Transformer switch on saturation

[R.G.]

In the first plot there is no load on the transformer as before. This transformer has about 2.2 ohms primary resistance. The plot shown is near to the highest measured; one measurement that I did not record peaked at about 21 amps. However, no measurement came close to the much higher values one would expect if true saturation was achieved. So there are two indications that, although there are current spikes short of complete saturation, the peaks do not really get there:
1. The peak values are too low.
2. Tranaformer action, although modified during the transient, does not cease.

You really do have to control turn on and turn off time in the half cycle to get the maximums. That's why I was after a non-zero-crossing SSR in my thought-design. For maximum remanence, you want a voltage (... er, or was it current?) peak turn of time. For maximum V-T integral, you need to turn on right at a zero crossing. For maximum entry into saturation, you want the remanence in the same direction as the current half-cycle is trying to drive the core.

And what maximum you get depends on the line voltage and how close to the edge of saturation the original designer decided to run the iron. Normal 4% silicon steel has a sloppy, slushy saturation region, so it's tolerant of minor overvoltages. Well, unless the designer tried to wring the last cent out of the iron and copper.

The maximum depends to some extent on the winding configuration, as well. Leakage inductance, defined as the inductance of magnetic flux that does not travel through the core, but instead through the air (or beeswax, or varnish, or vacuum, or asphalt) surrounding the copper coils. There's always some leakage, and air (or beeswax) does not saturate. Neither does the air gaps that are inevitable in E-I laminations. This is another reason that toroids are better for size, weight, and magnetizing current, but more prone to saturation.

This is a very heavy transformer; that is, it has a lot of steel.

The interaction in a transformer design used to be dominated in a three-way contest between iron cost, copper cost, and insulation class. You can use less iron if you use more copper (i.e. more turns) and a higher insulation class because the copper gets hotter. You can use less copper and lower insulation class if you use more iron. You can use less iron AND copper if you just let the thing get HOT and use Class F or H insulation - which is expensive too.

The second plot shows the results of a full wave rectifier supplying a 100 microfarad capacitor. Note that:
1. It takes several cycles to charge the capacitor.
2. The secondary voltage (5 volt winding) is reduced during the charging process, but this indicates that the primary voltage is reduced..

This brings up an interesting question. It is the primary voltage that is reduced during the charge. Saturation is critically dependent on the primary voltage. So, in a different transformer, where saturation is an issue with no load, does the capacitor charging eliminate the possibility of saturation?

Good question. Primary voltage is decreased because of the voltage lost to leakage inductance and primary wire resistance. Only what remains after these are accounted for can work the iron. There's a bit of complication in accounting for this. You have to use an unloaded winding to measure the equivalent referred primary voltage, because loaded secondaries also cause secondary voltage losses through their own leakage and resistive losses in their current flow.