Auto-Transformers

An auto-transformer has but one winding from which two or more tap leads are made available, as in Fig. 17-14. It is not economically adapted to high-ratio transformation, and the voltage difference between the high-voltage and low-voltage sides seldom exceeds 50 per cent of the higher value. Auto-transformers are commonly used in thermionic battery-charging circuits and transformer-type starters for induction and synchronous motors. The secondary circuit is not insulated from the primary circuit, as in the two-winding transformer, and the auto-transformer is therefore dangerous to life for some purposes.

If the losses in an auto-transformer are ignored, it may be said that the volt-amperes input is equal to the volt-amperes output. An ordinary two-winding transformer, such as the one shown in Fig. 17-6 with a permanent connector as in Fig. 17-15 (a), may be used as a step-up or step-down auto-transformer. By connecting terminals H₁ and H₂ to a 2200- volt line, 2420 volts may be obtained at terminals H₁ and X₁. Also, by connecting terminals X₂ and X₁ to a 220-volt line, 2420 volts may again be obtained from terminals H₁ and X₁.

It should be noted that, when the secondary is loaded as in the simplified diagram of Fig. 17-15 (b), part of the load current comes to the load "conductively" from the line downward toward the load tap. The remainder of the load current comes to the load "inductively" from the secondary coil upward toward the load tap. Ihe portion

Fig. 17-16. Transformer in Example 17-3

of the load current in the secondary winding produces a counter magnetomotive force which tends to reduce the mutual flux. There must then be a corresponding rise in primary magnetomotive force to counteract that of the secondary, if the mutual flux is to be kept constant. The rise in primary amperes flowing through the primary coil is said to produce "cancellation" ampere-turns. For this arrangement,

Example 17-3. - If the transformer of Fig. 17-16 is rated 100 kv-a as a two-winding transformer, how many amperes may be delivered to a 2200-volt load connected to terminals H₂ and H₁ without overloading either winding when the applied voltage at H₁ and X₁ is 2420?

Solution. - In a two-winding transformer the full-load current of the 2200-volt winding is

This then is the maximum permissible value of I₂. Hence, or and

Also,

With this loading, neither the primary nor the secondary winding is overloaded