Source: Nanjing Evergrowing Resources Co.,Ltd

A current transformer is actually a small current converter. It converts large currents in a circuit into small currents that can be used for measurement and metering. It is mainly suitable for current monitoring and electric meter metering, and can also provide working power for relay protection, such as thermal relays.

A current transformer consists of a closed iron core and two windings wound on the same iron core, divided into primary windings and secondary windings. The primary side winding has very few turns and needs to be connected in series in the measured circuit. The secondary side winding has more turns and is connected in series in the measuring instruments and protection circuits. When a large current passes through the primary side winding, the secondary winding wound on the same concentric core induces a small current required for current measurement and circuit protection according to the principle of electromagnetic induction.

The current we usually see in ammeters, watt-hour meters, numerical values, and the operating current of thermal relay protection is provided by the secondary side of the current transformer, which is a small current. Some transformers can be directly connected in series in the measured circuit, while others cannot. Because the current in the circuit directly connected in series with the measured circuit is relatively small and the cable is relatively thin, it can be directly connected to the primary side winding of the current transformer. However, if the current of the measured circuit is very large, tens of amperes, hundreds or even thousands of amperes, and the capacity of some ammeters is too small, they cannot be directly connected in series in the measured circuit. In this case, the wire of the measured circuit needs to pass through the annular iron core of the current transformer, and the wire at this time is the primary winding.

If using a current transformer in conjunction, how should the current be calculated?

Current transformers all have a certain transformation ratio, such as 50:5, 200:5, or 400:5, etc. Similarly, when selecting an ammeter, its ratio must be the same as that of the transformer, so that the reading of the ammeter can be more accurate.

From: Alvin Zhu

E-mail:alvin.zhu@evergrowrs.com