It is possible to obtain transformer action by means of a single coil, provided that there is a “tap connection” somewhere along the winding. Transformers having only one winding are called autotransformers, shown schematically in figure 1. An autotransformer has the usual magnetic core but only one winding, which is common to both the primary and secondary circuits.
The primary is always the portion of the winding connected to the AC power source. This transformer may be used to step voltage up or down. If the primary is the total winding and is connected to a supply, and the secondary circuit is connected across only a portion of the winding (as shown), the secondary voltage is “stepped-down.”
If only a portion of the winding is the primary and is connected to the supply voltage and the secondary includes all the winding, then the voltage will be “stepped-up” in proportion to the ratio of the total turns to the number of connected turns in the primary winding.
When primary current I1 is in the direction of the arrow, secondary current, I2, is in the opposite direction, as in figure 1b. Therefore, in the portion of the winding between points b and c, current is the difference of I1 and I2. If the requirement is to step the voltage up (or down) only a small amount, then the transformer ratio is small – E1 and E2 are nearly equal. Currents I1 and I2 are also nearly equal. The portion of the winding between b and c, which carries the difference of the currents, can be made of a much smaller conductor, since the current is much lower.
Under these circumstances, the autotransformer is much cheaper than the two-coil transformer of the same rating. However, the disadvantage of the autotransformer is that the primary and secondary circuits are electrically connected and, therefore, could not safely be used for stepping down from high voltage to a voltage suitable for plant loads.
The autotransformer, however, is extensively used for reducing line voltage for step increases in starting larger induction motors. There are generally four or five taps that are changed by timers so that more of the winding is added in each step until the full voltage is applied across the motor. This avoids the large inrush current required when starting motors at full line voltage. This transformer is also extensively used for “buck-boost” when the voltage needs to be stepped up or down only a small percentage.
One very common example is boosting 208 V up from one phase of a 120/208-V three-phase system, to 220 V for single-phase loads.