C-I Core

A Low Cost Solution to a New Generation in Transformer Design

Transformers used for the distribution and control of electrical energy consist of a primary and secondary copper wound coil carrying a magnetizing current surrounding an iron core which is used for the transferring of power from the primary to the secondary coil.

The iron core is normally made of thin gauge laminated steel either of low cost stamped laminations in the assembly form of EI’s or EE’s (see Figure 1) or similar laid up structures sometimes in a more costly tape wound double cut core assembly as shown in Figure 2.

CI-Core Figure 1 and 2

Stamped laminations are usually made of .006″ thick material and can go as high as .025″, the cost for producing the lamination is very low, but the high cost comes from the stamping dies used to stamp the laminations and also the final assembly by the customer. While cut cores can be made of very thin magnetic steel strip (such as .001″ and can go as high as .014″) wound over simple low cost mandrels, heat treated, impregnated, and finally cut into two pieces as a completed set and ready for customers use.

The new core configuration called the C-I Core ( Patent pending ) combines the advantage of low cost laminations requiring expensive tooling and those of cut cores which are more expensive to manufacture but requires only simple low cost tooling.

The C-I core configuration is made of one cut core (rectangular, toroidal or any other shape) and one laminated “I” bar as shown in Figures 3a & 3b.

CI-Core Figures 3a and 3b

The core in its final assembly has the same shape as an EL, EI, or F stamped lamination and is similar to the configuration of a double C-core.

The C-I core can be made of .001″ thick material or greater (up to .014″), using Grain Oriented Silicon iron, Nickel iron or other Crystalline steels. It offers the designer of transformers and inductors flexible dimensions, high frequency range up to 20KHz, and low cost tooling.


  1. Cost less than conventional double cut core. Producing one cut core and one “I” bar requires less tooling and is less time consuming than two cut cores, therefore the C-I core is less expensive to make than two cut cores.
  2. The ease of copper coil winding. Since the “‘I” bar is straight and has flat surfaces, the copper coil can be wound directly onto the “I” bar without having to make a bobbin. This will reduce the cost and simplify the production of prototypes significantly.
  3. Reduction of the winding resistance. Since the “I” bar is removable, it can be smoothed out around the four edges (optional ) where the copper wires are wound which would reduce the the wire length and ultimately reduce the resistance of the winding.
  4. Adjustable air gaps: For Inductors requiring air gaps, the insulation can be placed onto the “I” section to be used as air gaps.
  5. Tighter tolerance: The tolerance only applies to one cut core instead of two as in the shell type, and the “I” bar is made of stacked up lamination in which the tolerance can be more accurately controlled to fit the bobbin.
  6. Efficient assembly time: Normally the transformer using double cut cores required two clamping bands on each individual core as shown in Figure 2. The C-I core is held together by one clamping band as in Figures 3a & 3b.


The C-I core is made up of two “C” sections of a cut core and one laminated “I” bar put together as shown in Figure 4.

CI-Core Figure 4

The cut core is a normal cut “C” core with dimensions D, E, F, G, A, B as indicated in Magnetic Metals’ cut core catalog and shown below in Figure 5.

CI-Core Figure 5

The laminated “I” bar is made up of laminations stacked up and impregnated with the following dimension as shown in Figure 6.

CI-Core Figure 6

The stripwidth of the “I” bar D1 = 2E, the laminations are stacked up to a thickness of E1 = D, and the length of the “I” bar L = A (D, E, and A are dimensions of the cut core). For best mechanical and electrical performance, the dimensions of the “I” section will be built to the norminal D, E, and A dimensions plus there tolerance.

C-I cores have the same design equations as cut cores and laminations. The equations are: Power VA = K1AwAcB

Inductance L = K2AwAcu

Where Aw is window area, and Ac = 2ExD is the cross sectional area of the core.

If you have the need for additional technical information or samples regarding this concept, please call Magnetic Metals Corporation, Anaheim, CA at (714) 828-4625, or contact us at info@magneticmetals.com.

CI-Core EE/EI/F Lamination
EE/EI/F Lamination
6-25 mil50-400 Hz1Low3Very
Power X-formers,
Current X-formers,
material and parts, better control of tight tolerance
Limited thickness, expensive asssembly and tooling
CI-Core Shell Type Double C Core
Shell Type Double C
1-14 mil50-20
4High2FairPower X-formers, Current X-formers, Inductors, ChokesInexpensive tooling, easy assembly, available in many sizesMore expensive than laminations, wide tolerance
C-I Core
1-14 mil50-20
3Low or none (can wind directly on “I” bar)1LowPower X-formers, Current X-formers, Inductors, ChokesInexpensive tooling, available in many sizes, easy assembly, wind directly on “I” barWider Tolerance than lamination but better control than Shell Type C Core
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