Chapter 9 - Transformer Selection

Abstract

This chapter illustrates the properties of transformers. An ideal transformer does not allow primary current to flow if the secondary is an open circuit. In actual transformers, there is a small amount of current that flows when the secondary is open. This is the magnetizing current and is a result of the inductance of the primary winding. This magnetizing inductance should be as large as practical so that the magnetizing current is a very small fraction of the current when the transformer is delivering power. When delivering power, the ideal transformer appears as a very small resistance in parallel with the magnetizing inductance. The choice of core material directly affects magnetizing inductance. An ideal transformer core would have infinite permeability and give infinite magnetizing inductance. As with transformers, a ferrite core is the best choice. The primary reason for using a transformer is to provide safety isolation of the load from the power mains. Safety agencies have several specifications for the amount of isolation that is necessary.