A transformer is an electrical device that works on the principle of Faraday's law of electromagnetism and is used to increase or decrease the input voltage. Faraday's law predicts how a magnetic field will interact with an electrical circuit to produce an electromotive force. Explains the process of electromagnetic induction, which is the basis of many electrical devices and technologies.
Various types of transformers are available with unique features:
- Current transformer — reduces or multiplies an alternating current
- Power transformer – transmits electrical energy from one circuit to another without changing the frequency
- Potential transformer – converts voltage from a higher value to a lower value
- Pulse transformer – designed to transmit electrical pulses or high frequency signals with minimal distortion
- RF transformer – transfers energy from one circuit to another by electromagnetic induction
- Audio transformer — transmits and modifies incoming electromagnetic signals into output signals through inductive coupling
In this article, we will cover current transformers, which reduce high voltage currents to a lower value, often providing a convenient way to safely monitor a device's electrical current. These transformers can provide galvanic isolation in current sensing devices. Examples include a switched-mode power supply (SMPS), motor control, or electronic lightning ballasts.
Now that we know the basics of how a current transformer works, let's take a look at this little device.
A typical current transformer.
The marking on a current transformer is important, informing the number of turns on its secondary coil. Manufacturers use unique formats to brand each device. For example, the transformer above has 54XXXC, where three digits replace the X, representing the turns in the secondary coil. So 54050C would mean the secondary coil has 50 turns.
The package
A view of the transformer packaging.
This transformer has a polymer housing made from a material known as UL94 V-0, which meets RoHS standards. RoHS stands for Restriction of Hazardous Substances, and compliance means that an independent authority has tested a product for certain prohibited substances.
UL 94 refers to a plastic flammability standard published by Underwriters Laboratories. The standard classifies plastics, typically used in electronic devices, according to how they burn in various orientations.
The last two alphanumerics represent a flame rating. Therefore, V-0 means that the flames will dissipate in a maximum of 10 seconds, without burning the material.
The windings and pin structure
The windings inside a current transformer.
Copper wires are used as current conductors at the bottom of this current transformer. There are six terminals, of which two are connected to the secondary winding and four to the primary winding.
The primary winding has a larger cross-sectional area because the wire's resistance is inversely proportional to its cross-sectional area. The transformer input current must be greater than the secondary current. Therefore, the secondary coils have a smaller cross-sectional area to minimize the magnetic flux density.
There are two primary coils in this device with 1:1 ratio, which improves the safe current capacity of the transformer.
The pin structure
The structure of the current transformer.
This current transformer is a six-pin device. The pins serve as input and output terminals. A circular groove in the lower left corner of the transformer identifies the pin structure.
The primary windings are connected to pins 1 and 6, then 2 and 5, and then 3 and 4 of the secondary winding. The rotation ratio of the device is 1:1:200, where 200 is the rotation ratio of the secondary coil.
The internal structure
The internal structure of current transformer wiring.
The transformer's polymer housing was carefully opened to see how the copper windings overlap. The windings are placed concentrically to reduce flux leakage in the transformer.
The primary windings in a current transformer.
Core and coils
There are two single turn primary windings, which overlap the secondary winding. Insulators made of varnish or enamel separate the primary and secondary windings. The transformer also has internal insulation to prevent short circuits in the conductors.
The core and coils of the current transformer.
After removing some copper wires, it is possible to see “the core” of the transformer. This is a toroidal design, where the conductor tightly surrounds the central core with no air gap.
The toroidal ferrite core transfers energy through a magnetic field. It is used to reduce losses at high frequencies.
The toroidal ferrite core inside the current transformer.
