In practice, it is often necessary to know the electrical currents in a component or power cable. Standard ammeters usually cannot measure more than a few tens of amperes of current. While this is sufficient for many applications, it is easy to imagine scenarios where the currents are much higher:
As for this, current transformers (also known as current transducers) make it possible to carry out a measurement under such conditions. They convert a high current at the input into a smaller, but proportional current at the output. For example, 1000 A at the input (which are difficult to measure) can be converted into 2 A at the output (which are more easily measured with standard ammeters). In this case, the proportionality factor is 1000 A/2 A = 500. The principle of a current transducers thus similar to that of a voltage transformer. However, measurement technology is not the only use case of current transformers. Current transducers can be applied in control technology or for protection purposes. In such applications, the reduced current passes on to RCDs or control instruments.
The most common type a current transducer is the inductive current transformer. Its main use case is in converting alternating currents. Inductive current transformers work as described (Figure 1):
If one looks more closely at the underlying physical equations, it is possible to determine the ‘conversion ratio’. The conversion ratio indicates how much the current reduces by the current transducer (corresponds to the proportionality factor mentioned above):
As can be seen, the fraction of windings (N) of the primary and secondary circuits (N_primary/N_secondary) corresponds to the conversion ratio. In practice, there are further possibilities to realize a current transformer. As an example, well-known constructions are based on Hall-Probes or Rogowski-Coils.
Nevertheless, a current transformer is rarely perfect. Two major error variables of current transformers are:
DEWETRON is a company that offers its customers high-precision measuring and testing equipment. To complement our TRION and TRION3 measuring modules, we also provide high-quality current transformers. These offer the optimal solution for a wide range of different tasks. A characterizing feature of our models is the high conversion ratio of up to 2000 A/A. They offer an outstanding accuracy with transformation errors of as low as 0.002 % and error angles of up to less than 0.01°.
If you want to learn more about DEWETRON and our application areas, please feel free to contact us.