Mechanics and Manufacturing Methods -- part 2b

(cont from Part 2a)

3.10 MAGNETIC CORES

Stator core assemblies, shown in Figs. 3.24 and 3.25, are insulated stacks of laminations lined up to close tolerances. The laminations themselves can be held to tolerances within 0.001 in  0.0005 in) on the OD and ID. However, they must be assembled into a core in some fashion. Permanent-magnet motors have air gaps on the order of 0.015 to 0.040 in per side. Induction motors have air gaps on the order of 0.010 to 0.015 in. These gaps must be held very consistent to avoid performance and noise degradation. There are many methods for assembling stacks of laminations.

This section discusses some of the most common stacking methods with their positive and negative attributes.

Loose armature, induction rotor, or outer rotor brushless dc motor laminations may be pressed onto a sleeve or shaft. Other methods include heat shrinking, ring staking, or adhesive bonding.

3.10.1 Welded Cores

The lamination stacks are generally fixtured off the stator bore and welded along the OD of the stack. The tolerance is now a result of the welding fixture, which allows for some shift in lamination placement. The eddy current core losses are increased because the welding short-circuits to the laminations at the weld joints.

The core assembly may have to be machined to bring the OD back to an acceptable dimension. Field depth should be kept to a minimum, and welds should be positioned behind the poles or teeth. When possible, a laser weld is best in the cases in which it provides adequate strength.

ill. 3.24 Outer rotor brushless dc stator core assembly (FEMD).

3.10.2 Bonded Cores

These cores are built by coating the laminations with an adhesive, aligning them on a fixture, and heating the cores to set up the adhesive. Here the tolerances are a function of the fixture tooling, and the OD may have to be machined to get it to an acceptable dimension. If anything, the adhesive assists in reducing eddy current losses; however, it takes up space, and the effective core length may be somewhat less than expected.

ill. 3.25 Inner rotor brushless dc stator core assembly.

3.10.3 Cleated Cores

These cores are assembled by forcing steel straps into notches in the periphery of the laminations, as shown in ill. 3.26.

This method provides for good dimensional stability with very little increase in core loss. Cleats are typically folded over the ends of the cores and may have an adverse affect on electrical clearance.

Cleat and notch tolerances are determined as shown in ill. 3.27.

ill. 3.26 Cleated core.

3.10.4 Lamination Design Consideration

Perhaps the most significant single factor to be considered in automatic or semiautomatic winding systems is the lamination design. The second most important consideration is the winding specifications (wire size, turns, and winding configuration or pole configuration), which are covered in a later section in more detail. It is mentioned at this point only to emphasize the fact that winding specifications can be readily changed without a major penalty, whereas the lamination itself cannot be altered with-out paying a very large penalty in modifications to existing tooling or new and expensive toolings.

Lamination design is basically an electrical consideration; however, there are five major areas that should be considered when designing a lamination that directly affects the manufacturing process to achieve a completed stator assembly.

ill. 3.27 Cleat specs. Basic dimensions may vary, but tolerances should remain the same.

The OD shape or periphery of the lamination is always important from the standpoint of material savings, particularly when punching laminations from strip material. The shape of the cleat notch is an important consideration in that notch standardization should be maintained for all laminations, regardless of stator size.

The number of cleating notches and their location then become the only variables.

Cleating notches vary in number, ranging from 2 to 16 per stator depending on the size of the motor. Location of the cleat notches should be in line with a lamination tooth or slot opening, with lamination tooth alignment being preferable. On large stators having many cleats, it's essential that they be equally spaced about the periphery of the stator and always be two notches, 18 degrees apart.

It is always desirable to have the largest possible slot opening in a lamination; however, this can sometimes be in direct opposition to the desires of the designer from an efficiency standpoint and sometimes to considerations of noise in the final product. Hence, final selection of slot openings is always a compromise between the product parameters and the restrictions imposed on the manufacturing process. In the past, when hand winding was the only manufacturing process, the major consideration was the stiffness of the wire. Usually 17 AWG was considered the practical maximum wire size from an operator's standpoint.

The lamination designer then selected slot openings accordingly-usually in the

0.070- to 0.080-in category. With the advent of machine insertion, the operator restriction on wire size was removed. A reasonable maximum wire size was then 13 or 14 AWG; however, a portion of the placer tooling also occupies a part of the slot opening, as shown in ill. 3.28.These two items resulted in larger slot openings, to the point where a majority of laminations are now in the 0.095- to 0.125-in category.

Even though the slot opening has been made larger, there are still compromises on wire sizes relative to the tooling opening selected. As a general rule, the larger the slot opening, the greater versatility when considering wire sizes.

ill. 3.28 Slot configuration.

For automatic insertion of wedges, there is very definitely a desirable configuration.

ill 3.29 outlines this configuration for the bottom of the slot. Note that at this point we are not too much concerned about the overall shape of the slot, only the shape of the slot nearest the bore. ill 3.29 gives a few basic dimensions defining the desirable shape.

ill. 3.29 Blade gap illustration.

Referring to ill. 3.29, the depth of the slot is defined as the length from the bottom of the slot to the back of the slot, and the width is defined as the width at the bottom of the slot prior to converging toward the slot opening. For ideal conditions for inserting wire, we would like to see this ratio approach infinity. That is, we would like to see a slot that's extremely narrow in width and very long in depth. Ideally, a slot would have a width equal to the wire diameter and a depth equal to the number of turns to be put into that particular slot. One can readily see that this is extremely impractical from the standpoint of material usage and lamination design, and would result in a motor of extremely large diameter. From a practical standpoint, based on experience gained from over 800 designs of placer or inserter tooling, it has been found that 4 is a practical value for this ratio. As with all rules of thumb, the number 4 isn't sacred, but it's a practical rule in which any lamination design having a ratio in the area of 4 or greater can be expected to result in a relatively problem-free insertion. A lamination having a ratio of 3 or less, particularly when the ratio approaches 1, begins to present problems in process which are nonexistent in the higher-ratio designs. Most of these problems are involved with the wedge, such as wire behind the wedge, wedge dislocation, the wedge falling out of the slot, and many other undesirable characteristics.

The shape of the back of the slot isn't crucial for most practical purposes. It is generally found in two basic configurations, a square bottom and a round bottom.

The round bottom is the preferred shape due to the simplification of tooling.

The present basic practice in lamination design is to consider what is the best lamination design in order to obtain the desired electrical characteristics or performance of the motor. From an electrical and mechanical standpoint, there are some areas that must be different within a family of laminations which are important to the performance of the final product, such as having 24 slots for a 2-pole motor and 36 slots for a 6-pole motor. However, there are also certain other areas that can be standardized or grouped within a family of laminations, such as having two 36-slot 6 pole laminations, one for copper and one for aluminum windings, which differ only in the slot detail. Standardization of the slot opening and wedge area or base of the slot can result in identical placer toolings even though the laminations are not identical in all other respects.

The grouping of the family of laminations should begin with the bore, then further subdividing common bores into groups having the same number of slots. Examination of the laminations within the same subgroup will then show that there are usually very small differences in the significant areas, as previously mentioned-for example, the slot opening and the wedge entry area. Usually these small variations can be eliminated with little effect on the electrical performance of the final products.

Standardization of laminations therefore can significantly reduce product costs through standardization of toolings, increased machine utilization, and reduced labor by eliminating tool changeovers.

Winding Specifications. Winding specifications-for example, wire size, number of turns, and pole configurations-are the second most crucial criteria, as mentioned previously for automatic or machine insertion of stators. Compatibility of the selected tooling opening, or blade gap, which is dependent on the slot opening and the wire size to be used, is essential. However, unlike lamination changes, wire sizes and numbers of turns can be varied to a certain degree without incurring the penalties of major costs or degraded product performance. As mentioned in the previous subsection, the larger the slot opening, the greater the versatility in wire-size selection.

The slot opening versus wire size situation can be best explained by the chart shown in ill. 3.30, where the advantages and disadvantages become apparent.

There is an area on the chart that represents the locking-wire condition in which two wires attempting to pass one another lock. This condition can generate locking forces which can damage wire insulation, actually stall the insertion process, and in some cases cause tooling damage. The total locking force generated in the locking area depends on the number of turns (conductors) being inserted. Obviously, 1 turn cannot lock, 2 turns generates a small force, and a large number of turns, such as 50, generates a large force.

Empirical conclusions based on a large number of tooling designs and conditions indicate that, in general, a maximum turn count of 20 can be inserted per slot in the locking area without damage to wire or tooling.

The area between the locking-wire area and the maximum-wire-size curve is referred to as the precision wind area, so called as the wire must be in a single-layer or precision condition. This area also presents some restrictions on the number of turns due to column height and a condition similar to the previously mentioned locking situation. Usually, 35 turns for standard placer tooling and 50 to 55 turns with special tooling options are considered safe selections. Higher turn counts can result in wire damage and can involve extensive tooling development programs.

ill. 3.30 Blade gap chart. Note that allowance must be made for stack (core) skew or stagger. Maximum blade gap = iron gap = 0.030 in (0.8 mm). The area below the locking area represents the level wind area and is considered the ideal situation for coil insertion. Slot fullness is usually the limiting condition for this area, except for special slot configurations. Anything below 70 percent is considered good, with 76 percent a practical maximum for standard toolings. Further discussion concerning slot fullness is covered in a later section.

Many developments in tooling design and special features have improved these restrictions and the quality obtained, but have not entirely eliminated them.

Once the wire has been inserted into the stator, the major material costs have been incurred and the basic quality of the stator assembly has been determined except for lead connecting. Compatibility of the slot opening and the wire size there-fore becomes a major contributing factor to cost, through high or low scrap rates, which directly reflect the difficulty or ease of the manufacturing process and the final quality of the stator.

Pole Configuration. Pole configuration usually refers to the number of poles and the physical shape and location within a stator. There exist two major categories of pole configuration, lap winding and concentric.

The coil-insertion process has almost eliminated lap-type windings. Although it's possible to insert some lap windings, it's almost impossible to achieve the slot fills and production rates of equivalent concentric windings. Almost all of the lap-type motors, probably 90 to 95 percent, are hand wound.

This section therefore deals only with the single-phase and three-phase concentric-type windings. The vast majority of the single-phase motors produced today follow a standard two-, four-, or six-pole configuration, varying only in the number of con-centric coils per pole, which is limited, and slot fullness. The tendency today is for higher slot fills, which in a single-phase motor are much more difficult to achieve than in a three-phase motor due to the nature of the main winding.

Three-phase motors present a much greater opportunity for variation in pole configuration. The industry standard has been the three-layer uniform design, having no shared or some shared slots, which is accomplished by varying the number of concentric coils per pole. In the past, some manufacturers have used a two-coil-sides per-slot design in which all slots are shared, but this has not yet proven to be a popular approach.

The two-pole three-phase motors are fairly straightforward in a three-layer design, with some or all slots shared. This is fairly well limited due to the physical configuration inherent in a two-pole design.

The four-pole three-phase motors present a high degree of variation; however, the majority of the stators produced today are of the standard three-layer four-pole per-layer uniform design. One fairly popular design is the European or consequent pole design. The main reason is that the 2-layer three-pole-per-layer configuration, usually in a nonsharing slot condition, requires only two insertions to complete the motor. Winding time is less to generate 6 total coils for a complete motor, rather than the normal 12 poles. Phase insulation for this type of motor is much simpler due to having only two layers, instead of three, and in some cases is eliminated.

However, there are some disadvantages. It is usually considered to be a less efficient motor, it requires more interpole or lead connections, and , finally, it has been known to require more copper than the three-layer design.

The six-pole three-phase motor also presents the same type of variations as the four-pole, but perhaps to a lesser degree. A six-pole European or consequent design is also used, but is less popular. The same basic criteria exist as in the four-pole, two insertions instead of three, and in this case 9 poles rather than the normal 18. Some of the same disadvantages also exist as for the four-pole.

The standard three-layer designs for two-, four-, and six-poles can be produced in a gradient design. The gradient design is achieved by decreasing the circumference of the pole for each succeeding phase or layer. The first layer or phase inserted would be the longest, the second slightly smaller, and the third slightly smaller yet.

The net result is a savings in wire and better nesting of the end turns. This type of design is generally considered to be an imbalanced-phase winding and could have a negative effect on the performance. However, if properly designed, the advantages can outweigh the performance disadvantages.

Although gradient design isn't generally used across the industry, the more progressive manufacturers are using this approach or looking at it very seriously. It is beginning to appear as an approach that could become standard practice in the future.

Slot Fill. Slot fill is usually given as a percentage figure that expresses the amount of wire in a slot in relation to the total slot area. Unfortunately, there are several methods presently in use for calculating slot fill.

Widely varying slot-fill percentages can be obtained for the same situation depending on the method used. Two general methods are circular mils and square wire. Electrical designers would normally use the circular-mil method because they are more interested in the actual cross section of the conductor. Process engineers or equipment suppliers would be more apt to use the square-wire method, which more nearly reflects the actual conditions with which they must work.

Variations within each of these basic methods will also occur depending on the method used to determine the slot area. The insulation in the slot-for example, the slot cell and wedge-will occupy a portion of the slot area.

Some calculations will take into consideration the slot insulation, while others will ignore it. Taking into consideration the slot insulation results in an available slot area.

For purposes of this section, slot fill (SF), is calculated as follows (see ill. 3.31): SF, % = (wire diameter)^2  number of turns in available slot area Wire diameter = bare wire plus insulation

ill. 3.31 Percentage slot fill.

Available slot area is the slot area calculated from the punching, subtracting the insulation-occupied area and the area between the base of the wedge and the bore of the slot. This results in a true available slot area.

Today's trends and desires are to maximize slot fill. For purpose of brevity, we list some of the benefits here, and don't go into a lengthy explanation of reasons why these results are obtained.

Greater performance efficiency; Minimal material usage; Smaller package for same performance. A few years ago, a slot fill of 50 to 65 percent was acceptable and efforts were concentrated on reducing labor. As material costs and volumes rose along with a greater consciousness of power efficiencies, the requirements for higher slot fills increased.

Today, as a general rule, slot fills of 70 to 75 percent are fairly common and are produced in volume with relatively few production problems.

In some cases, slot fills of 80 to 81 percent have been achieved, but to do this requires special development and a concentrated effort in tailoring toolings for a particular application.

The desire for high slot fills is very great at the present time, and there are several theories or approaches under consideration which have not been developed, as yet, into practical production methods.

One area of effort is to develop a slot shape which approaches the ideal slot. As mentioned previously, the more nearly ideal slot from a production viewpoint involves a compromise of a larger-OD stator and therefore is less attractive than other approaches.

Perhaps the most attractive and promising approach is the compaction process.

This process is exactly as its name implies-compacting the wires in the slot. Some manufacturers have taken tentative steps in this direction.

The theory is very simple and consists of inserting a first layer of wire into a slot, compacting or deforming the wires into the back of the slot to fill all of the void spaces between wires, then inserting a second layer of wire. As a simplified example, if a 92 percent slot fill is desired, a first insertion of 46 percent or half the total wires is inserted. By the process of compaction, these wires are then forced to occupy only 40 percent of the slot. The remainder of the slot, 60 percent, is now available for the next insertion of 46 percent of the wire, which results in a 76 percent slot-fill insertion attempt on the second pass. The second insertion falls within the range of feasi-bility based on present practice.

The use of the General Electric Electro-press process could also contribute to the feasibility of achieving the desired results or increasing the total percentage slightly.

This can be accomplished by taking advantage of the fact that the Electro-press process has a tendency to straighten wires, eliminating the crossing in the slot that occupies additional space. Straightening of the wires prior to compaction would therefore assist in achieving higher slot fills.

The compaction and Electro-press slot-development approach should make slot fills in the 90 percent range accessible.

Phase Insulation. From a labor and materials standpoint, phase insulation is per-haps one of the most difficult, time-consuming, and expensive areas of stator manufacture.

Phase insulation can be accomplished through several different methods.

The main object of this particular section is to recommend the type of phase insulation and the point in the assembly line where phase insulation is to be installed.

This assumes that phase insulation is required. A major savings can be accomplished if phase insulation is eliminated. Some motor manufacturers have eliminated this type of insulation, but not without some problems. For the most part, they have been successful. Unfortunately, at this time, the manufacturers who have eliminated phase insulation are in the minority. It has always been felt that phase insulation is required in order to guarantee a quality product and is an insurance policy of sorts. However, with today's improvements in wire insulation, slot-cell insulation, and manufacturing methods, the whole area of phase insulation should be very seriously considered for potential cost reduction. We would recommend that a program be instituted to investigate the possibility of the elimination of phase insulation.

At the present time, various methods of phase insulation are being used. One of the most popular is the H-type paper insulation, which must be installed between the layers or phases of windings during the process of winding the stator. This involves winding one layer of the stator, hand-inserting the H-type paper insulation, and then winding the next layer of wire into the stator. In three-phase stators, one more layer is yet to be inserted; therefore, a second hand insertion of phase paper must be undertaken, and then the third and final layer of winding is inserted into the stator. This is at best a slow operation and also very labor intensive, not to mention that the H paper is generally made of a polyester-type material with very high scrap rates.

A second method is to insert the first layer and then proceed by hand to add an adhesive tape to the end turns which acts as a phase insulation. The second layer is then inserted and has to be taped by hand prior to the insertion of the third layer.

This is also a labor-intensive operation and requires special materials which some-times are not compatible with the products being manufactured (for example, the adhesive tape).

The third method, the method which we prefer, is an operation in which all three layers (three-phase) or two layers (one-phase) of the motor are inserted in a continuous flow operation without interruption for phase insulation. A variable-wedge length device is included on the equipment for notching wedges. In the end, the wedges constitute a part of the phase insulation. After the stator winding is completed, phase insulation is added into and between the end turns of the wire as required. This material can be of the same polyester as is used in the H paper; however, it's merely a strip of material, and scrap is virtually eliminated except for rounding some corners. There are several advantages to using this method:

The winding operation isn't interrupted.

The equipment isn't idle during phase insulation, and no unloading and reloading of the stator into the insertion flow system is required.

The elimination of polyester scrap is an obvious cost savings.

The time required to insert the phase insulation is substantially less than that required for either of the first two methods.

There are some disadvantages to this method, one of which is that it must be a hand operation. Second, it requires some manipulation of the end turns with a hand tool in order to separate the windings where the insulation is to be inserted, which could possibly result in a deterioration of the quality. However, this type of process has been and is being used successfully in some major motor plants around the country. In conclusion, the best approach to phase insulation is to eliminate phase insulation. If this cannot be accomplished, then the second best is to automatically machine-insert phase insulation. However, at the present time this type of equipment isn't yet very popular. In the interim, the process that's best utilized for minimizing cost is the hand insertion of polyester segments into the end turn at a station away from the winding and insertion of the stator.

Salient Pole Motors. Salient pole machines like universal motor fields, shaded-pole motor stators, and stepper motor stators are usually needle- or gun-wound.

They may be wound with shroud tooling, as shown in ill. 3.32, or they may be wound directly on molded plastic insulation which also serves as tooling, as shown in ill.

3.33. In these cases, the minimum slot opening must take the needle size and path into account. The typical needle path is shown in ill. 3.34. The slot opening must allow for this movement plus some clearance. The slot opening is determined as follows.

Select the maximum coated wire diameter that will be used in the intended application. Next, determine the minimum allowable needle bore, outside dimensions, and clearances per ill. 3.35.

ill. 3.32 Typical two-pole motor with shrouds.

ill. 3.33 Plastic insulation and winding shroud combination.

ill. 3.34 Typical needle winding path.

ill. 3.35 Needle, wire, and iron gap minimum dimensions and clearance relationships.

Stack-in-Die Cores. This method utilizes a punch in the edge to pierce a portion of each lamination about half of the way through the material, as shown in ill. 3.36.

Succeeding laminations are pressed into the previously punched lamination by inserting the protrusion of one into the recess of the other. Because of taper in the raw lamination steel, it's necessary to periodically rotate some of the laminations 18 0 deg to hold the stack square. This rotation usually isn't necessary until the stack length Lstk reaches 2.00 in or more. Using this method allows the stack OD to be held within  0.001 in up to about a 4.0-in stack length. As mentioned earlier, the laminations are annealed to promote grain growth, which reduces hysteresis losses. During this process an oxide is also generally put on the surface of the lamination, which increases interlaminar resistances.

This is extremely important in reducing eddy current losses, which are the predominant part of the core loss at higher frequencies. The laminations are stacked tightly together while in the punching die. Later, during the annealing process, some of the laminations in the center of the stack may not receive the same amount of coating as the laminations on the ends. This may result in higher than expected core loss. New processing methods tend to minimize this problem. A key to low core loss is the placement of the protrusions.

ill. 3.36 Stack-in-die laminations.

3.10.5 Manufacturing Rotor and Stator Stacks in the Stamping Die

Laminations of all types (see ill. 3.37) may be staked together into stacks of predetermined heights as they are stamped in progressive dies during the manufacturing process. This staking-in-the-die technique represents an opportunity for both improved quality and significant cost reduction to manufacturers of products that require rotor, stator, transformer, ballast, magnet assembly, back-pole counterbalance, and other types of laminations that subsequently are laminated into stacks.

In addition, symmetrical laminations, such as for rotors for electric motors, may be rotated in the die prior to staking in order to compensate for material thickness variations or to produce a skew angle, as found in many motor designs.

Producing finished stacks of rotor and stator laminations in the die has many advantages to the manufacturers of electric motors, particularly the elimination of downstream manual or mechanical assembly operations such as riveting or welding.

Staking in the die also can produce stacks of more uniform height.

A die capable of staking laminations together requires a specially designed die cavity and special staking punches. The staking punches create protrusions that cause the individual laminations to stick together as they become a stack in the die cavity. Whether as many as four or more staking punches should be used depends on the size and design of the rotor or stator part.

Absolute Count Stacking. The easiest way to control stack height is to program a microprocessor controller for a predetermined number of laminations in each stack.

E.g., a 1-in rotor using 0.025-in-thick material would require 40 laminations.

By programming the die's staking punches, a 1-in stack of rotors may be produced with every 40 strokes of the press. The staking process produces tight stacks which then are allowed to drop from the die cavity onto a low-profile conveyor.

The integrity of the 1-in stack height is completely dependent on the thickness consistency of the 0.025-in coil stock. If the material begins to run thick during the stamping process, 40 laminations could produce an unacceptably long rotor stack. To guard against this, the press operator must monitor the stack heights. If thick or thin material causes the stack to reach a height outside the acceptable tolerance range, the operator must adjust the lamination count accordingly.

A more accurate and reliable method of controlling stack height is to monitor the thickness of the coil material and let the controller dynamically calculate the number of laminations required for the proper stack height. The patented system uses a material thickness sensor that measures the inbound material and every lamination that will comprise the stack. By setting the control unit for a 1-in stack height (not on an absolute count of 40 laminations), the 1-in finished stack may contain 39, 40, or 41 laminations, depending on whether the 0.025-in material is running thick or thin.

There are several advantages to using a system of this sophistication. It produces rotor and stator stacks of consistently uniform height and frees the press operator from having to continuously measure stack heights to determine if they are within tolerance. The system also permits some motor manufacturers to use less uniform and less expensive steel stock because the controller determines the proper number of laminations needed to achieve the desired stack height.

Another consideration is that stack height can be changed on the control unit in a matter of seconds. This permits the manufacturer to shift production to a different motor length without turning off the stamping press.

ill. 3.37 Lamination stacks.

Rotating Laminations in the Die. A material thickness monitoring system like that shown in ill. 3.38 can compensate for stock variations throughout the length of the coil and automatically adjust the lamination count to maintain uniform stack height, but it cannot compensate for thickness variations across the stock width. E.g., if the left side of the coil strip were consistently thicker than the right side, the stator and rotor stacks produced would lose their perpendicularity and would lean to the right. These stacks would be of inferior quality and would complicate subsequent manufacturing operations.

ill. 3.38 Material thickness monitoring system.

To solve this problem, a method has been patented using dies that have been designed to rotate the lamination in the staking die cavity. As each new lamination enters the cavity, the existing stack is rotated a fixed number of degrees as a function of the number of rotor slots. A rotor with two staking points, for example, may only be rotated 18 0 deg with respect to the following lamination. A rotor with four staking points could be rotated 9 0 deg.

Oberg Industries uses a belt system driven by a high-speed motor to rotate the die cavity. The motor is controlled by the same controller that monitors the staking and stack-height functions.

Rotation isn't necessarily confined to rotors. If a stator is perfectly symmetrical, it also may be rotated in a staking die cavity. In addition, when motor manufacturers require loose rotor laminations, the laminations may be rotated without being staked together and loaded into stacking chutes. When these loose laminations finally are assembled, they will also exhibit improved perpendicularity and balance.

Many motor designs incorporate a skew angle in the rotor assembly to improve motor performance. In Oberg-produced dies that contain the rotating skewing cavity, the skew angle to the rotor stack is quickly set by entering the desired skew angle into the control system. Skew angles may be set in addition to the rotation or by themselves without any other rotation in the lamination.

When skewing, the consistency of material thickness throughout the length of the coil stock is a concern. Rotor stacks 1-in high of 0.025-  0.002-in laminations will have varied numbers of laminations to achieve proper height. To compensate for variations in coil-stock material thickness, the control unit adjusts the rotation on each lamination. The end result is a consistent skew offset, even though the stack may contain 39, 40, or 41 laminations.

Although staking, rotating, and skewing of the laminations is performed in the stamping die, the critical component of the proprietary system is the microprocessor control unit. The controller must have the capacity and speed to control the die's staking punches, the high-speed motor that drives the rotation cavity, and the material monitoring sensor, as well as permit the stamping press to operate at top speed.

Some lamination die controllers may have to eliminate features to avoid significantly slowing the press speed.

System Benefits. Whether the motor manufacturer stamps its own laminations or buys them from a lamination stamper, staking, rotating, and skewing in the die offers several benefits. One of the benefits of the technology is that it requires less material handling. Staked lamination stacks eliminate much of the handling and moving associated with loose laminations. Some motor manufacturers are able to send stacks on conveyors to the next process area, directly from the press.

For many types of motors, staked rotor and stator stacks may eliminate welding and riveting operations. The labor and costs associated with these operations is eliminated, and there also is no need to replace welding and riveting equipment when it wears out.

One motor manufacturer, faced with the replacement of an obsolete welding line, invested instead in a staking die and controller. The company calculated a two-month payback on the investment and , in addition, was able to access much-needed floor space when the welding line was removed. The manufacturer intends to eliminate all welding in the plant within two years.

The rotor and stator stacks produced with the staking-and-rotation technique are consistently of higher quality than those produced from loose laminations. Some manufacturers using staking dies have realized a reduction in the costs associated with balancing and other motor-finishing operations. Motor performance also has been improved.

For manufacturers that stamp their own laminations, the benefits related to production flexibility may be substantial. Stack height and skew angle are changed easily, and combinations of height, skew angle, and rotation can be varied and adjusted by the operator.

ill. 3.39 Sample lamination stacks.

Die and Controller Requirements. Rotation, skewing, and staking of laminations, the process used to produce the sample stacks shown in ill. 3.39, presents several challenges to the die or controller supplier. The basic accuracy requirements of the rotational motion are recognized when one considers that a 1 0 deg skew angle in a 40 lamination stack results from the rotation of each individual lamination by 0.2 5 deg .

Variations in material thickness can adjust that by 0.0000 5 deg or less. Also, extremely high accuracy is required in the stamping die to permit rotation of laminations while maintaining concentricity. The location of the stakes must be perfectly symmetrical for them to attach properly to the preceding rotated lamination. Rotation of square, rectangular, or other non-round shapes requires extreme accuracy of the rotational motion, since the punch is now penetrating a moving die section. It also requires mechanical devices that will prevent damage if the rotating chamber does not align with the punch.

The design and construction of the controller must take into account the fact that the system will be operated in a pressroom environment, and must minimize both additions to the operator's workload and intrusion into the already crowded work space. A simplified operator interface and a rugged, vibration-resistant package are basic to the operational success of a staking and stamping die.

Technology Limitations. There are areas within this staking and rotating technology where motor manufacturers face some limitations and cautions. First and most important, it should be noted that the staking process requires a sufficient open area on the face of the rotor and stator laminations to allow a stake protrusion to be made without distorting a critical dimension of the lamination. When designing a lamination with staking in mind, the advice of a die designer is essential.

Multipart dies are common in the motor industry, and staking dies have been built that produce as many as five rotors at a time.

Although staking and rotating dies for smaller-sized rotors may not require reduced press speeds, the rotation of the die cavity for larger-diameter rotors could force a press to run more slowly. Also, for annealed laminations, the electrical properties of staked stacks must be compared critically to those of stacks made from loose laminations. Motor manufacturers that stake stacks in the die generally have found few differences of consequence, but it's a factor to be considered when designing a motor.

Although the tightness and integrity of staked stacks usually are not problems, some handling precautions are advisable to prevent stack delamination. One manufacturer permits its stacks to drop almost 4 ft from the press into a collecting bin, but this kind of handling may not be suitable for some types of lamination designs.

The technology of staking, stacking, rotating, and skewing in the stamping die, along with other emerging technologies designed to reduce costs and improve quality, should position the small-motor industry to compete successfully in the global economy.

3.10.6 Electric Motor Stator Lacing

ill. 3.40 ( a) Laced stator and ( b) bare stator core.

Stator lacing is the process of tightly securing the field coil ends of an electric motor stator with a stitched cord (see ill. 3.40). Lacing is typically used on long-life-expectancy or high-efficiency motors where the cost of failure is high. Generally, low-cost "throwaway" motors are not laced. There are several reasons for lacing motor stators:

The lacing holds the thermal protector, coil ends, and leads in the proper position.

Lacing extends a motor's life by preventing the wires in the coils from vibrating and causing fatigue failure during operation.

Lacing may be used to hold the coils in position and provide loops to hang the stator from a conveyor during the dip-and-bake varnishing operation.

The most obvious reason for lacing a stator automatically by machine is increased productivity. The lacing machine can lace more stators per hour than a person doing it manually, and usually at a substantially lower cost. The lacing machine also provides improved lacing quality and consistency over long time periods.

Another important advantage of machine lacing is the avoidance of carpal tunnel syndrome, a debilitating hand and wrist injury caused by repetitive strenuous hand-work.

Several different styles of lacing machines are generally available to the motor manufacturer. The simplest machines lace one end of the stator at a time, and are referred to as single-end lacers (ill. 3.41). Others have two needles and lace both ends of the stator at the same time, and are known as double-end lacers (ill. 3.42).

Link Engineering Company of Plymouth , Michigan , pioneered the development of the automatic stator-lacing machine with interchangeable tooling in the mid-1960s.

Lacing machines may be constructed so the stator is vertical and the lacing needle is horizontal, or with the stator horizontal and the lacing needle vertical.

Most lacing machines index the stator about its axis during the lacing cycle, but a few machines have been designed to clamp the stator and rotate a lacing head with the needle around the stator.

Some machines use a closed needle with an eye, like a sewing machine, but the vast majority use an open needle, similar to a crochet hook, to form the stitch. In general, the open-hook needle produces a "diamond" stitch, with diagonal coverage of the coils, whereas the closed-eye needle produces more of a radial stitch.

There is also a wide variety of lacing cords available. These cords may be made from fibers such as cotton, polyester, nylon, or other synthetics, and may be formed by twisting or braiding into a round or flat-tape shape. Twisted round lacing cord is the most popular and least expensive type. Tensile strength of the cord is determined by its diameter, the material it's made from, and the weaving technique used to make it.

Most lacing cords are made from polyester, which shrinks when it's heated. The percentage of shrinkage may be specified from almost none up to about 15 percent.

The higher-shrinkage cords tend to produce a tighter lacing when shrunk.

The size and type of the lacing cord, as well as the cord tension during the lacing and knot-tying cycle, play a critical role in achieving consistent high-quality lacing.

Functional Characteristics. There are several important characteristics of a stator-lacing machine that determine its performance in a demanding plant environment.

One of the most important is speed. The faster a machine laces, the higher the throughput and the lower the cost per stator. To properly evaluate speed, however, the total lacing cycle must be considered. This includes loading a stator, positioning the leads if necessary, lacing, knot tying, removal of cord tails, and unloading the stator.

ill. 3.41 Single-end stator lacer.

ill. 3.42 Link Model 940 double-end stator lacer with servo index and knot-tying system.

Typical time to manually unload a laced stator and load an unlaced stator is about 10 s. Automatic loading/unloading devices can be used to speed up the handling to and from the lacing machine.

Lead positioning may be done by an operator or by a lead clamp or lead wiper incorporated into the lacer. As the number and length of the leads increase, so does the difficulty encountered in lacing the stator.

The actual stitching speed is usually a function of the coil end-turn size of the stator being laced. Large coils require greater movement of the relatively heavy needle mechanism and therefore require more time. Typical lacing speed for small stators is 2 stitches per second and for large stators about 1 stitch per second.

Manually tying a knot and burning off the tail typically takes about 5 s for each end of the stator. Automatic knot tying and tail burning typically takes about 4 s, and both ends are done simultaneously.

Setup time may be another important consideration in evaluating a lacing machine. If a line is dedicated to a single stator, or even if production runs are very long, with infrequent changeovers, it's not too important to be able to change from one stator to another quickly. But if runs are short, with only a few of each type of stator laced at one time, setup time can be more important than lacing speed.

Changes in ID, OD, stack height, coil end-turn height, number of slots, or stitch pat-tern may require from a few seconds to 10 or 15 min each to make programming or mechanical adjustments to the machine. New servomotor-driven lacing machines, with computer control systems, offer dramatically reduced setup time, as little as 7 s.

The servomotor lacing machines are ideal for motor manufacturers with small lot sizes which require frequent changeover.

The quality of the lacing is also a critical characteristic of the lacing machine. If the machine drops stitches, breaks the cord, makes loose stitches, forms loose knots, leaves long tails, or damages either the wire or lamination, it won't meet strict quality standards. Machine demonstrations and discussions with existing users can verify lacing quality.

The durability of the lacing machine is also important in determining its productivity.

If the machine fails often, is difficult to get parts for, or takes a long time to repair, it won't meet overall throughput goals. A proven machine design from a reputable company is the best assurance that the machine will deliver uninterrupted performance on the plant floor.

Typical Lacing-Machine Features and Options. Although a basic stator-lacing machine can be a big improvement over hand lacing, many features and options make the process faster or more flexible.

A time-saving option is automatic knot tying. This device forms a secure, tight knot, then burns off the tail and vacuums it into a waste container. A device is also available to fully automate the cord cutting and clamping at the end of the knot-tying cycle.

Automatic stator lifting and lowering raises and lowers the stator so the operator can easily grasp it. This feature is particularly useful for stators that have short stack heights or are particularly heavy.

Automatic stator loading and unloading can take the form of a manually assisted arm and gripper or a fully automatic robotic handling device. These loading/unloading systems can be integrated into a fully automated line to virtually eliminate the requirement for an operator.

Broken-cord and end-of-cord sensors enable detection of the end of a spool of cord or a break in a cord. This is especially useful in fully automatic lines that don't have an operator to observe such cord faults.

Computer-based touch-screen control systems offer simple programming, graphic displays, internal documentation, diagnostics, machine statistics, and large data-storage capacity.

Non-radial slot lacing allows manufacturers to lace stators with odd slots that are not in line with the center of the stator core.

A hanger loop option forms two long loops in the lacing cord at opposite sides of the stator, so the stator can be hung from a conveyor for processing through a varnish bath.

Roller casters may be placed under the legs of the lacer and the electrical box to enable easy movement of the machine from one location in the plant to another.

Summary. The automatic stator-lacing machine has a proven track record of being a productive, reliable, cost-effective tool for motor manufacturers seeking to pro-duce high-quality motors at a competitive price. Many evolutionary changes have led to a wide variety of models and options incorporating significant improvements in flexibility, reliability, and speed.