Materials of CT Cores
Current Transformers, or CTs are widely used in electrical devices and instruments. The core on which the secondary wire is wound plays a significant part in the performance of a CT. Core materials include silicon steel, ferrite, permalloy, nanocrystalline, amorphous.
Usually, we want the magnetic materials which have high permeability. In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself otherwise known as distributed inductance in Transmission Line Theory.
Nowadays, considering the high permeability and low coercity, more and more engineers choose nanocrystalline cores for AC transformers. An AC transformer is an electrical device that is used to change the voltage in alternating current (AC) electrical circuits.
An induction motor is like a transformer with the motor’s stator acting as the primary winding and the rotor the secondary. Nanocrystalline cores can be used in AC motor circuit to make the core loss very low.
An ammeter (from Ampere Meter) is a measuring instrument used to measure the current in a circuit. Moving iron ammeters are commonly used to measure current in industrial frequency AC circuits.
Fe-based Amorphous strip is a new soft magnetic alloy. It is produced through the advanced technology of rapid solidification of molten metal at a cooling rate of about a million K/sec. During this process the metal is rapidly quenched as a form of ribbon with 25-35 µm thickness, and the micro-structure of the alloy is the amorphous due to the high quenching rate.
Fe-based nanocrystalline alloy is made of Fe,Si,Boron,Nb,Cu,etc. Forming the very fine grains whose size is only 10 to 20 nm when the annealing temperature of Iron-based Amorphous alloy contains Cu and Nb is above the crystallization temperature,and the crystalline material is called Nano-crystalline alloy.Fe-based nanocrystalline alloy strip has the properties of high saturation magnetization and permeability,low coercive force and core loss,good stabilities on temperature and time effect,excellent abilities of anti-wearing and anti-rusty,and it shows the best property-value ratio among all of the soft magnetic materials.Substitution for permalloy and ferrite,it’s the ideal material of Mid-high frequency transformer and inductive components.
- EMC common mode, nornal mode filter chokes.
- Cores for main transformer of switch power supply and control transformer.
- Different accuracy CT/VT cores.
- Mag-amp and pulse killer cores.
|Silicon Steel||High Bs, High Iron Loss|
|Ferrite||Low Bs, Low Price|
|Permalloy||High Permeability, High Price|
|Nanocrystalline||High Permeability, Moderate Price|
|Amorphous||High Bs, Moderate Price|
The 1k107 iron-based, or Fe-based nanocrystalline alloy is an amorphous material formed by a rapid solidification process of an alloy composed of a small amount of Nb(Niobium), Cu(Copper), Si(Silicon), and B(Boron) elements. After heat treatment,extremely small crystals with a diameter of 10-20 nm can be obtained and dispersed on an amorphous matrix, which is called nanocrystalline or nano crystalline material.
|Saturation induction Bs(T)||1.25|
|Curie temperature Tc(℃)||570|
|Crystallization temperature Tx(℃)||500|
|Saturation magnetostriction λs||2.0×10-6|
Nanocrystalline materials have excellent integrated magnetic properties: high saturation magnetic induction (1.2T), high initial magnetic permeability (8×104), low Hc (0.32A/M), low frequency loss under high magnetic induction (P0 .5T/20kHz=30W/kg), the resistivity is 80μΩ/cm, which is higher than that of permalloy (50-60μΩ/cm). High Br (0.9T) or low Br (1000Gs) can be obtained by longitudinal or transverse magnetic field treatment. It is the best comprehensive material on the market at present; the applicable frequency range is 50Hz-100kHz, especially 20kHz-50kHz. It has been widely used in high-power switching power supply, inverter power supply, magnetic amplifier, high-frequency transformer, high-frequency inverter, high-frequency choke core, current transformer core, leakage protection switch, common mode inductor core.
Nanocrystalline cores are widely used in automotive applications, car manufacture, offering real benefits in terms of comfort, protection and energy conservation.
Dual cores, which contain both nanocrystalline and amorphous, are also widely used in DC Immune Current Transformers. The type of core determines price and accuracy. We choose amorphous & nanocrystalline core to make DC immune and Anti-Magnetic current transformers. Accuracy is comprised by the actual input to output transfer ratio, as well as linearity and phase shift. While phase shift is of little significance for current measurements, in the measurement of power, an uncompensated phase shift will lead to large errors in measurement of real power and power factor.