GB2023459A

GB2023459A - A method of manufacturing the rotor of a cast metal squirrel-cage asynchronous electric machine

Info

Publication number: GB2023459A
Application number: GB7921179A
Authority: GB
Original assignee: Cegelec SA
Current assignee: Cegelec SA
Priority date: 1978-06-27
Filing date: 1979-06-18
Publication date: 1980-01-03
Also published as: BE876853A; BR7904053A; SE7905589L; DE2924853A1; FR2430127B1; JPS555094A; FR2430127A1; IT7968359A0; IT1119009B

Abstract

The squirrel-cage 17 of the rotor of an asynchronous motor is made by casting aluminium into slots provided in a stack of magnetic laminations. Two kinds of laminations are provided in the stack. One kind comprises a disc 5 having bar-forming slots and ventilation holes 8. The other kind comprises an annulus 13 with a central void and radially extending ventilation slots 11 positioned between bar-forming slots. After casting, the rotor is machined externally to open the outer ends of the radial ventilation slots to provide air flow paths 19. <IMAGE>

Description

SPECIFICATION A method of manufacturing the rotor of a cast metal squirrel-cage asynchronous electric machine The present invention relates to a method of manufacturing squirrel-cage rotors.

Producing such cages using cast aluminium or aluminium alloy is a well-known operation, the aluminium or aluminium alloy being cast by gravity, injection, cetrifuging, vibration or other known means.

Since this manufacturing method is particularly economical, it has been applied to larger motors; however, in large motors it is necessary to form radial ventilation vents constituted by radial passages which lead to axial passages and this complicates casting the cage.

Indeed, when casting, slots are formed in the laminations to constitute a mould for casting the bars of the cage, but the radial passages consitute interruptions in the continuity of the mould thus formed.

Solutions have been found to provide continuity of the mould when casting the cage; in particular, stacks of intermediate disks have been formed, said disks being slotted with larger slot cross-sections than the slot cross-sections of the magnetic laminations; these stacks of intermediate disks are placed between two consecutive stacks of magnetic laminations. The cage is then cast and the intermediate disks are destroyed to form the radial passages. their destruction being by burning or by dissolving or even by melting, depending on the material of which the disks are made. The spacing of the various stacks of magnetic laminations is maintened by the enlarged bars between these stacks of laminations. Such an embodiment is described for example in US patents Nos 2 368 296, 2 368 295,2 370 458 and 2 607 969.

However, such a method has drawbacks; indeed, if the intermediate disks are destroyed by burning or by melting, there can be incompatibility with production of the cage; for example, preheating can be prevented or casting temperatures can be limited. If these disks are destroyed by dissolving, it seems difficult to find a material which simultaneously withstands the casting of molten metal and is soluble in a solvent; such disks are therefore expensive to manufacture. If they are made of sand or of a material like plaster which can be more easily destroyed after casting, manufacture is expensive and tricky and the disks are fragile.

Preferred realization of the present invention mitigate these disadvantages and allow qreater liberty in the choice of casting method which may optionally include a wide choice of casting temperatures, preheating (even at very high temperatures), vibration, centrifuging, etc.

The method according to the invention can also be very economical and need not use any techniques which are unfamiliar to those who manufacture rotating machines.

The present invention provides a method of manufacturing the rotor of a cast metal sqirrel cage asynchronous electric machine, the method comprising the steps of: providing a plurality of stacks of generally disc shaped magnetic laminations, said laminations including radially closed bar-forming slots which, when stacked, form channels in which the bars of the squirrel cage will subsequently be cast;; providing stacks of generally annular special laminations between the stacks of disc-shaped laminations, the "special" laminations having the same bar-forming slots as the disc-shaped laminations to ensure continuity in casting of the said bars and additionally including ventilation slots in between their bar-forming slots, said ventilation slots being open at their radially inner ends and being closed at their radially outer ends, which ends are situated at substantially the same radial distance from the centre of the laminations as the outer ends of the barforming slots; forming a rotor member by casting the squirrel cage in the combined stack of both kinds of lamination in such a manner that the bars of the squirrel cage are formed in the channels provided by the stacked bar-forming slots; and removing the outer surface of the rotor member thus formed until the radially outer edges of the cast squirrel cage bars are exposed and until the radially outer ends of the ventilation slots are opened.

Preferably the special laminations are cut out zas follows: firstly, laminations which are identical to those of the stacks of magnetic laminations are provided in the same way as the latter with closed slots which are disposed identically to those of the magnetic laminations; parts are cut away by slotting between the closed slots so that said cut away parts extend from the vicinity of the outer periphery of the closed slots to a position lower than the bases of the closed slots; and lastly, the central part of the lamination is removed, the radius of the central part extending between the bases of the closed slots and the bases of said cut-away parts.

Alternatively the special laminations may be initially provided having an outside diameter greater than that of the magnetic laminations which constitute stacks, in which case the peripheral ends of the voids situated between the slots advantageously extend beyond those of the slots, this facilitating the opening of these voids when the rotor is subsequently turned.

In either case a few distance pieces of thickness corresponding to the thickness of the stack of special laminations may be placed between two consecutive stacks of magnetic laminations.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which: figure 1 shows a complete rotor; figure 2, 3 and 4 show the method of producing the special laminations; figure 5 is a partial cross-section showing the assembly of the magnetic circuit; figure 6 is a cross-section along VI--VI in figure 5; figure 7 is a cross-section along VIl-VIl in figure 5; figure 8 is a partial cross-section of a finished rotor; figure 9 is a cross-sectional along IX-IX in figure 8 figure 10 is a partial view of another example of special laminations; figure 11 is a partial cross-section of magnetic circuitry which uses laminations such as shown in figure 10; figure 12 is a cross-section along Xll-Xll in figure 11; and figure 13 is a cross-section of another embodiment of a magnetic circuit.

Figure 1 illustrates a complete rotor which includes a shaft 1 , a magnetic circuit composed of several stacks 2 of metal sheets (i.e. laminations), radial ventilation passages or vents 3 and a squirrel-cage represented in this figure by its short-circuit rings 4 only.

Figure 2 shows a lamination of the magnetic circuit such as can also be used for forming radial ventilation vents after extra cuts have been made in it. The lamination 5 has closed slots 6 for casting the bars of a squirrel-cage, said slots having narrowed outwardly directed leakage portions 7. It also has ventilation holes 8, a central bore 9 for assembly on a shaft and a key slot 10.

To make the radial channels, a lamination 5 such as that shown in figure 2 is taken and extra voids 11 are cut out, then the central portion of the lamination is removed round a circumference referenced 12; a special lamination such as 13 which is partially illustrated in figure 4 is thereby obtained.

Figures 5, 6 and 7 illustrate the manner in which the magnetic circuit is realised: firstly a number of laminations 5 are stacked on an assembly and casting mandrel to form a first length of magnetic circuit. Distance pieces 1 5 whose length correspond to the anticipated widths of the radial ventilation passages are welded to the end lamination 5. A number of special laminations 13 are positioned by means of a few rods such as 1 6 to form a stack whose height is equal to or slightly greater than that of the distance pieces 1 5.

Laminations 5 are stacked again to form a second length of magnetic circuit and the previously described operations are repeated until the complete magnetic circuit is formed.

It is then possible to cast the cage by a method known per se.

When the cage has been formed, the mandrel 14 can be dismantled and the magnetic circuit can be installed on the shaft. Likewise, the ventillation channels 11 are cleared at the outer periphery by a turning or other machining operation, often carried out to form the narrowed leakage portions 7 of the slots 6.

Figures 8 and 9 who partial cross-sections of a finished rotor: the figures show the cast cage 1 7 which ends in short-circuit rings 18. The arrows 1 9 show the ventilation circuit schematically.

The distance between the slots 6 and the ventilation channels 11, i.e. the breadth of the special laminations where they surround the bars 1 7 and separate them from the channels 11 can be very small, i.e. about 1.5 mm or even less. In this way, there is little hindrance in the heat exchange between the ventilation air and the cage.

It is therefore seen that this method of producing a rotor with a cast cage is very simple and economical and that it does not require any unusual techniques.

Of course, with this manufacturing method, it is possible to produce rotors whose cage bars have any cross-section and to produce double cage rotors.

Figure 10 illustrates another example of a special lamination 1 3 produced in the same way as that in figure 4 but for a double cage rotor. In this example, the central void has been formed so as to leave a greater height of lamination 20 below the slots 6 so as avoid having to place distance pieces.

Therefore, as seen in figures 11 and 12, the distance pieces 1 5 have not been installed; in contrast, if it is necessary to provide good support, rings 21 can still be installed. These rings 21 are cut out from the same laminations as the special laminations, when the central portion is being removed.

Also, as shown in figure 13, laminations of larger outside diameter than that of the laminations 5 of the magnetic circuit can be used as special laminations. This provides special laminations which are much more rigid, taking into account the voids of these laminations. Also channels 11 can be cut between the closed slots 6, the ends of the channels extending beyond those of the slots. This facilitates cutting these channels open, during subsequent turning for radial ventilation. Due also to the fact that the outside diameter of the special laminations is greater than that of the ordinary magnetic laminations and forms protruding rings, the magnetic circuit can be better positioned and held and, more particularly, deformation can be avoided during the preheating and casting operations.

Also, when the channels 11 are being cut in the special laminations 13, they can be formed with a wider and/or more rounded shape so as to improve the admission of air such as schematically shown by the arrows 1 9 in figure 9.

Claims

1. A method of manufacturing the rotor of a cast metal squirrel-cage asynchronous electric machine, the method comprising the steps of: providing a plurality of stacks of generally discshaped magnetic laminations, said laminations including radially closed bar-forming slots which, when stacked, form channels in which the bars of the squirrel cage will subsequently be cast;; providing stacks of generally annular special laminations between the stacks of disc-shaped laminations, the "special" laminations having the same bar-forming slots as the disc-shaped laminations to ensure continuity in casting of the said bars and additionally including ventillation slots in between their bar-forming slots, said ventilation slots being open at their radially inner ends and being closed at their radially outer ends, which ends are situated at substantially the same radial distance from the centre of the laminations as the outer ends of the bar-forming slots; forming a rotor member by casting the squirrelcage in the combined stack of both kinds of lamination in such a manner that the bars.of the squirrel cage are formed in the channels provided i by the stacked bar-forming slots; and removing the outer surface of the rotor member thus formed until the radially outer edges of the cast squirrel cage bars are exposed and until the radially outer ends of the ventilation slots are opened.

2. A method of manufacturing a rotor according to claim 1, wherein the special laminations are cut out as follows: firstly, laminations which are indentical to those of the stacks of magnetic laminations are provided in the same way as the latter with closed slots which are disposed identically to those of the magnetic laminations; parts are cut away by slotting between the closed slots so that said cut away parts extend from the vicinity of the outer periphery of the closed slots to a position lower than the bases of the closed slots; and lastly, the central part of the laminations is removed, the radius of the central part extending between the bases of the closed slots and the bases of said cut-away parts.

3. A method of manufacturing a rotor according to claim 1 , wherein the special laminations are produced as follows: firstly, laminations with an outside diameter greater than that of the stacked magnetic laminations are slotted in the same way with closed slots which are disposed identically with the same distance in relation to the centre.; parts are cut away by slotting between the closed slots so that they extend beyond the tops and bottoms of the closed slots; and lastly, the central part of the lamination is removed, the radius of the central part extending between the bases of the closed slots and the bases of said cut-away parts.

4. A method according to one of claims 1, 2 or 3, wherein distance pieces of thickness corresponding to the thickness of the stacks of special laminations are also placed between two consecutive stacks of magnetic laminations.

5. A method of manufacturing the rotor of a cast metal squirrel-cage asynchronous electric machine, substantially as herein described with reference to the accompany drawings.

6. A rotor manufactured according to the method of any preceding claim.