Transformer saturation tests

[Chris Wilson]

I have a big high voltage single phase mains transformer. Two bobbin type, bobbins in parallel. I want a tad more volts from the secondary. I am running on the 0 and 240V primary taps. Mains here is variable, as I am out in the sticks. I know when it's cold, or dinner time, the mains can drop to 220V. The transformer has -10, 0, 180, 220, and 240 volt taps. If I mess with the taps 10 volts at a time, how can I tell if it's close to saturation? I considered a couple of loose turns of insulated wire around the outside of one of the bobbins, connected to my scope. is this a viable way to see if the waveform is beginning to flat top? It's duty cycle will be low, but it was actually built for CCS, so I feel I have some leeway. Thanks.

[megajocke]

Saturation will start to happen just before the zero crossings of the voltage so not much will happen to the tops. I'd suggest looking at the primary current. It tends to increase sharply as saturation is reached.

[moepower]

Yeah it is not clear exactly what the specifications of that transformer is.  However, the previous post is right.  Evidence of saturation is when your current waveform, under no-load, exhibits sharp "peaky" type wave-shape.   Under no-load, the current should be around 5% of it's rating, which is the magnetizing current.  If your current is peaking much above 5% of its rating (e.g., 5% of sqrt(2) * rated current(rms)), then you're getting into saturation.

[SeanB]

Why not just wind a few extra turns on the secondary side using the same size wire as the existing wire? Simple to do and will not result in saturation of the core at all. Most transformers are run pretty close to saturation unless they were designed and built 20 years ago when copper prices were lower so manufacturers would not skimp on copper or iron losses much. If there is not enough room then I would suggest pulling off the secondary covering to get some room.