SATURATION: What’s All the Flux About?
Transformers have saturation problems that limit their capabilities at low frequencies.

In fact, a transformer that is doing a great job at 100 Hz can be an amplifier killer just an octave lower.

What happens when a distribution transformer saturates? What does saturation sound like? Does this mean you have to high pass all your HV systems?

These and other questions are answered below.

What Happens When a Transformer Saturates?
Transformers transfer power from winding to winding by coupling through mutual magnetic fields. This transfer of power is amazingly efficient, and it happens with or without a core.

However, the iron core plays two essential roles:
1) The core contains the magnetic fields. Without a core, significant portions of the magnetic fields balloon out around the windings, reducing mutual coupling and potentially causing interference problems.
2) The magnetic field in the core itself opposes uncoupled current flow in the primary. This is why the transformer primary, even though it is made of heavy copper wire, does not normally act like a short.

NOTE: Without a core, the primary does act like a short, and a saturated core is not much better than no core.

Here are the primary voltage (top) and current (bottom) oscilloscope traces for a distribution transformer entering saturation toward the end of each half cycle of input. Both waveforms should be sinusoidal, but the spikes on the current waveform are due to saturation.

Notice that the power amplifier, in this case, is still doing a good job of delivering a sine wave, in spite of the current spikes. A less-robust power amplifier would show noticeable distortion coinciding with each spike. The coupling between primary and secondary is not much affected by core saturation. However, during saturation, the DC resistance of the primary suddenly appears in parallel. The power amplifier tries to maintain its output voltage, but the load impedance has taken a sudden dive.

Core saturation happens when the magnetic field in the core reaches its maximum possible density, which is what happens when the applied voltage polarity remains the same for too long.

NOTE: Saturation has nothing to do with power delivery: the onset of saturation depends only on the voltage waveform applied to the primary.

To reinforce this point, the next graphic shows amplifier voltage and current with an unloaded (open circuit) secondary. The current waveform stays near zero until the volt-seconds limit of the primary is reached.

For audio signals, core saturation is more likely as you lower the frequency and raise the applied voltage. In the preceding photo, the input was 20 Vrms at 25 Hz. The onset of saturation depends on voltage and time, so expect to see a similar problem for 40 Vrms and 50 Hz.

NOTE: The saturation voltage of a transformer rises linearly with frequency. This means that power handling capability declines with the inverse square of the frequency.

It should be no surprise that DC voltages are a serious problem for transformers, since DC is like zero frequency. Indeed, dc offsets of a few tens or hundreds of millivolts can asymmetrically saturate a transformer, meaning that saturation current spikes will occur primarily on one signal polarity.

For this reason, some power amplifiers are a poor match to distribution transformers, This is also one reason why some manufacturers recommend installing resistors and capacitors in series with distribution transformers, further impacting low frequency response. Well-designed power amplifiers have low offsets, and well-designed transformers tolerate a reasonable level of offset.