How to deal with stator core damage

RichardNailen Electrical Apparatus

Jun 30, 2006 20:00 EDT

"RUSSIA IRON," "STOVE-PIPE IRON" OR JUST plain "sheet iron" -such were the materials

used to make laminated cores for electromagnetic apparatus before development of the

"electrical steels" we know today, with their carefully controlled additives of silicon,

aluminum, cobalt, or other performance-enhancing elements. Hence the term iron loss

remains in common use. However, "core loss" better describes the energy wasted in

magnetizing and demagnetizing a core assembly.

Core loss takes only two forms. One is hysteresis. That's the energy required to align and

realign the molecular structure of steel with the movement or change in the magnetic field

passing through the material. Hysteresis loss is determined by the metallurgy of the steel

itself. The punching process creates internal stress within the material that can increase this

loss. That's why laminations (or assembled cores) are often annealed, to relieve that

"punching stress."

In the 1980's, an English manufacturer of large rotating machines reported that tests

"suggested" core loss around teeth and slots exceeded those in unpunched steel laminations

by about 10% because of "work hardening" caused by the punching process.

The second type of loss is an I^sup 2^R phenomenon, created by the flow of eddy currents

through the steel's electrical resistance, driven by voltage that the magnetic field induces

within the laminations. The purpose of laminating the core, with some type of insulation

coating between layers, is to minimize that current flow.

Both hysteresis and eddy current losses increase with the frequency of field alternation. Both

also depend upon the magnetic characteristics of the material. However, another major

influence on the eddy current loss alone is the electrical resistance to current flow between

adjacent laminations. That so-called interlaminar resistance in turn depends upon three

variables: 1) the resistivity of the insulating coating or coreplate on the lamination surfaces; 2)

the nature and location of structural "bridges" or contact areas that short-circuit the coreplate;

and 3) the tightness with which the laminations are pressed together in the core stack.

In an a-c machine stator, these two losses occur in three places:

* Below the slots, in the "core," back iron, or yoke.

* In the teeth, between slots.

* At the inner surface or stator bore.

The losses at the stator bore, involving tooth surfaces, are variously described as pole face,

surface pulsation, or zigzag losses. They result from higher-frequency harmonic fluxes that

interlink stator and rotor across the air gap, influenced by the respective surface

discontinuities caused by the slotting. In many small motors, bridged rotor slots eliminate the

discontinuity on one side of the gap.

Stray core loss

The nature of the pole face loss sometimes leads to its being described as a "stray core loss."

Strictly speaking, stray load losses are functions of rotor current, so they do not appear at no-

load. The often-used formula for calculating surface pulsation loss is shown in the box on the

following page.

This loss is not directly measurable. It does not exist unless the rotor is in place. However,

total measured "core loss" in a machine is about double the sum of the calculated tooth plus

back iron losses (the multiplier ranges from 1.75 to 2.2).

The division of core loss into its two components is important because, as is not always clearly

understood, most causes of core damage have no effect on hysteresis loss. One exception is a

winding fault that produces local overheating and arcing in the slots, sufficient to vaporize

copper and melt steel. Otherwise, steel metallurgy is unaffected either by temperatures below

about 1,500

If such damaged areas are small enough, as in Figure 1, repair may be possible by carefully

grinding or drilling away the fused material. Otherwise, the core must either be scrapped or

restacked with enough new laminations to restore the original structure.

Other sources of core damage will affect only eddy current loss. One is the well-known

"stripping oven" over temperature, in which the core is heated enough to char or soften

insulation in the existing winding to permit its removal and replacement. Depending upon the

core configuration and the oven operation, the excessive heat may destroy the inter laminar

insulation throughout the core, or only in certain areas (the teeth being most vulnerable).

If damage is severe enough, increased core loss and local heating will jeopardize the

replacement winding insulation as well as seriously decreasing machine efficiency

(particularly at light load).

Another source of stator core damage is the rotor "rub" resulting from a bearing failure or

magnetic instability (Figure 2). Friction between the spinning rotor surface and the stator ID

may not be sufficient to thermally damage the winding itself, but the grinding action will

"smear" together the surfaces of lamination teeth, with consequent large increase in surface

pulsation loss. Machining the stator bore to "clean up" such smearing can not only be

ineffective, it can make matters worse.

Such smearing is usually detected by visual inspection. If the line separating one lamination

edge from the next is clearly visible, separation is probably adequate. If only a single,

unbroken mirror-like surface can be seen, however, inter laminar contact is sure to be

troublesome. That can sometimes be corrected by etching the stator bore surface with a mild

phosphoric acid solution.

Done either at the factory or during rebuilding, slot filing will also increase eddy current loss.

The problem should not occur in the repair shop unless the core has been taken apart and

restacked without sufficient care to maintain slot alignment. Once a core is assembled with

excessive "stagger" or misalignment of slots from one lamination layer to the next, proper fit

of coils in the slots becomes impossible. The only remedy is to broach or file the slots to

remove the high spots. That creates the same kind of "smear" just described.

Core tests

Establishing the existence of core defects except right at some external surface can be difficult.

Proponents of the "El Cid" test developed for that purpose have compared several diagnostic

approaches as shown in the table at right. But even the best method should be considered only

as an adjunct or follow-up to a ring or loop test (as detailed in the Appendix to IEEE Standard

56).

To insulate laminations from one another, several different coreplate insulation materials

have been used by the manufacturers of lamination steel. For many years, "C-3" was most

common. This is an organic varnish material, easily destroyed by temperature above 600°F to

700°F. More common today is "C-5," a largely inorganic material containing "ceramic fillers

or film-forming components," that will withstand annealing temperatures. It will not normally

be damaged in a stripping oven. A "C-4" coating is formed by chemically treating or

phosphating the steel surface. A more recently developed "C-6" material is a silicate-resin

compound for semi-processed lamination steel, capable of withstanding 1,500°F with post-

annealing resistance superior to C-5.

Unfortunately, manufacturing records do not necessarily indicate which coreplate coating may

be present in a particular motor. Visual inspection is useless in making that determination.

Nor can surface appearance of any lamination reveal the quality (or even the presence) of

suitable coreplate. For many years, the only standardized procedure has been American

Society for Testing & Materials No. A-717, known as the Franklin test.

The Franklin test involves clamping lamination material samples under a set of electrodes

imposing 300 pounds per square inch pressure. A standard test voltage is then applied to the

electrodes and the resulting current measured. From that measurement, the resistance across

one surface of the material can be calculated in "ohms per square." Unit size of the square is

immaterial, because any dimensional change increases both square width (which lowers

resistance) and square depth (which raises it by the same amount).

Note that this procedure doesn't directly measure the interlaminar resistance between

surfaces of two laminations within a core, at whatever the core clamping pressure may be.

Hence the adoption in 1996 of ASTM A-937, titled "Standard Test Method for Determining

Interlaminar Resistance of Insulating Coatings Using Two Adjacent Test Surfaces," which

does involve such a direct measurement.

For any machine, using allowances for variation that have been determined through

experience, motor designers and manufacturers can calculate losses to be expected in the first

two of the three locations mentioned earlier (core and teeth). Various tests (including the

"Epstein" method for hysteresis) support the calculations involved. Subsequently, total tooth

plus core loss in the assembled machine can easily be checked by the standard no-load test

procedure of IEEE Standard 112.

However, judging the accuracy of the calculation procedure requires some means of

separating the hysteresis and eddy current components. That's done by measuring total core

loss at two different frequencies, such as 50 and 60 Hz. Suppose total measured loss in watts

is P^sub 1^ at 50 Hz, and P^sub 2^ at 60 Hz. Then, the equation in the box at left applies.

If such testing isn't feasible, a common assumption is that hysteresis and eddy current losses

are equal. However, the relationship can vary considerably. Typical values of the hysteresis-to-

eddy ratio have ranged from 0.889 to 1.8 in 449 frame two-pole machines, for example.

Mechanical considerations

What are the mechanical features of stator core construction that can influence eddy current

loss? The most important one is the burr formed by the punching or stamping process at all

cut edges in each lamination. (see Figure 3.) As dies wear during production, that burr will

increase in height.

A common practice of large motor manufacturers has been burr grinding of laminations. After

punching, each lamination is passed through a belt sanding station that smoothes off the

burrs. Coreplate must then be added later to cover all bare metal areas left by the sanding.

Such deburring has been found to reduce total core loss by 4% or 5%.

Subsequently, care is normally taken to stack all laminations with the slot punching burrs in

the same direction. That's important because the region at the tooth tip is the most sensitive to

interlaminar current flow in general and to "surface pulsation" in particular. The sharp edges

of the burrs on adjacent laminations will cause good electrical contact if opposed, which is

minimized if they "nest "together-see Figure 4.

Laser-cut laminations are not subject to this precaution. However, cost and production time

constraints still support punch-and-die manufacture of most laminations, particularly for

mass-produced ratings.

Another influence on eddy current loss is core stacking pressure. Interlaminar resistance will

decrease, driving up the eddy current loss, as that pressure is increased (Figure 5). Making

sure the stack is compressed tightly-yet not too tightly-is a challenge for any manufacturer or

rebuilder.

The problem area is around the teeth. Despite the use of many different types of "fingers" to

clamp teeth together, some flaring or splaying is inevitable. One reason is that laminations are

seldom perfectly flat or of uniform thickness throughout. The dimensional variations involved

here are extremely small, and seldom accurately measurable outside the laboratory.

Nevertheless, the buildup over a long core stack can be considerable with typically 51

laminations per inch of length. The surface oxide coating on the steel could reach 0.00009

inch. The coreplate coating thickness may be taken as 0.00022 inch (total for both sides). Add

0.0004 inch for burrs, and another 0.0001 for deviation in flatness. All that makes a total

possible addition per lamination of 0.000729 inch.

The usual result of all these variations is a finished stack length longer at the ID than at the

OD, by 1/8 to 3/8 inch. Consequently, what the retained pressure will be throughout the stack

can never be known. Subject as they are to continuous vibratory forces in the axial direction at

twice line frequency, teeth not tightly restrained can abrade coil insulation, or eventually

break loose to cause more serious damage.

Deciding whether the finished core is "tight enough" is necessarily somewhat subjective. One

criterion is the "knife test," in which the attempt is made to insert a knife blade between

laminations, and the degree of core tightness is judged by how deeply the blade may

penetrate. Obvious drawbacks to such a test are:

* What should be the blade thickness?

* What depth of penetration determines that the stack is too loose?

* Should the test be made at the O.D. or the I.D. of the core?

Each repair shop must determine the appropriate answers. The stator core restacking

specification issued by a large public utility describes a similar test this way: "It should not be

possible to insert a 0.020 inch feeler gauge between any laminations." (Although it may seem

unrealistic, that at least answers questions 1 and 2.)

Rebuilding cores

The first step in core building or rebuilding, however, is to decide on a stacking pressure

expected to produce a tight stack. The second step, for long cores in large machines (and

"long" has its own subjective definition) is to specify appropriate "intermediate pressing." To

align the laminations properly, any core must be stacked on some kind of supporting structure

-a mandrel fitting against lamination ID or OD; guide bars or "drifts" in certain slots; or keys

or studs on the OD.

As laminations are stacked up, increasing friction against those supports will resist the

clamping pressure. The result is variation of core tightness throughout the stack length, from a

maximum near the top of the stack to minimum near the bottom. To equalize the compaction

throughout, clamping pressure is applied to the first quarter, third, or half of the total stack.

The next section is then added, pressure applied again, and so on until the finished height is

reached.

Pressing force often ranges between 100 and 150 pounds per square inch of gross lamination

surface area. No standard applies. The area is generally taken as the overall difference

between OD and ID, without regard to the material removed by slotting. Some manufacturers,

however, consider only the area below the slots.

The definitive study of core pressing is a 1964 paper presented to the British Institution of

Electrical Engineers, titled "Pressing and Clamping Laminated Cores." Its authors

experimentally developed the relationships shown in Figures 6 and 7. Their conclusion was

that a stacking pressure of only 40 psi ensures adequate core tightness, provided that such

pressure is maintained throughout the core for its entire life.

Probably the most uncertain condition in stator core building is the effect on lamination

short-circuiting of the manner in which the pressed core is held together. Segmented cores are

stacked on studs or dovetail key bars, with relatively poor electrical contact along the OD

(Figure 8). Smaller, one-piece laminations may be held together by welding or dealing (Figure

9).

Certainly the most solidly connected laminations will be in a core that is welded together on

the OD In small sizes, weld beads are deposited directly on the laminations. One difficulty is

that the heat involved can cause undesirable metallurgical changes in the lamination material

(a TIG process is recommended). Also, a good bond is difficult to achieve with silicon steel.

Larger cores will be welded against supporting steel bars to which clamping end plates are

attached. Limited experiments have shown that core losses do vary with the pattern of such

welding, but the range is neither great nor readily predictable. Theoretically, for any machine,

the greater the back iron depth in the core, the farther the OD is removed from the most

intensely magnetized region closer to the winding, and the less will be the influence of

interlaminar contact.

In one 500 hp 3,600 RPM machine, tests of a "fully welded" core showed 22% more core loss

than in an unwelded version; that amounted to less than two-tenths of a percentage point

drop in efficiency. However, the result was not conclusive because of other differences in core

construction that also influenced losses. In contrast, two 350 hp 720 RPM stators built with

widely different amounts of welding exhibited identical core losses and temperature rises (see

Figures 10 and 11).

When unacceptably high core loss is observed in a small or medium motor, a simple remedy

may be what some have called watt knocking, thumping, or slamming. In its simplest form,

this involves striking the unwound core sharply with a hammer, or pounding it a time or two

against a hard surface. The mechanical shock tends to break apart interlaminar "sticking"

caused by core annealing or machining. A more sophisticated and better-controlled process

applies the impact of an air-powered vibrating hammer for a minute or so.

Large cores require more elaborate methods of repair. Before deciding what to do, you should

study a core carefully to judge the nature and extent of damage. Loop or ring testing, with

infrared or other measurement of temperature on all accessible core surfaces, is the only way

to do that. The "El Cid" test, employing the same basic principle, is suited to the largest

machine sizes and will highlight damaged areas well away from external core surfaces.

In a loop test to evaluate stator core condition during the repair process, both hysteresis and

eddy current losses are combined in the result. Two serious limitations affect the

measurement. First, only the back iron or core region is fully magnetized. Second, and more

important, those losses will reasonably match those in the assembled machine only if the

magnetic field strength during the test matches the design value. Assuming a "typical" figure

cannot be expected to produce losses at the operating level (assumed core flux density for

years has been in the range of 60 to 100 kilolines per square inch-about 1.5 Tesla. Yet motors

of all sizes today often use actual back iron flux densities of 110 to 130 KL/square inch).

The proper indication of core condition is not the total temperature observed after some

period of time, but the rate at which temperature rises. Readings need to be taken at frequent

intervals as the test proceeds (for at least 20 minutes; some recommend 40 to 60 minutes).

One good rule of thumb: temperature in a "good" core should rise no more than 5°C to 10°C

within half an hour, whereas in a bad core the rise can be 15°C to 20°C higher. If damage is

extensive, the difference may be much greater-see Figure 12.

In a large motor (at least 1,000 hp at 3,600 RPM down to 400 hp at 514 RPM) economics will

usually dictate restacking a damaged core. Depending upon its construction and the location

of the damage, that can be done in several ways.

One method is either to reinsulate existing laminations or to partially or completely

disassemble the stack and replace them with new ones. Existing laminations can be re-coated

on one side only. This decision has to be a judgment call based on experience. The coreplate

material need not have any specific dielectric withstand capability, because interlaminar

voltage seldom exceeds a few volts.

However, two precautions are in order that may prove difficult in the service center. One is

that a single layer of the material used should provide at least as much electrical resistance as

the two layers present originally (one on each of the abutting lamination surfaces). Secondly,

the insulation layer must not be thicker than the original, unless the number of lamination

layers recoated is fairly small. Otherwise, the core stack length can be unacceptably increased.

Also, the material used for recoating obviously has to withstand motor operating temperature.

Removing and restacking segmented laminations requires great care, because the segments

may be of a number of different configurations that must be re-stacked in a particular order.

(See "The importance of slot combination in a-c motor design" in EA June 2005.)

The second method of repair has been loosening of the core clamping, then giving the core a

low-viscosity VPI treatment (retightening the stack before the resin is cured). One drawback

to this method is the risk of destroying that insulation if the winding fails again later and must

be removed in a burnoff oven. An alternative is slipping thin mica sheets between laminations.

That's of limited value for badly damaged cores, because of the difficulty of placing the mica

over sizable areas, and the thickness of the material (measured in thousandths of an inch

whereas coreplate coatings are much thinner).

All these procedures are not only labor-intensive; they will be impossible with some machines.

When large motor laminations have been effectively short-circuited together only in or around

the ID or slots, particularly by machining or a rotor rub, acid etching is a common means of

repair. For the bore itself, careful grinding of the surface may remove smear and adequately

separate the lamination edges. This cannot, however, reach down into the teeth themselves.

Several methods have been suggested, usually involving a weak solution of phosphoric acid as

the etching agent. Following is the draft of one of the more elaborate procedures as proposed

for an IEEE standard 20 years ago:

"Make up a solution as follows:

20% by weight of 85% phosphoric acid

20% by weight of denatured alcohol

60% by weight of water

Make up an applicator from dacron or wool felt fitted to a steel rod with an electrical contact

and insulating handle. Soak felt with solution and apply to core by wiping the felt in the

direction of the laminations. Apply variable voltage from a 120 volt a-c source between

applicator and core; increase voltage until current - 2-3 amperes per square inch of applicator

area. Continue application for 60-90 seconds followed by 30 seconds' cooling time to extend

applicator life. Continue until clear demarcation between laminations is visible using 5% to 7%

magnification. Wipe off excess acid solution with dry cloth."

Chemical processes

The IEEE motor repair standard, No. 1068, describes several methods of core repair but

doesn't include any such process. Etching with acid ("orthophosphoric") alone has also been

done, with the caution that "care must be taken" and "only experienced personnel" should do

the job. Today, of course, safe handling practices and personal protective equipment

requirements go well beyond what was typical even in the 1980's. And experienced repair

personnel have warned, "one out of three of these grinding or etching exercises makes the core

look prettier but actually worsens the fault."

For many years, the chemical process known as bonderizing has been used as surface

preparation for painting steelwork exposed to outdoor or corrosive environments. Also

sometimes termed phosphatizing, this process involves the chemical conversion of a steel

surface into a coating to which paint bonds exceptionally well. One version of the process,

known as Parkerizing after its developer, creates a rust resistant surface of a characteristically

blue gray color.

The typical "phosphate coating" process has been described as applied "by brushing, spraying

or prolonged immersion in an acid orthophosphate solution . . . composed of four parts:

detergents, metal phosphates (such as zinc or manganese phosphate), phosphoric acid, and

additional agents (such as reaction catalysts or chemicals to tie up byproducts). . . . The

phosphoric acid provides for minor acid etching or pickling allowing for better paint and

lubrication adhesion. The metal phosphates react with the surface . . . to form the corrosion-

resistant phosphate coating."

In a recently tested stator core repair method based on that concept, a solution containing

acid with metallic compounds in solution and heated to 80°C to 100°C is applied to the

similarly preheated core either by dipping (if the core is small enough) or spraying (see

Figures 13-15). In a typical test on a 20 hp stator that had been through a burnoff oven, the

initial core test showed a loss of 5.9 kilowatts per pound. After treatment, followed by a water

rinse and dryout, the tested loss dropped to 4.1.

After concern was voiced about the possible effects of conductive residue within the

laminations, the core was thoroughly cleaned again, and the loss dropped further, to 3.3.

Surface resistance tests on individual laminations have shown that the treatment does restore

interlaminar resistance to a degree warranting continuing investigation.

Significant damage to cores of the size in Figure 13, of course, will seldom be repaired. Motors

of that size are typically considered throwaways unless they're of special design. Large

machines, on the other hand, will be repaired despite high cost. A 12,500 hp motor stator

containing more than 34,000 individual lamination segments in a core 85 inches in ID and 80

inches long was found to have numerous unacceptably hot spots prior to rewind. The damage

apparently took place when a torch was used to remove some insulation, followed by blasting

with a conductive abrasive.

Normally, the entire core would have required restacking. The new acid treatment process was

successfully used to largely eliminate the hot spots (as shown by infrared thermography) and

lower the tested core loss from 2.3 to 1.9 watts per pound, with a one-third reduction in hot-

spot temperatures. The process can be completed in a day, whereas disassembly and

restacking could take weeks.

In summary: most motor failures involving more then superficial winding damage can be

suspected of having caused stator lamination damage. No rewind should be undertaken

without careful checking of core condition, and economic evaluation of several possible repair

methods.

© 2006 Barks Publications Provided by ProQuest LLC. All Rights Reserved.

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