Why do we need Current Transducers?
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:
- Electric motors (about 150 A)
- Overhead lines for railroads (about 1000 A)
- Generator currents in power plants (several kA)
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 transduceris 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.
How do Current Transducers work?
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):
- A ‘magnetically good conducting’ material connects two circuits, the primary and secondary circuit. This material is often a ferromagnetic ceramic, a so-called ferrite. Primary and secondary circuits are then spirally wound around this ferrite. However, there are also designs in which the primary circuit only consists of one cable running through the center of the ferrite.
- The alternating current to be measured flows in the primary circuit (but is too high for our ammeter). This alternating current generates a magnetic field according to the so-called ‘Amperè’s law’. This magnetic field changes its polarity continuously, similar to the alternating current flowwing in the circuits.
- The magnetic field mentioned is present in the whole ferrite, because ferrites are magnetic ‘conductors’. It is therefore also present in the secondary circuit. According to another physical law, the law of induction, this ‘changing’ magnetic field generates a current in the secondary circuit. In the case of a current transducer, the secondary current is smaller than the primary current.
Figure 1: Schematic construction of a current transducer
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.
Imperfections of real Current Transducers
Nevertheless, a current transformer is rarely perfect. Two major error variables of current transformers are:
- Transformation error: The real transformation ratio can deviate from the ideal transformation ratio. The transformation error indicates this difference as a percentage value.
- Error angle: The phase between the primary and secondary currents can deviate (the currents do not change polarity synchronously). The error angle indicates the difference of this phase in units of degrees.
Current Transducer at DEWETRON
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°.
Figure 2: Current Transducer PA-IT-65-S from DEWETRON
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