Metal working – Method of mechanical manufacture – Electrical device making
Patent
1989-10-02
1991-04-30
Hall, Carl E.
Metal working
Method of mechanical manufacture
Electrical device making
29732, H02K 1502
Patent
active
050106384
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the manufacture of rotary electrical machines and has been developed for the manufacture of hermetic motor-compressor units for refrigerant fluids.
It relates, in the first place, to a method for assembling a stator, formed by a stack of core laminations and windings therefor on a respective head support of a small rotary electrical machine in a position concentric with a rotary shaft mounted in the support. The periphery of one end face of the stack of laminations rests on peripheral coplanar surfaces or the support which define a reference plane. A centering device is interposed between the shaft and the stack and the stack and support are held together by means of peripheral bolts inserted in regions corresponding to the coplanar surfaces of the support, perpendicular to the surfaces, and are screwed into the support.
A method of this type is known from the document IT-A-996.643. In this document, a centering device is described, inter alia, which includes two concentric, "tulip"-type collets, of which the inner one is designed to contract around the shaft and the outer one is designed to expand against the surface of the bore in the stack of laminations.
This collet ensures very accurate centering of all the laminations in the stack relative to the shaft before the stack is clamped by means of the bolts. When the bolts are tightened, however, the clamping torque is transmitted, by friction, from the heads of the bolts to the immediately-adjacent lamination, causing the first lamination to be forced radially against the outer collet; this force is transmitted by friction to successive laminations so that the laminations are forced along a generatrix against the outer collet and, when this is released, the accuracy of centering obtained thereby is cancelled out. This inaccuracy means, on the one hand, that larger air gaps have to be provided in the design stage and this reduces the efficiency of the electrical machine. What is worse in the case of a non-synchronous motor, is that the mutual displacement of the laminations caused by tightening of the bolts causes irregularities in the air gaps resulting in the motor being unable to reach its starting torque under load. This problem is particularly serious in the case of small hermetic motor compressors for refrigerators, particularly considering the fact that testing of the motor is carried out only on the finished compressor, that is when the compressor motor is enclosed in its hermetic casing. In this case, rejection of the compressor due to defects in its motor involves a cost corresponding to that of the finished product.
Currently, in order to avoid displacement of the laminations during tightening of the bolts, in small electric motors, before the construction of the windings, the stator stack is impregnated with a thermosetting resin whose hardening results in a monolithic stack in which the laminations can no longer be displaced by the tightening torque. Methods of this type are known, inter alia, from the documents U.S. Pat. No. 3,356,360 and U.S. Pat. No. 3,573,129.
Impregnation is a fairly expensive operation both in capital expenditure and in time: from the latter point of view, it constitutes a lengthy additional step in the manufacture of an electrical machine.
According to another method, known from the document FR-A-1,452,093, a stator stack of laminations is fitted around an expandable collet of the "tulip"-type. After the collet has been expanded in the bore of the stack, the latter is submitted to axial compression. While the stack is compressed the laminations are welded together on the periphery of the stack to obtain a monolithic stack.
The document FR-A-2,093,227 discloses a method similar to that of FR-A-1,452,093. According to FR-A-2,093,227 use is made of a non-expandable mandrel and the laminations of the stack under axial compression, with the mandrel fitted in the bore of the stack, are welded along welding lines adjacent to the bolt holes.
The methods according to FR-A-1,452,093 and FR-A-2,093
REFERENCES:
patent: 3490143 (1970-01-01), Hull
patent: 3864822 (1975-02-01), Eash et al.
patent: 4290291 (1981-09-01), Burns
Aspera S.r.l.
Hall Carl E.
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