GB2066585A - Rotors of synchronous electric motors - Google Patents

Abstract

The invention concerns a synchronous electric motor having a rotor with solid salient poles supplied by indirect converters having natural commutation. At least a portion of the outer surface of the poles 2, Fig. 1, for example the frontal surface 3a and the lateral surfaces 3b and 3c of the polar pole shoes 3, is covered by a layer of metal of high electrical conductivity, for example copper, by an electrolytic method. The invention enables losses by Foucault currents in the periphery of the rotor to be reduced to a large extent. Copper pieces 5 or rings may be added on both sides of the rotor. In another embodiment, Fig. 3, in which the rotor is contained within a hoop 10 of steel, a layer of copper 11 is provided over the circumferential surface of the hoop and preferably also the side surfaces. <IMAGE>

Description

SPECIFICATION Synchronous electric motor The invention relates to a rotor for synchronous electric machines having solid salient poles.

It is becoming more and more frequent to supply variable frequency motors through the agency of static converters. In this case, the use of synchronous motors is favourable since the major portion of the necessary reactive power is provided by stator windings. For the high frequencies, indirect static converters are generally used operating by natural commutation. In this case, the current in each phase of the motor has the shape of a succession of rising and falling steps along a sine curve.

Consequently, for the greater part of the time, the rotor is turning opposite a fixed field stator. If the machine comprises a solid rotor, that is to say non-laminated, which is often the case with rapid synchronous motors, considerable additional losses may thus be created by the Foucault currents at the periphery of the rotor, losses which are occasionally the cause of inadmissible heating and a reduction in the efficiency of the machine.

The periphery of the rotor in question is generally constituted by polar pole shoes. It may also be constituted, as described in French patent application No. 78 23720 of 1 itch August 1978 in the name of the applicant, by a hoop having high mechanical resistance disposed around the poles and comprising portions having high magnetic permeability situated opposite the poles and portions of magnetic metal situated between them.

The object of the present invention is to reduce to a considerable extent the losses in the rotor so as to permit supplying by indirect converters functioning by natural commutation.

To this end, the rotor of the synchronous electric machine having solid salient poles in accordance with the invention is characterised in that at least a part of the outer surface of the poles is covered by a layer of metal having high electrical conductivity.

This layer constitutes a screen for the harmonics of the magnetic field of the stator between the said stator and the surface of the rotor which faces it.

It is known that the Foucault currents which are developed in a screen, for example, of copper and of a sufficient thickness, for example 3 to 6 mm, lead to losses very much less than those which would be created by Foucault currents in pieces of magnetic steel or to losses due to hysteresis and Foucault currents which would develop in pieces of steel having high magnetic permeability and non-laminated.

The layer of metal, in particular copper used in the present invention must be provided so as to resist mechanical forces due to the centrifugal force or to the different expansion between the steel support and the applied layer of metal, and it must also have a thickness sufficient for acting as a screen. An electrolytic depositing process enables these conditions to be met.

The layer of copper may be extended over the two lateral surfaces of the poles or, if the rotor is provided with a hoop as in the French patent application referred to above, over the two lateral surfaces of the said hoop so as to increase the cross-sections of the paths offered to the currents circulating around each pole and from one pole to the other.

The layer of copper may also be provided with grooves so as to permit a better removal of the losses by a cdoling fluid. These grooves must have approximately the orientation of the currents and be less deep than the layer of copper so as not to increase the resistance too much of the passage offered to the said currents.

The invention will be described with the aid of the accompanying drawings in which: Figure 1 represents diagrammatically, in perspective, a portion of a rotor according to the invention.

Figure 2 represents, in perspective, a rotor of a homopolar machine according to the invention.

Figure 3 represents in section a rotor of a synchronous machine provided with a hoop.

Figure 4 represents the course of the Foucault currents produced in the hoop of the rotor of Figure 3, developed in the plane of the figure.

In Figure 1, a spider referenced 1 is provided with solid poles 2 and polar pole shoes 3. The windings are referenced 4.

According to the invention, the frontal surface 3a of the polar pole shoes 3 is covered with a laye,r of metal having high electrical conductivity, for example copper, preferably by an electrolytic method. It may also be necessary to extend the said layer of copper over the lateral surfaces 3b of the polar pole shoes facing adjacent poles.

Moreover, so as to facilitate the circulation of currents on both sides of the polar pole shoes, it is also advisable to cover the lateral surfaces 3c of the shoes with copper.

In order to complete the screening effect of the layer of copper and to reduce subtransitory reactants, pieces of copper such as those shown at 5 may be added on both sides of the rotor, only one of which has been shown for clarity of the figure, and which ensure the electrical continuity of the screen over the whole of the circumference of the rotor. The cross-section of the pieces 5 must be sufficient for the ohmic losses which are produced therein to be kept low.

For a better mechanical action of the pieces 5, they may be replaced with advantage by continuous rings over the entire circumference of the rotor and arranged axially on both sides of the rotor.

Figure 2 shows a solid rotor for a homopolar machine, comprising lobes 6a and 6b forming the poles. The entire outer surface of the poles is covered with a layer of copper 7a, 7b. The latter may with advantage be extended over a portion of the end surfaces in the regions 8a and 8b defined by chain dotted lines 9a and 9b.

In Figure 3, which shows a rotor provided with a mixed hoop such as described in patent application 78 23720 already referred to, the spider is referenced 1, the poles 2 provided with their windings 4 and the hoop is referenced 10, as in Figure 1, the portions 1 Oa of which situated opposite the poles 2 are of steel having a high magnetic permeability and the portions 1 Ob of which situated between the poles 2 are of magnetic steel.

A layer of copper 11 sufficiently thick for Foucault currents to be allowed to develop in it without creating too much loss, is disposed around the hoop 10 and preferably also over the side surfaces. The layer of copper 11 disposed over the side surfaces enables the cross-sections of the passage offered to the currents circulating around each pole and from one pole to the other, to be increased, as illustrated diagrammatically in Figure 4.

In this manner, a very low resistant damping circuit and having low magnetic leakage is produced which constitutes a screen between the stator field and the steel surface of the rotor and which considerably diminishes the losses and the heating at the periphery of the machine. The addition in the layer of copper of grooves as referred to above, in particular of grooves in the peripheral sense and adjacent to the lateral surfaces of the hoop 10 likewise permits improvement, if necessary, in the removal of losses due to the circulation of Foucalt currents.

Another advantage of the described solution is that the damping screen constituted by the layer of copper with its low leakage and it substantially isotropic structure has the effect of reducing at the same time the subtransitory longitudinal and transverse resistances and thus the reactants of commutation, which is very favourable for the functioning of the converter.

In the case of a homopolar machine with a mixed hoop, such as described in the French patent application No. 78 23720 referred to, the production of the layer of copper is simiiar to that of Figure 1. The difference resides in the fact that, in this case, the poles are not provided with windings.

Of course, the invention continues to be valid if only a portion of the surfaces of the polar pole shoes of the poles or of the hoop facing the stator or a portion of the lateral surfaces of the polar pole shoes or of the hoop is covered with copper.

From the foregoing, it will be appreciated that the described rotor is in use incorporated with a stator to define a synchronous electric motor, which is driven by a static converter, typically an indirect converter functioning by natural commutation, with the advantage of a mucn more efficient operation than in the aforesaid prior art.

Claims (6)

1. A synchronous electric machine having a rotor with solid salient poles and supplied with a driving current by a static converter, the peripheral surface of the poles being covered with a continuous layer of metal which is a good conductor, said layer having been deposited by electrolytic deposition in a manner providing high electrical conductivity and very strong adherence to the poles.

2. A machine according to claim 1 in which the poles are provided with polar pole shoes wherein the surface covered by the metal of high electrical conductivity comprises the surface of the polar pole shoes situated opposite the stator of the motor as well as at least a portion of lateral surfaces of the polar pole shoes.

3. A machine according to claim 2 wherein the lateral surfaces of the polar shoes covered with metal of high electrical conductivity and disposed on both sides of the shoes in the axial sense are connected together by pieces of metal of high electrical conductivity ensuring an electrical peripheral surface between adjacent said shoes.

4. A machine according to claim 1 comprising a hoop of high mechanical resistance disposed around the poles and comprising portions of high magnetic permeability situated opposite the poles and portions of magnetic metal situated between the poles wherein the surface of the hoop facing the stator is covered at least in part by the said layer of metal of high electrical conductivity.

5. A machine according to claim 4 wherein the lateral surfaces of the hoop are also covered by the said layer of metal of high conductivity.

6. A rotor for an electric machine, substantially as hereinbefore described with reference to the accompanying drawings.