WO2024132937A1

WO2024132937A1 - Method for operating a vacuum pump

Info

Publication number: WO2024132937A1
Application number: PCT/EP2023/086077
Authority: WO
Inventors: Dirk Schiller
Original assignee: Leybold Gmbh
Priority date: 2022-12-23
Filing date: 2023-12-15
Publication date: 2024-06-27
Also published as: GB2625786A; TW202432956A; GB202219655D0

Abstract

Method for operating a vacuum pump (14) including: Operating the vacuum pump in a first operating state; Determining one or more pump parameter (17) of the vacuum pump; Determining a forecast for a vacuum pump temperature from the one or more pump parameters for the condition of changing the operation of the vacuum pump from the first operating state to a second operating state; Reduce thermal load of the vacuum pump if the forecast exceeds a predetermined threshold.

Description

METHOD FOR OPERATING A VACUUM PUMP

The present invention relates to a method for operating a vacuum pump in particular to provide a dynamic load limitation for a vacuum pump. In addition, the present invention relates to a controller for a vacuum pump implementing such method, a vacuum pump comprising such a controller and a software storage device.

Common vacuum pumps comprise a housing having an inlet and an outlet. A rotor assembly is rotatably supported within the housing. The rotor assembly comprises a rotor shaft driven by an electromotor and at least one pump element interacting with a stator of the vacuum pump or, in the case of a two shaft vacuum pump, interacting with a pump element of another rotor assembly, in order to convey a gaseous medium from the inlet to the outlet of the vacuum pump.

Vacuum pumps in general require a load limitation to avoid thermal overload which would reduce the lifetime of the machine or even cause immediate damage. The load limitation can be realized in different ways. The most common ones are:

- Switching the pump off above a defined operation pressure level; Differential pressure limitation via bypass-valve;

- Shaft torque limitation by for example a hydrokinetic drive;

- Motor current limitation by reduced rotational speed.

In most cases these limitations are based on fixed thresholds that are defined for each product. A threshold must cover worst case thermal conditions which leads to rather conservative limitations that in many cases do not use the full capability of a machine. This can be partly compensated by providing thresholds that are dependent on certain boundary conditions like for example the expected compression ratio that the vacuum pump needs to handle. Some of the thermal conditions, however, are not known in advance or not constant over time which makes it difficult to select an ideal threshold.

Recent developments have shown that more of the thermal potential of a machine can be used under transient conditions if the conditions are known in advance or are cyclic and can be analyzed by an algorithm.

These solutions are optimized for certain applications like load lock cycles with rather rigid boundary conditions. However, if there are no strict periodicity in the processes or if there is an abnormal interruption of the cycles, these solution are not able to prevent reliably thermal overload of the vacuum pump.

It is an object of the present invention to provide a more flexible method for operating a vacuum pump in order to reliably prevent thermal overload of the vacuum pump.

This problem is solved by a method for operating a vacuum pump according to claim 1, a controller according to claim 8, a vacuum pump according to claim 11 and a software storage device according to claim 12.

The method according to the present invention for operating a vacuum pump includes the steps:

Operating the vacuum pump in a first operating state;

Determining one or more pump parameter of the vacuum pump;

Determining a forecast for a vacuum pump temperature from the one or more pump parameters for the condition of changing the operation of the vacuum pump from the first operating state to a second operating state;

Reduce thermal load of the vacuum pump if the forecast exceeds a predetermined threshold.

Thus, when operating the vacuum pump in a first operating state one or more pump parameters of the vacuum pump are determined. From these pump parameters a forecast is determined of the vacuum pump temperature under the condition that the operation of the vacuum pump changes from the first operating state to the second operating state. Thus, a prediction is made by the forecast regarding the thermal load of the vacuum pump if the operating state of the vacuum pump is changed. Therein, the vacuum pump may remain operating in the current first operating state and does not actually change its operating state. If the forecast exceeds a predetermined threshold the thermal load of the vacuum pump is reduced. Thus, according to the present invention, when operating the vacuum pump in a first operating state the consequences for the thermal load of the vacuum pump when the operating state would change from a first operating state to a second operating state is considered by the forecast. Thereby, a possible thermal overload of the vacuum pump when changing the operation of the vacuum pump from the first operating state to the second operating state can be avoided. Hence, it is not necessary to control the actual thermal load by a fixed threshold. Instead, a determined forecast is used in order to make sure that even upon change of the operating state of the vacuum pump the vacuum pump is operated within its thermal limits. Thus, a more flexible method for operating the vacuum pump is provided. Operation of the vacuum pump does not depend any more on fixed thermal limits or periodic cycles of the process.

Preferably, the vacuum pump is a roots pump, a claw pump, a screw pump, a molecular drag pump or the like.

Preferably, the forecast is an extrapolation of the one or more pump parameters of the vacuum pump. Thus, for determining the forecast the one or more pump parameters are monitored over a certain time and from this time development a forecast for the vacuum pump temperature can be determined. Therein the extrapolation might be a linear extrapolation, a polynomial extrapolation or a fitting of a predetermined functional relationship between the one or more pump parameters and the vacuum pump temperature. For example, a functional relationship between the vacuum pump temperature and the pump parameters may be proportional to e'^A/t wherein A specifies the one or more pump parameters and t the time. Therein, extrapolation or fitting can be performed by measurement of the one or more pump parameters in a reference vacuum pump by the manufacturer and determine the functional relationship with the reference vacuum pump temperature by measuring the vacuum pump temperature in the reference vacuum pump for example by sensors. The vacuum pump of the customer then may have no sensors for determining the vacuum pump temperature directly and only may use the extrapolation determined by the reference vacuum pump.

Preferably, the forecast is determined by a trained neural network for example implemented in the controller, wherein the neural network receives the one or more pump parameters as input and provides the vacuum pump temperature as output. Therein, the neural network can be trained on the basis of measurements in a reference vacuum pump of the manufacturer, wherein for training of the neural network the reference vacuum pump may comprise sensors or other measures to determine the one or more pump parameters and sensors to measure the vacuum pump temperature directly, wherein the output of the neural network to be trained is correlated to the actual measurements of the sensor for the vacuum pump temperature. In the vacuum pump delivered to the customer or used in real processes, no such sensors for measuring the vacuum pump temperature directly are present anymore. The vacuum pump only make use of the neural network trained by the reference vacuum pump.

Preferably, the pump parameters are one or more of a stator temperature, a rotational speed and a motor current. Therein the rotational speed of the electromotor and the motor current of the electromotor may be provided by a motor controller. The stator temperature is the temperature of the stator or the housing of the vacuum pump and may be measured for example by a temperature sensor placed at or in or at least in the vicinity of the stator. Preferably, the vacuum pump temperature is one or more of a stator temperature, a rotor temperature and a difference between the stator temperature and the rotor temperature.

Preferably, reducing the thermal load includes one or more of reducing the motor current, reducing the rotational speed, increasing the cooling for example by an external cooling device or a coolant flow, increasing the pressure difference or the pressure ratio at the vacuum pump, switching the vacuum pump off or changing the gas type pumped by the vacuum pump. Therein the pressure difference is the difference of the pressure between the inlet and the outlet. Increasing the pressure difference may be performed by opening a bypass valve.

Preferably, the first operating state may be a pump down phase or a continuous operation phase. Therein the pump down phase refers to a phase in which the vacuum pump reduces pressure at the inlet and creates a vacuum in the connected vessel or vacuum chamber. The phase of continuous operation relates to an operation in which the vacuum pump holds or keeps the vacuum in the vessel or vacuum pump chamber. Therein, the phase of continuous operation is usually characterized through a same or similar thermal load of the vacuum pump.

Preferably, the second operating state is operation under end pressure, switch- ing-off of the vacuum pump or continuous operation. Therein the end pressure relates to the ultimate pressure of the vacuum pump and may occur under closing the main valve at the inlet of the vacuum pump disconnecting the running vacuum pump from the vessel or vacuum chamber. Alternatively, the end pressure may be reached at the end of the pump down phase.

Preferably, the first operating state is the pump down phase and the second operating state is the continuous operation. When pumping down the vessel or vacuum chamber it is checked by the method according to the present invention in form of the determined forecast whether, when switching to the continuous operation, the forecast exceeds the predetermined threshold. Thus, during pump down, the vacuum pump can run with an increased rotational speed in order to accelerate the pumping down. Thus, during this boost phase or accelerated pump down, the thermal load is increased. However due to observing the forecast determined from the pump parameters, even if it is returned to continuous operation, there is no thermal overload of the vacuum pump since the thermal load is reduced if the forecast exceeds a predetermined threshold.

Preferably, the first operation state is a continuous operation and the second operating state is operating under end pressure or switching-off of the vacuum pump. Thus, under continuous operation during the first operating state a thermal load is evolving into a thermal equilibrium. If the pressure difference is reduced to zero, i. e. the gas load is reduced to zero, the temperature difference between the stator and the rotor may raise due to a different fast decrease of the rotor temperature and the stator temperature. Thus, by the different decrease of the rotor temperature and the stator temperature, critical temperature differences between the stator and the rotor may occur which exceed a predetermined threshold. Thus, upon continuous operation in the first operating state it is checked whether returning to the end pressure is still possible without exceeding the thermal load of the vacuum pump. If it is determined, that the temperature difference between the rotor and the stator would exceed a predetermined threshold upon changing the pressure difference to zero, the thermal load during the first operating state, i. e. the continuous operation, is reduced in order to avoid this situation.

Preferably, the thermal load is reduced upon detecting that the forecast will exceed the predetermined threshold. Alternatively, the thermal load is reduced at the timepoint the forecast predicts thermal overload if it would be switched to the second operating state. Preferably, the remaining time is displayed until the forecast exceeds the predetermined threshold. If it is determined that the threshold will be exceeded, the time difference between the actual time and the timepoint when the forecast will exceed the predetermined threshold is displayed. Thus, it is indicated for the operator how long the vacuum pump will operate in the current operating state without reducing the thermal load. In particular, the remaining time is displayed on a display of a controller as described in more detail below.

Preferably, the thermal load is reduced if the vacuum pump temperature exceeds a second predetermine threshold. Thus, a second predetermined threshold can be defined wherein the thermal load is reduced if the vacuum pump temperature itself, and not its forecast, exceeds the second predetermined threshold. Thus, the absolute values of the vacuum pump temperature itself can be used in addition to avoid thermal overload of the vacuum pump.

In another aspect of the present invention a controller for a vacuum pump is provided, wherein the controller is configured to implement the method as descried before.

Preferably, the controller is connected to a motor control to determine a rotational speed and/or motor current as pump parameter.

Preferably, the controller is connected to a temperature sensor to determine a stator temperature of the vacuum pump as pump parameters.

In another aspect of the present invention a vacuum pump is provided comprising a controller as described before.

Preferably, the vacuum pump is a roots pump, a claw pump, a screw pump, a molecular drag pump or the like. In another aspect of the present invention a software storage device is provided storing instructions which when executed by a processor of controller performing the steps of the method described before.

In the following the present invention is described in more detail with reference to the accompanying figures.

The figures show:

Figure 1 a schematic flow diagram of the method according to the present invention,

Figure 2 a vacuum system according to the present invention,

Figure 3 a diagram of the temporal behavior of the vacuum pump according to the present invention,

Figure 4A a diagram indicating the temporal behavior of the vacuum pump according to the present invention with a first threshold and

Figure 4B a diagram indicating the temporal behavior of the vacuum pump according to the present invention with a second threshold.

The operation limit of a vacuum pump is very much determined by the temperatures of the rotor and the clearance between rotor and stator.

The rotor must not exceed a fixed temperature to protect rotating parts from damage. Additionally, the temperature difference between rotor and stator must be limited to a fixed value to assure a safe running condition without a risk of seizure of the pump. These both parameters can only be measured directly with expensive measurement equipment. Such tests are typically done as type test with reference vacuum pumps in specialized laboratories but due to cost considerations not on vacuum pumps running in real applications.

Both temperatures are determined by:

Pressure level

- Pressure ratio

- Rotational speed

- Gas type

- Cooling conditions which in combination are referred to as "thermal load".

Knowing these parameters allows a precise prediction of the operation limits under stationary conditions. As vacuum pumps are heavy machines also the transient behavior must be considered to enable a maximum usage of the thermal capabilities of a unit.

Lab tests can be conducted to characterize a machine under both stationary and transient conditions. With sufficient data a thermal model can be generated that predicts in a forecast the temperatures of rotor and stator over time based on a measurement of stator temperature, motor rotational speed and motor current.

The invention is the generation of a thermal model that reliably simulates rotor temperatures in real time based on the input data mentioned above and controlling the thermal load of the pump in a way that eliminates a risk of pump damage while at the same time using the maximum thermal capability of the pump. In addition, information generated by the algorithm like remaining time to operate under current conditions is made available to the operator. This allows an assessment of the thermal load of the machine and potential room for process optimization.

Differentiation from state of the art is that the roots vacuum pump can be operated under boost conditions with variable limits based on the thermal history of the pump, meaning that at least one temperature and the rotational speed plus motor current is permanently measured and a thermal model is used to predict in the forecast how much overload the pump can handle until it reaches a thermal limit. When the thermal limit is reached or is anticipated to be reached in the future, a controller automatically reduces the thermal load, e.g. the rotational speed and with it the thermal load, to an acceptable level. Parameters which lead to a reduction of the rotational speed are made available to the operator.

Referring to Figure 1 showing a schematic flow diagram of the method according to the present invention. The method comprises the steps:

In step SOI, a vacuum pump is operated in a first operating sate.

In step S02, one or more pump parameters of the vacuum pump are determined.

In step S03, a forecast is determined for the vacuum pump temperature from the one or more pump parameters for the condition for changing the operation of the vacuum pump from the first operating state to a second operating state. In step S04 thermal load of the vacuum pump is reduced if the forecast exceeds a predetermined threshold.

Referring to Figure 2 showing a system 10 according to the present invention. The system 10 may comprise a vessel or vacuum chamber 12. The inlet 26 of a vacuum pump 14 is connected to the vacuum chamber 12. A backing pump 22 is connected to the outlet 28 of the vacuum pump 14 having an additional electromotor 24 and might be connected to the controller 16 as well. The vacuum pump 14 is operated by a motor 20. The motor 20 is connected to a motor control 18. Further the system 10 comprises a controller 16 which is connected to the motor control 18 and further connected to a temperature sensor 17. Via the motor control 18 for example the rotational speed and/or the motor current of the motor 20 can be determined as pump parameter. Via the temperature sensor 17 the stator temperature of the vacuum pump 14 can be determined as pump parameter. Therein, the controller 16 is configured to perform the steps of the method described before. In particular, from the determined pump parameters the controller 16 determines a forecast for the vacuum pump temperature under the condition of changing the operation of the vacuum pump from the first operating state to a second operating state. Thus, the forecast is determined only for the possibility of changing the operating state of the vacuum pump without actually changing the operation of the vacuum pump. Thus, thereby different scenarios for changing the operation of the vacuum pump can be considered by the forecast and thermal overload of the vacuum pump can be prevented before these scenarios occur. If the calculated forecast exceeds the predetermined threshold for the case that the operating state of the vacuum pump would be changed from a first operating state to the second operating state, while operating the vacuum pump in a first operating state, the thermal load of the vacuum pump is reduced. Thus, by reducing the thermal load of the vacuum pump while operating the first operating state, even if the vacuum pump is switched to a second operating state, no thermal overload would occur due to the previously reduced thermal load of the vacuum pump.

Referring to Figure 3 showing an example of the method. Figure 3 shows the development of the temperature T and the rotational speed rpm of the vacuum pump for two different cases over time t. Therein, in the example of Figure 3 the first operating state is the pump down state in order to create a vacuum in the vacuum chamber 12. The second operating state is the continuous operation in order to maintain the pressure or even further reduce the pressure without thermal overload of the vacuum pump. Temperature 32 refers to the temperature of the vacuum pump and in particular the rotor temperature in the first case. Rotational speed 30 refers to the rotational speed of the electromotor for the first case. The predetermined threshold 31 is indicated for the temperature T. During pump down the rotational speed 30 is increased. Thermal load increases which leads to an increased temperature 32. The increase of the rotational speed 30 is mainly controlled to avoid that the temperature 32 exceeds the predetermined threshold 31. After increase of the rotational speed 30 a constant or almost constant rotational speed is used during continuous operation.

Also referring to Figure 3 showing the rotational speed 34 and the temperature 36 for the second case. The same predetermined threshold 31 applies to the second case. However, according to the present invention, the rotational speed 34 of the vacuum pump increases in order to accelerate the pumping down of the vacuum chamber 12. Hence, efficient pump down can be achieved. However, also the temperature 36 increases faster than in the first case due to the increased rotational speed of the vacuum pump. However, the temperature 36 increases slower than the rotational speed 34 due to the thermal capacity of the vacuum pump. If it is determined by the controller 16 that the forecast of the temperature 36 exceeds the predetermined threshold 31, the rotational speed 34 of the vacuum pump is reduced. Thus, when reaching continuous operation of the vacuum pump with a similar or same rotational speed as in the first case, the temperature 36 of the vacuum pump did not exceed the predetermined threshold 31. Thus, the pump down phase is boosted and can be shortened due to the increased or overshooting rotational speed 34 of the vacuum pump.

Referring to figures 4A and 4B showing two different cases. The temperature of the stator 40, the temperature of the rotor 42 and the difference 44 of the stator temperature and the rotor temperature is depicted over time t. Also Figure 4A shows the pressure difference or pressure difference between the inlet and the outlet, i.e. the gas load. In Figure 4A a first threshold 46 is shown. When increasing load after 18h the rotor temperature as well as the stator temperature increases. Due to the different thermal capacity of the stator and the rotor, the temperature difference 44 decreases and then returns to a new value after thermal equilibrium of the vacuum pump has been reached. When after 30h the gas load is decreased to zero, the temperature of the rotor as well as the stator decreases. Due to the slower decrease of the temperature of the rotor, the difference 44 increases but still is kept below the threshold 46. Referring to now to Figure 4B showing a decreased threshold 46'. After 18h the load is increased similar to the situation of Figure 4A. The temperature of the rotor and the temperature of the stator increases to a new equilibrium which is slowly approached. However, the forecast of the temperature difference 44 shows that at time ti (approximately 26h) the pressure difference 44 would exceed the new threshold 46, if the gas load 38 would return to zero. Consequently, the gas load is slowly decreased as shown by curve 48 leading to a slow decrease of the thermal load, i.e. the rotor temperature 42 and the stator temperature 40. Consequently, if the vacuum pump is switched from the first operating state under gas load to the second operating state after 30h under end pressure or without gas load, a sudden decrease of the temperature of the stator as well as the temperature of the rotor occurs. However, in comparison to the increase of the temperature difference 44 of figure 4A, in the situation of the Figure 4B, the increase of the temperature difference 44 is still below the lower threshold 46 due to the formally reduced thermal load 48.

Hence by the method of the present invention dynamically adjustment of the thermal load is feasible. Thermal load is not controlled by fixed and predetermined thresholds, but by determining a forecast and considering the forecast when adjusting the thermal load. Hence, optimal and more efficient operation of the vacuum pump is possible without thermal overload. Therein, the method can be applied flexible to different pump situations without the need of too restrictive thresholds to avoid thermal overload.

Reference list:

10 system

12 vacuum chamber

14 vacuum pump

16 controller

17 temperature sensor

18 motor control

20 electromotor

22 backing pump

24 electromotor

26 inlet

28 outlet

30 rotational speed

31 threshold

32 temperature

34 rotational speed

36 temperature

38 gas load

40 stator temperature

42 rotor temperature

44 difference temperature

46, 46' threshold

48 thermal load reduction

Claims

1. Method for operating a vacuum pump including:

Operating the vacuum pump in a first operating state;

Determining one or more pump parameters of the vacuum pump;

2. Method according to claim 1, wherein the pump parameters are one or more of a stator temperature, a rotational speed and a motor current.

3. Method according to claims 1 or 2, wherein the vacuum pump temperature is one or more of a stator temperature, a rotor temperature, a difference between the stator temperature and the rotor temperature.

4. Method according to any of claims 1 to 3, wherein reducing the thermal load includes one or more of reducing the motor current, reducing the rotational speed, increasing the cooling, increasing pressure difference [ratio] at the vacuum pump.

5. Method according to any of claims 1 to 4, wherein the second operating state is operation under end pressure or switching of of the vacuum pump or continuous operation.

6. Method according to any of claims 1 to 5, wherein the remaining time is displayed until the forecast exceeds the predetermined threshold.

7. Method according to any of claims 1 to 6, wherein the thermal load is reduced if the vacuum pump temperature exceeds a second predetermined threshold.

8. Controller for a vacuum pump configured to implement the method according to any of claims 1 to 7.

9. Controller according to claim 8, wherein the controller is connected to a motor control to determine a rotational speed and/or a motor current as pump parameter.

10. Controller according to claims 8 or 9, wherein the controller is connected to a temperature sensor to determine a stator temperature of the vacuum pump as pump parameter.

11. Vacuum pump comprising a controller according to any of claims 8 to 10.

12. Software storage device storing instructions which when executed by a processor of a controller perform the steps of the method according to claims 1 to 7.