Core Materials

Electronic transformers make use of a large variety of core materials. In this chapter, the more useful magnetic properties of several grades of core materials are presented for reference and comparison. To guard against possible ambiguity, definitions of magnetic terms are first reviewed.

Fig. 18. A-c and d-c hysteresis loops.

Referring to the typical hysteresis loop of Fig. 18, curve OB_m is the manner in which completely unmagnetized steel becomes magnetized by a magnetizing force H gradually increasing up to value H_m. Flux density or induction is not proportional to H but rises more gradually as it approaches H_m, B_m. Once the material reaches this state, it does not retrace curve OB_m if H is reduced. Instead, it follows the left side of the solid-line loop in the direction of the arrow until, with negative H_m, it reaches the maximum negative induction -B_m. If H is now reversed, the induction increases as indicated by the right side of the loop, which is symmetrical in that the upper and lower halves are equal in area and have the same shape.

In laboratory tests of magnetic material, the changes in H are made slowly by means of a permeameter. The solid curve of Fig. 18 is then called the d-c hysteresis loop. If the changes are made more rapidly, for example at a 60-cycle rate, the loop is wider, as shown by the dotted lines. If a higher frequency is used, the loop becomes still wider, as shown by the dot-dash lines. At any frequency, energy is expended in changing induction from B_m to - B_m and back to B_m; this energy is called the hysteresis loss and is proportional to the area of the B-H loop. Increase in loop width with frequency is usually attributed to eddy currents which flow, even in laminated cores, to some degree.

If a closed magnetic core is magnetized to induction B_m and then the magnetizing force completely removed, induction decreases to residual induction B_r and remains at this value in the absence of magnetizing force, or for H = 0. The value of H required to reduce B to zero is called the coercive force (H_c). From Fig. 18 it is evident that B_r and H_c may change with frequency for the same B_m and grade of core material, and the design of transformers and reactors may be affected by the influence of frequency on core steel properties.

According to equation 10, the core flux is proportional to effective alternating voltage for a given frequency and number of turns, and so is flux density in a given core. Therefore the largest loop of Fig. 19 corresponds to a definite effective voltage and frequency, applied across a coil linking a definite core, and magnetizing it to maximum flux density B_m.

Fig. 19. Normal induction.

If effective voltage is reduced 20 per cent a smaller B-H loop results, with lower maximum flux density B'_m. If effective voltage is reduced further, still lower maximum flux density B"_m is reached. The locus of points B_m, B'_m, B"_m, etc., is drawn in Fig. 19, and is called the normal induction curve. It is similar in shape to, but not identical with, the virgin curve OB_m of Fig. 18. Each time the maximum flux density is lowered, a short time elapses before the new loop is traced each cycle. Thus the loops of Fig. 19 represent symmetrical steady-state or cyclic magnetization at different levels of maximum induction.

Fig. 20. Normal permeabilities.

A normal induction curve is drawn in Fig. 20. The ratio of B to H at any point on the curve is the normal permeability for that value of B. For the maximum flux density B_m, the normal permeability is

μ = Bm/Hm

[19]

It is the slope of a straight line drawn through the origin and B_m. A similar line drawn tangent to the curve at its "knee" is called the maximum permeability and is the ratio μ_m = B'/H'. The slope B_o/H_o of normal induction at the origin (enlarged in Fig. 20) is the permeability for very low induction B_o; it is called initial permeability and is usually much less than μ_m.

Maximum permeability as here defined is really the average slope of the normal induction curve up to induction B''. Actual slope from 0 to B' is greater at some points than maximum permeability, because the curve is steepest below B'. The slope at any induction is called differential permeability.

From inspection of Fig. 19 it will be noticed that, for H = 0, the sides of the B-H loop are steeper than any part of the normal induction curve and hence the slopes exceed μ_m. This fact has practical significance in the design of magnetic amplifiers.

In the foregoing, symmetrical magnetization has been assumed. If a core is magnetized with d-c magnetizing force H_dc as in Fig. 21, and a-c magnetization ΔH is superimposed, the cyclic magnetization follows a minor loop AB_m. Decreasing induction follows the left side of a major loop whose maximum induction is B_m, down to induction A = B_m - ΔB. Increasing induction follows a line which joins the right side of the major loop. The area of this loop is small, but so is the average slope, or incremental permeability. This permeability is important in reactor design. It is defined by

μΔ = ΔB/ΔH

[20]

and is generally smaller than μ_m. The dotted line in Fig. 21 is the normal induction curve, the locus of the tops of minor loops as H_dc is decreased.

Fig. 21. Incremental permeability.

Returning now to Fig. 19, if H_m is increased, an induction is finally reached at which unit increase of H produces only unit increase in B_m. This is known as saturation induction B_s. The value of H at which B_s is first reached is very large compared to H_c for most core materials.

A striking development has been the production of core materials with rectangular hysteresis loops. In such materials B_s is reached at small values of H, as shown in Fig. 22.

Fig. 22. Rectangular hysteresis loop.

Core material having a rectangular hysteresis loop is especially useful in magnetic amplifiers, and is discussed in Chapter 9.

The volt-amperes per pound or apparent core loss (P_a) of a magnetic material is the product of rms induced voltage and rms exciting current drawn from the source when a pound of the material is subjected to sinusoidally varying induction of a specified maximum value B_m and of a specified frequency f. Exciting current is non-sinusoidal, as can be seen from Fig. 5, Chapter 1. The power component of P_a is the core loss P_c. The reactive component is usually the larger and is called VARS per pound. It is related to permeability in the following way:

Let it be assumed that for conditions B_m, H_m in a core the magnetizing current is approximately sinusoidal, of effective value I_m, drawn from a supply of frequency f and effective voltage E. If we combine

[21]

[22]

[23]

[24]

convert to inches, and put density p = 0.27 lb/in.⁽³⁾, then

[25]

At 60 cycles,. Because of the non-linearity of I_M, this equation is approximate. Moreover, there is no allowance for core gap.

In usual electronic transformer practice, it is necessary to avoid reaching saturation flux densities, because high exciting currents produce high winding IR drops, high losses, low efficiency, and large size. Curves of induction and core loss are available from manufacturers of laminations. Grades and thickness are designated by numbers such as Armco Trancor M15 and Allegheny Transformer A. A wide choice of silicon-steel laminations is available in 0.014-in., 0.019-in., and 0.025-in. thicknesses, with silicon content of approximately 3 to 4%, and with core losses ranging from 0.6 to 1.2 watts per pound at 10,000 gauss, 60 cycles (64,500 lines per square inch).

Fig. 23. Core loss at high induction. Armco Trancor M15 grade, 29 gage.

Fig. 24. Exciting rms volt-amperes per pound, Armco Trancor M15 grade, 29

Figures 23 and 24 are core-loss and exciting va/lb for a widely used grade of electronic transformer core steel at 60 cycles.

Much work has been done in developing grain-oriented core materials. These materials have a composition similar to that of older, non-oriented core material, but grains in the material are oriented by cold-rolling in the direction illustrated by Fig. 25.

Fig. 25.

Magnified sections of laminations are shown in this figure; (a) shows the random directions of "easy" magnetization in grains of non-oriented silicon steel. When magnetic flux is established in the lamination, the grains must be aligned in the same direction, as in Fig. 25(b). If the grains are already oriented in this direction during the rolling process, much smaller magnetizing force is required to produce the desired flux. Coercive force and hysteresis loss are smaller than in non-oriented steel; permeability is greater, and so is B_r, so that the rectangular loop of Fig. 22 is approached in grain-oriented steel.

Grain-oriented core materials are of two major types: silicon-steel and nickel-iron alloy. Electronic power transformers (i.e., plate and filament supply transformers) formerly comprised only hot-rolled silicon-steel cores. The development of grain-oriented silicon steel has had a marked effect on size and performance of such transformers. To illustrate this effect, a comparison is made below between the older non-oriented steel (termed, for simplicity, silicon steel) and Hipersil, a cold-rolled steel in which grain orientation is carried out to a high degree. If core flux flows in the grain-oriented direction, high core inductions may be realized. Type C cores fulfill this requirement, because the strip is wound in the same direction as the flux path.

The material is rolled in three major thicknesses:

No. 29 gage (about 12 to 14 mils thick) for frequencies up to 400 cycles.

5 mils thick for frequencies 400 cycles and higher.

2 mils thick for frequencies in the low and medium r-f bands.

Probably the most remarkable property of this material is its high saturation point.

Fig. 26. Induction and core-loss curves of silicon steel and Hipersil at 60 cycles.

In Fig. 26 the comparison is given in terms of a hypothetical 60-cycle working induction using high-grade, conventional silicon steel. If this value is assumed to be 100 per cent, the induction obtained with grain-oriented steel is 130 to 150 per cent, with no increase in magnetizing force. Another way of expressing this improvement is shown in Fig. 27 as a comparison of the permeability of the two steels.

Fig. 27. Permeability of silicon and grain-oriented silicon steel.

The permeability of grain-oriented steel is much higher at the maximum point, and has the same percentage increase as in Fig. 26 for normal working inductions. iron loss in Hipersil is less than in silicon steel, as Fig. 26 shows. The decrease in iron loss is chiefly due to a reduction in hysteresis loss; the eddy-current loss is less affected by grain orientation. Future comparisons may widen these differences.

The increase in induction is beneficial in several ways. First, it permits a reduction of core area for the same magnetizing current. Second, it results in a smaller mean length of turn and thus in a reduction in the amount of copper needed. In distribution and power transformers, for maximum benefit the iron and copper losses are repropor-tioned. In small electronic transformers, the iron loss is usually a small part of the total loss, and the reduction in copper loss is of greater significance. Within certain limits, the sum of the two losses determines the size of a transformer, and here the usefulness of grain-oriented steel becomes most apparent.

The foregoing was written with 60-cycle applications particularly in mind. At higher power supply frequencies, such as the 400- and 800-cycle supplies encountered in aircraft and portable equipment, the results are somewhat different. The decrease in iron loss is not so marked, because the eddy current loss forms a larger proportion of the total iron loss. However, it is usual practice to use thin-gage laminations at these frequencies, and much better space factor can be obtained in wound cores than in stacked cores. The increase in permeability is just as effective in these higher frequency applications as at 60 cycles. The net result is a smaller transformer than was formerly possible, though for different reasons and in different proportions.

Reactors which carry direct current are usually smaller when made with grain-oriented than with ordinary silicon steel. At low voltages, where low inductions are involved, grain-oriented steel has greater incremental permeability, and maintains it at high flux densities also. Consequently, a reduction of 50 per cent in weight is often feasible.

Grain-oriented silicon steel does not replace high nickel-iron alloys for audio transformers, when they work at low inductions, and with little or no direct current. Some nickel-iron alloys have higher permeability at low flux densities, and their use for this purpose continues. But at high inductions, or where considerable amounts of direct current are involved, grain-oriented silicon steel is used. Lower distortion, extended frequency range, or small size is the result, and sometimes a combination of all three occurs.

Hipersil can be used for transformers in various applications in the low and medium r-f bands, at power levels ranging up to hundreds of kilowatts. The same is true of video and pulse transformers, which may be regarded as covering an extended frequency range down into the audio range and up into the medium r-f range. Such transformers are grouped rather loosely together as r-f transformers in the diagram shown in Fig. 28.

Fig. 28. Use of Hipersil in various frequency zones.

In this figure the several classifications, r-f, audio, and power transformers, are shown with respect to their frequency ranges and the approximate gage of the material used for these ranges. The gage is indicated by the symbol number in Table II.

Table II. Hipersil Core Data

Hipersil	Thickness	Typical Hipersil Space Factor ((1))	Typical Space Factor for Silicon Steel ((1))
C-97	0.013 in.	95%	90%
C-95	0.005 in.	90%	80%
C-91	0.002 in.	85%	70%

Core-loss and exciting va/lb for 29-gage Hipersil are plotted in Figs. 29 and 30. Joint reluctance is neglected in Fig. 30.

Fig. 29. Core loss in C-97 Hipersil cores (29 gage).

Fig. 30. A-c excitation curve, typical data. C-97 Hipersil cores (29 gage).

An example of specialized core materials is the development of a new grain-oriented silicon steel especially for weight reduction in components for 400-cycle applications. By means of large reduction in core loss at 400 cycles and still larger increase in permeability at high induction, a 0.004-in.-thick core material was developed which operates satisfactorily in many instances at 17,000 gauss, 400 cycles. As a result, 40 per cent of the weight was eliminated in transformers designed to take advantage of the 0.004-in-thick core material. At lower inductions the core loss of this material tends to be larger than in the older 0.005-in.-thick material. Hence it is only where 17,000 gauss is a practicable working induction that the weight reduction is possible.

Grain-oriented steel alloys of approximately 50% nickel content are extensively used in saturable reactors. Electrical properties of cores wound from these materials are spoiled if the strip is bent or constrained mechanically. Usually the nickel-alloy strip is wound into cores in the form of a toroid, annealed, and enclosed in an insulating box to protect it from damage. Special machinery is then used to wind turns of wire around the core. With the proper precautions, it is possible to realize the advantages of a very rectangular, narrow hysteresis loop in the finished reactor. These properties have been found useful also in pulse transformers, and are discussed in Chapters Magnetic Amplifiers and Pulse and Video Transformers in detail.

In audio- or higher-frequency low-loss reactors or transformers, it may be desirable to use powdered iron or nickel-alloy cores. These cores are made of finely divided particles, coated with insulating compound, which separates them and introduces many fine air gaps in the magnetic path. The cores are molded into various shapes suitable for the application. Effective permeability of such cores is reduced to a figure much lower than that of laminations made from the same material.

Magnetic ferrites likewise are used at higher frequencies. These substances are characterized by high resistivity so that neither laminations nor powder particles are necessary to reduce eddy-current loss. Cores are molded and sintered at high temperature. After sintering they have ceramic hardness but relatively low Curie temperature.⁽²⁾ Ferrites are useful at very high frequencies.

Some of the principal core materials are listed in Table III.

Table III. Core Materials

Approximate Description	Trade Names	Typical Maximum Permeability μm	Maximum Operating Flux Density Bm (gauss)	Coercive Force D-C Loop (oersteds)	Chief Uses
Silicon steel	Transformer Trancor M15 Power 58	8,500	12,000	0.5	Small power and voice frequency audio transformers
Grain-oriented silicon steel	Hipersil Trancor 3X	30,000	17,000	0.4	Larger sizes of power and wide-range audio transformers; low-frequency r-f transformers; saturable reactors
50% nickel steel	Hipernik Allegheny Electric Metal Nicaloi	50,000	10,000	0.06	Small, wide-range audio transformers and reactors (may have small d-c induction)
50% nickel steel, special heat treatment	Conpernik	1,400	((3))		Extremely linear and low-loss transformers
Grain-oriented 50% nickel steel	Hipernik V Orthonol Orthonik Deltamax Permenorm	50,000	14,500	0.15	Saturable reactors
80% nickel steel	Permalloy Mumetal Hymu	100,000	6,000	0.05	Small or wide-range audio transformers (no d-c induction)
80% nickel steel, special heat treatment	Supermalloy	200,000	6,000	0.01	Very small or wide-range transformers (no d-c induction)
Powdered iron	Crolite Polyiron	125	((3))		Wave filter reactors; low and medium r-f transformers
Ferrite	Ceramag Ferramic Ferroxcube	1,000	2,000	0.2	Sweep circuit transformers; r-f transformers and reactors

(1)	Refers to percentage of core volume occupied by metal. The Hipersil figure is for type C cores, and the silicon steel figure is for punched laminations.
(2)	The temperature at which a ferric substance loses its intrinsic permeability.
(3)	These materials are used for low flux density, low-loss applications.