtures. The choke and the mounting plate are fitted on the device’s outside and the connecting cables pass through a sealed opening.

For use in drive control, power electronics, power generation, wind power and photovoltaic installation as well as instrumentation and control, SMP supplies chokes and filters for frequencies up to 200 kHz and current ratings up to 1000 amperes. Depending on their application, the inductive components are constructed either as single-conductor chokes for high-current applications, individual chokes, choke modules or LC filters. These components offer a high energy storage capacity at low volume, reduced losses, good EMC characteristics and a cost-conscious design. SMP manufactures all components to customer specifications using in-house developed powder composites. All products are RoHS and REACH-compliant and the materials used are UL-listed. To allow for a wide range of requirements, components can be made to all common standards.