High rotational speed vacuum pump

Abstract

A high rotational speed vacuum pump (10) includes a compressor (12), a drive motor (14) for driving said compressor (12), and a motor control unit (16) for operating said drive motor (14) at a constant nominal rotational frequency. Depending on the basic conditions, undesirably strong oscillations may occur at the compressor (12) during compressor operation at the nominal rotational frequency. These undesirably strong oscillations cause the bearings to be excessively stressed. To reduce the oscillation stress of the compressor and/or the vacuum pump, an activatable rotational frequency correction device (18) is associated with the motor control unit (16). The correction device constantly corrects the nominal frequency by a rotational frequency correction value of max. 10% of the nominal rotational frequency when the rotational frequency correction device (18) is activated. This allows the rotational frequency of the drive motor (14) to be changed in a simple manner when strong natural oscillations of the compressor (12) and/or the vacuum pump (10) occur such that resonance effects and the like can be avoided.

Description

BACKGROUND

The invention relates to high rotational speed vacuum pumps, for example turbomolecular pumps.

In the present case, the term high rotational speed vacuum pump relates to vacuum pumps with nominal speeds as from 10,000 revolutions per minute. High rotational speed vacuum pumps, for example turbolmolecular pumps, are normally operated at a constant nominal rotational speed or nominal rotational frequency. The nominal rotational frequency is determined such that the resonant frequency or the resonant frequencies of the vacuum pump are not excited. While the nominal rotational frequency at which the motor control unit operates the drive motor virtually remains unchanged and constant, the resonant frequencies of the vacuum pump may change depending on numerous basic conditions. The resonant frequency of the vacuum pump is in particular influenced by changes of the vacuum pump mass, suspension and damping. In plants comprising a plurality of high rotational speed vacuum pumps which are all mounted on a single frame and are thus mechanically interconnected, the vacuum pump oscillations having the same frequency may combine into oscillations with a large amplitude, and/or the resonant frequency of one or a plurality of vacuum pumps may change such that the respective vacuum pump is excited by a vacuum pump operating at the nominal rotational frequency. Studies have shown that wear of vacuum pump bearings used in this type of plant is to a large extent caused by the effects described above.

It is an object of the invention to provide a possibility of reducing the wear caused by natural oscillations of high rotational speed vacuum pumps.

SUMMARY

According to one aspect, an activatable rotational frequency correction device is associated with the motor control unit, said correction device constantly correcting the nominal rotational frequency by a rotational frequency correction value of max. 10% of the nominal rotational frequency when the rotational frequency correction device is activated. Provision of the rotational frequency correction device allows the per se constant rotational frequency to be slightly changed. In this manner, the rotational frequency of the drive motor can be slightly adjusted preemptively or according to requirement, which normally is an adequate measure for eliminating resonance behavior of the vacuum pump at the nominal rotational frequency, or prevent the generated oscillations from summing up and building up when a plurality of vacuum pumps are mounted on a common frame. For this purpose, a rotational frequency correction value of max. 10% of the nominal rotational frequency is completely sufficient. Normally, even a rotational frequency correction value of 0.5-2% of the nominal rotational frequency suffices for preventing the occurrence of resonance and/or a build-up of oscillations. In this manner, in particular wear of the bearings is reduced.

Preferably, a rotational frequency correction activator is provided which may be connected to the rotational frequency correction device for activating the latter. The rotational frequency correction activator may be an external electronic storage element, for example, which is adapted to be electrically connected to the rotational speed correction device. Once the rotational frequency correction activator is connected to the rotational speed correction device, the nominal rotational frequency is reduced by a value stored in the rotational speed correction device or in the rotational frequency correction activator.

The external electronic storage element may be configured as a dongle, for example, which is connectable to an electric interface of the motor control unit and/or the rotational frequency correction device. Connecting the dongle to the interface causes the rotational speed to be corrected in a simple manner. Storage elements and/or dongles with different rotational speed correction values may be provided such that the value and the sign of adjustment of the nominal rotational frequency can be selected.

According to a preferred aspect, an oscillation sensor connected to the rotational frequency correction device is associated with the compressor, wherein the rotational frequency correction device is activated when the oscillation sensor detects that a predetermined oscillation amplitude of the compressor has been exceeded. Provision of an oscillation sensor and evaluation of the oscillation signal provided by said oscillation sensor allow the vacuum pump to be permanently monitored. Thus the vacuum pump is virtually maintenance-free with regard to prevention of strong natural oscillations. Further, it is thus ensured that a rotational frequency correction is performed when the resonant frequency of the vacuum pump changes in a critical manner only after startup of the vacuum pump.

Preferably, at least two different rotational frequency correction values are stored in the rotational frequency correction device. In this manner, a second rotational frequency correction can be performed, in particular in plants comprising a plurality of vacuum pumps mounted on a common frame, if the first rotational frequency correction turns out to be inadequate.

According to a preferred aspect, a control device is associated with the rotational frequency correction device, said control device receiving the controlled variable from the oscillation sensor. The rotational frequency is the manipulated variable. The control device ensures, in particular in complex systems comprising a plurality of vacuum pumps, that a rotational frequency correction is performed even under temporarily changing basic conditions, said rotational frequency correction ensuring the smallest possible oscillation of the vacuum pump and/or the compressor.

Generally, the control device may be provided both in the vacuum pump itself and, in particular in plants comprising a plurality of vacuum pumps, centrally in a control unit common to all vacuum pumps.

A method according to the invention for operating a high rotational speed vacuum pump having a compressor, a drive motor for driving the compressor, and a motor control unit for operating the drive motor at a constant nominal rotational frequency, comprises the following method steps:

• • determining the natural oscillations of the compressor,
  • when the natural oscillations of the compressor exceed a predetermined limit value: changing the rotational frequency by a value less than 10% of the nominal rotational frequency of the drive motor,
  • operating the drive motor at the changed rotational frequency.

The method according to the invention allows the natural oscillations of the compressor to be automatically monitored and the frequency of the oscillations generated by the drive motor to be corrected, if necessary.

BRIEF DESCRIPTION OF THE DRAWINGS

Two embodiments of the invention will now be described in greater detail with reference to the drawings which are for illustratrative purposes only and are not to be taken as limiting the invention.

FIG. 1 shows a first embodiment of a vacuum pump according to the invention, wherein the rotational frequency correction is activated by a dongle, and

FIG. 2 shows a second embodiment of a compressor according to the invention, wherein the oscillation behavior is monitored by an oscillation sensor.

DETAILED DESCRIPTION

FIGS. 1 and 2 each show a high rotational speed vacuum pump 10;30 configured as a turbomolecular pump. The high rotational speed vacuum pump may also be configured as a radial compressor. The vacuum pump 10;30 has a nominal rotational speed of 30,000 revolutions/min., for example, which corresponds to a rotational frequency of 500 Hz. The vacuum pump 10;30 may be used as a stand-alone unit, but may also form part of a plant of 100 to 120 vacuum pumps of the same type mounted on a common frame. Such plants are employed for glass coating purposes, for example.

The vacuum pump 10;30 comprises a turbomolecular compressor 12 driven by a drive motor 14. The drive motor 14 may be a direct current or an alternating current motor.

The drive motor 14 is driven by a motor control unit 16 which supplies current to the motor windings. The motor control unit 16 operates the drive motor 14 at a constant rotational frequency.

The vacuum pump 10 shown in FIG. 1 further comprises a rotational frequency correction device 18 adapted to be activated by a rotational frequency correction activator 20. The activated rotational frequency correction device 18 causes the motor control unit 16 to adjust the nominal rotational frequency, e.g. to adjust the rotational frequency by a rotational frequency correction value of 10 Hz.

The rotational frequency correction activator 20 is an electronic storage element configured as a so-called dongle. The dongle is connected to a parallel or serial interface 22, when necessary, said interface 22 being connected to the rotational frequency correction device 18. In the rotational frequency correction activator 20 the rotational frequency correction value is electronically stored.

When the rotational frequency correction activator 20 is not connected to the interface 22, the rotational frequency correction device 18 is not activated such that the motor control unit 16 operates the drive motor 14 at the nominal rotational frequency. Once the rotational frequency correction activator 20 is connected to the interface 22 of the rotational frequency correction device 18, the rotational frequency is reduced by the stored value.

In the embodiment shown in FIG. 2 of a vacuum pump 30, the oscillation behavior of the vacuum pump 30 is controlled by controlling the rotational frequency of the drive motor 14. For this purpose, an oscillation sensor 36 is fixed to the compressor 12, said sensor 36 sensing the frequency and the amplitude of the natural oscillations of the compressor 12. The oscillation sensor may also be arranged at a housing portion of the vacuum pump 30.

The oscillation sensor 36 is connected to the rotational frequency correction device 34 which evaluates the oscillation information supplied by the oscillation sensor 36 and causes the rotational frequency to be corrected when the oscillation sensor 36 detects that a predetermined oscillation amplitude of the compressor 12 has been exceeded. If the oscillation sensor 36 detects too strong oscillations, the rotational frequency correction device 34 performs an upward or downward correction of the nominal rotational frequency by a rotational frequency correction value of 10 Hz to obtain a new motor rotational frequency. If the oscillation sensor 36 detects inadmissibly high oscillations when the drive motor 14 is operated at the new motor rotational frequency, the nominal rotational frequency is changed by a second rotational frequency correction value, and the drive motor 14 is operated at the second new motor rotational frequency.

In the rotational frequency correction device a control device may be provided which receives the controlled variable from the oscillation sensor 36. The motor rotational frequency may also be infinitely variable such that the motor rotational frequency always lies as close to the nominal rotational frequency as possible.

The invention has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be constructed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Claims

1. A high rotational speed vacuum pump comprising

a compressor,

a drive motor for driving said compressor,

a motor control unit for operating said drive motor at a constant nominal rotational frequency, and

an activatable rotational frequency correction device associated with said motor control unit, said correction device constantly correcting the nominal rotational frequency by a rotational frequency correction value of max. 10% of the nominal rotational frequency when said nominal rotational frequency correction device is activated.

2. The high rotational speed vacuum pump according to claim 1, further including: a rotational frequency correction activator which is connectable to the rotational frequency correction device for activating the latter.

3. The high rotational speed vacuum pump according to claim 2, wherein the rotational frequency correction activator includes an external electronic storage element which is electrically connectable to the rotational frequency correction device.

4. The high rotational speed vacuum pump according to claim 3, wherein the rotational frequency correction activator includes a dongle which is connectable to an electric interface of the rotational frequency correction device.

5. The high rotational speed vacuum pump according to claim 1, further including: an oscillation sensor connected with the rotational frequency correction device and associated with the compressor, wherein said rotational frequency correction device is activated when said oscillation sensor detects that a predetermined oscillation amplitude has been exceeded.

6. The high rotational speed vacuum pump according to claim 5, wherein the rotational frequency correction device stores at least two different rotational frequency correction values.

7. The high rotational speed vacuum pump according to claim 5, further including: a control device associated with the rotational frequency correction device, said control device controlling the motor rotational frequency and receiving the controlled variable from the vibration sensor.

8. A method for operating a high rotational speed vacuum pump having a compressor, a drive motor for driving said compressor, and a motor control unit for operating the drive motor at a constant nominal rotational frequency, the method comprising the following method steps:

determining natural oscillations of the compressor by means of an oscillation sensor,

when the natural oscillations of the compressor exceed a predetermined limit value, changing the rotational frequency of the drive motor by a value less than 10% of the nominal rotational frequency of the drive motor and,

operating the drive motor at the changed rotational frequency.

9. A method for operating the high speed rotational vacuum pump of claim 1 comprising:

determining natural oscillations of the compressor by means of an oscillation sensor,

when the natural oscillations of the compressor exceed a predetermined limit value, changing the rotational frequency of the drive motor by a value less than 10% of the nominal rotational frequency of the drive motor and,

operating the drive motor at the changed rotational frequency.

10. A high speed rotational vacuum pump comprising:

a turbomolecular device;

a drive motor which drives the turbomolecular device;

a motor control unit which controls the drive motor to operate at a constant rotational frequency;

a rotational frequency correction device which causes the motor control lunit to decrement or increment the constant rotational frequency to inhibit the constant rotational frequency from corresponding to a resonant frequency of the turbomolecular device.

11. The high speed rotational vacuum pump according to claim 10, further including:

an oscillation sensor which senses oscillations indicative of operation approaching a resonant frequency, the rotational frequency correction device causing the motor control unit to decrement or increment the constant rotational frequency in response to sensed oscillations indicative of operation approaching a resonant frequency.

12. The high speed rotational vacuum pump according to claim 10, further including:

an electronic storage element which selectively instructs the rotational frequency correction device to cause the motor control unit to increment or decrement the constant rotational frequency.