Current Transformer Basics: What You Need To Know About Current Transformers

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A current transformer is a device that is designed to create an alternating current from a direct current. The device will measure the direct current in its primary winding and convert it to a proportional alternating current in its secondary winding. They are used to reduce a high voltage current into a much smaller and easier to manage current that can be easily monitored.

Typical Current Transformer

While there are several different types of current transformers, the basic construction is much the same. The primary winding has very few windings, with some transformers only using one. This primary winding can consist of one single flat turn, a coil of wire wrapped around a core or a single bus bar placed in the center of the transformer, depending on the style of transformer chosen. Current transformers are also commonly referred to as series transformers.

The secondary winding of a current transformer will generally consist of a large number of turns centered around a magnetic material core. The core material can vary and knowing how much current will be “stepped down” will determine which transformer is right for the job.

There are three basic types of current transformers: wound, toroidal and bar.

Wound current transformers – Wound current transformers have a primary winding that is directly connected to the conductor in a series. The conductor is the device that measures the actual input current. The resulting “stepped down” current is completely dependent on the turns ratio available between the primary and secondary windings in the transformer chosen.

Toroidal current transformers – Toroidal current transformers are slightly different in that they don’t have a primary winding at all. Rather the conductor is threaded directly into the transformer via a hole or window. This is a configuration that is commonly seen in split core transformers because it allows them to be opened and closed without disrupting the circuit they are attached to.

Bar type current transformers – A bar type current transformer doesn’t have a primary winding of its own. It uses the actual cable or bus bar of the existing circuit as its primary winding, giving it a single turn type configuration. These are often used in high voltage circuit situations because of their ability to be 100% insulated from high operating voltage environments and ability to handle larger currents.

As mentioned previously, the goal of a current transformer is to reduce current levels. They are used in circuits that carry thousands of amps to create a standard, usable output of 1-5 amps. Not only do they create “usable” currents, these stepped down currents can be more accurately monitored and controlled by devices because the devices are shielded from the high voltage lines. Without this insulation, readings are often skewed due to electrical interference. Common metering devices used include watt maters, power factor meters, and more.

While there are a range of different current transformer capabilities, most will come with a standard secondary rating of 5 amps. These are expressed as 100/5, indicating that the current at the primary winding being 100 times greater than the output at the secondary winding. Currents can be reduced even further, often down to 1 amp by simply increasing the number of secondary windings. This is because the number of turns in the primary and secondary windings are inversely proportional.


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