Ferromagnetic Core Design Amp; Application Handbook

[Eri Pfaff]

The MLX91217 is part of the Gen 2.5 high speed current sensor IC portfolio, targeting primarily inverter/converter applications. With the Conventional Hall technology - using an external ferromagnetic core for signal concentration - currents can be measured in the range of 50 A to 5000 A.

The MLX91217 is part of the Gen 2.5 high speed current sensor IC portfolio, targeting primarily inverter/converter applications. With the Conventional Hall technology - using an external ferromagnetic core for signal concentration - currents can be measured in the range of 50 A to 5000 A. The magnetic flux density inside the airgap of the core is measured with embedded Hall-effect technology and amplified to a 5 V full-scale ratiometric and fast analog output. The novel CMOS Hall-effect sensor is offered with different factory trimmed offset, sensitivity and filtering settings, but are customer programmable using the Melexis programming equipment PTC-04.

The main changes over the Gen 2 MLX91209 are:

The SIP4-VA package allows for insertion of the sensing elements into tight slots of the core, and several Trim&Form options of the sensor leads provide designers with higher flexibility for mechanical integration & assembly.

The current focus on electric vehicules is boosting the whole e-mobility industry. It is especially beneficial for the 2 and 3 wheelers which also require to solve similar constraints: from battery management (sufficient power, longevity and safety) to drive train efficiency and personalization.

Combining exercise and mobility was never so easy thanks to E-bikes. Because they support you while moving, they are an attractive option. Modern technology optimizes the pedal assist by the electrical engine while minimizing the battery usage. Thanks to Melexis ICs you control the calories and the battery energy you burn.

The function of the onboard charger is to efficiently, reliably and safely convert AC electricity from the electrical grid (e.g. household plug) to DC electricity so it can be stored in the battery of the electrified vehicle (xEV).

In electric vehicles, the inverter converts DC power from the battery to AC power for the motor drive, which in turn transforms it into propulsion power.The efficiency of this system has a direct impact on the all-electric range, road performance and comfort.

A transformer core is a structure of thin laminated sheets of ferrous metal (most commonly silicon steel) stacked together, that the primary and secondary windings of the transformer are wrapped around.

A transformer core is composed of limbs and yokes that are joined together to form a single structure around which the coils are placed. The manner in which the respective yoke and limbs join together will depend on the type and design of the core.

In the above example, the limbs of the core are the vertical sections which the coils are formed around. The limbs can also be located on the exterior of the outermost coils in the case of some core designs. The limbs on a transformer core can also be referred to as legs.

The densely packed laminated sheets that cores are constructed from, reduce overheating and circulating currents in the core. This lowers the transformers energy loss. Removing any air gaps between the laminations will result in a higher efficiency as well. A core design with large gaps between laminated sheets will yield higher no load or iron losses and a lower efficiency.

The earliest transformer cores utilized solid iron, however, methods developed over the years to refine raw iron ore into more permeable materials such as silicon steel, which is used today for transformer core designs due to its higher permeability. Also, the use of many densely packed laminated sheets reduces issues of circulating currents and overheating caused by solid iron core designs. Further increases in core design are made through cold rolling, annealing, and using grain oriented steel.

Silicon steel already has a very high permeability, but this can be increased even further by orienting the grain of the steel in the same direction. Grain oriented steel can increase flux density by 30%.

With a shell-type configuration, the core surrounds the windings. This creates a closed pathway surrounding the windings for magnetic flux to flow. This design also typically yields less energy loss than a core type design. A shell type design is the classification for most distribution class padmounts and substations with a wrapped 5-legged core.

A core type design is where the windings surround the core steel. In this design, there is no return path (or closed loop) for the magnetic flux around the coils. This design typically yields more energy losses, and it requires more copper or aluminum winding material than a shell-type configuration.

Due to the absence of an outer limb(s), the three legged core alone is not suitable for wye-wye transformer configurations. As the picture below shows, there is no return path for the zero sequence flux which is present in wye-wye transformer designs. The zero sequence current, with no adequate return path, will attempt to create an alternate path, either using air gaps or the transformer tank itself, which can eventually lead to overheating and possibly transformer failure.

One way this problem is solved is by adding a buried delta tertiary winding which provides a place for the flux to circulate freely. Other solutions for wye-wye transformers would include utilizing a 4-limb or 5-limb core as described below.

Rather than employ a buried delta tertiary winding, a four limb core design provides one outer limb for return flux. This type of core design is very similar to a five limb design as well in its functionality, which helps to reduce overheating and additional transformer noise.

Five-legged wrapped core designs are the standard for all distribution transformer applications today (regardless of whether or not the unit is wye-wye). Since the cross sectional area of the three inner limbs surrounded by the coils is double the size of the three limb design, the cross sectional area of the yoke and outer limbs can be half that of the inner limbs. This helps conserve material and reduce production costs as well.

There will be some loss of energy with any core configuration, but the better the core design, the less the no load losses. A well built transformer core help offset operating costs of the transformer and extend its lifespan.

Core designs have come a long way since the early days of transformer manufacturing, but the basic principles remain the same. Fill out the form below if you have any questions about transformer core design or are looking for a quote on a specific transformer.

A magnetic core is a piece of magnetic material with a high magnetic permeability used to confine and guide magnetic fields in electrical, electromechanical and magnetic devices such as electromagnets, transformers, electric motors, generators, inductors, loudspeakers, magnetic recording heads, and magnetic assemblies. It is made of ferromagnetic metal such as iron, or ferrimagnetic compounds such as ferrites. The high permeability, relative to the surrounding air, causes the magnetic field lines to be concentrated in the core material. The magnetic field is often created by a current-carrying coil of wire around the core.

The use of a magnetic core can increase the strength of magnetic field in an electromagnetic coil by a factor of several hundred times what it would be without the core. However, magnetic cores have side effects which must be taken into account. In alternating current (AC) devices they cause energy losses, called core losses, due to hysteresis and eddy currents in applications such as transformers and inductors. "Soft" magnetic materials with low coercivity and hysteresis, such as silicon steel, or ferrite, are usually used in cores.

An electric current through a wire wound into a coil creates a magnetic field through the center of the coil, due to Ampere's circuital law. Coils are widely used in electronic components such as electromagnets, inductors, transformers, electric motors and generators. A coil without a magnetic core is called an "air core" coil. Adding a piece of ferromagnetic or ferrimagnetic material in the center of the coil can increase the magnetic field by hundreds or thousands of times; this is called a magnetic core. The field of the wire penetrates the core material, magnetizing it, so that the strong magnetic field of the core adds to the field created by the wire. The amount that the magnetic field is increased by the core depends on the magnetic permeability of the core material. Because side effects such as eddy currents and hysteresis can cause frequency-dependent energy losses, different core materials are used for coils used at different frequencies.

In some cases the losses are undesirable and with very strong fields saturation can be a problem, and an 'air core' is used. A former may still be used; a piece of material, such as plastic or a composite, that may not have any significant magnetic permeability but which simply holds the coils of wires in place.

"Soft" (annealed) iron is used in magnetic assemblies, direct current (DC) electromagnets and in some electric motors; and it can create a concentrated field that is as much as 50,000 times more intense than an air core.[1]

Iron is desirable to make magnetic cores, as it can withstand high levels of magnetic field without saturating (up to 2.16 teslas at ambient temperature.[2][3]) Annealed iron is used because, unlike "hard" iron, it has low coercivity and so does not remain magnetised when the field is removed, which is often important in applications where the magnetic field is required to be repeatedly switched.

Due to the electrical conductivity of the metal, when a solid one-piece metal core is used in alternating current (AC) applications such as transformers and inductors, the changing magnetic field induces large eddy currents circulating within it, closed loops of electric current in planes perpendicular to the field. The current flowing through the resistance of the metal heats it by Joule heating, causing significant power losses. Therefore, solid iron cores are not used in transformers or inductors, they are replaced by laminated or powdered iron cores, or nonconductive cores like ferrite.

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