# Transformer ratings

1. What physical factors one needs to take into consideration (like maximum core field and saturation) to rate the "voltage rating" of a transformer?
2. Suppose that the voltage rating of a transformer isn't known (or you aren't allowed to look at its nameplate), is there an experimental way to determine the voltage ratings of the transformer?
3. As far as I understand, transformers are usually operated at their rated voltages. Also, if supply exceeds the rated voltage, current starts leaking through insulations, the core gets heated up and so on. Why then is it so that transformers are operated at a verge, voltages beyond which beyond could possibly harm the transformer?
• 1. Core saturation. 2. Measure the magnitizing current. When it starts to distort/duck tail at the end of the sine wave, you have reached the limit. 3. Incorrect. You reach saturation and magnitizing current becomes excessive, tripping some protection, fuse or breaker. Commented Jul 15, 2019 at 6:06

1) Core saturation limits the maximum voltage.time product you can apply. Therefore at a higher frequency, you can use a higher voltage.

2) If you have a variable voltage supply, say a variac, or a sine-wave generator and a power amplifier, then measuring the magnetising current while increasing the voltage into a single winding on an unloaded transformer is the best way. Plot the measured current against the applied voltage. The graph will suddenly kick upwards at the onset of saturation. Exactly where you want to operate in the vicinity of this 'knee' is up to you.

If you don't have a variable voltage supply, then a constant or limited current supply from a high voltage is a good second. Use a low power filament lamp in series with a high voltage to limit the current into a low voltage winding. Measure the voltage developed. Note this is likely to be already in saturation, so you should operate comfortably below this.

3) Operating at the rated voltage, core heating occurs through hysteresis loss (function of the core magnetic properties alone) and through eddy current losses (function of its electrical conductivity and lamination thickness). If supply exceeds rated voltage, magnetising current rises dramatically and increases heating in the primary winding. Core heating increases a little due to the slightly increased field.

Transformers are operated at winding voltages suitable for their core saturation, and at maximum voltages between windings given by the insulation between the windings. If you exceed either of these, the transformer will fail.