# Modeling A Current Transformer in FEMM

I would like to accurately design and model a current transformer in FEMM. I'm hoping this will enable me to accurately estimate the secondary current in a current transformer, including it's phase relative to the primary current. The model will also help to size the core.

I've used the FEMM software previously to design magnetic concentrators for hall effect sensors. The problem I'm running into is that I cannot induce a current in the secondary winding in FEMM, it seems I have to guess and specify the secondary current.

I've followed David Meeker's example on transformer design, but I cannot follow it exactly because I'm not designing a voltage transformer, i.e. the secondary current is never zero.

These are the 2 equations I'm trying to solve for Is (secondary current):
(source: gsu.edu)
:

I know (or can choose) the following:

1. Ip - Current in the primary
2. Rb - Burden resistance
3. w - Natural frequency of the primary
4. Vp - Voltage in the primary, reflected burden resistance (from FEMM)
5. All relevant material properties
6. All relevant physical properties

this leaves me with 4 unknowns, specifically:

1. Rp+jwLp - primary coil impedance
2. jwM - mutual impedance
3. Rs+Rb+jwLs - secondary coil impedance + burden resistance
4. Is - current in secondary winding

Currently considering trying the following:

1. Use super position to independently determine Rp+jwLp and Rs+Rb+jwLs (i.e. remove the each coil and separately calculate these as constants)
2. Independently calculate values for R, L, M based on idealized equations.

Both of these would be followed by regression to get Is, I'm not certain how to formulate this yet either.

Any help would be greatly appreciated, thanks!