Transformers
A transformer transfers electrical energy from one or more primary circuits to one or more secondary circuits by means of electromagnetic induction. It consists of at least one primary winding and one secondary winding of insulated wire on a common core. No electrical connection exists between any primary or input circuit and any secondary or output circuit, and no change in frequency occurs between the two circuits.
Figure 1-20 Transformer schematic symbols: (a) step-up transformer, (b) step-down transformer, and (c) multiple-wound transformer.
If an AC voltage is applied to the primary winding of a transformer, an electromagnetic field forms around the core and expands and contracts at the input frequency. This changing field cuts the wires in the secondary winding and induces a voltage in it. The voltage that appears across the secondary winding depends on the voltage at the primary winding and the ratio of turns in the primary and secondary windings. Schematic diagrams for three commonly specified transformer configurations are shown in Fig. 1-20.
A step-up transformer, as shown in Fig. 1-20a, has twice the number of turns in its secondary winding as it has in its primary winding, so the voltage across the secondary winding will be twice that of the voltage across the primary winding. Similarly, a step-down transformer, as shown in Fig. 1-20b, has half as many turns in its secondary as in its primary, so the secondary voltage will be half that of the primary voltage. A multiple-winding transformer, as shown in Fig. 1-20c, provides three separate output voltages that also depend on the ratios between primary and secondary windings.
All of these transformer configurations obey the law of conservation of energy. In transformers this can be interpreted as the equality of the products of voltage and current or power in both primary and secondary windings, except for losses. Thus, the power input at the primary winding is nearly equal to the power output at the secondary winding or the sum of the secondary windings if there are more than one.
If, for example, the voltage at the secondary terminals of the transformer is twice that
of the primary terminals, the current at the secondary terminals must be about half that at the primary terminals to keep the product of voltage and current, which is equal to power, constant. An ideal transformer would be 100 percent efficient because the power output would be equal to the power input. But, because losses reduce the efficiency of most transformers to about 90 percent, output power is about 10 percent less than input power. The total loss is the sum of ohmic resistance loss, eddy-current induction loss, and hysteresis (molecular friction) loss, all caused by the changing polarity of the applied current.
Most transformers transform voltage or current up or down, but an isolation transformer provides secondary voltage and current that are essentially the same as the primary voltage and current (except for resistive losses) because both windings have the same number of turns. These transformers prevent the transfer of unwanted electrical noise from the primary to the secondary windings, thus providing isolation.
The transformers closely associated with electronics are the power, audio, pulse, and RF transformers. They are rated according to the products of their secondary voltages and current in voltamperes (VA) or watts. The transformers specified for most electronic applications are rated for less than 100 VA or 100 W, but some switching power supplies have transformers rated to 1 kW.
Military Standard MIL-T-27 is the mandatory guide for workmanship on mil-spec transformers, but it is also widely used as a guide in the manufacture of commercial units. Commercial transformers that are connected to the AC power line are usually certified by a national organization for conformance to recognized safety guidelines because faults or failures in these transformers could cause electrocution or fires.
