ELECTRONIC TRANSFORMERS AND CIRCUITS

for structural

other class A material is permissible in a class В coil reasons, but it should be kept to a minimum.

In general, the vital difference between these classes of insulation is one of operating temperature. Glass-covered wire is preferable to asbestos for space reasons; it is available in approximately the same dimensions as cotton-covered wire. Built-up mica is the usual insulation wrapper material. With special bonds it is flexible enough to

TAPE ANCHOR

OUTER WRAP

PAD-

COIL FORM

-l лчг1.1

OUIL -- L.fa4X.X.UOrrTT

-SEPARATOR INSULATION OUTER INSULATION WINDING EXTENSION.

Fig. 35. Start-lead insulation in multiple-wound coils.

wind over coils or layers of wire. Stiff mica plate for lead insulation and mica tubing for coil forms are usually bonded with heat-resistant varnish. Class В insulating material is more expensive than class A and is used only when other advantages outweigh the cost.

The necessity for small size in aircraft or mobile apparatus is continually increasing the tendency to use materials at their fullest capabilities. As size decreases, the ability of a transformer to radiate a given number of watts loss also decreases. Hence, it operates at higher temperature. Transformers for 400- and 800-cycle power supplies can be made in smaller overall dimensions by using class В insulation (see Section 20). As a result, from 30 to 50 per cent decrease in size (as compared with class A insulation), in addition to increased ability to withstand extremes of ambient temperature, humidity, and altitude, is obtained. Class В insulation is thus of special importance in

aircraft apparatus. Usually at 60 cycles enough room is available to use class A insulation, but mica may be used to reduce the size of high-voltage units.

A third class of insulation is the silicones, organic silicates with remarkable thermal and mechanical properties. These materials are coming into use at operating temperatures approaching 200°C. Sili-cone-treated cloth, silicone rubber, and silicone varnish are already in use. Under development are silicone wire enamel and silicone-bonded mica. They are generally designated as class H insulation.

For apparatus having long service life, AIEE Standard 1 limits the hottest spot temperature of impregnated coils as follows:

Class A insulation 105 °C Class В insulation 130C Class H insulation 200 °C

Life test data are plotted in Fig. 36 for class A and class В insulation. The temperature scale is special, based on T. W. Dakins data, showing that insulation life is proportional to the reciprocal of absolute temperature. The tw lines indicate how operating temperature may be increased for a given life when class В insulation is used. Equal life is obtained when class A insulation is operated at 105°C maximum (40°C ambient, 55°C rise, 10°C hottest spot gradient), and when class В insulation is operated at 130°C maximum (40°C ambient, 80°C rise, 10°C hottest spot gradient). Intermittent load temperatures may be high for short periods. These periods are additive. For example, class A insulation has approximately the same life whether it is operated at 115°C continuously or half the time at 123°C and half the time at 25°C. Figure 36 shows only the influence of temperature on insulation life. Life is further reduced by moisture, vibration, and corona. It is therefore important that insulation be protected against damage caused by all these factors. Such protection is discussed in Section 20.

18. Dielectric Strength. The usual figure given for dielectric strength is the breakdown value in rms volts at 60 cycles in a 1-minute test. It is not possible to operate class A insulation anywhere near this value because of the cellular structure of all organic materials. Even after these materials are treated with varnish, many small holes exist throughout a coil structure which ionize and form corona at voltage

1 For the definition of impregnation, see Section 20.

2 See Electrical Insulation Deterioration Treated as a Chemical Rate Phenomenon, by T. W. Dakin, Trans. AIEE, 67, 113 (1948).

far below breakdown. With class A insulation (organic materials), the designer must be governed more by resistance of the insulation to corona over a long period than by breakdown strength of the insulation in a 1-minute test. For example, a 20-mil thickness of treated cloth will withstand 10,000 volts for 1 minute. However,

260 240 220

Ш 200

Ш

cc о

Ш

о

< 160

100 1,000

LIFE IN DAYS

10,000

Fig. 36. Approximate life expectancy of electrical insulation.

corona starts at 1,250 volts, and operation at any higher voltage would puncture the insulation in a few weeks. It is much wiser to keep a reasonable margin, say 20 to 30 per cent, below the corona limit than to use a fraction of the 1-minute breakdown test. Approximate voltages at which corona is audible are plotted in Fig. 37 as a function of insulation thickness.

Differences in hearing ability between persons make a corona measurement desirable. This is done by means of the standard NEMA circuit of Fig. 38. With the transformer connected as shown, receiver

1 See Radio Influence Characteristics of Electrical Apparatus, by P. L. Bel-laschi and C. V. Aggers, Ттш. ALEE, 57, 626 (November, 1938).

20,000

10,000

1,000

.01 .02 .04 .06 .08 .1 .2 .4 .6

TOTAL INSULATION THICKNESS IN INCHES

.8 I.

Fig. 37. Corona Hmit for treated cloth and paper.

Class В insulation can be worked much closer to the ultimate dielectric strength, but the latter is less a factor in determining size than creepage distance to the core. For mica an approximate working voltage rule is 100 volts rms per mil thickness.

Insulated coils in air are subject to a two-dielectric effect that is peculiarly troublesome. If the path of electric stress is partly through solid material and partly through air, the air may be overstressed because it has the lower dielectric constant (unity, compared with 3 to 5 for most coil materials). If this condition exists, it is usually imprac-

output meter is adjusted to half-scale by a volume control potentiometer in the recei\er. Next, the transformer is replaced by a modulated 1-mc signal generator, the output of which is varied until the noise meter output is again half-scale. The signal generator output in microvolts is read on an attenuator; this is then a measurement of the corona present.

ticable to increase the air distance and so reduce the volts per inch to a value below the corona limit. The addition of more solid insulation over the whole coil may make it too large. Often the only feasible remedy is to fill the air space with more solid material, either in the form of filling compound or strips of insulation like micarta or press-board.

It is important, when dealing with insulation voltage, to make a

с

TESTING TRANSFORMER TRANSFORMER UNDER TEST COUPLING CAPACITOR

R INPUT RESISTOR (600 OHMS)

NM NOISE METER (RECEIVER WITH

METER OUTPUT) RFC RADIO FREQUENCY CHOKE

Fig. 38. Standard NEMA radio-influence measuring circuit.

distinction between test voltage and operating voltage. Of the two, operating voltage is the better value to specify.

19. Creepage Distance. Although solid insulation dielectric strength is important, the usual bottleneck for high voltage is creepage distance, such as margins between wire and core along the layers of insulation, or margins between lead joints and frame along the leads and coil sides. A common way of increasing the direct creepage distance across the margins is to use an insulating channel as in Fig. 39(a). This is especially helpful when the part of the coil adjacent to the core tongue is at low potential and the upper part is at high potential, as in some plate transformers. When the whole coil is at high potential it may be insulated by taping the coil, but taping is expensive and is avoided wherever creepage safely provides the necessary insulation strength.

Creepage distances over treated cloth or other organic material in air are shown in Fig. 40 for breakdown voltages up to 100 kv. The primary purpose of these curves is to find the proper margins for coils adjacent to the core.

CORE TONGUE

MARGIN

INSULATING CHANNEL

COIL FORM

Fig. 39. (a) Use of insulating channel; (b) taped coil.

is brought across the margin and up the side of the coil. In such a case, the only creepage distance is the thickness of the coil form. In low-voltage coils this may be enough; in higher-voltage coils, a barrier of insulating material is needed between the coil form and the core, under the spot where the lead is brought out of the coil. Such a

Insulation between the start (or finish) turn of the first layer and the core consists of creepage along the margin plus the thickness of the coil form. This is not a relevant distance, however, if the coil lead

barrier is provided by outer insulation in Fig. 35. Dimensions of the insulating barrier should be such that a distance at least equal to the coil margin should intervene between the start lead and the core in all directions and the thickness may be the same as the coil form.

THICKNESS X 10 INCHES

Fig. 40. Creepage curves in air over smooth organic insulation.

In any coil where the finish lead is at the top of the coil, there is less difficulty in insulating the finish lead. The finish lead has a longer creepage distance to the core if the height of the coil is a greater distance than the margin. It is necessary to avoid using materials on the sides of the coil which would result in any decrease of dielectric strength. In this respect, the creepage strength of some materials with high puncture strength is not good. The last layer of wire may he insulated from the core with a channel as in Fig. 39(a).

When practical coil margins, even with barriers, are insufficient to

WINDING, N0.1

fMARGIN*

LINES OF ELECTRICAL STRESS

WINDING N0 2

INSULATION A THICKNESS

INSULATION THICKNESS

INSULATION-

Fig. 4L Adaptation of Fig. 40 for insulation between coils.

by an imaginary center line. With equal margins in the two windings, the voltage stress is symmetrical about this center line. Margins should be such that there is sufficient creepage distance, in conjunction with one-half the insulation thickness, to withstand one-half the test voltage between these adjacent windings. That is, when full test voltage is applied between the windings, only half of it appears between the first layer of winding 1 and the center line of the insulation between the windings. If the margins arc unequal, the sum of the two margins, in conjunction with the total insulation thickness, should be large enough to withstand the full test voltage, in accordance %vith Fig. 40.

Coils may be divided into part coils or sections, to reduce insulation stresses, but such coils should be closely integrated with the circuit. For this reason, part coils are discussed in later chapters.

20. Impregnation. After a coil is wound the best practice is to impregnate it in some sort of insulating liquid which hardens after filling. This is done for several reasons. First, it protects the wire from movement and possible mechanical damage. Second, it prevents the entrance of moisture and foreign matter which might corrode the wire or cause insulation deterioration. Third it increases the dielectric

support the induced or applied voltage, coils are taped as in Fig. 39(6). Taping is the most time-consuming but the safest method of insulation. Separate secondaries may be taped and then assembled over the primary. If the whole transformer winding is taped, the coil form must be large enough to allow room for the taping between the core and coil form. It is also important that the leads be taped, to prevent breakdown from joints to ground.

Ordinarily, a winding is separated from the winding under it by wraps of Kraft paper or other insulation. In the coil of Fig. 41 the insulation thickness between winding 1 and winding 2 is shown divided

strength of fibrous insulating materials. Fourth, it assists in heat dissipation from the coil. Single-layer coils may be dipped in the liquid, drained, and dried, but deeper, thicker coils require the use of vacuum to remove air from the coil and admit the liquid to all parts of the interior. The best mechanical result is obtained when coils are assembled with cores before treatment.

Insulation is considered to be impregnated when a suitable substance replaces the air between its fibers, even if this substance does not completely fill the spaces between the insulated conductors.

Coils having little or no temperature rise in normal use are impregnated with chemically neutral mineral wax. The wax is melted in a sealed tank and is drawn into another tank in which preheated coils have been placed, and a vacuum is maintained. Coils are removed from the tank, drained, and allowed to cool. Wax treatment provides good dielectric qualities and moisture protection. It is a quick, simple process.

Transformers having operating temperatures of 65°C or higher are impregnated with varnish. Varnish of good grade and close control is essential to achieve thorough filling and dry coils after impregnation. Oleoresinous varnishes, which polymerize to a hard state by baking, are notably useful for the purpose. A high degree of vacuum, fresh varnish, and accurate baking temperature control are necessary for good results. Plasticizers are sometimes added to the varnish to prevent brittleness in finished coils. Varnish may attack wire enamel {which itself is a kind of varnish), and so the soaking and baking time periods must be regulated carefully.

Varnishes for impregnation of electrical coils have until lately been diluted by solvents to lower the viscosity so as to permit full penetration of the windings. When the coils are baked, the varnish dries and the solvent is driven off. The drying leaves very small holes through which moisture can penetrate and in which corona may form. Eventually, the insulation deteriorates. It is, therefore, necessary to allow large clearances for high voltages or to immerse the coils in oil. Either of these alternatives increases the size of a high-voltage transformer in relation to that of a low-voltage transformer. For this reason, solventless resins have come into use as filling compounds for dry-type coils. They are known by trade names such as Fosterite, Para-plex, and Stypol. These resins have the advantage of changing from a liquid to a solid state by heat polymerization, so that small holes formed by drying of the solvent are eliminated. Filling of the coil

may be accomplished by casting the transformer in a mold, or by encapsulation. Encapsulation is readily adapted to irregular coil surfaces and is accomplished by a leak-proof coat before filling. In either process, a good vacuum is necessary to insure complete filling.

Silicone materials are moisture-resistant. Basic insulation should be inorganic, or silicone-treated cloth, tape, laminated sheets, and tubes. Through the use of silicones, some transformers may be designed to have very small dimensions for their ratings. This may be achieved most successfully if the coil insulation comprises only silicone or inorganic materials, including impregnation with silicone varnish. Dielectric strength of silicones is about the same as class A materials. Hence the thickness of silicone coil insulation is similar to that for organic materials.

Continual development improves all classes of insulation; present A, B, and H insulation classes may be superseded eventually by new classes based entirely on functional evaluation. Life tests have been proposed which classify a transformer according to its ability to withstand the effects of voltage, moisture, and vibration, as well as temperature.

In encased high-voltage units, air around the coils, bushings, and leads is especially subject to the formation of corona. To reduce this tendency, the containers are filled with asphaltic compound which replaces the air with solid, non-ionizing material. A similar compound is often used to fill containers of low-voltage transformers to avoid the need for mechanically fastening the core to the case. This is a permissible practice if the melting point of the compound is higher than the highest operating temperature and if its cracking point is below the lowest operating temperature.

21. Oil Insulation. Although, in electronic apparatus, there is a tendency toward the use of dry-type transformers, frequently voltages are so high that air clearances are impracticable and oil-filled containers must be used. In Fig. 42 the curves show rms breakdown voltage versus creepage distance under oil. An example will show the advantage of oil filling. From Figs. 40 and 42 it will be seen that 10-in. creepage distance is required in air to withstand a 1-minute breakdown test of 60 kv on insulation 0.5 in. thick, whereas in oil only 2-in. creepage distance is required,

1 See Functional Evaluation of Insulation for Small Dry-Type Transformers Used in Electronic Equipment, by R. L. Hamilton and H. B. Harms, AIEE Tech. Paper 54121.