Hipersil, the myth and the truth.
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 ot 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 pre-annealed. It is then bonded so it
cant come apart. Next, the core is transfered 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 mis-information 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 rectanglar 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 trys 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 quantuty 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 ti
ght 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.
Best,
Will
If you would like a PDF copy, please e-mail me.
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