Cut Cores throughout the core manufacturing industries are currently being produced with a straight cut along the horizontal plane as core shown in Figure 1 below.

The core is cut in this manner so that a coil designed with specific winding turns and dimensions can be placed onto the leg of the core as part of an assembly unit for a transformer (see Figure 2).

Cut cores require large magnetizing current that is necessary to magnetize the core and also to jump across the air gap between the cut segments of area D x E This current is often referred to as the Excitation Current (Iex ) and is defined as:

Equation 1



Equation 2

wt =	Weight of the core in pounds
VA/# =	Volt Ampere / pound of core material
V =	Voltage applied to the winding N
B =	Flux Density
eg =	Effective air gap between the cut segments
SF =	Stacking Factor
N =	Number of Turns of primary winding
Imat =	Current to magnetize material
Igap =	Current to drive flux over air gap

Equations (I) and (2) show that the excitation current Iex is dependent on the material quality (VA/#), and the effective air gap eg between the cut segments.

For many applications such as Power Transformers, AC Inductors, and Current Transformers etc., it is necessary to minimize the current Iex of the core. Therefore, besides selecting the right material for the application, the air gap of the cores has to be kept as small as possible to achieve a maximum efficiency rating. Usually core manufacturers lap the mating core surfaces to reduce the air gap, but lapping has its limitations, and will greatly increase the cost of the core.

Example: A core weighing 0.5 to 2.0 lbs normally has air gap of .001". By precision lapping the two segments of the core, the typical gap achievable would be .0005". This is only a small difference in the gap, but the added cost for the lapping is significant.