Theory & Practice of Electromagnetic Design of DC Motors & Actuators
George P. Gogue & Joseph J. Stupak, Jr.
Part of the design process is making choices which ultimately affect the manufacturing process and make the difference between an economical and expensive motor. These choices have been discussed in Chapter 7 and guidelines for a good design were established.
After the design choices are made, there are manufacturing considerations to be made regarding the process by which the subassemblies of the motor are made. These considerations influence the following items:
2. stator stack
4. magnet magnetization
5. bearing assembly
The following discussion is intended to describe some of the available options which have been widely used in the industry. There are many more ways of building motors than can ever be documented, especially in a publication like this. Novel designs and applications usually require new manufacturing methods, and the following discussion should assist in that direction.
In most motors, the thickness of the steel laminations is usually between 0.014 or 0.025 inches. Thicker laminations are rare but thinner laminations are sometimes used for improved performance. A wide range of thickness, however, is acceptable in several manufacturing methods. The accuracy in dimensions and the perpendicularity of the edges depend on the method used in manufacturing. Cost is also a consideration in selecting a particular method, with regards to the suitability of a particular method for the production quantities required. The thinner the laminations are, the greater the quantity required. This pushes the cost up.
The most widely used methods are punching, chemical etching and laser cutting. The accuracy of all three methods is typically +/- 0.001".
8.2.1 Die punching:
In this method, dies are made to the shape and profile of the lamination and are then used with an appropriate press to punch the lamination out of a sheet of the steel material. The dies are made of high quality, hardened steel to exact dimensions and are good for millions of cycles of use. A single lamination or multiple lamination can be punched in each cycle depending on the number of die sets available. The initial investment in this method is high but the cost of producing the parts is low. It is, therefore, only suitable for high production quantities where the initial investment can be amortized. Once the dies are made, modifications to the design are difficult and expensive, rendering this method inappropriate if the design is not finalized.
The finished parts are of high quality in dimensional accuracy and the resulting edges through the material are straight. There is, however, a slow but inevitable change in dimensions as the dies wear out over time. The process of punching results in stresses building up at the edges of the part which in turn cause changes in the magnetic characteristics of the material. Annealing the finished parts at high temperature must then be done to relieve these stresses and regain the magnetic characteristics.
Punching by die may result in slightly raised edges, especially if the dies are not sharp or do not fit each other properly. This may not be a problem if the laminations consistently face one way when assembled in a stack.
8.2.2 Chemical etching:
This method requires a very small initial investment in the form of photographic artwork. The images of the laminations are then used in chemically etching sheets of steel to remove the unwanted areas. The quality of the finished parts is good and the edges are burr-free and straight throughout the material. There could, however, be some variations between batches of laminations due to variations in the chemical process. Compared to the die punching method, chemical etching yields parts which are more expensive but without the high initial investment. It is possible, therefore, to make changes to the design without incurring a large expense. The process does not result in any stresses at the edges of the finished part and the need for annealing is avoided.
The time required to reach production stage, in this method, is much less than that required to make dies in the previous process. This method is, therefore, suitable for small and moderate quantities where the design is still likely to change.
8.2.3 Laser cutting:
The initial investment is the lowest in this method. The information on the design of the lamination is computerized and can easily be changed. The process involves a laser beam tracing the profile of a lamination on a sheet of steel where one piece at a time is made. The dimensional accuracy is good but the edges are beveled through the material. Some deterioration to the magnetic characteristics may occur because of the stress of the cut but it is usually too small to require annealing. The speed of cutting determines the smoothness of the cut, but it also increases the cost per unit if a slow cut is required for smooth edges. This method is ideal for small quantities of laminations and is also the quickest in yielding parts, which makes it suitable in the prototyping stage of development. Lately, however, this method has become economically feasible for medium quantities of production as control over the speed of cutting has improved. Of course, the parts are consistent in quality as a result of the repeatable nature of the process.
In selecting a vendor for laser cutting, it is important to choose one with specific experience in this field and with the right equipment. The required tolerances (+/- 0.001") may not be achievable with some types of equipment. It is also necessary to prevent buildup of minute amounts of metal beads on the side away from the laser. The presence of these beads would result in forced separation or staggering of the laminations at the edges upon assembly. Metal beads only 0.001" across would cause a 6% increase in length for a stack consisting of 0.015" thick laminations.
Electrical discharge machining (EDM) is a similar process to the one described here in its results. It uses instead the principle of removing tiny particles of unwanted material with electrical sparks.
8.3 Stator stack:
It is highly desirable that lamination stacking is done in the shortest possible time and to a high level of accuracy. To achieve these objectives, one of several methods of stacking is chosen with consideration given to the particular motor. Some of the available methods are: riveting, bolting, welding, bonding and using dimples.
Riveting and bolting require giving up a small portion of the lamination in the form of holes. The heads of the rivets and bolts can be an obstruction to the end turns of the winding at both ends of the stack. These methods are economical and reliable in stacking the laminations. Welding is also an economical and reliable method of stacking but it may require follow-up smoothing and cleaning of the welded surface to make the stack fit on its mechanical support. Bonding requires fixturing the assembled stack while allowing the adhesive to cure. This latter method does not require any specialized equipment or additional parts as riveting and bolting would, but it lacks the physical strength needed in large stacks. Coating the stack with epoxy is usually done to enhance the strength of the stack after bonding. Placing dimples on the individual laminations, so that they nest inside each other when the laminations are stacked, is a convenient way of assembling a stack. But, as in the bonding method above, coating the stack with epoxy is required to add mechanical strength to the assembled stack.
The epoxy referred to here is a hard glossy substance which is primarily intended to protect the winding from the sharp edges of the steel. It replaces the slot liners sometimes used for this purpose but it also strengthens the stack by encapsulating it. It is applied in one of several methods. The stack is preheated to a particular temperature and then either dipped in a tank of fluidized powder or sprayed directly with the powder. The particles of the powder melt upon contact with the metal forming a smooth layer over its surface. The material then solidifies as the temperature is lowered, forming a hard, insulating, protective coating. The process can also be performed by relying on electrostatic charges to deposit the powder uniformly on the stack. In all these methods, it was found that a minimum thickness of 0.008" (0.2 mm) of the epoxy is needed to ensure adequate coating on the edges.
A few winding patterns were discussed in Section 7.6 and many more are available for the wide combination of numbers of poles and slots. Even after a particular pattern is chosen, there is some flexibility in the choice of wire diameter and in the series/parallel connections of the coils. If the number of turns per coil is small, hand winding is acceptable for economic reasons. The need to use a large diameter wire would also make hand winding more practical.
For a large number of turns, however, machine winding is needed to cut manufacturing costs. By using winding machines, the winding time per unit is significantly reduced and the finished windings are much more consistent compared to the hand wound variety. There are, however, some limitations on winding machines imposed by the wire gage used. Large diameter wires are difficult to bend around the edges of the lamination stack, whereas small diameter wires may break frequently due to the tension experienced during the process. Thus machine winding should only be considered if the wire gage is between 26 and 38 AWG (approximately).
Another necessary compromise in using winding machines is the lower packing factor achieved in comparison with hand winding. Due to the speed at which machine winding is done, it is not possible to lay the wires in the slots as neatly as by hand. It is, therefore, preferable to wind by hand if the space available for the winding is very limited.
Some tooling is needed as a part of the preparation for production using a winding machine. This consists of a mandrel with suitable flying heads to guide the wire into the appropriate slots, without damage to the insulation. A mechanism for developing a steady but adjustable tension on the wire is also required. The tension adjustment process is critical to the success of the winding process and an acceptable level can only be achieved with repeated trials.
8.5 Magnet magnetization:
Magnetized magnets are difficult to ship without extensive packaging and shielding. (It's illegal too, because of the damage to other magnetic media in the mail, plus the dangers of attraction of other objects.) They are, therefore, usually magnetized at the facility of the manufacturer rather than the supplier. Additionally, it is important to magnetize the magnet at a specific point in the assembly procedure to avoid contamination of the magnets with metal particles and accidents caused by the attraction to metal objects.
It is, therefore, desirable to magnetize each magnet after it has been assembled on the iron backing ring which supports it. This, however, is not always possible, especially with high energy magnets. The reason for this is that it is often required to use a magnetizing coil on both inner and outer surfaces of the magnet (see Chapter 5). The magnets, either in the form of continuous rings or in several arcs, are magnetized in a specialized fixture and then utilized in the assembly process. Except for the low coercivity magnets like Alnico, there is no need for a metal keeper in transferring magnetized magnets, since there is no perceived demagnetization of the magnets in air.
Once the magnets are positioned correctly in the assembly, adhesives are commonly used for bonding to the metal surface they are mounted on. In cases where the magnets are subjected to a large centrifugal force, causing them to fly off the metal surface, tape is strapped around the magnet or a ring is slipped onto their outer surface.
It is also possible to injection-mold a plastic material (such as PPS) over a rotor and magnet in a manner to bond the two and also protect the magnet surface.
8.6 Bearing assembly:
Once the stator and rotor subassemblies are built, all that remains is the final assembly, which results in a complete motor. The bearing assembly facilitates the relative rotation between stator and rotor. Ball bearings and bushings are commonly used in the industry although there are many other methods of supporting the rotor. Whatever the means, there is usually the requirement to permanently build-in an axial force, called "preload", between the two bearings. This force ensures that the amount of axial movement of the rotor with respect to the stator is held to a minimum. In the absence of such a force, a large axial movement could be experienced in the form of axial runout, which may not be acceptable in some applications. The amount of preload force is determined by how stiff the bearing assembly needs to be, mechanically. However, a large amount of preload results in a large amount of drag in the bearing, which is seen as additional current to the motor. Moreover, as the amount of preload force is increased, the expected lifetime of the bearing is shortened. This lifetime expectancy is expressed in the mean time to failure for a particular number of rotational cycles. For example: an MTBF of 100,000 hours for a million cycles at a preload of 5.0 lb., or a 10% failure rate of 70,000 hours for 2 million cycles at a preload of 8.0 lb. Bearing manufacturers can provide a lot of assistance in life calculations. They can recommend values of preload and should be consulted at the time of selection of bearings.
Bearing life may also be expressed as 'B10' or 'L10' life, meaning the percentage of bearings expected to fail is 10%, in that length of time. Sometimes other percentages, e.g. L2, are used. It should be noted that life for a pair of bearings is less than the life for each separate bearing.