How to laminate the electrical motor stator stacks by progressive die stamping?
Motor stator core Laminations of all types (see Following pictures ) may be staked together into stacks of desired heights as they are stamped in progressive dies during the manufacturing process. This staking-in-the-dietechnique represents an opportunity for both improved quality and significant cost reduction to manufacturers of motor stator stack products that require rotor, stator,transformer, and other types of laminations that subsequently are laminated into Stator stacks.
In addition, symmetrical motor laminations, such as for stator stacks of electric motors, may be rotated in the die prior to staking in order to compensate the material thickness variations from one mateiral coil or to produce a skew angle, as found in many motor designs.
Producing finished motor stator stacks of laminations in the die has many advantages to the manufacturers of electrical motors, particularly the elimination of downstream manual or mechanical assembly operations such as manual riveting or welding after the lamination. Staking in the die also can produce motor stator stacks with 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 can 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 micro processor controller for a predetermined number of laminations in each stack. For example, a 20mm motor stator stacks using 0.2mm-in-thick material would require 40 laminations. By programming the die’s staking punches, a 20mm 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 20mm stack height is completely dependent on the thickness consistency of the 0.2mm coil stock.If the material begins to run thick during the stamping process, 40 laminations could produce an unacceptably long rotor stack.
Lamination of motor stator stacks.
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 motor stator stack height.
There are several advantages to using a systemof 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 alsopermits some motor manufacturers to use less uniform and less expensive steelstock because the controller determines the proper number of laminations neededto 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.
Rotating Laminations in the Die. A material thickness monitoring system like that shown in Fig. 3.38 can compensate forstock variations throughout the length of the coil and automatically adjust thelamination count to maintain uniform stack height, but it cannot compensate forthickness variations across the stock width. For example, if the left side ofthe coil strip were consistently thicker than the right side,
the stator and rotor stacks produced would losetheir perpendicularity and would lean to the right. These stacks would be ofinferior quality and would complicate subsequent manufacturing operations.
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 180° with respect to the following lamination. A rotor with four staking points could be rotated 90°.
we uses a belt system driven by ahigh-speed motor to rotate the die cavity. The motor is controlled by the samecontroller that monitors the staking and stack-height functions.
Rotation is not necessarily confined to rotors.If a stator is perfectly symmetrical, it also may be rotated in a staking diecavity. In addition, when motor manufacturers require loose rotor laminations,the laminations may be rotated without being staked together and loaded intostacking chutes. When these loose laminations finally are assembled, they willalso exhibit improved perpendicularity and balance.
Many motor designs incorporate a skew angle inthe rotor assembly to improve motor performance. In Oberg-produced dies thatcontain the rotating skewing cavity, the skew angle to the rotor stack isquickly set by entering the desired skew angle into the control system. Skewangles may be set in addition to the rotation or by themselves without anyother rotation in the lamination.
When skewing, the consistency of materialthickness throughout the length of the coil stock is a concern. Rotor stacks1-in high of 0.025- ± 0.002-in laminations will have varied numbers oflaminations to achieve proper height. To compensate for variations incoil-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 contain39, 40, or 41 laminations.
Although staking, rotating, and skewing of thelaminations is performed in the stamping die, the critical component of theproprietary system is the microprocessor control unit. The controller must havethe capacity and speed to control the die’s
staking punches, the high-speed motor thatdrives the rotation cavity, and the material monitoring sensor, as well aspermit the stamping press to operate at top speed. Some lamination diecontrollers may have to eliminate features to avoid significantly slowing thepress speed.
System Benefits. Whether the motor manufacturerstamps 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 thetechnology is that it requires less material handling. Staked lamination stackseliminate much of the handling and moving associated with loose laminations.Some motor manufacturers are able to send stacks on conveyors to the nextprocess area, directly from the press.
For many types of motors, staked rotor andstator stacks may eliminate welding and riveting operations. The labor andcosts associated with these operations is eliminated, and there also is no needto replace welding and riveting equipment when it wears out.
One motor manufacturer, faced with thereplacement of an obsolete welding line, invested instead in a staking die andcontroller. The company calculated a two-month payback on the investment and,in addition, was able to access much-needed floor space when the welding linewas removed. The manufacturer intends to eliminate all welding in the plantwithin two years.
The rotor and stator stacks produced with thestaking-and-rotation technique are consistently of higher quality than thoseproduced from loose laminations. Some manufacturers using staking dies haverealized a reduction in the costs associated with balancing and othermotor-finishing operations. Motor performance also has been improved.
For manufacturers that stamp their ownlaminations, 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.
Die and Controller Requirements. Rotation,skewing, and staking of laminations, the process used to produce the samplestacks shown in Fig. 3.39, presents several challenges to the die or controllersupplier. The basic accuracy requirements of the rotational motion arerecognized when one considers that a 10° skew angle in a 40-lamination stackresults from the rotation of each individual lamination by 0.25°. Variations inmaterial thickness can adjust that by 0.00005° or less. Also, extremely highaccuracy is required in the stamping die to permit rotation of laminationswhile maintaining concentricity. The location of the stakes must be perfectlysymmetrical
Sample lamination of motor stator stacks.
for them to attach properly to the precedingrotated lamination. Rotation of square, rectangular, or other nonround shapesrequires extreme accuracy of the rotational motion, since the punch is nowpenetrating a moving die section. It also requires mechanical devices that willprevent damage if the rotating chamber does not align with the punch.
The design and construction of the controllermust take into account the fact that the system will be operated in a pressroomenvironment, and must minimize both additions to the operator’s workload andintrusion into the already crowded work space. A simplified operator interfaceand a rugged, vibration-resistant package are basic to the operational successof a staking and stamping die.
Technology Limitations. There are areas withinthis staking and rotating technology where motor manufacturers face somelimitations and cautions. First and most important, it should be noted that thestaking process requires a sufficient open area on the face of the rotor andstator laminations to allow a stake protrusion to be made without distorting acritical dimension of the lamination. When designing a lamination with stakingin 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 forsmaller-sized rotors may not require reduced press speeds, the rotation of thedie cavity for larger-diameter rotors could force a press to run more slowly.Also, for annealed laminations, the electrical properties of staked stacks mustbe compared critically to those of stacks made from loose laminations. Motormanufacturers that stake stacks in the die generally have found few differencesof consequence, but it is a factor to be considered when designing a motor.
Although the tightness and integrity of stakedstacks usually are not problems, some handling precautions are advisable toprevent stack delamination. One manufacturer permits its stacks to drop almost4 ft from the press into a collecting bin, but this kind of handling may not besuitable for some types of lamination designs.
The technology of staking, stacking, rotating,and skewing in the stamping die, along with other emerging technologies designedto reduce costs and improve quality, should position the small-motor industryto compete successfully in the global economy.
