Current transformer vs voltage transformer, many experienced electricians have a vague concept!
A current transformer is similar to a step-up transformer. The primary coil is the power line, usually with only one through turn, while the secondary coil has tens to hundreds of turns depending on the transformation ratio;And the Voltage Transformer is actually a step-down transformer, with the primary coil with more turns connected to the high-voltage system, and the secondary coil with fewer turns connected to the secondary system with lower voltage.
When in use, the primary coil of the current transformer is the power line, which can be directly passed through the transformer without any direct electrical connection between the two. The function of a current transformer is to convert high current into low current, so it only requires a current signal, not a voltage signal. Based on this, the resistance values of the secondary coil of the current transformer and the current coil of the connected instrument are very small, similar to short-circuit operation.
Seeing this, some people may ask why the voltage of the secondary coil of the current transformer is very low, even though the voltage of the power line passing through the transformer is not low and there are so many turns of the secondary coil? The answer is actually very simple.
For example, in a low-voltage 220V single-phase circuit, only the live wire passes through the transformer. This means that the two ends of the primary coil passing through the transformer are actually two points on a live wire.
Obviously, the voltage between these two points is 0V. Therefore, even if the number of turns of the secondary coil of the transformer is much more than that of the primary coil, the secondary voltage will not be too high.
In addition, if the same current needs to be measured, the current coils of each instrument connected to the secondary coil of the current transformer are in series, as shown in the two ammeters in the above figure, because the current in the series circuit is equal.
When a voltage transformer is in use, it converts a high voltage into a low voltage. The primary coil is connected in parallel with the primary system, and the voltage to be measured is connected in parallel with whoever needs to be measured
Due to the fact that voltage transformers only require voltage signals and there is no need for current, the resistance between the secondary coil and the connected voltage coil is very high, similar to no-load operation. In addition, when obtaining the same voltage signal, the voltage coils are in parallel because the parallel circuits have equal voltages.