Split Core Current Transformer Design
Just like the typical current transformer, the split-core current transformer measures alternating current flowing through a conductor. The distinguishing feature of the split core current transformers is that their design permits them to be assembled around a buss bar without disconnecting the buss bar.
The typical current transformer is usually a toroidal coil, which is slipped over the end of a buss bar, hence requires disconnecting the buss bar. “C” core and “U” core structures are commonly used for split-core current transformers because they are relatively easy to take apart and put back together around the buss bar. Some sort of bracketry or band clamps and holds the assembled pieces of the split-core current transformer together. Historically, this has not been as practical (but is possible) for toroidal coils, where the bracketry is more complicated. Typically, the coil(s) must be sector wound on the toroid before cutting the core in half, whereas the U and C core structures of the typical split-core current transformer permit use of bobbin wound coils which can be wound independently of the core. There are now some flexible toroids which permit the split-core feature of installing it around a buss bar.
The electrical performance of split-core current transformers is not as good as that of the continuous toroidal coil. The circle like (or ring like) shape of the toroid usually offers a shorter magnetic path length than other cores. Since the toroids are continuous, they do not add any air gap to the core structure. Split-core current transformers (including toroidal split- cores) add some air gap to the core structure. Consequently, the split-core current transformers will draw more magnetizing (exciting) current than a continuous toroidal current transformer made of the same core material (assuming comparable size and/or weight). The toroidal shape provides better magnetic coupling and less leakage inductance than the C and U core structures commonly used in split-core current transformers.
Split-core current transformers for lower frequency applications (power frequencies) typically use grain oriented silicon steel or nickel alloys for the core material. There are some more exotic materials available. The material is cut into strips and then wound on an arbor (mandrel) to form a core. The core is then cut in half. These are known as tape-wound cores because their construction resembles a roll of tape. Strip thickness ranges from 0.025 to 0.0005 inches. The thinner strips have less core loss at higher frequencies hence they are used in higher frequency applications up to about 10 kilohertz. High accuracy current transformers require low core losses hence they either utilize the thinner strip thickness, the lower core loss materials such as the nickel alloys, or both. Ferrite materials are usually used for very high frequency designs, up to several megahertz. Some very specialized applications may require a core-less (air-core) coil.