Vibration damping in rotor assembly

Abstract

The present invention relates a rotor assembly for an electrical machine (e.g. motor or generator) where a tuned vibration absorber adapted to provide radial damping is mounted directly to the rotor shaft.

Claims

1. A rotor assembly for an electrical machine comprising: a rotatable shaft; and a tuned vibration absorber configured to provide radial damping to the rotatable shaft, the tuned vibration absorber being mounted to the rotatable shaft by a mechanism including a rigid mass and a plurality of spring assemblies; wherein the rigid mass forms part of a fan configured to cool the rotor assembly during operation; and wherein the plurality of spring assemblies and the tuned vibration absorber are fixedly secured to an annular flange part of the rotatable shaft and radially secured to an inner surface of an annular root part of the rigid mass, and wherein the annular root part includes a plurality of fan blades mounted thereon.

2. The rotor assembly of claim 1, wherein a damping constant of the mechanism is selected to restrict the response magnitude of the rotatable shaft.

3. The rotor assembly of claim 1, wherein the tuned vibration absorber includes a substantially annular rigid mass.

4. The rotor assembly of claim 3, wherein the spring assemblies are a plurality of circumferentially-spaced discrete springs.

5. The rotor assembly of claim 3, wherein the viscoelastic material is a polymeric material.

6. The rotor assembly of claim 1, further comprising a rotor body.

7. The rotor assembly of claim 1, wherein the tuned vibration absorber is mounted to an annular flange of the rotatable shaft.

8. An electrical machine, comprising: a stator assembly; and a rotor assembly including: a rotatable shaft; a tuned vibration absorber configured to provide radial damping to the rotatable shaft, the tuned vibration absorber being mounted to the rotatable shaft via a mechanism that includes a rigid mass and a spring; wherein the rigid mass forms part of a fan configured to cool the rotor assembly during operation; and wherein the spring and the tuned vibration absorber are fixedly secured to an annular flange part of the rotatable shaft and radially secured to an inner surface of an annular root part of the rigid mass, and wherein the annular root part includes a plurality of fan blades mounted thereon.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Exemplary embodiments of the invention will now be described, with reference to the accompanying drawings, in which:

(2) FIG. 1 is a schematic drawing showing a tuned vibration absorber;

(3) FIG. 2 is a cross sectional view showing a rotor assembly according to the present invention;

(4) FIG. 3 is a perspective view of the rotor assembly of FIG. 2; and

(5) FIG. 4 is a graph showing response magnitude versus operating speed for a rotor assembly according to the present invention that incorporates a tuned vibration absorber. A comparison graph shows the response magnitude versus operating speed for an identical rotor assembly without a tuned vibration absorber.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) With reference to FIG. 1 a rotating primary system consists of a rigid mass (having mass M) supported by a spring and damper mechanism having a spring rate k and which provides damping c. The primary system has a first (lowest) critical speed. A tuned vibration absorber (TVA) for radial damping consists of a smaller rigid mass (having mass M.sub.a) mounted to the rigid mass of the primary system by a spring and damper mechanism having a spring rate k.sub.a and which provides damping c.sub.a. The spring rate k.sub.a and the mass M.sub.a of the TVA are chosen to match the first critical speed of the primary system. Damping c.sub.a is provided to restrict the response magnitude or displacement at critical speeds. The effect of the TVA is to replace the critical speed of the primary system with two separate critical speeds. Typically the structural resonances at those critical speeds are of a lower magnitude but this does not have to be the case.

(7) FIGS. 2 and 3 show how a TVA can be implemented practically for use with a rotor shaft 2 of a rotating electrical machine (e.g. a motor or generator). The TVA consists of an annular mass 4 supported by a layer of viscoelastic material 6, which acts as a damper, and axially spaced spring assemblies 8a, 8b. Together the spring assemblies 8a, 8b and the viscoelastic material layer 6 form the spring and damper arrangement that is shown schematically in FIG. 1. The annular mass 4 is not connected directly to the rotor shaft 2 but is connected to it indirectly by means of the spring assemblies 8a, 8b and the viscoelastic material layer 6.

(8) Each spring assembly includes a plurality of discrete springs 8c arranged around the radially inner circumference of the annular mass 4 to provide the required axial, torsional and radial stiffnesses. The springs 8c are preferably of a fail-safe design so that the annular mass 4 remains secured to the rotor shaft 2 under all conditions. The spring assemblies 8a, 8b give good stability against axial forces but have the correct stiffness in the radial direction to enable the TVA to match the first critical speed of the rotor shaft 2.

(9) The viscoelastic material layer 6 has a relatively high damping characteristic c.sub.a to provide resonance control at the two separate critical speeds. The viscoelastic material can be a polymer or a plastics material. The viscoelastic material layer 6 can be shaped and positioned to provide radial and axial stability in addition to its damping effect. The mass M.sub.a of the annular mass 4 and the spring rates k.sub.a of the springs 8 are chosen according to the mass M of the rotor shaft 2 (and optionally also the mass of the supporting bearings and structures which are not shown) to match the first critical speed of the rotor shaft.

(10) The annular mass 4 forms part of a rotor air-cooling fan and a plurality of fan blades 12 are integrally mounted to it. The springs 8c and the viscoelastic material layer 6 are fixedly secured to an annular flange part 10 of the rotor shaft and to a radially inner surface of an annular root part 14 of the annular mass 4 to which the fan blades 12 are integrally mounted.

(11) In a working example the rotor shaft shown in FIGS. 2 and 3 can be used as part of a high-speed motor with an operating speed range of 0 to 6600 rpm. A conventional rotor shaft for the same motor would have a first critical speed of about 3100 rpm. The addition of TVAs each with a mass (M.sub.a) of 57 kg at two axially spaced rotor air-cooling fan positions provides a reduction in structural resonance vibrations of 99.97% at a rotational speed of about 3100 rpm. Two new critical speeds are created at about 2600 rpm and about 3700 rpm but the structural resonances at those critical speeds have a magnitude lower than that for the conventional rotor shaft. The addition of two axially spaced TVAs means that the rotor shaft can meet American Petroluem Institute (API) vibration standards without the need for a more complicated rotor construction.

(12) The vibration magnitude versus operating speed for the conventional rotor shaft (i.e. the rotor shaft without TVAs) is represented in FIG. 4 by the dashed line. It shows a maximum vibration magnitude at the first critical speed of about 3100 rpm. The vibration magnitude versus operating speed for the rotor shaft that incorporates the TVA is represented in FIG. 4 by the solid line. It shows maximum vibration magnitudes at the critical speeds of about 2600 rpm and about 3700 rpm.

(13) The benefits of integrating a TVA within a rotating electrical machine to provide vibration damping and control the critical speed(s) of the rotor provides the following technical benefits: Vibration levels are reduced over all operating speed ranges Fatigue stresses are reduced There are no limits to operating speed Stiff and massive rotors are not required leading to reductions in weight and cost Cheaper materials can be used The TVA can be retrofit to existing rotor designs The TVA does not require a control system, lubrication or renewable components