Tower Bridge, London

Voor het novembernummer 2010 van Update, de Client Newsletter van ArcelorMittal FCE schreven wij onder meer dit artikel over het gebruik van elektrisch staal in transformatoren. Het artikel werd in het Engels geschreven op basis van een interview en uitgeschreven informatie.

Increased insight in the use of
electrical steels in transformers

ArcelorMittal FCE is a leading supplier of iron silicide (FeSi) and iron-cobalt (FeCo) alloys. These materials are used as magnetic cores for electrical machines such as motors, generators and transformers. Manufacturers of auxiliary transformers for various applications in aviation (ventilation, heating up passengers’ meals...) recently launched several re-engineering projects aimed at weight, volume and cost reduction. Researchers at ArcelorMittal FCE seized this opportunity to investigate the optimal choice of soft magnetic material for specific applications.

Is FeCo still the obvious choice?

Auxiliary transformers used in aircraft systems – also known as aeronautic transformers – obviously need to provide power, maintain consistent voltage levels and prevent load losses. Moreover, they should be as small and light as possible and produce little or no noise in use, while keeping the costs down.

If only volume and weight reduction are the critical design parameters, FeCo alloys are the obvious choice. Compared to FeSi alloys they allow the working point of the transformer to be shifted to higher polarisation levels, resulting in smaller magnetic cores and hence lower transformer volume and weight. The use of FeCo alloys also allows keeping the eddy current losses low, so higher efficiency can be reached.

Although FeSi alloys have lower saturation polarisation than FeCo alloys, they can be designed for high resistivity. And through the use of specific production methods it is possible to optimise the permeability levels and magnetic performances of FeSi electrical steels. So much so, that in some cases they may become viable alternatives for the more expensive FeCo electrical steels.
Sigrid Jacobs is one of ArcelorMittal FCE’s electrical steel specialists and in charge of Customer Relations & Strategy. ‘Several of ArcelorMittal’s steel mills are involved in the production of a wide range in FeCo and FeSi chemistries,’ she says. ‘Consequently we are exceedingly well placed to investigate the pros and cons of both these chemistries and to help our customers make informed material choices in terms of transformer performance, dimensions, noise production and cost.’

The comparative study

‘We started our investigation in early 2010,’ Sigrid Jacobs continues. ‘We decided to focus on a three phase transformer used for auxiliary electrical supply. We took a transformer with an FeCo core and 0.20 millimetre lamination thickness as our reference point. The lamination’s thickness is an important design parameter as it is a significant factor in the heating up of the steel core. Transformers’ cores are indeed not made of solid steel to avoid the circulation of large currents causing losses.’

As can be seen in the table below, eight different magnetic materials were compared:

Alloy type FeCo FeSi
Texture
(composition)
Non-Oriented
(49% Co)
Grain-Oriented
(27% Co)
Non-Oriented
(3% Si)
Grain-Oriented
(3% Co)
Thickness (mm) 0.20 0.34 0.20 0.34 0.20 0.35 0.20 0.35

For their comparative study, the researchers started from the presumption that aluminium foil would be used as a conductor. They then started to investigate how much they would have to modify the dimensions and the volumes of the transformers to maintain the same apparent power and full load losses, regardless of the materials that the magnetic cores would be made of.

‘We chose for a numerical approach,’ Sigrid Jacobs explains. With respect to weight and volume the FeCo core transformer we used as a reference point, was clearly superior to all other transformers. So, we used the values we measured in this transformer as benchmarks. By comparing the values we measured in all other combinations of materials and dimensions against the benchmarks, we were able to establish which alternatives to the reference transformer would be viable with regards to cost and weight.’

In recent years engineers have been able to reduce the total weight of modern areoplanes. According to Sigrid Jacobs this opens up new possibilities for the use of transformers with FeSi magnetic cores. ‘These are indeed a bit bulkier an heavier than the more expensive FeCo transformers,’ she admits. ‘But since modern fuselages have become lighter, it doesn't matter so much anymore that certain auxiliary instruments and appliances have become a bit bigger and heavier, especially if they contribute to the comfort of pilots and passengers. This is certainly the case with FeSi tranformers, which – unlike FeCo transformers – don’t produce buzzing sounds.

Summarised into a handy list

ArcelorMittal FCE’s comparative study has arrived to some interesting conclusions, which Sigrid Jacobs summarises into a handy list. ‘Mind you, these conclusions don’t take into account punching and assembly costs, which are of course also related to material choice,’ she warns.

  1. If the aim is to build a very compact transformer and buzzing noises (magnetostriction) are not an issue, 0,2mm thick high cobalt-containing non-oriented electrical steels (FeCo) are still the best option.
  2. If a weight increase of about 20% is acceptable and the material cost needs to be drastically reduced (to 7% of the most expensive option), it’s advisable to use 0,23mm grain oriented FeSi GO. This material will only moderately increase noise associated with magnetostriction.
  3. Materials which cause a 33% weight increase, do not allow further material cost reduction.
  4. For the cheapest transformer, use 0,35mm non oriented FeSi steel. This reduces the material cost to only 6% of the most expensive option. On the plus side, this material produced very little noise (low magnetostriction). On the minus, it causes a weight increase of 46%.

ArcelorMittal supplies practically all types of electrical steels for manufacturing cores of high frequency transformers. ‘Thanks to this new study, our technical and commercial team can help customers make the best possible material choice for any kind of transformer application. Our research has indeed given us increased insight in the effects of material choice on cost, volume, weight, performance and noise.’