Unofficial
FEMM
documentation
by Dr Stan Zurek

LUA script for calculating magnetic coupling coefficient between two windings. The windings have to be set up as described in the comments in the code below.

-- Short Lua script for calculating coupling factor of two coils.
-- It presumes the problem is already defined in FEMM, and
-- that the two windings are defined by circuits named "pri" and "sec".
-- The problem may be DC or AC at a frequency of your choice.
-- Intended for circuits with series property.
-- Execute with the FEMM menu item File/OpenLuaScript.
clearconsole()
showconsole()

-- mi_modifycircprop takes three parameters
-- 1st = circuit name to be changed, 2nd = 1 = change current value, 3rd = new current value (A)
-- change the curent value to your own, here it is assume that prim=0.01A, sec=10.0A

-- Simulate first with only secondary current:
mi_modifycircprop("pri", 1, 0)   
mi_modifycircprop("sec", 1, 10)   -- change "10" to your own I2 in "sec"
mi_analyze()
mi_loadsolution()
print(mo_getcircuitproperties("sec"))  -- Optional display of intermediate results
print(mo_getcircuitproperties("pri"))  -- Optional display of intermediate results
i, Vsec, Lsec = mo_getcircuitproperties("sec") 			-- extract inductance
i, Vpri_sec, Lpri_sec = mo_getcircuitproperties("pri")	-- extract inductance

-- Simulate next with only primary current:
mi_modifycircprop("pri", 1, 0.01)   -- change "0.01" to your own I1 in "prim"
mi_modifycircprop("sec", 1, 0)
mi_analyze()
mi_loadsolution()
print(mo_getcircuitproperties("sec"))  -- Optional display of intermediate results
print(mo_getcircuitproperties("pri"))  -- Optional display of intermediate results
i, Vsec_pri, Lsec_pri = mo_getcircuitproperties("sec")   -- extract inductance
i, Vpri, Lpri = mo_getcircuitproperties("pri")    		 -- extract inductance

-- Calculate and print coupling factor:
coupling_factor = sqrt(Lsec_pri * Lpri_sec / (Lpri * Lsec))
print("Coupling factor is:", coupling_factor)