A transformer is a device which consists of two windings called primary and secondary. It transfers energy from one side to another with suitable change in the level of current or voltage.
Related: Working Principle of Transformer
A current transformer (CT) is a type of transformer that is used to measure AC current. It produces an alternating current (AC) in its secondary which is proportional to the AC current in its primary. Current transformers, along with voltage or potential transformers are Instrument transformer.
Current Transformer Symbol / Circuit Diagram
A current transformer (CT) basically has a primary coil of one or more turns of heavy cross-sectional area. In some, the bar carrying high current may act as a primary. This is connected in series with the line carrying high current.
The secondary of the current transformer is made up of a large number of turns of fine wire having small cross-sectional area. This is usually rated for 5A. This is connected to the coil of normal range ammeter.
Working Principle of CT
Construction of Current Transformer
- Wound Type CT
- Torroidal (Window) Type CT
- Bar Type CT
1. Wound Type Current Transformer
The construction is shown below.
In a low voltage wound type current transformer, the secondary winding is wound on a bakelite former. The heavy primary winding is directly wound on the top of the secondary winding with a suitable insulation in between the two. Otherwise the primary is wound completely separately and then taped with suitable insulating material and assembled with the secondary on the core.
The current transformers can be ring type or window type. Some commonly used shapes for the stampings of window type current transfonncrs are shown in the figure below.
The core material for wound type is nickel-iron alloy or an oriented electrical steel. Before installing the secondary winding on core it is insulated with the help of end collars and circumferential wraps of pressboards. Such pressboards provide additional insulation and protection to the winding from damage due to the sharp corners.
2. Bar Type Current Transformer