TEC manufactures auto-transformers across the whole range of models available. An auto-transformer can offer an appreciable cost, weight and size saving where electrical separation of the primary and secondary windings is not essential. However this reduction in size is solely dependent on the transformation ratio required: the nearer unity the greater the saving.

In general the maximum economical transformation ratio is 5:1 or 2:5. Calculation of the frame size is also dependent on the transformation ratio, and for this reason no fixed guide can be given. The effect of the ratio can be seen in the following examples:


Consider two 500 VA transformers:

(a) 250/200V 50 Hz

Frame VA = 500 × ((250 - 200)/250))

Frame VA = 100 VA

(b) 500/220V 50 Hz

Frame VA = 500 × ((500 - 220)/500))

Frame VA = 280 VA

In a tapped auto-transformer the maximum VA must be calculated from the voltage and current of each tapping and the frame size calculated accordingly. If the tappings are required for simultaneous operation, the design becomes involved and each transformer requires individual consideration. The auto-transformer may be step up or step down.

For more information contact us with your requirements and we will specify the appropriate auto transformer for you.

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The Claude Lyons Group
2 Tower House, Hoddesdon, Herts, EN11 8UR, England
Spares are obtainable from who are stockists of surplus Claude Lyons units and spares .

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Company registered in England No 222791
Registered office as above

VAT Reg No. GB 213 5042 15


UK Established Since 1918

The Claude Lyons Group was founded in the UK in 1918 and is still in private ownership. Since our early days we have been innovators in areas related to voltage and power control and more recently in energy saving and harmonic mitigation. Operating as a global exporter with many years experience we can justifiably call ourselves a world leader in the field of voltage stabilisers, power conditioning and uninterruptible power supplies (UPS)