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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:

FRAME VA = AUTO VA × ((HIGH VOLTAGE - LOW VOLTAGE)/HIGH VOLTAGE))

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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