Transformers

Unlike the generator, a transformer cannot be used to convert mechanical energy into electrical energy, it being able to transform electrical energy from one circuit at the same or some other voltage.

Essentially, a transformer consists of two coils, not electrically connected to each other, but wound over a common core. The core may also be of open type, or it may be merely a tube of some insulating material, the latter being referred to as an air core.

If a varying voltage be applied to the primary coil, the electromagnetic field set up around the coil will rise and fall in accordance with the e. m. f. variations applied. This moving field cuts the turns in the secondary coil and induces an e. m. f. therein. The value of this induced e. m. f. depends upon the strength of the applied e. m. f. and the ratio of se­condary turns to primary turns. Should there be twice as many turns in the secondary as in the primary, the voltage in the secondary would be twice that applied to the primary. If there were half as many turns in the secondary, the voltage would be half that applied to the primary. This voltage step up or step down in proportion to turn ratio will hold good for all combinations. Where the voltage is raised, however, amperage is lost in the same proportions, and vice versa. Therefore, the power in watts supplied to the transformer is the same as that drawn from it, assuming the transformer to be 100 per cent efficient. The copper losses, or ohmic re­sistance of the windings, and the core losses due to the induc­tion of eddy currents in the core material, as well as hyste­resis or molecular friction caused by changing polarity of the current applied, all combine to reduce modern transfor­mer efficiency to about 90 per cent.

Transformers are classed according to the use they are de­signed for. Where it is desired to step up a low-voltage a. c. supply to a value useful for radio receivers and transmitters, a power transformer is used. The windings are sufficiently heavy to carry the current without undue heating, and the secondary may consist of two or more separate windings to provide various voltages from one input source.

As the iron core increases, the inductive reactance of the transformer increases, and inasmuch as this type of reactance also increases with the frequency of the applied alternating current, there is a limit to the frequencies that can be efficiently used in transformers with metallic cores. Where very high-frequency alternating current is used such as in the r-f circuits of receivers and transmitters, air core transformers must be used to eliminate prohibitive core losses.