MM0704, Lesson 4
inductance at between 90 and 270. Note that at 180 or when the coils are directly opposing each other, the
inductance is at a minimum. This is because the windings on L1 and L2 are in opposite directions (figure 4-13). For
90 rotation or 270 rotation, the total inductance is equal to L1 + L2. For 180, it is L1 + L2 - 2M. For 0 and 360, it
is Lt = L1 + L2 + 2M.
Figure 4-13. Plotted Angle Affect on Coupling.
Transformer Losses
Radar equipment is usually operated from special power units. Transformers are a vital part of these units. A
transformer supplies the high voltage and current required for the operation of radar equipment.
To study a typical power transformer, you need a unit that gives you maximum transfer of energy from the primary to
the secondary with minimum power loss and dissipation. In order to do this, the primary and secondary coils must be
wound closely together for coupling. A core is inserted to direct the flux through the windings, thus minimizing flux
leakage.
There are several factors to be taken into consideration in connection with the core of a transformer. Although you
reduce flux leakage by providing a path for the magnetic lines of force, there are power losses in the transformer that
must be minimized in order to obtain maximum efficiency.
There is a power loss in the primary due to the resistance of the windings. This is the normal I2R loss. In addition, the
resistance of the wire in the secondary gives I2R loss in the secondary wiring. To keep the I2R losses to a minimum,
copper wire (as large as possible without making the transformer too bulky) is used.
There are other losses called iron losses. These are caused by the effects of primary and secondary currents in the core
material (figure 4-14). The first of these losses is due to eddy-currents. When AC is applied to the windings, a voltage
is induced in the core. This voltage sets up currents in the core that are more or less circular and that are called eddy-
currents. Since the solid iron core has a large cross-sectional area and offers very little resistance, large values of eddy-
currents flow and produce heat.
89
Previous Page
