A boost-and-buck transformer has two windings that are tied together in such a way that they act like a transformer with a single tapped winding. This figure shows the schematic for this type of transformer. If you apply 208 V to the input terminals, the voltage at the output terminals will be 240 V. This type of transformer is popular for applications in which small compressors are run at low voltages, and the voltage needs to be “boosted.” Of course, if a lower voltage were needed, the transformer could be connected as shown in this illustration, with an input voltage of 240 V and a resultant output voltage of 208 V. This transformer’s “bucking” operation is caused by the counter electromotive force (CEMF) developed in the windings.
A control transformer has several different applications in the HVAC/R industry. The upper figure shows what the schematic connections look like for a standard control transformer. The most common application for this type of transformer is to reduce the line voltage ( 440 V or 220 V) to either 220 V or 120 V. This is especially important where cost is a factor. Since it must handle greater currents, this type of transformer is rated in kilovolt-amperes (kVA). The lower figure shows how a control transformer can be connected for a 440-V primary and a 220-V secondary. The terminals marked “H” are usually the primary windings, located on the high-voltage side of the transformer. The terminals marked “X” are usually the secondary windings, located on the low-voltage side of the transformer.
An autotransformer is similar to the boost-and-buck transformer in that the primary and secondary do not appear to be separate and distinct windings. The primary difference is that an autotransformer has a variable output. Note in this illustration that a single coil is “tapped” to produce the electrical equivalent of a primary and secondary winding. Neither autotransformers nor boost-and-buck transformers reverse the phase, as transformers with separate (primary and secondary) windings do. The movable tap can be used with the fixed winding to select an output voltage that can be adjusted from 0 V to a voltage above the input voltage.
Extremely high currents at low voltages can be achieved with a high-current secondary transformer, a type of transformer in which the secondary winding is wound with heavy wire and just a few turns. There are many applications for high-current secondary transformers, including welding equipment and soldering guns. This figure is a representation of a typical high-current secondary transformer.