Hipersil, the myth and the truth

Hipersil, what’s behind?

by Will Matney

I was asked about a transformer using nothing but a Hipersil transformer. I offered an EI type, M6 material transformer of the same specs, but it wouldn’t do. Well folks, this gave way to me writing this for all on what exactly is a Hipersil transformer.

Hipersil is a trademark owned by Westinghouse Electric for a type of transformer core and material they manufactured at one time. Hipersil, the material, is mearly a cold rolled, grain oriented, silicon steel with about 3.5% silicon added. That’s it, nothing more. M6 material is the same material offered by several steel manufacturers, and is what Westinghouse bought in large coils. M6 is the AISI (American Iron and Steel Institute) number for 3.5% silicon steel which has been cold rolled, with the grain oriented in the rolling direction.

What is cold rolled, grain oriented (CRGO)?

This is a type of steel, which after being rolled out to a strip, or sheet, is cold rolled again to where the grain of the steel orients itself in a certain direction. What this does is drop the reluctance of the steel to the magnetic flux, which in turn lowers its losses, nothing more.

Why add silicon?

Silicon is added to control residual magnetism (core staying magnetized). The thinness of the laminations controls eddy currents. There is different grades of silicon steel, with varying amounts of silicon. On the lower end is non oriented silicon steel (CRNO) which wasn’t cold rolled for the grain to line up like M50. On the upper end is M6 which is about the top of the heap for CRGO steel without other alloying agents being added such as cobalt or nickel.

What is a Hipersil core?

Well, there are many “Hipersil” cores if you want to call them that, including EI cores, but the steel manufacturers just use the “M” numbers to designate the steel. The proper question is what is a “cut core” or a “C-core”? A C-core is made by winding a thin strip of CRGO or CRNO steel around a rectangular or square mandrel in a lathe. The steel strip has an adhesive on it so it will bond together after it is rolled to a certain thickness. This strip can be several thicknesses where 14 mils is a very common one. The very thin ones like 2 mils is for higher frequency work in the audio frequency range. 14 mils is commonly used in power transformers for 50 and 60 Hz.

After the core has been wound to the proper thickness, it is generally annealed to improve its properties, or some are preannealed. It is then bonded so it cant come apart. Next, the core is transferred to a band saw and cut in half. Each half is marked and then the cut faces are either machined or ground for a close fit when put back together. This helps eliminate the air gap if any.


Why is a Hipersil core touted to be better than an EI core?

Well, there’s a lot of myth and misinformation here. The only difference between a cut core and an EI core using the same material is weight. This weight savings is due simply to the corners being rounded off where an EI core is rectangular overall, that’s it, nothing more. The weight savings you’ll get is from 15% to 20% just because the corners are rounded off. No matter what, each type core has to have the same core area (A) in each type.

A cores power handling ability (in watts or volt amperes) comes from its ability to cram all the magnetic lines of force (flux) into a small core area (flux density). Every core has a maximum flux density (Bmax) which is determined by the cross sectional area of the core (A) and the material the core is made from, nothing more. If this limit is exceeded, the core goes into saturation. This means that any more increase in current does not cause any more flux in the core. It also means that the cores permeability drops off sharply at this point. The waveform becomes distorted at the point of saturation and beyond too. A core is designed to not saturate when maximum power is drawn from the transformer (this is not the case in an inverter). Generally, a flux density is selected which is just below Bmax at design time. Lets say the material gives a Bmax of 14,500 gauss. A working flux density of maybe 14,000, or 13,500 might be used. However, this increases the weight of the core because more iron has to be added to it to keep the flux density down. This is why when selecting a transformer, you should figure the maximum current in amperes to be drawn for each winding including losses. Losses in most filtered power supplies run around 5% or so. The reason for this is the designer tries to make the transformer as light as possible and this in turn means they design the transformer to run as close to saturation as possible without going over. On designing power chokes, the DC portion has to be considered along with the AC to keep away from saturation. The core can be gapped to correct some of this, but that is more than what can be covered here at this time.

What are the disadvantages to a C-Core?

First and foremost is cost. The cost of pre-made cores is expensive unless buying a large quantity of each size. The mounting hardware costs more than does on an EI core of the same size. The last, and an important one is that a C-core runs hotter than an EI core of the same size. This is due to the mass of iron being smaller than on an EI core, plus the fit of the bobbin to the core. The only plus to a C-core is labor savings, and a minor weight savings. It doesn’t take as long to put together a C-core as it does to stack an EI core. However, an experienced builder can go pretty darn fast stacking the lams of an EI core, even being interleaved. Actually, a tigther fit can be achieved of the coil to the core on an EI core than a C-core, because of adding the lams, one at a time to the exact thickness needed. With a C-core, you get the one thickness, that’s it. To make it fit, extra paper or wedges is used a lot. This also makes one heat up more if the bobbin doesn’t have a tight fit to the core. This raises the cost of the mandrel used to wind the coil as it’s tolerances are tighter.

Is an EI core better than a C-core?

In my opinion, Yes! The reason being is the heat. Heat is the killer of all electrical and electronic components. I wouldn’t care if the core weighed 20% more as long as it lasted longer.

The above is just my opinion on the subject, and hope more have been brought into the light on the differences between these two types of transformers.

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