An induction motor

Abstract

An induction motor comprising a rotor assembly 10, the rotor assembly comprising a pair of cage end rings 12 and a plurality of rotor bars 14 connecting each of the pair of cage end rings to each othe

Description

The present invention relates to an induction motor, in particular to the cage end rings and the rotor bars of the rotor assembly of the induction motor. Induction motors typically employ a series of rotor bars embedded around the periphery of the rotor which must electrically and mechanically connected at their ends via a cage end ring. One known method of connecting the rotor bars to the cage end rings is to butt up the rotor bars to the cage end ring and then braze the rotor bars in place. Another known method of connecting the rotor bars to the cage end rings is to insert the rotor bars into a closed cavity or aperture formed in the cage end ring and then braze the rotor bars in place. Whilst both the above known methods typically provide an adequate electrical connection between the rotor bars and the cage end rings, the mechanical connection is not always strong enough, particularly for high-speed induction motors. For induction motors in which the rotor bars are inserted into the closed cavity of the cage end ring, the gap therebetween can be inconsistent, either too large or too small, preventing capillary action, leading to a failure to braze in that location. The latter not only results in inconsistent electrical conductivity but is a particular problem for highspeed rotors causing points of fatigue and catastrophic failure. An object of the present invention is to provide an improved mechanical connection between the rotor bars and the cage end ring of a rotor assembly of an induction motor. A further objective of the present invention is to make the mechanical connection more amenable to semi-automated process control thus eliminating the need for, or reducing input to oversight from, a skilled brazing technician. Semi-automated process control is a key enabler to establishing a quality control regime where non-destructive post process examination is unavailable to demonstrate adequate integrity of brazing. Thus, according to the present invention, there is provided an induction motor comprising a rotor assembly, the rotor assembly comprising a pair of cage end rings and a plurality of rotor bars connecting each of the pair of cage end rings to each other, each cage end ring includes a plurality of apertures, each aperture having an interior surface, in which the interior surface includes a plurality of inwardly facing projections which are sized so as to form an interference fit with each of the rotor bars and create a gap between the interior surface of the aperture and an exterior surface of the rotor bar, the gap enabling a capillary draw through braze connection to be achieved between the rotor bars and the cage end rings to connect the rotor bars to the cage end rings. Advantageously, the inwardly facing projections enable the rotor bars to be inserted into the cage end ring and brazed together with a uniform or consistent gap as well as a known gap of appropriate size to guarantee the fit between the rotor bar and cage end ring to the optimum bond line thickness all around the junction between the rotor bar and the cage end ring. This ensures both maximum strength at the junction and maximum, uniform electrical conductivity. Furthermore, the inwardly facing projections facilitates semi-automated process control of the rotor bars and cage end ring assembly which further enhances quality control and life expectancy of the rotor assembly in a location that is otherwise difficult to post inspect non-destructively. Preferably, each aperture is a through aperture, that is an aperture that extends through the cage end ring as opposed to a blind hole or a cavity. Advantageously, this enables the rotor bar to be inserted through both cage end rings and more easily axially located during assembly using external tooling, compared to known rotor assemblies in which rotor bars are inserted into cage end rings with blind apertures. Blind apertures require different geometry to prevent trapped gasses and cannot maintain the same close tolerance uniformity of fit. Furthermore, the through aperture in the cage end ring allows the braze to be drawn through the carefully controlled gap by capillary action, and for the braze to be applied from the outboard end of the cage end ring resulting in a better quality braze and easier assembly of the rotor bars. Preferably, the gap is substantially uniform which creates an even braze bond line and therefore stronger join or connection. Typically, the gap is between 0.15mm to 0.25mm. Preferably, the inwardly facing projections are all the same size to ensure a uniform gap is created. Preferably, the projections create an interference fit with the rotor bars, thus eliminating any variation in the gap due to a clearance or transition tolerance fit. Preferably, the profile of the projections is designed to score into the rotor bars, rather than remain on the surface of the rotor bars such that the interference is accommodated by local surface migration of material during insertion and not by gross distortion of the cage end ring as a whole. Preferably, by means of the interference fit, scoring and migration of material the uniform gap is accurately maintained within designed limits and not affected by tolerance build up. This supports the conditions that permit process-controlled brazing and consequently enable process-based quality control. Preferably, the plurality of inwardly facing projections are equally spaced around a periphery of the interior surface. Advantageously this facilitates location of the rotor bar into the cage end ring when being inserted as well as equally distributing the force created by the interference fit. Preferably, the plurality of inwardly facing projections are six inwardly facing projections. The number of projections is chosen so as to centralise the rotor bar which can come in a variety of sections. Preferably, the rotor bars extend outwardly beyond an outer surface of the cage end rings. According to another aspect of the present invention there is provided an induction motor comprising a rotor assembly, the rotor assembly comprising a pair of cage end rings and a plurality of rotor bars connecting each of the pair of cage end rings to each other, each cage end ring includes a plurality of through apertures, in which the rotor bars extend through the through apertures and outwardly beyond an outer surface of the cage end rings. According to another aspect of the present invention there is provided a method of assembling rotor assembly comprising the steps of providing a pair of cage end rings, each cage end ring includes a plurality of through apertures, each through aperture having an interior surface, in which the interior surface includes a plurality of inwardly facing projections, providing plurality of rotor bars, inserting each of the rotor bars into each through aperture such that each rotor bar engages with the inwardly facing projections to form an interference fit between the rotor bar and the inwardly facing projections, and creates a gap between the interior surface of the aperture and an exterior surface of the rotor bar and inserting a braze in the gap to form a brazed connection between the rotor bars and each of the cage end rings. Preferably, the step of inserting a braze in the gap to form a brazed connection comprises inserting the braze from an outboard side of the cage end rings. Preferably, the step of inserting each of the rotor bars into each through aperture further comprises the step of inserting each of the rotor bars into each through aperture such that the rotor bars extend through the through apertures and outwardly beyond an outer surface of the cage end rings. Preferably, an end surface of each rotor bar is machined such that each rotor bar is substantially equal in length. According to another aspect of the present invention there is provided a rotor assembly produced according to the aforementioned method. According to another aspect of the present invention there is provided an induction motor including a rotor assembly produced by the aforementioned method. According to another aspect of the present invention there is provided a rotor assembly comprising a pair of cage end rings and a plurality of rotor bars connecting each of the pair of cage end rings to each other, each cage end ring includes a plurality of apertures, each aperture having an interior surface, in which the interior surface includes a plurality of inwardly facing projections which are sized so as to form an interference fit with each of the rotor bars and create a gap between the interior surface of the aperture and an exterior surface of the rotor bar, the gap enabling a braze to be introduced between the rotor bars and the cage end rings to connect the rotor bars to the cage end rings. The invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a perspective view of an induction motor according to the present invention, Figure 2 is a perspective view of part of the induction motor of Figure 1, Figure 3 is a perspective view of part of the induction motor of Figure 1, Figure 4a is a perspective view of part of the induction motor of Figure 1 with the rotor bar inserted into the cage end ring, Figure 4b is a perspective view of part of the induction motor of Figure 1 with the rotor bar removed from the cage end ring, Figures 5 and 6 are side schematic views of part of a prior art induction motor, Figures 7 to 9 are schematic views of part of the induction motor of Figure 1, Figures 10 and 11 are photographs of part of the induction motor of Figure 1, Figure 12 is a front sectional view of part of the induction motor of Figure 1, Figure 13 is a perspective view of part of the induction motor of Figure 1, and Figure 14 is a perspective view of part of the induction motor of Figure 1. In Figures 1 and 2, an induction motor 11 comprises a casing 13 within which a rotor assembly 10 and a stator 15 are housed and arranged in a known way. The stator 15 includes multiple motor windings (not shown) which are connected to a power supply (not shown) via a terminal block 17. Applying power to the motor windings causes the rotor assembly 10, and a shaft 21 connected thereto, to rotate. The rotor assembly 10 is of the so-called squirrel cage type and includes a pair of copper annular cage end rings 12 connected by a plurality of copper elongated rotor bars 14 The rotor assembly 10 further includes a solid core made from stacks of steel laminations 19 which include slots 23 through which the rotor bars 14 can pass through (Figures 2, 10 and 11). In Figures 3, 4a, 4b and 12 to 14, each cage end ring 12 includes a plurality of equally spaced through apertures 16, that is, apertures which extend through a thickness T of the cage end ring 12. Each aperture 16 has an interior surface 18, which is precision machined to create a plurality, in this embodiment, six, inwardly facing projections 20 are machined. The projections 20 are substantially the same size with a length L and depth D. The depth D is chosen to form an interference fit with the rotor bar 14 when it is inserted and create a required gap G between an exterior surface 22 of the rotor bar 14 and the exterior surface 22 of the apertures 16 of the cage end ring 12 to enable brazing as will be described below. The gap is 0.2mm. The length L is chosen to be robust enough to withstand insertion of the rotor bar 14. The rotor bars 14 and the apertures 16 of the cage end ring 12 have substantially the same cross-sectional profiles to facilitate the creation of the uniform gap G. The rotor assembly 10 is assembled as follows: In Figures 3 and 4a, each rotor bar 14 is inserted into a corresponding aperture 16 of the cage end ring 12. Each rotor bar 14 engages with, and is located laterally (Y direction in Figure 9) and vertically (Z direction in Figure 9) in the cage end ring 12 by projections 20 to form an interference fit between the projections 20 and an exterior surface 22 of the rotor bar 14, with a uniform gap G between the interior surface 18 of the aperture 16 and the exterior surface 22 of the rotor bar 14 (Figures 8 and 9). It can be seen in Figures 8 and 12 that the rotor bars 14 extend through the through apertures 16 and outwardly beyond an outer surface 24 of the cage end rings 12. Once inserted, a braze filler B is inserted into the gap G from an outboard side OS, and heated and cooled in a known way to form a brazed connection between the cage end rings 12 and the rotor bars 14 (Figure 7). A comparison of the assembly of Figure 7 of the present invention with the prior art assemblies of Figures 5 and 6 firstly shows that the braze B can be more easily applied from the outboard side OS compared to the inboard side IS where access is more restricted. It can also be seen from the comparison of Figure 5 (prior art) and Figure 7 that the gap G is not uniform in Figure 5 due to the absence of the projections 20 in the prior art assembly. Such a non-uniform gap leads to a non-uniformed braze and therefore weaker brazed connection, and also the potential for areas of failed and hidden failed braze. It can further be seen from the comparison of Figure 6 (prior art) and Figure 7 that the rotor bar in Figure 6 butts up against the cage end ring as opposed to being inserted into the aperture 16 of the present invention before the braze B is applied, resulting in a weaker brazed connection. It can yet further be seen from the comparison of Figures 5 and 6 (prior art) and Figures 7 and 8 that the rotor bar in Figures 5 and 6 butts up in a longitudinal direction X against the cage end ring or the end of an aperture in the cage end ring. This contrasts with the present invention in Figure 7 where the rotor bar 14 is free to move in the longitudinal direction X which aids assembly and inspection. The rotor bars 14, steel laminations 19 and cage end rings 12 are assembled together prior to the brazing process, and are held in a jig in their correct relative positions. During the brazing process, the rotor bars 14 heat up more, causing them to expand more than the rest of the rotor assembly. As the braze cools, it connects with the rotor bars 14 while they are their expanded length, and as the rotor bars 14 cool they contract and apply a compressive load to the rotor assembly. After applying the braze B, the heating effect of the brazing, and the differential thermal expansion caused in the rotor bars 14 which are made of copper and the steel laminations 19, results in a compressive load on the steel laminations 19. As the braze B cools, the cage end ring 12 cools to apply a load and form a connection joint with the rotor bar 14. After the assembly has cooled, the axial end surface 25 of each rotor bar 14 is machined such that each rotor bar 14 is of substantially equal in length L2 so that the assembly 10 is balanced. Finally, the assembly 10 is installed into the induction motor casing 13. It will be understood that the rotor assembly 10 of the present assembly is easier to assemble and inspect, and results in a brazed connection not only of higher strength compared to known rotor assemblies, but also, crucially reliably of constantly higher integrity, and therefore a known unpredictable fundamental weakness in the braze process has been eliminated due to the provision of the inwardly facing projections 20. More specifically, the projections 20 enable reliable, repeatable the insertion of the rotor bars 14 into the aperture 18, and control the gap G between the exterior surface 22 of the rotor bars 14 and the interior surface 18 of the aperture 16 to design specification without skilled oversight of further inspection. The interference and scoring of the projections into the rotor bars ensure the gap is controlled to a degree that would otherwise would not be achievable so that it is substantially uniform and of a specific size to enable a capillary draw through effect during the brazing process to provide a strong and reliable join, and at the same time the projections 20 permit the insertion and the brazing operation to be a closed loop machine parameter-controlled brazing process.

Claims

1. An induction motor (11) comprising a rotor assembly (10), the rotor assembly (10) comprising a pair of cage end rings (12) and a plurality of rotor bars (14) connecting each of the pair of cage end rings (12) to each other, each cage end ring (12) includes a plurality of apertures (16), each aperture (16) having an interior surface (18), in which the interior surface (18) includes a plurality of inwardly facing projections (20) which are sized so as to form an interference fit with each of the rotor bars (14) and create a gap (G) between the interior surface (18) of the aperture (16) and an exterior surface (22) of the rotor bar (14), the gap (G) enabling a capillary draw through braze connection to be achieved between the rotor bars (14) and the cage end rings (12) to connect the rotor bars (14) to the cage end rings (12).

2. An induction motor (11) according to claim 1 in which each aperture (16) is a through aperture.

3. An induction motor (11) according to claim 1 or 2 in which the gap (G) is substantially uniform.

4. An induction motor (11) according to claim 1 or 2 in which the inwardly facing projections (20) are the same size.

5. An induction motor (11) according to any preceding claim in which the plurality of inwardly facing projections (20) are equally spaced around a periphery of the interior surface (18).

6. An induction motor (11) according to any preceding claim in which the plurality of inwardly facing projections (20) are six inwardly facing projections (20).

7. An induction motor (11) according to any preceding claim in which the rotor bars (14) extend outwardly beyond an outer surface (24) of the cage end rings (12).

8. An induction motor (11) comprising a rotor assembly (10), the rotor assembly (10) comprising a pair of cage end rings (12) and a plurality of rotor bars (14)connecting each of the pair of cage end rings (12) to each other, each cage end ring (12) includes a plurality of through apertures (16), in which the rotor bars (14) extend through the through apertures (16) and outwardly beyond an outer surface (24) of the cage end rings (12).

9. An induction motor (10) according to any preceding claim in which the gap (G) is between 0.15mm to 0.25mm.10.An induction motor (10) according to any preceding claim in which the projections 20 are formed by machining the interior surface (18) of the aperture (16)11. A method of assembling a rotor assembly (10) comprising the steps of:a. providing a pair of cage end rings (12), each cage end ring (12) includes a plurality of through apertures (16), each through aperture (16) having an interior surface (18), in which the interior surface (18) includes a plurality of inwardly facing projections (20),b. providing plurality of rotor bars (14),c. inserting each of the rotor bars (14) into each through aperture (16) such that each rotor bar (14) engages with the inwardly facing projections (20) to form an interference fit between the rotor bar (14) and the inwardly facing projections (20), and creates a gap (G) between the interior surface (18) of the aperture (16) and an exterior surface (22) of the rotor bar (14), andd. inserting a braze in the gap (G) to form a brazed connection between the rotor bars (14) and each of the cage end rings (12).

12. A method of assembling a rotor assembly (10) according to claim 11 in which the step of inserting a braze in the gap (G) to form a brazed connection comprises inserting the braze from an outboard side (OS) of the cage end rings (12).

13. A method of assembling a rotor assembly (10) according to claim 11 or 12 in which the step of inserting each of the rotor bars (14) into each through aperture (16) further comprises the step of inserting each of the rotor bars (14) into each through aperture (16) such that the rotor bars (14) extend through the throughapertures (16) and outwardly beyond an outer surface (24) of the cage end rings (12).

14. A method of assembling a rotor assembly (10) according to claim 13 in which an end surface (25) of each rotor bar (14) is machined such that each rotor bar (14) is substantially equal in length (L2).

15. A rotor assembly (10) produced by the method of any one of claims 11 to 14.

16. An induction motor (11) including a rotor assembly (10) produced by the method of any one of claims 11 to 14.

17. A rotor assembly (10) comprising a pair of cage end rings (12) and a plurality of rotor bars (14) connecting each of the pair of cage end rings (12) to each other, each cage end ring (12) includes a plurality of apertures (16), each aperture (16) having an interior surface (18), in which the interior surface (18) includes a plurality of inwardly facing projections (20) which are sized so as to form an interference fit with each of the rotor bars (14) and create a gap (G) between the interior surface (18) of the aperture (16) and an exterior surface (22) of the rotor bar (14), the gap (G) enabling a braze to be introduced between the rotor bars (14) and the cage end rings (12) to connect the rotor bars (14) to the cage end rings (12).

18. An induction motor (10) according to any one of claims 1 to 10 in which the rotor bars (14) and the apertures (16) of the cage end ring (12) have substantially cross-sectional profiles to facilitate the creation of the uniform gap (G).

19. An induction motor (10) according to any one of claims 1 to 10 in which the rotor bars (14) and the apertures (16) of the cage end ring (12) have substantially corresponding, prismatic cross-sectional profiles to facilitate the creation of the uniform gap (G).

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