MM0704, Lesson 4
There is another type of RF transformer in which a higher coefficient of coupling is desired. To get it, a powdered iron
core is inserted in the windings. This core is nothing more than small particles of iron suspended in a plastic material
so that they are insulated from each other. Although there is no complete magnetic path for this flux, this type of core
does offer a higher degree of coupling than air. Usually these cores can be adjusted, in and out of the windings, to
In your study of magnetization curves, you noted that, if the magnetizing force is increased enough, a point known as
saturation would be reached. The ordinary power transformer is operated well below saturation. However, there are
special circuits in which magnetic saturation is used. A transformer used in a circuit where saturation is desired is
called a saturable transformer.
After the point of magnetic saturation is reached, the flux density is practically constant even though the force causing
the flux is increased. Also, a stationary flux cannot induce a voltage. With these facts in mind, the operation of a
saturable transformer will be easy to understand.
In the unsaturated transformer, the current in the primary lags the voltage across the primary by 90. Figure 4-25
shows this phase relationship between Ep and Ip. Also, the value of current will depend upon the magnitude of the
Figure 4-25. Phase Relationship Between Current and Voltage on the Primary
of an Unsaturated Transformer.