WO2005088133A1

WO2005088133A1 - Vacuum pump

Info

Publication number: WO2005088133A1
Application number: PCT/GB2005/000871
Authority: WO
Inventors: Ingo Stephen Graham; Mark David Gardiner; Neil Turner
Original assignee: The Boc Group Plc
Priority date: 2004-03-12
Filing date: 2005-03-08
Publication date: 2005-09-22
Also published as: GB0405527D0

Abstract

A vacuum pump (1) comprises first and second Northey mechanism rotor assemblies (4, 5). Each rotor assembly is mounted on a drive shaft (2, 3), each drive shaft being independently driven by a respective synchronous motor (8, 9). The shafts contra-rotate in use such that the rotor assemblies intermesh. A controller (12) is provided for controlling the frequency of the voltage supplied to each motor.

Description

VACUUM PUMP

The present invention relates to the field of vacuum pumps, more particularly to vacuum pumps with a twin shaft configuration.

Conventional twin shafted pumps have a single motor. One of the shafts is connected to the motor, the shafts being coupled together via timing gears so that the shafts rotate at compatible speeds but in opposite directions. Such timing gears require lubrication to maintain their smooth and effective operation.

The use of lubricant means that it is possible for the lubricant to be dispersed into the working section ofthe pump. This would not only contaminate the process gasses within the pump, potentially leading to an increased likelihood of deposits being formed within the pump, but could lead to lubricant migrating back upstream of the vacuum pump into the process chamber. Contamination of the process chamber can cause loss of the payload of the system. A further consequence of this loss of lubricant is the necessity to service the pump to replenish the level of lubricant in the gears at regular service intervals such that the gears do not deteriorate due to lack of lubrication. Mitigation of these problems through improved sealing systems is not only expensive but leads to significant increase in the complexity and design time of the pump.

It is an object of the present invention to seek to solve this and other problems. According to the present invention there is provided, a vacuum pump comprising first and second Northey mechanism rotor assemblies, each rotor assembly being mounted on a respective drive shaft, each drive shaft being independently driven by a respective synchronous motor so that, during use of the pump, the shafts contra-rotate and the rotor assemblies intermesh, and a controller for controlling the frequency of the voltage supplied to each motor.

Due to the lack of contacting gears, vibration of the pump can be reduced resulting in a reduction in noise associated with use of the pump. Power requirements can be reduced as some of the friction-related losses are removed with removal of the conventional contacting gears. This reduced power consumption is especially apparent when the pump is running at low inlet pressure. It is also beneficial at low exhaust pressure as experienced in boosters and secondary pumps. With the elimination of contacting gears, and the use of "sealed for life" grease-packed bearings, the aforementioned problems associated with leakage of liquid lubricant from bearings, gears and their associated sealing systems, are eliminated.

The use of a Northey mechanism for the pump rotor assemblies enables angular misalignment, occurring during use of the pump, to be accommodated. This is because the Northey rotor components only intermesh for a small portion (about 20%) of a rotational cycle. During this intermeshing portion, no pumping activity takes place and therefore there is more flexibility in the design of the geometry of the intermeshing sections of the rotor components. This flexibility and the greater tolerance associated with it allows for the accommodation of angular misalignment.

The pump may comprise means for monitoring a condition of the pump and outputting a signal indicative of the monitored condition to the controller, the controller being arranged to receive the signal and control the frequency of the voltage and/or current supplied to each of the motors in dependence on the received signal. The monitored pump condition may be a condition of one of the motors, for example of the rotor of the motor. In particular it may be the rotational position of the rotor of one of the motors. The pump may comprise a motor rotor position sensor, for example, a Hall effect sensor, arranged to monitor the rotational position of the rotor of said one of the motors. Use of synchronous motors is particularly advantageous in that such feedback may be required from only one of the motors, providing apparatus that is significantly reduced in complexity when compared to a control system that actively manages the speed and phase relationship between the two motors.

The synchronous motors are preferably permanent magnet motors, or switched reluctance motors.

The pump preferably comprises means for preventing clashing of the rotor assemblies in the event of a power failure. For example, a gear arrangement comprising a back-up gear may be used to prevent the assemblies clashing . The back-up gear may be made of a material that does not require lubrication such as a polymer, alternatively the material used for the back-up gears may be coated in such a way as to remove the need for lubrication.

The invention is described below in greater detail by way of example only with reference to the accompanying drawings, in which:

Figure 1 is a schematic representation of a vacuum pump according to one embodiment of the present invention;

Figure 2 is a schematic representation of the drive and motors of the pump of Figure 1; and

Figure 3 is a schematic representation of an example of a Northey pumping mechanism which may be suitable for use with the pump of Figure 1. Figure 1 illustrates a pump (1), having two shafts (2, 3) each with a Northey (or "Claw") mechanism rotor assembly (4, 5) formed thereon. The rotor assemblies (4, 5) are each formed from a series of individual Northey rotor components. A cooperating pair of Northey rotor components forming a single pumping stage is illustrated in Figure 3.

A primary motor (8) is mounted on one drive shaft (2) and a secondary motor (9) is mounted on the other drive shaft (3). These synchronous motors (8, 9), typically permanent magnet motors, are each connected to respective power amplifiers (10, 11) as represented in Figure 2, these power amplifiers (10, 11) being further connected to a drive controller (12).

In its simplest form, the drive controller (12) is an open loop phase controller which merely ensures that voltage of the same frequency is provided to each of the motors. In a more sophisticated embodiment, the drive controller (12) is a closed loop phase controller that determines the frequency of the voltage to be provided to both of the motors based on monitored parameters within only one of the motors - typically the primary motor. This is discussed in more detail below.

In a conventional twin shafted pump both shafts would typically be driven by a single motor. This motor would be directly connected to the first shaft and timing gears would be introduced to transfer this rotation motion to the second shaft. In contrast, according to the invention, each shaft (2, 3) is independently rotationally driven by its respective motor (8, 9) and so timing gears and their associated lubrication are not required. In use, the shafts (2, 3) are driven in opposite directions by their respective motors (8, 9) to cause the Northey rotor components to intermesh within a stator (14) of the pump. The interaction of these components (4, 5) with the stator (14) draws process gas into the pump (1) from a process chamber (not shown) located upstream of the pump, through an inlet (6). The process gas is then displaced as it is trapped between each subsequent pair of cooperating Northey rotor components (4, 5) and the stator. The gas is finally forced to exit the pump through exhaust (7). Thus the process chamber is evacuated.

When the loading experienced by the two shafts is similar, ideally where rotor assemblies (4, 5) and motors (8, 9) are loaded equally, a pump with independently driven shafts operates optimally. The Northey pumping mechanism produces no imbalance in pressure load between the two shafts during use of the pump and is therefore ideally suited to being driven in this way. Such a pump is, therefore, especially suitable for clean applications, where low levels of accumulation of deposits that might cause imbalanced shaft loading, are found. Pumping down a load lock chamber is one such application and is also an application which is particularly demanding on the oil seal systems (in terms of the pressure variations experienced) where the benefits of this invention would be most apparent.

In less clean applications, it is possible that the two shafts (2, 3) could experience unbalanced loading for example if one of the pump components in the swept volume were to be subject to formation of residue from the process gas passing through the pump (1) such that the friction between the rotor component (4, 5) and the stator (14) of the pump (1) were to be, perhaps temporarily, different in magnitude.

The synchronous motors (8, 9) used by the present embodiment are permanent magnet motors (pmm) as known in the art and hence not described in detail here. It would not be appropriate to use the, generally, larger induction motors since the operating speed of such induction motors is governed by the loading to which the motor is exposed. Consequently, in the event of unbalanced loading of the rotor assemblies, there would be a difference between the shaft speeds, which would eventually lead to clashing of the assemblies unless the speed of the motors and/or the loading of the rotors was constantly monitored and corrected by a controller. Significant expense and complexity is generally associated with such a controller for induction motors. In contrast, the steady state operating speed of such permanent magnet motors is dependent only on the frequency of the supply voltage rather than the load. This is used to advantage by the present invention, as discussed below.

During installation of the motors (8, 9) into the pump (1), the field of the secondary motor (9) is rotationally aligned with that of the primary motor (8) by setting the angular position of the motor stator. This alignment can be expected to be maintained during normal operation of the pump (1) where the motors (8, 9) each receive an identical supply frequency so that the motors (8, 9) run at the same rotational speed and therefore maintain a substantially constant phase relationship between each another.

If, during use of the pump, the two shafts experienced unbalanced loading, this results in an angular misalignment of the shafts (2, 3) of the pump; the shafts continue to rotate at the same angular velocity. Consequently, the rotors of the motors can be subject to a phase (but not speed) difference, determined by the load differential experienced by the shafts. Thus, during use, the secondary motor rotor can either lead or lag the position ofthe primary motor rotor depending on the relative loading on the two shafts.

The Northey rotor/stator port configuration and the magnetic stiffness of the motors can be adjusted to accommodate the variation in the angular position of the rotors arising from the loading difference and thus ensure that the Northey rotor components do not clash.

Where it is necessary to optimise torque delivery from the motors (8,9), a position sensor (13), such as a Hall sensor, is provided within the primary motor (8) to monitor the angular position of a rotor of the primary motor (8) during use of the pump. Alternative means for monitoring the position of the motor rotor, for example, using a transducer or by measuring motor parameters, may be used. The position of the primary motor rotor is fed back to the drive controller (12), as shown in Figure 2. This feed-back enables the condition of the pump to be monitored so that the drive controller (12) can determine how the frequency of the voltage supply to both motors should be modified to enable maximum torque delivery to the drive shafts (2, 3) of the pump (1) to be maintained throughout normal operation.

Of course, in circumstances where it is not of such importance to optimise the torque delivery to the drive shafts, a pump can be provided that does not incorporate any motor rotor position sensing facility.

As already described, a consequence of an imbalance in loading is that some misalignment of the rotor assemblies may be experienced. This needs to be accommodated in the mechanism by increasing profile clearance between meshing components. Some pumping mechanisms, such as Roots or screw mechanisms, are particularly intolerant of large rotor meshing clearances which can lead to significant levels of leakage back through the pump and consequently a reduction in pumping performance. The Northey rotor component only intermeshes with its cooperating rotor component for a small portion of the cycle as indicated in Figure 3. Crucially, during this period of intermeshing the pumping stage is not performing any pumping action.

Consequently, there is some freedom to adapt the geometry of the claw-root section of the rotor component such that the meshing clearances are increased (to further avoid clashing) whilst not having a detrimental effect on the pumping performance. Thus, the Northey mechanism can readily be adapted to accommodate an angular misalignment of, say, > + 5°.

Internal back-leakage issues become particularly significant where a smaller pump is to be provided. Where a substantial pump is to be provided, the larger motors required to drive the shafts may have sufficient stiffness to ensure that any angular misalignment will be minimal. Additionally, leakage losses due to any requirement for meshing clearance increase may be compensated for by the overall capacity of the pump to a certain extent. However, where a smaller pump is to be provided, the associated motors become less stiff such that the potential angular misalignment is more significant. Consequently, the meshing clearances, which are in any case, proportionally greater when compared with the pump displacement, need to be further increased to accommodate the greater level of angular misalignment and the leakage becomes so great that it is not practical to implement a typical screw or Roots mechanism. By incorporation of a Northey mechanism, as required by the present invention, the increased angular misalignment can be more readily accommodated without degrading pumping performance. An improved performance, smaller capacity pump can be provided to serve applications such as backing scientific turbomolecular pumps, evacuation of loadlock and transfer chambers, and some lighter semiconductor processes.

If the power to the pump were to be interrupted, a larger angular misalignment could potentially be experienced temporarily. Such a large-scale misalignment could cause the cooperating rotor assemblies to clash and consequently cause damage. To prevent this from happening, in the preferred embodiment a back-up gear (15) is provided, as shown in Figure 1. This back-up gear (15) will typically have very few teeth and will only become active (i.e. contact a corresponding/cooperating gear on the other shaft) if the angular misalignment exceeds a particular value to prevent clashing of the rotor assemblies. The back-up gears will therefore not require liquid oil lubrication and could be formed of a polymeric material. Such a gear has the advantages of being cheap, quiet, will not fuse with its cooperating gear on contact and will not create loose particulate matter within the pump. This back-up gear (15) may also become active to prevent clashing if the pump were to seize or due to human intervention, for example, during maintenance. Open loop phase control of the secondary motor, as provided by the present invention, simplifies a twin shaft pumping system with independently driven shafts. Elimination of continuous contact, timing gears enables a pump to be provided without the risks of oil mist forming. Without the presence of oil, costs ofthe pump are reduced in terms ofthe sealing requirements, lubrication circulation systems (and therefore reduced parts count and complexity), maintenance associated with preserving lubrication levels and the costs of the lubricant itself. A lower power machine is, therefore provided that achieves significant gains at low pressure conditions.

Claims

1. A vacuum pump comprising first and second Northey mechanism rotor assemblies, each rotor assembly being mounted on a respective drive shaft, each drive shaft being independently driven by a respective synchronous motor so that, during use of the pump, the shafts contra- rotate and the rotor assemblies intermesh, and a controller for controlling the frequency of the voltage supplied to each motor.

2. A vacuum pump according to claim 1 , comprising means for monitoring a condition of the pump and outputting a signal indicative of the monitored condition to the controller, the controller being arranged to receive the signal and control the frequency of the voltage supplied to each of the motors in dependence on the received signal.

3. A vacuum pump according to claim 2, wherein the monitoring means is arranged to monitor a condition of one of the motors.

4. A vacuum pump according to claim 3, wherein the monitoring means is arranged to monitor a condition of a rotor of said motor.

5. A vacuum pump according to claim 4, wherein the monitoring means is arranged to monitor the rotational position of a rotor of said motor.

6. A vacuum pump according to Claim 5, wherein the monitoring means comprises a motor rotor position sensor arranged to monitor the rotational position of the rotor of said motor.

7. A vacuum pump according to Claim 6, wherein the rotor position sensor is a Hall effect sensor.

8. A vacuum pump according to any preceding claim, wherein the controller is arranged to optimise the torque delivered to the drive shafts.

9. A vacuum pump according to any preceding claim, wherein the motors are permanent magnet motors.

10. A vacuum pump according to any of Claims 1 to 8, wherein the motors are switched reluctance motors.

11. A vacuum pump according to any preceding claim, comprising means for preventing clashing of the rotor assemblies.