ABSTRACT
One of the major problems with large power transformers is audible noise that can be loud enough to be very annoying. Essentially all transformer noise is due to a phenomenon called magnetostriction. When a strip of steel is magnetized, it contracts very slightly. At the flux densities used in large power transformers, the amount of magnetostriction is only about 60 µ per meter of length. For a 60 Hz transformer, this small change in dimension occurs 120 times per second. Due to the fact that magnetostriction is not linear with respect to the flux density B there are also harmonics of 120 Hz present in the noise. If any part of the transformer is in resonance with any of the harmonics, the noise can be amplified hundreds of times. Therefore, part of the core design and the overall transformer design is an analysis of the resonant frequencies.
We are now prepared to complete our prototype transformer by adding a second winding. It should now be apparent that by coiling the primary conductor around the core using many turns, a considerable voltage can be induced with only a tiny magnetizing current if the peak flux density is kept below the saturation value. Now suppose a secondary coil is wound around the same core, surrounding the same magnetic flux as the primary coil as shown in Figure 1.15. Since the secondary coil encircles the same flux as the primary
coil, the induced voltage per turn is the same in both the primary and secondary coils. Let Ep applied primary voltage, Es induced secondary voltage, Np number of primary turns, and Ns number of secondary turns. Since the induced voltage in the primary coil equals the applied voltage and since the induced volts per turn is the same for both primary and secondary,
