GB2153484A

GB2153484A - Resonance damper for a rotating body

Info

Publication number: GB2153484A
Application number: GB08402519A
Authority: GB
Inventors: Douglas James Rogers
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1984-01-31
Filing date: 1984-01-31
Publication date: 1985-08-21
Also published as: GB8402519D0

Abstract

A resonance damper for a rotating body 6, 7 (e.g. the overhung rotor of an electric motor) comprises an auxiliary rotor 21 attached to the rotating body by a number of resilient members 22 (e.g. spring strip). A number of damping members 23 (e.g. blocks of rubber) are secured between the rotating body and the auxiliary rotor 21 to absorb vibrations transferred to the auxiliary rotor 21 by the resilient members 22. The axial and radial moments of inertia of the auxiliary rotor 21, and the compliances of the springs and dampers are so chosen that the two dynamic resonant frequencies of the rotating body are damped. <IMAGE>

Description

SPECIFICATION Resonance damper for a rotating body This invention relates to a resonance damper for a rotating body and in particlar for a high-speed motor having an external overhung rotor.

The majority of rotating bodies have a rotor supported on a shaft supported by two bearings for rotation inside a stator. Most, if not all, of the mass of the rotor is usually located between the bearings. This ensures that, if any vibration of the rotor should occur it is adequately restrained by the bearings.

Some types of high-speed motor have an external rotor rotating outside the stator, and in some situations it is necessary to support this rotor on one end of a shaft passing through the centre of the stator. The rotor is such a situation is far more sensitive to vibrations and external forces which, if unrestrained, could cause the stator and rotor to touch or at least cause the alignment of the motor to be impaired. Motors of this types posses one simple resonant frequency when stationary, but when rotation occurs two different resonant frequencies exist, one being higher than the static resonant frequency and the other being lower. The difference between these two resonant frequencies is proportional to the ratio of the axial inertia of the rotor to its radial inertia.

It is an object of the invention to provide a resonance damper for a rotating body which damps the above-mentioned resonant frequencies.

According to the present invention there is provided a resonance damper for a rotating body which includes an auxiliary rotor attached to the rotating body for rotation therewith by a number of resilient members and having a first moment of inertia measured along the axis of rotation and a second moment of inertia measured radially of said axis, and a number of damping members secured between the rotating body and the auxiliary rotor, the ratio between the first and second moments of inertia of the auxiliary rotor and the compliences of the resilient members and of the damping members being such that energy stored in the rotating body due to the resonant frequencies thereof is transferred to the auxiliary rotor by the resilient members and absorbed by the damping members.

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a sectional diagram showing the construction of a known type of electric motor having an external overhung rotor; Figure 2 is a similar view of a motor incorporating the invention; and Figure 3 is an end view of the motor of Figure 2, Referring now to Figure 1, the known motor comprises a stator 1 secured to a base 2. Inside the stator 1 are two bearings 3 and 4 supporting a central shaft 5. On this shaft is mounted the rotor comprising a disc portion 6 and a flange portion 7, the latter substantially covering the stator 1. The method of construction will clearly allow some movement of the rotor about an axis passing approximately through the centre of the outer bearing 4, due to resilience of the shaft 5.The rotor has an axial inertia, measured about the axis of rotation, and a radial inertia measured perpendicular to the axis, and the ratio between these two determines the difference between the two resonant frequencies referred to above. The forces resulting from these resonant frequencies may cause sufficient movement of the rotor to allow it to touch the stator when rotating or to cause excessive misalignment, clearly an undesirable situation.

Figures 2 and 3 show how the resonance damper of the invention may be added to the motor of Figure 1 so as to reduce the susceptibility to forced resonances to an acceptable level. The existing motor is modified by the addition of an auxiliary rotor in the form of a tube 21 attached to the flange 7 of the rotor by means of a number of resilient members 22, three being shown in Figure 3.

In addition a number of dampers 23 are provided, these may be blocks of resilient material secured between the tube 21 and the flange 7 of the rotor.

Again, three are shown in Figure 3 equally spaced between the resilient members 22. The resilient members may conveniently be some form of diaphragm spring or spring strip, whilst the damping members may be blocks of resilient energy-absorbing material such as rubber.

The auxiliary rotor 21 should have substantially the same static resonant frequency as the rotor of the motor, and this is obtained by suitable adjustment of the compliance of the resilient members 22.

The compliance of the resilient members may be varied by changing their shape or dimensions, whilst the damping effect of the damping members depends upon their dimensions and the material used.

In operation, since the rotor and the auxiliary rotor 21 have substantially the same resonant frequency, when resonance occurs the auxiliary rotor draws energy from the rotor and dissipates it in the dampers 23. It is, of course, well known to provide a damper which will effectively damp a single resonant frequency in a vibrating body. However, the invention described in this specification makes it possible to damp out simultaneously the two resonant frequencies which exist in any rotating body.

The auxiliary rotor 21 may take a different form, though the tubular shape shown in the drawings is one of the simplest to produce. Alternatively a pair of spaced parallel rings may be used with much the same effect.

The inertias of the auxiliary rotor and of the main rotor need not necessarily be in the 5:1 ratio referred to above. However, if the ratio is much above 10:1 then tuning the resonant frequency of the auxiliary rotor becomes difficult. If the ratio is reduced as low as 1:1 then the need to use a tuned damper is unnecessary and the effect could be achieved by simply placing a damping material between the rotor and auxiliary rotor. In situations where the rotor of the motor might be expected to spin at speeds in excess of 10,000rpm, it is necessary to maintain the ratio of axial to radial inertias of the complete rotating assembly to a value greater than 1.5 to 1, and the ratio of axial to radial inertias of the auxiliary rotor to within 0.1 of this.

In addition, the resonant frequency of the auxiliary rotor at zero spin speed should be between 70% and 110% of that for the complete rotor assembly.

The above description has described a motor in which the auxiliary rotor is attached to the end of the rotor nearest to the disc portion 6. However, with suitable adjustment of its mass and of the compliance of the resilient members 22, the auxiliary rotor may be attached to any desired region of the rotor.

A resonance damper of similar form may be used with other types of motor than those with external rotors. Conventional motors with the rotor inside the stator may have substantial masses located outside the bearings and may therefore require a similar sort of resonance damper. Similarly, the same form of damper may be used with rotating bodies other than electric motors.

Claims

1. A resonance damper for a rotating body, which includes an auxiliary rotor attached to the rotating body for rotation therewith by a number of resilient members and having a first moment of inertia measured along the axis of rotation and a second moment of inertia measured radially of said axis, and a number of damping members secured between the first and second moments of inertia of the auxiliary rotor and the compliences of the resilient members and of the damping members being such that energy stored in the rotating body due to the resonant frequencies thereof is transferred to the auxiliary rotor by the resilient members and absorbed by the damping members.

2. A damper as claimed in Claim 1 in which the auxiliary rotor is in the form of a tube supported coaxially with the rotating body.

3. A damper as claimed in either of Claims 1 or 2 in which the auxiliary rotor is attached to the rotating body by at least three resilient members and at least three damping members.

4. A damper as claimed in any one of Claims 1 to 3 in which the rotating body is the rotor of an electric motor

5. A damper as claimed in any one of the preceding claims in which the motor has an overhung rotor supported at one end by bearings, the auxiliary rotor being attached to the rotor of the motor at the end of the rotor nearest to said bearings.

6. A damper as claimed in any one of the preceding claims in which the ratio of the axial moment of inertia of the rotating body to the radial moment of inertia of the rotating body is greater than 1.5 to 1.

7. A damper as claimed in Claim 6 in which the raio of the axial moment of inertia of the auxiliary rotor to the radial moment of inertia of the auciliary rotor is with 0.1 of the ratio of the inertias of the rotating body.

8. A resonance damper for a rotary electric motor substantially as herein described with reference to Figures 2 and 3 of the accompanying drawings.