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Why electric motor stator lacing is necessary?
Stator lacing is the process of tightly securing the field coil ends of an electric motor stator with a stitched cord,motor stator braiding.
Lacing is typically used on long-life-expectancy or high-efficiency motors such as some induction motor 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 handwork. 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. Others have two needles and lace both ends of the stator at the same time, and are known as double-end lacers. 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 (insulation thread) 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 is made from, and the weaving technique used to make it.
Most lacing cords are made from polyester, which shrinks when it is 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. 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 sta-tor 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 is 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 pattern 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 will not 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 will not 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 do not 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 datastorage capacity. Nonradial 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 produce 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.