The Nordyne 621709R Transformer is used to reduce higher voltage 208-240V to a safer voltage of 24V for use in different HVAC systems. The working of transformers is based on the electromagnetic induction only. When the direct AC flows through the primary winding, which is 208-240V, then a magnetic field is induced. The secondary winding of the 24V side is induced with a voltage through this fluctuating magnetic field. The amount of turns in primary and secondary coil decides the voltage transformation. In this case, the transformer scales the higher voltage to 24V which is a common control voltage for most HVAC parts.