All You Would Want To Know About Current Transformers

A current transformer is a type of transformer that measures alternating current (AC) by producing an AC in its secondary which proportional to the AC in its primary. When the current to be measured or the system voltage of the circuit is too high, current transformers are used to provide an isolated lower current in its secondary, which is proportional to the current in the primary circuit. The induced secondary current produced is then suitable to measure instruments and process electronic equipment. This is the reason why current transformers are popularly used for metering and protective relays in the electrical power industry.


Current transformers are current sensing units of a power system. They are used to transform currents or voltages from a higher value to a value that is easy to handle for relays and instruments. In this way, they provide a convenient method of safely monitoring the actual electric current flowing in the AC transmission line using a standard ammeter. These transformers are also used to insulate the metering circuit from the primary high voltage system. There are a variety of metering applications and uses of current transformers like power factor meters, watt-hour meters, Watt-meters, protective relays, and trip coils in magnetic circuit breakers.

Construction of current transformers

Current transformers, unlike power or voltage transformers, consist of only one or very few turns in the primary winding, which can be a single flat turn or a soil of heavy duty wire wrapped around the core/conductor/bus bar placed through a central hole. Because of this type of arrangement wherein the primary winding is in series with the current carrying conductor, current transformers are also referred to as series transformers. Conversely, the secondary winding in current transformers may have a coil with large number of turns wound around a laminated core of low-loss magnetic material having a large cross-sectional area. This results in a low magnetic flux density that uses much smaller cross-sectional area wire, depending upon the amount of current that needs to be stepped down.

Types of current transformers

There are three main types of current transformers available – toroidal, wound, and bar, which are described below.

Toroidal current transformer – These transformers do not contain a primary winding. Instead, the line that carries current flowing within the network is threaded through a hole within the transformer. These transformers can be opened, closed, and installed, without disconnecting the circuit to which they are attached. For the best-in-class toroidal transformer in India, you can get in touch with Miracle Electronics, who can provide you with RoHS Compliant, REACH Compliant, and CE and UL marked transformers.

Wound Current Transformer – These transformers have their primary winding connected in series with the conductor that carries current flowing within the circuit. In these transformers, the magnitude of secondary current depends upon the turns ratio of the transformer.

Bar Current Transformer – As the name suggests, these transformers have the bus bar in place instead of the core or conductor as the primary winding, meaning a single turn. They are completely insulated from the high operating voltage of the system, and are attached to the current carrying device.


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