Re: [femm] Current transformer ...

[David Meeker <dmeeker@xxxxxxxxxxxxxxxxx>]

jrcamacho@xxxxxx wrote:

> Dear femm friends,
> I am doing an analysis on femm of a six current transformers
> connected to a generator neutral, due lack on information I would
> like to know magnetic properties of two materials used in these
> transformers.
> First: What magnetic material is generically used in the core? M36-
> Steel?
I often use 0.014 inch thick M19 laminations as sort of a "generic" but
quality steel.  This type of steel is described as:
"at the top of the scale for desirable core-loss properties. 
Very good permeability at low and intermediate inductions.  The list
of applications includes high-efficiency rotating equipment, relays, and
dry-type transformers.  Especially applicable when highest possible
energy efficiency is desired or when efficiency increase is to be obtained
without change of shape, size, or weight of motor."
> Second: The magnetic property of Epoxy? The information
> on
> "sigma" is needed for this material.
For the purposes of a magnetics finite element analysis, it is probably
OK to assume that the epoxy has a permeability of 1 and an electrical conductivity
of 0.  However, there are special-purpose types of epoxy that are
intentionally doped to make them electrically conductive for the purpose
of electrostatic shielding, or filled with powdered iron to make solid
ferromagnetic cores that aren't subject to eddy current losses (see here
for example).  However, I'd guess that your in your case, the epoxy
is just used to pot your transformer, rather than for some other special
purpose.
>  
> Third: What is the range of current density generically used in the
> secondary of these transformers?
>  
The current density at which you can operate has a strong dependence on
thermal considerations (how much heat you can reject).  That being
said, however, you often see bulk current densities (i.e. averaged over
the entire cross-section of occupied by the coil consisting of the wire,
insulation, and the space between wires) in electric machines of between
1 and 5 MA(rms)/m^2, with something like 2 MA(rms)/m^2 being fairly typical. 
It is probably realistic to assume about a 0.5 fill factor (fraction of
the bulk coil cross-section actually occupied by copper) for a "typical"
transformer winding--a carefully made bobbin-wound coil is like 0.65-0.7
>  
> Any clues will be welcome.
>  
David Meeker
 

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