No study of transformers is complete without a discussion of core saturation. The ferromagnetic cores of all transformers are constrained by the limits of the flux densities they can support. At saturation, increases in the magnetic field (mmf) no longer result in proportional increases in magnetic field flux (ɸ).
Common causes of saturation include:
- Excessive applied voltage, which will cause the core to saturate during the peak of each cycle.
- Operating at frequencies outside of the transformer design parameters (for example connecting a 50 Hz source to a 60 Hz transformer).
- Poor power factor due to reactive loading.
- The presence of high harmonic content.
- The presence of DC in the primary.
Core saturation is often incorrectly cited as the reason for voltage drop in over-loaded power transformers. The logic is that too much demand will result in an increase in flux on the primary which will eventually lead to a situation where the core can no longer support additional power transfer. This theory disregards the simple fact that the magnetic flux is essentially “consumed” by the secondary and so additional demand does not, therefore, result in core saturation. In fact, power transformers can deliver significantly more than their rated currents if the coils can be kept cool and voltage drop will not become an issue until the internal impedance of the transformer becomes a significant portion of the circuit impedance.