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.
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Switched-mode power supply advantages
Volume reduced by 50+%
Weight cut by 80+%
Stable output voltage (