New Materials

Like all electronic apparatus, transformers are subject to continual change. This is especially so since the introduction of new materials such as

• Grain-oriented core steel.
• Solventless impregnating varnish.
• Inorganic insulating tape.
• Improved wire enamel.
• Low-loss, powdered iron cores.
• Ferrite cores.

Through the application of these materials, it has been possible to

• Reduce the size of audio and power transformers and reactors.
• Increase the usefulness of saturable reactors as magnetic amplifiers.
• Reduce the size of high-voltage units.
• Design filters and reactors having sharper cut-off and higher Q than previously was thought possible.
• Make efficient transformers for the non-sinusoidal wave shapes such as are encountered in pulse, video, and sweep amplifiers.
• Extend the upper operating frequency of transformers into the high-frequency r-f range.

Occasionally someone asks why electronic transformers cannot be designed according to curves or charts showing the relation between volts, turns, wire size, and power rating. Such curves are very useful in designing the simpler transformers. However, this idea has not been found universally practicable for the following reasons:

(a)  Regulation. This property is rarely negligible in electronic circuits. It often requires care and thought to use the most advantageous winding arrangement in order to obtain the proper IX and IR voltage drops. Sometimes the size is dictated by such considerations.

(b)  Frequency Range. The low-frequency end of a wideband transformer operating range in a given circuit is determined by the transformer open-circuit inductance. The high-frequency end is governed by the leakage inductance and distributed capacitance. Juggling the various factors, such as core size, number of turns, interleaving, and insulation, in order to obtain the optimum design constitutes a technical problem too complex to solve on charts.

(c) Voltage. It would be exceedingly difficult, if not impossible, to reduce to chart form the use of high voltages in the restricted space of a transformer. Circuit considerations are very important here, and the transformer designer must be thoroughly familiar with the functioning of the transformer to insure reliable operation, low cost, and small dimensions.

(d)  Size. Much electronic equipment is cramped for space, and, since transformers often constitute the largest items in the equipment, it is imperative that they, too, be of small size. An open-minded attitude toward this condition and good judgment may make it possible to meet the requirements which otherwise might not be fulfilled. New materials, too, can be instrumental in reducing size, sometimes down to a small fraction of former size.

In succeeding chapters the foregoing considerations will be applied to the performance and design of several general types of electronic transformers. The remainder of this chapter is a brief review of fundamental transformer principles. Only iron-core transformers with closed magnetic paths are considered in this introduction. Air-core transformers, with or without slugs of powdered iron, are discussed in a later chapter on high-frequency transformers. Most transformers operate at power frequencies; it is therefore logical to begin with low-frequency principles. These principles are modified for other conditions in later chapters.

Fig. 1. Transformer coil and core.

A simple transformer coil and core arrangement is shown in Fig. 1. The primary and secondary coils are wound one over the other on an insulating coil tube or form. The core is laminated to reduce losses. Flux flows in the core along the path indicated, so that all the core flux threads through or links both windings. In a circuit diagram the transformer is represented by the circuit symbol of Fig. 2.

Fig. 2. Simple transformer.