10000

о о

(с

о >-

о

>-о

о

1000

1.0 ю

watts PER pound

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

1000

10000

O.Ol 0.1 1.0 10 fOO 1000

apparent watts per pound

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

ponents 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 9 and 10 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. Ferrites are useful at very high frequencies.

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

16. Windings. Current density in the winding copper is sometimes estimated for design purposes by rules such as 1,000 cir mils per amp. These rules are useful in picking out a first choice of wire size for a given current requirement but should not be regarded as final. In-

1 The temperature at which a ferric substance loses its intrinsic permeability.

Approximate Description

Silicon steel

Grain-oriented silicon steel

Trade Names Transformer Trancor M15 Power 58

Hipersil Trancor 3X

Table HI. Core Matekials

Typical Maximum Coercive

Maximum Operating Force

Permeability Flux Density D-C Loop

Im Вт (gauss) (oersteds)

8,500 12,000 0.5

50% nickel steel Hipernik

Allegheny Electric Metal Nicaloi

50% nickel steel, Conpernik special heat treatment

Grain-oriented Hipernik V

50% nickel Orthonol

steel Orthonik

Deltamax Permenorm

80% nickel steel Permalloy Mumetal Hymu

Ferrite

Ceramag Ferramic Ferroxcube

30,000

80% nickel steel, Supermalloy 200,000

special heat treatment

Powdered iron CroUte

Polyiron

125 1,000

17,000

Chief Uses

Small power and voice frequency audio transformers

50,000 10,000 0.06

1,400

Larger sizes of power and wide-range audio transformers; low-frequency r-f transformers; saturable reactors

Small, wide-range audio transformers and reactors (may have small d-c induction)

Extremely linear and low-loss transformers

50,000 14,500 0.15 Saturable reactors

100,000 6,000 0.05

6,000 0.01

2,000 0.2

Small or wide-range audio transformers (no d-c induction)

Very small or wide-range transformers (no d-c induction)

Wave filter reactors; low and medium r-f transformers

Sweep circuit transformers; r-f transformers and reactors

* These materials are used for low flux density, low-loss apphcations.

stead, the temperature rise, regulation, or other performance criterion should govern the final choice of wire size. Regulation is calculated as in Section 11, and temperature rise as in Sections 22 and 23. In Fig. 31 the circular mils per ampere are plotted for small enclosed dry-type transformers with Hipersil cores and a winding temperature rise of 55 centigrade degrees; it can be seen to vary appreciably over this range of sizes.

Space occupied by the wire depends on the wire insulation as well as on the copper section. This is especially noticeable in small wire sizes. Table IV gives the bare and insulation diameters for several common kinds of wire and Table V the turns per square inch of wind-

600 ы

< 500

ОС Ы Q.

(o 400

о

5 200

VOLT AMPERES

Fig. 3L Wire size in windings of small enclosed 60-cycle transformers.

wound coil. Table VI gives the minimum paper thickness based on this consideration.

Space factor may refer to linear spacing as across a layer, or to the total coil section area. It is more convenient to use linear space factor in designing layer-wound coils and area space factor in random-wound coils. The values in each case depend largely on the method of winding. For example, it is possible to wind No. 30 enameled wire with 97 per cent linear space factor by hand, but with only 89 per cent on an automatic multiple-coil winding machine. (See Fig. 33.) Moreover, values of space factor vary from plant to plant. An average for multiple-coil machines is given in Table VI.

ing space. Space usually can be saved by avoiding cotton or silk wire covering, and instead using enameled wire with paper layer insulation as in Fig. 32. Thickness of layer paper may be governed by layer voltage; it is good practice to use 50 volts per mil of paper. In coils where layer voltage is low, the paper thickness is determined by the mechanical strength necessary to produce even layers and a tightly

Table IV. Insulated Wiee Sizes

Diameter of Insulated Wire

Single Cotton Enamel

Single Silk Enamel

Single Cotton

.0095

.0102 .0109 .0117 .0127 .0137

.0148 .0162 .0175 .0192 .0210

.0234 .0256 .0282 .0310 .0344

.0385 ,0425 ,0469 .0521 .0576

.0640 .0711

.0075

.0082 .0089 .0097 .0107 .0117

.0128 .0142 .0155 .0172 .0190

,0211 .0233 .0259 .0287 .0319

.0090

.0096 .0103 .0111 .0120 .0129

.0140 .0153 .0166 .0182 .0199

,0355 ,0395 ,0439 ,0491 ,0546

.0610 .0681

.0222 .0244 .0269 .0296 .0330

.0370 .0409 .0453 .0503 .0558

.0621 .0691

Double Cotton

.0130

.0136 .0143 .0151 .0160 .0169

.0180 ,0193 .0206 .0222 .0239

.0262 ,0284 .0309 .0336 .0370

,0410 .0449 .0493 .0543 .0608

.0671 .0741

Single Silk

.0070

.0076 .0083 .0091 .0100 .0109

.0120 ,0133 ,0146 .0162 .0179

.0199 ,0221 .0246 .0273 .0305

.0340 .0379 .0423 ,0473 .0528

.0591 ,0661

Double Silk

.0090

.0096 .0103 .0111 .0120 .0129

.0140 .0153 .0166 .0182 .0199

.0219 .0241 .0266 .0293 .0325

,0360 .0399 .0443 .0493 .0548

.0611 .0681

Area in

Circular

Mils

4.00 4.84 6.25 7.84

9.61 12.25 16.00 20.30 25.00

31.40 39.70 50.40 64.00 79.20

100 128 159 202 253

320 404 511 645 812

1,020 1,300 1,600 2,030 2,600

3,250 4.100 5,180 6,530 8,235

10,380 13,090 16,510

Ohms per

1000

Feet at

25°C

2,700 2,150 1,700 1,350

1,103 864 659 522 424

338 266 210 165 134

106 83.1 66.4 52.5 41.7

33.0 26.2 20.7 16.4 13.0

10.3 8.14 6.59 5.22 4.07

3.26 2.58 2,00 1,59 1,26

1,00 .792 .628

Feet per

Ohm at

25°C

.3850 .4670 .6050 .7630

.9550 1.204 1.519 1.915 2.414

3,045 3.839 4.841 6.105 7.698

9.707 12.24 15.43 19,46 24.54

30.95 39.02 49.21 62.05 78.25

98.66 124.4 156.9 197.8 249.4

314,5 396.6 499.3 629.6 794.0

1,001 1,262 1,592

TRANSFORMER CONSTRUCTION. MATERIALS, RATINGS 37 Table V. Turns рев Square Inch of Insulated Wire

Mean length of turn must be calculated for a coil in order to find its resistance in ohms. This may be found by referring to the side view of Fig. 32. Note that there is a small clearance space between core

-COflE TONGUE

WIRE-

-WPER

d = TONGUE WIDTH W* STACK

r - COIL TUBE RADIUS

A - MARGINS В-WINDING TRAVERSE С - OVERALL LENGTH D-BUILD UP

E - INSIDE DIMENSION OF TUBE F - OUTSIDE DIMENSION OF COlU G - TUBE THICKNESS

Fig. 32. Paper-insulated coil.

and coil form or tube. Let d be the core tongue and w the stack. Suppose there are several concentric windings. The length of mean turn of a winding V at distance r from the core and having height D, is

(-1)

MT = 2w-2d-[- 27Г \r +

= 2{w + d) + 7r(22D -f- D) (26)

where is the sum of all winding heights and insulation thicknesses between winding V and the core.

The mean turn of the winding V just below V ordinarily is calculated before that of winding V. This fact simplifies the calculation of winding V, the mean turn of which is

MTv = MTu + <Du + Dv + 2c) (27)

where с is the thickness of insulation between V and V.

Allowance must be made, with many coil leads, for bulging of the coil at the ends and consequent increase of mean turn length.

The placement, insulation, and soldering of leads constitute perhaps the most important steps in the manufacture of a coil. When coils

Table VI. Paper-Insulated Coil Data (Courtesy Phelps-Dodge Copper Products Corp.)

are wound one at a time, the leads can be placed in the coil while it is being wound. The start lead may be placed on the coil form, suitable insulation may be placed over it, and coil turns may be wound over the insulation. Tap leads can be arranged in the same way. Finish leads must be anchored by means of tape, string, or yarn, because

wfM win

Fig. 33. Winding 20 coils in multiple machine: layer paper at right.

there are no turns of wire to wind over them. Typical lead anchoring is shown in Fig. 34.

In multiple-wound coils, the leads must be attached after the coils are wound. Extra wire on the start turn is pulled out of the coil and run up the side as shown in Fig. 35, with separator insulation between wire extension and coil. Outer insulation covers the wire extension up to the lead joint. A pad of insulation is placed under the joint, and one or more layers of insulation, which insulate and anchor the joint, are wound over the entire coil and the lead insulation. Electrical-grade

TREATED CLOTH

COIL

START LEAD

FISHPAPER TREATED CLOTH FISHPAPER SOLDERED JOINT

WHEN FIRST PLACED ON TUBING

FIRST LAYER OF WIRE

START LEAD

JOINT

INSULATION

AFTER FIRST LAYER IS WOUND

Fig. 34. Start-lead insulation in hand-wound coils.

In high-voltage transformers it would often be possible to seal the windings if there were no leads; hence lead placement calls for much care and skill. Leads and joints should also be mechanically strong enough to withstand winding, impregnating, and handling stresses without breakage.

17. Insulation. Three classes of insulation are used in dry-type transformers. Class A insulation is organic material such as paper, cotton, silk, varnish, or wire enamel. Class В insulation is mica, asbestos, glass, porcelain, or other inorganic material with organic binders such as varnish for embedding the insulation. A small amount of

scotch tape is widely used for anchoring leads. It is important to avoid corrosive adhesives.

Leads should be large enough to introduce only a small amount of voltage drop and should have insulation clearances adequate for the test voltage. These clearances can be found as explained in Section 19.