INNOVATIONS in ferrous alloys are giving rolling stock designers valuable weight and space savings through improvements in transformer technology. Siemens’ subsidiary Vacuumschmelze (VAC) is offering a range of high-power transformers aimed at railway applications, including a cluster of units delivering 70 kW on ICE2 vehicles for air-conditioning and lighting power supplies; this weighs just 30 kg, compared to 800 kg for the previous design.

The VAC design is based on tape-wound transformer cores built up from a nano-crystalline alloy, Vitroperm¨. This is combined with high-performance IGBTs to give a switched-mode power supply, with switching frequency typically above the audible limit of 20kHz.

Power can be drawn from AC supplies of widely-ranging voltage. This is rectified and then chopped by IGBTs into a high-frequency square wave and fed to the power transformer. A control circuit gives accurate regulation of output voltage, independent of the input voltage.

Vitroperm¨ allows operation at much higher flux densities, with losses reduced over a wide range of temperature. Whereas most ferrites have minimum loss in the 60 to 80°C range, core losses in Vitroperm¨ are similar at these temperatures, remaining roughly constant with lower temperature, and decreasing with higher temperature. Thus a Vitroperm¨ core gives higher efficiency while the transformer warms up, and optimum maximum temperature is only limited by heat resistance of insulating materials.

Vacuumschmelze GmbH

Hanau, Germany

Reader Enquiry Number 140

Switched-mode power supply advantages

Volume reduced by 50+%

Weight cut by 80+%

Stable output voltage (