Induction heating coils for high frequency induction heating can be made in a wide variety of styles, shapes, and sizes. Their design usually is governed by the nature of the component to be heated and by the type of generator used. Briefly, induction heating coils can be of the single-turn or multiturn type, fabricated, multiple, series-connected, or cast to shape. However, some high frequency induction heating generators are limited to certain specific types of coils, because of the impedance of their tank circuits which may limit their use to multiturn induction heating coils. In view of this condition, the power source must be taken into account before a heating coil is designed. Other generators, however, are provided with means to match practically any type of induction coil so that a much broader selection of induction heating coil designs is made possible.
In describing different types of induction heating coil for inductive operations, it would be difficult to cover all the different types of condition which arise, such as the shape and size of the work piece, the nature of the surface to be heated, and its relation to the heating coil. However, from the variety of induction heating coil shown, representing the designs most commonly used, together with some of the basic principles involved, a good conception of the importance of induction coil construction will be gained. The designs represent coils that will service the need for a wide variety of heating operations, and that with slight modifications will cover the majority of heating applications encountered.
Perhaps nothing is more important to induction heating than the coil itself. Next is the source of power, which must be suited to the particular operation contemplated. It is here that frequency and volume of power show their possibilities as well as their limitations. But with suitable high frequency power supply it is on the coil that we depend for the correct distribution of heat within a metal part.
If we consider an induction heating coil as a means of transferring heat by eddy currents, and then consider the shape of the induction coil in relation to the work's surface, we can see that the heat pattern will closely resemble the shape of the coil, more so with a close coupling than where the work and the heating coil are farther apart. The problem therefore is to make the induction heating coil of correct shape to surround a surface to be treated and then space it according to the amount of heat needed. As the induction coil is placed farther from the work, the eddy currents spread out in wider form and cover a larger surface. Coupling, therefore, is an important consideration.
Not always is maximum heat transfer the most desirable factor. Often a slow, even heat distribution is needed. For general hardening purposes a quickly and rapidly heated surface usually is desired. For soldering a slower penetrating heat might be more desirable. For brazing a
Fig. 37 - A variety of heating coils used in connection with induction heating equipment, including those of the single and multiturn types.
somewhat in between heat might be required, since rapid heat might cause blistering and a slow heat would result in lost time or the heating of unwanted areas.
Types of Coils. In Fig. 37 are illustrated some of the more commonly used coils as applied to induction heating equipment. The general purpose type most used is one shown at A representing the multiturn design, which is would or formed from copper tubing, either symmetrical in contour or shaped to suite the outline of the part to be heated. Solid-type induction coil, like the one shown at B, are also widely used and are particularly suitable for heating of parts where a restricted heat zone is desired. The heating coil shown at C is of series design, which makes possible the heating of several pieces at onetime. The induction heating coil at D is made of flat strip, which, as may be seen, may be used in a variety of heating operations.
Regardless of the types of induction heating coil used, it is necessary to provide cooling, which in the case of the multiturn coil is accomplished by circulating a flow of water through the coil itself. With the single-turn inductors, cooling is accomplished by adding a copper tube to the outside or by other suitable cavities for the passage of water.
Fig 38. - Multiturn coils can be formed to various shapes and usually have a wide latitude of applications in connection with induction heating work.
All heating coils, regardless of their design or shape, should be made of copper having a conductivity of 90 per cent or more. Pure copper unquestionably serves as the best material for coil construction.
Multiturn Induction Heating Coils. In making a multiturn induction heating coil of copper tubing, a wide variety of shapes is possible, as shown in Fig. 38. The most common is a cylindrical coil, as at A, which is suited to surface-heating of shafts and round parts. The rectangular or square coil at B, as well as the cam-formed coil at C, also is used for heating the outer surfaces of bars or shafts and can be easily formed over a wooden block. The pancake coil at D is used for heating flat surfaces, such as clutch jaws, or ends of shafts, while the spiral-helical coil E is used for heating conical surfaces, such as bevel gears. The coil at F is of the internal type and is used for heating inner surfaces of holes.
Other suggested types of induction coil made of copper tubing, arranged for heating irregular surfaces, may be seen in Fig. 39. The induction heating coil at A heats the surface around the slot, while the one at B heats the end section of a formed steel bar. Coil C, somewhat similar in shape, heats on edge and the fillet of a plate, one view of which illustrates the coil in position during heating. With irregular surfaces such as these, requiring coils of intricate shape, it is often necessary to experiment until
Fig. 39 - Copper tubing can be formed into various odd-shaped coils, as shown, for the localized heating of metal parts.
the correct heat pattern is obtained. Sometimes only the spreading of the coil, one way or another, might offer the results desired, whereas in other cases a completely new shape may have to be wound.
In Fig. 40 suggestions are shown for the construction of copper-tube induction heating coils. Such induction coils usually are made of tubing ranging from 1/8 to 1/4 in. in diameter. The 1/8 in. size should be used very sparingly, however, because of its small area and since the flow of cooling water is likely to be too small to prevent overheating. For short heating cycles, where a slight heating of the coil may have no effect, this size could be considered. However, the two larger sizes are preferable.
Round copper tubing can be used for many types of heating coils, as shown in A, although it is usually preferable to flatten the tubing as illustrated at B. Another practical form is the square or rectangular shape shown at C. It also is possible to use a larger diameter tubing, such as 5/8 or 3/5 in., as shown at D, and to produce a flat coil similar to a solid inductor previously mentioned.
In making helical copper-tube coils where a restricted heat zone is desired, the coils should be made with an offset, as shown at E, so that uniform heating can be provided. At F is shown
Fig. 40 - Constructional details of copper-tube heating coils of the multiturn types are shown here.
a means of providing a brace for a large-diameter induction heating coil, which might have a tendency to sag or go out of shape. These braces provide unusual stiffness and, as shown, are attached by brazing small rivets to the induction coil section and heading the rivet over on the outside of the braces. These braces can be made of thin asbestos board, or a laminated plastic material.
A suggested form of tube flattener for use in the making of heating coils requiring square, oval, or other formed sections, is illustrated in Fig. 41. This device consists of two formed rolls, through which the copper tubing is passed. Feeding of the tubing through the rolls is done by means of the hand crank. The upper roll is mounted on a hinged bracket, which is adjusted. to correct relation with the lower roll by means of the hand knob B. At C is shown the type of roll which can be used for forming a square-shaped section from a round tube, whereas at D is illustrated the roll used for providing an oval - or flat - shaped tubing.
Fig. 41 - A hand - operated tube flattener, provided with forming rolls, can be used in the construction of heating coils requiring flat tubing.
Heating Rates. Because of the path of magnetic flux around a cylindrical induction coil like that at A, Fig. 42, the greatest field strength will be obtained within the coil itself, rather than on the outside. For this
Fig. 42 - Three most popular types of multiturn coils are the external, pancake, and internal designs shown here.
reason outside surface-heating of parts, such as steel shafts, can be preformed exceedingly fast. With a flat- or pancake - type induction heating coil, as at B, heating will be somewhat slower per square inch of area, or roughly 75 per cent that of the example at A. With internal-type coils, like those shown at C, heat transfer is still slower, because less magnetic flux is concentrated on the work surface, and may be heated only 50 to 60 per cent as fast as outside heating. These estimates are based on the use of a high frequency generator of a given output power. for non-ferrous materials heating times will be somewhat greater than for ferrous materials, usually in the order of 1 1/2 or 2 to 1.
Internal Induction Coils. since the density of the magnetic flux is less on the outside than on the inside of an induction coil, and because the greatest strength lies next to the coil, it is advisable to make internal coils in
Fig. 43 - Constructional features of internal coils are shown here. Coils of this type should be made as thin as possible and should be located close to the work's surface.
such a way that the over-all distance from the surface of the hole being heated to the inner surface of the induction heating coil is held to an absolute minimum. In A, Fig. 43, is shown a multiturn induction coil of usual design, which, when applied within a hole, results in the dissipation of heat energy because of the excessive over-all depth between the work surface and the inner diameter of the induction heating coil.
At B the condition is greatly improved, since the coupling between the work and induction coil is reduced and since the coil is flattened. This means that the over-all depth is held to a minimum, thus assuring maximum heat energy, the magnetic flux being more constricted to the heated area. The coupling for internal heating coils should be made no more than 1/16 in., and less if possible.
Internal induction heating coils for hole heating usually are limited in size because of their mechanical construction, and usually a 5/8 in. hole is the smallest that can be heated with a multiturn induction coil, as shown at C. Another form of internal coil is the double-turn heating coil, illustrated at D, in which the tubing is flattened to provide for maximum heat transfer. In making coils of this type, care should be taken to avoid closing in of the tubing, so that the water passage will not be restricted. Also, because the hat pattern within the hole is likely to resemble four longitudinal bands, the work piece should be rotated during heating. The induction heating coil at E provides still another design using formed tubing, thus permitting the coil to be small in diameter.
The hairpin coil at F also is practical for small-hole heating, but the part must be rotated during heating to assure uniformity of heat transfer. Usually internal coils of the types shown are limited to surfaces in which the height of the heated surface is not more than twice the diameter. Where the length is in excess of the ratio progressive heating should be provided if possible. In this case the heat is concentrated to a small area of the hole, while the work itself feeds at a uniform rate, depending upon available output capacity of the generator, the size of the hole, and the area being heated.
When heating a tapered surface, the induction heating coil is usually made to conform with the taper, although exceptions may be considered as at A, Fig. 44, which shows a fixed-diameter induction coil arranged so that the pitch of the winding is wider at the large end than at the small end. With this design, a greater concentration of heat will be provided at the small end, because of the variation in pitch.
It is well to remember that with a fine-pitch induction coil the penetration of heat will be deeper than with one having a coarser pitch. Usually the spacing between coils should not exceed half the diameter of the induction heating coil, while smaller spacing is preferable.
In heating conical parts, such as bevel gears, a heating coil with a consistent spacing or coupling will result in a more intense heat at the small end, as shown at B. To compensate for this condition, the induction heating coil should be made with a wider angle at the small end, as shown at C, so that heating will be uniformly distributed.
The same procedure is followed when using a solid induction coil, shown at D. In heating flat surfaces, like that illustrated at E, a coil made parallel to the face will have a tendency to create the greatest heat toward the center. Induction heating coils for such surfaces, therefore, should be made slightly conical or angular, as shown at F.
Fig. 44 - Coils provided with variations in spacing between turns can be arranged to transfer heat uniformly to tapered surfaces. Usually in heating tapered surfaces the coils should be made with a wider angle than that of the work piece.
Distribution of Heat. An example of a multiturn copper-tube induction coil used for hardening a projectile on which differential-hardness readings are desired is illustrated in Fig. 45. The induction heating coil is made so that the hardness readings from the point of the projectile to the base will taper gradually from 60 to 35 Rockwell C. In operation the part is heated and then quickly dropped into a quench. The variation in heat and hardness is attained by a variation in coupling, and in the pitch of the coil turns, which
Fig. 45 - A coil provided with variations in turn spacing, which is used for the hardening of a projectile on which a gradient of hardness ranging from 35 to 60 Rockwell C is obtained.
as can be seen are wider at the bottom than at the top. This type of coil provides for maximum heat where the hardest area is desired, and gradual diminishing of heat for less hardness.
Fig. 46 - A single-turn heating coil used for brazing a steel tube and an insert together. It is necessary to arrange the coil so that the transfer of heat is concentrated on the heaviest section of the assembly.
In designing coils for heating purposes, it is often necessary to analyze the application to determine the best means of heat distribution. For the application shown in Fig. 46, which requires the brazing of a steel insert to a drawn-steel shell, the coils as shown at A might cause overheating of the then material before the proper amount of heat could be conducted to the heavier insert. This being the case, the outside surface also might blister and become badly warped.
To overcome this condition, an induction heating coil such as that shown at B would be desirable, since the generation of heat would be distributed more to the heavier section, particularly at the bottom, and then would travel by conduction to the remaining surfaces directly above. For heating operations of this kind, a solid inductor of the single-turn, or even-series, type, is to be preferred. The induction heating coil can be made somewhat higher than might be necessary. Then after observation of the resulting heat the induction coil can be trimmed down one way or the other until the exact heat pattern has been produced.
Fig. 47 - It is not good practice to make long multiturn coils, because uneven heating is very likely to take place. In heating long areas, it is better to use a short induction heating coil and feed the work progressively through it.
Length of Induction Heating Coil. There is a limit to the length of surface which can be heated at one time, and while no fixed rule applies, a helical induction heating coil usually should not be more than three to four times its diameter, whereas a single-turn induction coil will be found more effective when the length is not more than half the diameter. The example shown at A in Fig. 47 obviously is too long, since too great a surface is being heated at one time, causing possible deflection as well as an uneven distribution of heat. For a surface of this type, assuming that hardening is required, it would be much better to use a shorter heating coil and progressively feed the part through it, such as is shown at B. The available output power of a generator also is relative to the amount of surface that can be heated at one time. In this same example a generator with considerable power would be required to heat the entire shaft at one time, as compared with one of nominal power for the progressive-heating method.
Multiturn coils exceeding the proportions given can be sued for the heating of this sections, such as steel wire and rod, especially in annealing operations. For hardening and brazing requirements,
Fig. 48 - In connecting heating coils to generators, it is well to keep the leads as close together as possible in order to avoid inductance losses, which would detract from maximum heating in the work.
however, exceptionally long heating coils may offer difficulties. Sometimes it is this consideration that decides whether a job can best be performed by inductive heating or some other method.
Inductance Losses. When connecting the leads of an induction heating coil to a generator, especially those of the quick-change type, it is desirable to keep them as close together as possible, in order to avoid inductance losses between the leads, as might be represented by the example in Fig. 48. The spacing, as represented at A, would result in some dissipation of the high - frequency current, so that maximum heating of the work within the induction coil would not be attained. By providing leads as shown, where the space B is held to a minimum, there is a better assurance of maximum heating of the work within the induction coil.
This same condition would be true in a series-connected induction heating coil, where substantial losses will occur if the proper technique is not provided. The coil shown at A in Fig. 49, for example, represents a very poor design, where owing to its shape undesired inductance is set up between the induction coil sections. On the other hand, the induction coil at B, which represents the same general type, is very well laid out and would offer the assurance of maximum heat concentration to the work pieces located in the openings.
Fig. 49 - In connecting series-type coils, it is essential to make the connections in such a way that minimum inductance losses will occur.
Pitch of Induction Coil Windings. In applying multiturn coils for the heating of steel shafts and other round parts, the pitch of the turn windings will have a direct relation to the depth of heat penetration. With a find-pitch induction heating coil, such as that illustrated at A in Fig. 50, the magnetic flux penetration will be more highly concentrated on the surface of the work, with a resultant deeper layer of heat. With an open-pitch induction coil, where the space between windings is increased as at B, the penetration of heat to the surface of the work will be somewhat less affected by the fact that the magnetic flux spreads out over a wider area and causes certain losses. Usually in making a multiturn induction coil it is best to arrange the windings so that the space between turns will not be more than one-half the thickness of the tubing itself. Beyond this limit, or with a loosely wound induction heating coil, non-uniform heating is likely to occur, and unless the piece being heated is rotated the heat pattern on the work's surface will follow the spiral of the heating coil.
When a steel part, such as a shaft, is heated at a comparatively fast rate by a multiturn induction heating coil, there is likely to be a variation in temperature on the work's surface because of the field set up by the spiral windings. The more open the windings of the induction coil, the more pronounced will be the bands of heat, to a point where the heated surface will follow the spiral form of the heating coil. For this reason, the coupling, or spacing from the work to the heating coil is an important consideration and, likewise, bears a direct relation to the pitch winding of the induction heating coil.
Fig. 50 - Coils provided with fine-pitch windings have a tendency to throw a deeper layer of heat than those would with loose windings.
With an induction heating coil like the one shown at A in Fig. 51, where the coupling is close and the pitch winding of the induction coil loose, the heated zone will take on a bright and dark pattern, following the exact shape of the coil windings. By increasing the induction heating coil coupling, as at B, the heating becomes more uniform, though it increases somewhat because of less flux-density transfer to the work's surface.
This heating effect is an important consideration when using multiturn induction heating coils. It is most obvious when the spacing between coil turns is about equal to the coupling, and diminishes as the tube spacing are reduced, in proportion to coupling. The best procedure is to rotate the work within the coil so that uniform heating is assured. This can be done in many cases by hand, if the part permits, or by a motor-driven spindle, using a speed as low as 25 r.p.m., or just enough to break up the uneven distribution of current density.
Fig. 51 - Multiturn coils should be correctly proportioned as to space between windings and their distance from the surface of the work, so that an even distribution of heat will be assured.
Naturally, the induction coil spacing around a part to be heated should be uniform, especially if a uniform distribution of heat is desired. In some cases, however, a slight amount of eccentricity between
Fig. 52 - If an eccentric coil is used to heat a section of round steel, the variation in depth of heat penetration will be proportionately greater with narrow coils than with high.
the work and the induction heating coil is not objectionable and may have no effect on the heated area. This is usually the case when heating a small part and where an ample amount of power is available. Where the rate of heating may be slower, however, eccentricity of the coil may produce a variation in heat, the side nearest the coil receiving the greatest amount.
With a high induction coil, like that shown at A in Fig. 52, a slight amount of eccentricity will be hardly noticeable. However, with a narrow induction coil, as shown at B, any appreciable amount of eccentricity or unevenness in coupling will be more apparent in the form of heat variations to the work's surface. This is quite noticeable in heating such parts as gears, especially when the thickness is small in proportion to the diameter. Rotation of the work, of course, offsets irregular heating due to slight
Fig. 53 - When heating coils overlap the edge of a piece of work, there is likely to be overheating on the end surface. Placing the coil slightly below the edge will produce a more uniform heat pattern.
variations in coil couplings, as would be the case with an eccentrically located inductor.
Heating Effects. When a cylindrical-type heating coil is placed around the end of a shaft in such a way that it projects over the edge, as indicated at A in Fig. 53, there will be a pronounced heating over the face as indicated at B. If, however, the induction heating coil is arranged as shown at C, in which case the edge is even with or slightly below the end of the work, the heat pattern on the surface will be more uniform, as shown at D.
This same condition exists when heating a disk, or the flanged area of a piece, as shown at E. The heat pattern will be such that the depth of penetration will be less in the middle of the blank than at the ends. With some parts it may be necessary to use a formed induction heating coil, as shown at F, with the inner face made convex. This design provides for a uniform layer heat, as shown at G, and eliminates the overheating of the edges which might occur with a straight cylindrical-type induction coil. These same conditions apply to use of multiturn heating coils.
In applying heat to a surface which may have a shoulder or flange at each end, as illustrated in Fig. 54, the spacing of the heating coil in relation to the various surfaces becomes an important consideration. If the induction heating coil were proportioned as shown at A, where the edges of the induction coil are closer to the flanges than the central part is to the body, some heat would be applied to the edges. This
Fig. 54 - In applying heat to flanged surfaces, the relationship of the coil to the surface of the work is an important consideration.
would result in the absorption of much of the flux density, so that the body of the part would receive only a small amount of heat.
For such a surface, therefore, the induction heating coil should be made as shown at B, arranged so that the induction coil is closest to the body requiring heat. In any case, a split-type inductor would be required in order that the heat-producing surface might be properly placed in relation to the surface requiring heat. For such a part there is the possible use of a larger induction heating coil such as shown at C, through which the work is inserted, and then positioned eccentrically as shown. Rotation of the work is thus provided. With such a setup the flanges are more liable to absorb some of the eddy currents, although the body will become more predominantly heated.
Two-piece Induction Heating Coil. A split-type induction heating coil like that used to heat a part having flanges on each side of the bearing surface is illustrated in Fig. 55. In each section of the induction coil are separate cooling tubes, which in turn are series connected for the continuous flow of water. Both sections are held together by a clamp, although various other means of connecting may be used. One of the pieces to be heated is shown at the right of the heating coil.
A hinged-type inductor of solid design used for the hardening of crankshaft bearings is illustrated in Fig. 56. The induction heating coil is arranged so that after the work has been properly located and centralized within
Fig. 55 - A typical single-turn heating coil of the split type, which is used to heat the small diameter of a flanged part.
it, the upper half of the induction coil is brought into contact with the lower half, thus completing the electrical connection. Both heating coil sections are held together with pressure during the heating operation. Also, both sections of the induction coil are provided with internal passages for cooling purposes, with suitable quenching chambers to make the quenching water pass directly through the area of the coil's heating surface. The cycle for the operation is controlled by a timer and functions as follows:
1. The induction heating coil is closed.
2. High-frequency heat is generated to the work's surface.
3. The quench is applied.
4. Finally, the induction heating coil is opened so that the work can be removed.
When applying high-frequency current to surfaces which include keyways, or holes like those illustrated in Fig. 57, the eddy currents concentrate at the corners, probably casing some overheating. With the keyways as shown the edges will heat rapidly and a slight burning action might take place. To overcome this, the part can be fitted with a copper key to bridge this gap, thus making the heating more uniform.
Fig. 56 - A well proportioned hinged-type induction heating coil used for hardening crankshafts bearings.
In case a part having a hole through the surface is to be heated, as shown at A, the magnetic flux from the induction coil will tend to enter the hole and develop maximum heat around the edges. To reduce this condition the hole can be provided with a copper plug which shunts the opening and thus reduces the absorption of eddy currents at this point.
In heating a slotted part within a cylindrical induction heating coil, as shown at A in Fig. 58, the circulating current on the surface of the work piece naturally is interrupted by the slot, but the current will continue to flow
Fig. 57 - Overheating of sharp corners, such as keyways and holes, often takes place and the use of copper inserts may be advisable to shunt the path of high frequency current.
on the inside because of the nature of high frequency, which must form a closed circuit. When treating pats of this design, therefore, a somewhat higher heat usually is produced at the edge of the slot, as indicated at S. With the multi-slotted part as shown at B, the circulating current around
Fig. 58 - In heating shaped parts such as those shown in this example, the high frequency current tends to crowd into the slots resulting in overheating.
the work piece will have a tendency to crowd into the slots T, and again an overheating condition is most likely to occur on the corners. The thinner the coil in relation to its diameter, the more pronounced the heating at the slots. On the other handle, with the use of a heating coil having a height greater than its diameter, this condition will be less pronounced.
Fig. 37 - A variety of heating coils used in connection with induction heating equipment, including those of the single and multiturn types.
Fig 38. - Multiturn coils can be formed to various shapes and usually have a wide latitude of applications in connection with induction heating work.
Fig. 39 - Copper tubing can be formed into various odd-shaped coils, as shown, for the localized heating of metal parts.
Fig. 40 - Constructional details of copper-tube heating coils of the multiturn types are shown here.
Fig. 41 - A hand - operated tube flattener, provided with forming rolls, can be used in the construction of heating coils requiring flat tubing.
Fig. 42 - Three most popular types of multiturn coils are the external, pancake, and internal designs shown here.
Fig. 43 - Constructional features of internal coils are shown here. Coils of this type should be made as thin as possible and should be located close to the work's surface.
Fig. 44 - Coils provided with variations in spacing between turns can be arranged to transfer heat uniformly to tapered surfaces. Usually in heating tapered surfaces the coils should be made with a wider angle than that of the work piece.
Fig. 45 - A coil provided with variations in turn spacing, which is used for the hardening of a projectile on which a gradient of hardness ranging from 35 to 60 Rockwell C is obtained.
Fig. 46 - A single-turn heating coil used for brazing a steel tube and an insert together. It is necessary to arrange the coil so that the transfer of heat is concentrated on the heaviest section of the assembly.
Fig. 47 - It is not good practice to make long multiturn coils, because uneven heating is very likely to take place. In heating long areas, it is better to use a short induction heating coil and feed the work progressively through it.
Fig. 48 - In connecting heating coils to generators, it is well to keep the leads as close together as possible in order to avoid inductance losses, which would detract from maximum heating in the work.
Fig. 49 - In connecting series-type coils, it is essential to make the connections in such a way that minimum inductance losses will occur.
Fig. 50 - Coils provided with fine-pitch windings have a tendency to throw a deeper layer of heat than those would with loose windings.
Fig. 51 - Multiturn coils should be correctly proportioned as to space between windings and their distance from the surface of the work, so that an even distribution of heat will be assured. 
Fig. 52 - If an eccentric coil is used to heat a section of round steel, the variation in depth of heat penetration will be proportionately greater with narrow coils than with high. 
Fig. 53 - When heating coils overlap the edge of a piece of work, there is likely to be overheating on the end surface. Placing the coil slightly below the edge will produce a more uniform heat pattern.
Fig. 54 - In applying heat to flanged surfaces, the relationship of the coil to the surface of the work is an important consideration. 
Fig. 55 - A typical single-turn heating coil of the split type, which is used to heat the small diameter of a flanged part. 
Fig. 56 - A well proportioned hinged-type induction heating coil used for hardening crankshafts bearings.
Fig. 58 - In heating shaped parts such as those shown in this example, the high frequency current tends to crowd into the slots resulting in overheating.
