The choice of magnetic material used in a current transducer impacts the cost, size, weight as well as performance aspects such as linearity, phase-shift and stability over temperature. Consequently, the choice of material depends on the application. Split-core current transducers usually use either ferrite, FeSi or FeNi.
FeSi split-core current transformers
FeSi current transformers are relatively low cost but suffer from poor linearity and drift, mainly due to the air gaps induced by the split-core architecture. They are also heavy and bulky – thus, not very suitable for environments with limited space. Poor linearity, especially at low currents, and large phase shift limits the use of FeSI to low cost applications not requiring a high degree of accuracy. A typical application is branch current monitoring in panel boards to detect overload risk and trigger an alarm or load balancing.
FeNi split-core current transformers
FeNi has been the best material for split-core current transformers for a long time, offering good performance, but at a relatively high cost. FeNi offers a good alternative to the FeSi material when accuracy and phase shift are important, or when transformers need to measure small currents. Apart from the price, FeNi current transformers have some other limitations.
Ferrite split-core current transformers
Although ferrite materials have been well known for years, their poor performance in terms of saturation level and magnetic permeability did not allow their use at frequencies as low as 50/60Hz. However, new types of ferrite have significantly improved permeability and can be implemented in 50/60Hz current transformers as a substitute for FeSi or FeNi cores, despite the low magnetic saturation level. Split-core current transformers implementing these new types of ferrite can perform accurate measurement of AC signals in an extended frequency range that includes the 50/60 Hz application domain.
Ferrite provides high accuracy and excellent linearity even at very low current levels. The material also offers low phase-shift between input and output currents. The hard and dense core allows for the minimization of air gaps and, in contrast to FeSi or FeNi, is virtually insensitive to ageing and temperature changes. Ferrite is also available at low cost, which makes high performance split-core ferrite current transformers an attractive choice. However, the large ferrite cores required for higher currents are not easy to manufacture. Consequently, for higher currents FeNi transformers or Rogowski Coils are typically a more appropriate choice.