Transformers may be manufactured as either isolating or auto-wound.
An isolating transformer has primary and secondary windings which are not in any way electrically connected to one another, they are purely coupled by magnetic effects in the iron core. This has the benefit that the secondary windings may be electrically connected to a different ground system to the primary, or not grounded at all, simply left floating with respect to ground.
An auto-transformer has a point which is electrically common to the input and output of the unit. It does not, therefore, provide isolation between input and output, the circuits are electrically connected. The benefits of using an auto-winding are a reduction in the size and cost, as the transformer does not actually have to be large enough to handle the full load power, it merely has to provide the balance of the power. Hence, if the difference between the supply voltage and output voltage is small the transformer can be small.
e.g. A single phase supply voltage of 230 volts is available and it is desired to connect a 240 volt load rated at 24 kW what part of the load is the auto-transformer required to handle?
Ans. Calculate the load current, 24 000 W / 240 V = 100 A
Calculate the difference between supply voltage and load voltage, 240 V - 230 V = 10 V
Calculate the transformer loading, 100 A x 10 V = 1 000 W = 1 kW
The transformer has to handle 1 kVA (assuming a load power factor of unity. See Total Load Rating of Transformer for further information).
When the Total Load Rating of the Transformer is specified in the form (for an auto-transformer) it is essential that the actual load rating is given, NOT the portion of the load power handled by the transformer, i.e. enter only the larger of the two values, 24 kVA in this example.