Beckhoff SCT Series, Documentation

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Product overview
SCT5xxx 9 Version: 1.0
2.2 Basics current transformers
Basic information on the technology area of current transformers is given in the following. The information is
of general nature; so it is imperative to check the extent to which these notes apply to your specific
application.
Function and design
A current transformer is a transforming device that transforms an input current into a processable current
signal at the output. A current transformer is mainly used to transform currents of large magnitudes to directly
measurable, smaller values in the milliampere or small ampere range. With a classic current transformer, the
input current is proportional to the output current. Due to the physical principle and the mechanical
construction, the current signal is transmitted galvanically isolated to the evaluation electronics.
A current transformer basically consists of a small number of windings on the primary side and a larger
number of windings on the secondary side. The current to be converted flows through the primary side. The
windings are usually wound on an alternating magnetic ferrite ring core.
A typical transformer type is the ring core or ring-type current transformer. The current rail or current-carrying
cable is often used as the primary winding, which is guided through the toroidal core of the transformer.
Thus, the rail or line forms the primary winding with one turn. The secondary winding is located on the ring
core. The transformation is determined by the ratio of the number of primary and secondary windings. The
classic structure of a ring-type current transformer is shown in the following figure.
Fig. 2: Principle ring-type current transformer
Another classic type is the coiled current transformer. In this type of transformer, the primary winding is a
current-carrying wire which is wound around the ring core on the primary side. In this case, the primary
winding number is > 1, but smaller than the secondary winding number. The principle is shown in the
following figure