How to stop the vacuum pump

Abstract

The present invention provides a method for stopping rotation of a rotor in a vacuum pump, the method comprising rotating the rotor at an intermediate RPM with substantially no probability of the rotor colliding with the stator for a dwell time sufficient to allow the vacuum pump to fall below a threshold temperature, and then coasting down the rotation of the rotor until rotation stops.

Description

[Technical field]

[0001] The present invention relates to vacuum pumps, and in particular to a method for stopping (or stalling) the rotation of a rotor of a vacuum pump. [Background technology]

[0002] When manufacturing vacuum or compressor pump components, as with all manufactured components, slight variations in physical dimensions may occur between individual units of the same component. These manufacturing tolerances are generally inherent and unavoidable, but are generally reduced as much as possible. Manufacturers can balance the production costs and efficiencies of providing components with minimal manufacturing tolerances with the benefits this brings to the final assembled pump or compressor. Acceptable manufacturing tolerances may vary depending on the component and its application.

[0003] Reducing manufacturing tolerances is particularly important for components that move relative to other components when the pump or compressor is in use, such as the rotor blades or lobes of a pump. Reducing tolerances allows moving components to be placed closer together, improving the efficiency of the pump.

[0004] This is particularly important in dry-type vacuum pumps such as screw, roots and claw pumps, as the main pumping chamber between the rotor and stator is not lubricated. The absence of lubrication is beneficial in reducing contamination of the vacuum system, but the absence of lubrication requires even smaller clearances between the rotor and stator to improve pumping efficiency.

[0005] The smaller the clearance, the greater the chance of a rotor colliding with the stator or another rotor, which can not only stall the pump but can also cause damage to the associated rotor and / or stator.

[0006] The inventors have found that in many pumps, for example multi-stage Roots pumps, additional spacing may be required between relatively moving components due to the thermal cycles the pump undergoes during use. Heat provided by the pump causes thermal expansion of the pump's components. Because different components within the pump comprise different materials, the components may have different thermal expansion rates. The effect of the different thermal expansion rates of the components within the pump is that the spacing between adjacent components within the pump may change with changes in temperature, causing the components to effectively move relative to one another.

[0007] When the pump is "cold started", i.e. when the rotor is started to rotate by the vacuum pump at ambient temperature (e.g. 25°C), the rotor is substantially displaced in the upstream direction of the pump. This displacement is partly due to torque effects and settling of the bearings in which the rotor shaft is rotatably mounted. This displacement is also partly due to an internal pressure gradient that occurs in the pump chamber along the pump direction. Typically, the displacement is less than about 100 μm, preferably less than about 50 μm, e.g. less than 20 μm.

[0008] Thus, to prevent the rotor of each stage from colliding with the adjacent upstream partition (i.e., the stator), the upstream axial clearance may be greater than required for optimum pump efficiency. Typically, therefore, the greater upstream axial clearance allows for greater rotor displacement during "cold start."

[0009] As mentioned above, as the pump is used, the temperature of the pump may increase, causing thermal expansion of components within the pump. The inventors have found that as the temperature of the pump increases, the rotor may be displaced substantially toward the downstream pump relative to the stator. This may be due in part to differences in thermal expansion of the pump components caused by differences in the geometry of the components. It may also be due in part to different components comprising different materials having different thermal expansion coefficients. Typically, the rotor and stator comprise different materials, for example, the stator comprises aluminum and the rotor comprises iron. Thus, the thermal expansion coefficient of an aluminum stator may be greater than the thermal expansion coefficient of an iron rotor.

[0010] By way of example, the inventors have found that in some pumps operating near their thermal limits, the rotor may be displaced substantially toward the downstream pump relative to the stator by up to about 150 μm, or even up to about 250 μm.

[0011] When the pump is turned off, the rotor's rotational speed may slow down until it stops. This slowing down to a stop is sometimes called coasting down. Typically, the rotor is free to spin to a stop, but braking (e.g., motor braking) may occur. Coasting down may take less than about 120 seconds, preferably less than about 60 seconds, for example about 20 seconds.

[0012] When the pump coasts down, the reduction in the rotor's rotational speed may be non-linear. Immediately after the pump is turned off, the rate of change of the rotor's rotational speed may initially be small because a pressure gradient still exists in the pump and the rotor's rotational inertia remains relatively high. However, as the pressure gradient decreases, the rate of reduction in the rotor's rotational speed may become larger as the rotor drag increases. Finally, as the rotor approaches a stop, the rate of reduction in the rotor's rotational speed slows again.

[0013] As the pressure gradient in the pump chamber decreases, the rotor may be displaced substantially toward the downstream pump.

[0014] If the pump is operating such that the temperature of the pump causes the rotor to displace substantially downstream relative to the stator, as the rotor coasts down, the reduced pressure gradient may cause the rotor to displace further downstream. This further downstream displacement of the rotor may cause the rotor to collide with the downstream stator. A collision between the rotor and stator may be defined as direct contact between the rotor and stator and may cause damage to the rotor and / or stator. One or more components of the pump may require repair and / or replacement, and as a result, the collision may prevent further rotation of the rotor until such repair / replacement is performed.

[0015] When used in certain applications, for example, semiconductor manufacturing processes, some pumps may exhibit a buildup of particulate matter in the pump chamber during use. When the pump is stopped and components thermally contract, this particulate matter may become compressed between the rotor and the adjacent stator in the pump chamber. When the pump is "cold started," the torque required to initiate rotation of the rotor may increase due to increased friction between the compressed particulate matter and the rotor. In some cases, the required torque may exceed the running torque of the pump, causing it to fail to start.

[0016] To solve this start-up problem, various mechanisms are known for purging particulate matter during pump shutdown. For example, WO 2004 / 038222 discloses an automated shutdown sequence that includes shutting down the pump mechanism, monitoring the temperature of the pump mechanism, starting the pump mechanism at preselected temperature intervals to purge contaminating particulate matter from within the pump chamber, and finally shutting down the pump mechanism.

[0017] Shutting down in this manner allows any particulate matter that has built up in the pump chamber to be purged, thereby ensuring that such matter is not compressed between the rotor and stator of the pump mechanism, thereby avoiding start-up problems.

[0018] EP 1 900 943 A1 discloses an alternative method for removing particulate matter in the pump chamber. This method involves reducing the rotational speed of the pump mechanism below a preset threshold speed, maintaining the rotational speed for a period of time to purge accumulated particulate matter from between the rotor and the adjacent stator, and then stopping the rotation of the pump mechanism. Again, this prevents particulate matter from accumulating and being compressed in the clearance between the rotor and the stator. Thus, the increased torque required during cold start of the pump mechanism is avoided.

[0019] EP 2048365 A1 discloses a mechanism for rotating the pump rotor in a forward and / or reverse direction according to a predetermined timing pattern after a pump stop operation has been performed and before stopping the pump rotation. This mechanism also purges particulate matter in the clearance between the rotor and the stator to prevent its compaction.

[0020] Each of these disclosures is directed to preventing the torque increase start-up problem of vacuum pumps caused by compression of particulate matter in the clearance between the rotor and the stator, but neither disclosure identifies the more serious problem of the rotor colliding with the stator during rotation shutdown.

[0021] Currently, to reduce the risk of collision during use, the nominal spacing between components takes into account both tolerance stack-up and thermal cycling / expansion. However, increasing the spacing between components, especially between the rotor and stator, significantly reduces pump efficiency. These issues are relevant, especially for small vacuum pump sizes, as efficiency losses have a significant impact on overall pump performance. This can negatively impact pumping performance and reduce the final pump pressure achievable by the pump. [Prior art documents] [Patent documents]

[0022] [Patent Document 1] International Publication No. 2004 / 038222 [Patent Document 2] European Publication No. 1900943 [Patent Document 3] European Publication No. 2048365 Summary of the Invention [Problem to be solved by the invention]

[0023] Thus, there is a continuing need to address the effects of thermal expansion on pump efficiency. [Means for solving the problem]

[0024] The present invention is directed to solving these and other problems of the prior art.

[0025] Accordingly, in a first aspect, the present invention provides a method of arresting (i.e. stopping) rotation of a rotor of a vacuum pump.The vacuum pump comprises a pumping chamber including a rotor and a stator.

[0026] Prior to beginning the method, the rotor is rotating at an operating RPM (revolutions per minute), which is greater than a threshold RPM such that the rotor cannot coast down from the operating RPM to a standstill (i.e., stopped) without a relatively high probability of the rotor colliding with the stator.

[0027] The method includes rotating the rotor at an intermediate RPM where there is substantially no chance of the rotor colliding with the stator for a dwell time sufficient for the vacuum pump to reach or below a threshold temperature, and then coasting the rotor down until it stops rotating.

[0028] The intermediate RPM is less than or equal to the threshold RPM.

[0029] Advantageously, the inventors have found that the method according to the invention significantly reduces the risk of rotor-stator collision when the vacuum pump is subsequently turned off. Rotating the rotor at an intermediate RPM during the dwell time according to the invention ensures that a sufficient pressure gradient is maintained within the pump while the pump temperature is reduced, thereby preventing significant downstream displacement of the rotor during thermal component displacement that could cause collision. Furthermore, because the effects of this temperature-dependent collision mechanism are substantially avoided, the clearance between each rotor and the adjacent downstream stator can be reduced, improving pump efficiency.

[0030] Preferably, by using the method of the present invention, the rotor and stator of a vacuum pump can be positioned such that the minimum distance (i.e., clearance) between them is such that if the rotor rotates above a threshold RPM during operation and coasts down to a stop, there is a relatively high probability that the rotor will collide with the stator.

[0031] In the present invention, the threshold RPM is the maximum rotational speed at thermal steady state at which the rotor can coast to a stop without substantial probability of colliding with the stator. Those skilled in the art will appreciate that the threshold RPM may depend on a number of factors, such as the particular type of vacuum pump, the orientation of the rotor relative to the stator, the minimum clearance between the rotor and the stator, the materials of the vacuum pump components, and the cooling rates of the vacuum pump components.

[0032] The threshold temperature is the temperature of the vacuum pump when the rotor is rotating at the threshold RPM in thermal steady state.

[0033] In the present invention, stopping the rotation of the rotor of a vacuum pump means slowing down the rotational speed of the rotor until it is substantially stationary, i.e., rotating at a speed of about 0 RPM. Approximately 0 RPM rotation is considered to be stopped. Such stopping of rotation may generally be associated with turning off the pump, such as for maintenance or between uses.

[0034] Typically, during coastdown, the rotor spins freely to a stop, but braking may be performed. Typically, when coasting down, the motor will no longer be driving the rotor.

[0035] For purposes of the present invention, "rotor colliding with stator" may be defined as direct contact between the rotor and stator. Generally, the rotor is allowed to rotate when colliding with the stator. Upon collision, friction and collision between the rotor and stator causes the rotor to stop rotating. A rotor colliding with a stator may cause damage to the rotor and / or stator. The collision may require repair and / or replacement of one or more components of the pump.

[0036] A collision between the rotor and the stator may cause a locking engagement between the rotor and the stator. Such a locking engagement may result from the rotor contacting and binding to the stator, causing seizure of the rotor rotation. If a locking engagement occurs during a collision, it may cause irreparable damage to the rotor and / or stator, leading to significant downtime of the vacuum pump. Rotors and / or stators that include aluminum or aluminum alloys are particularly susceptible to a locking engagement during a collision.

[0037] In the present invention, a "relatively high probability of the rotor colliding with the stator" can be defined as a probability of the rotor colliding with the stator greater than about 1% when coasting down from that particular RPM, preferably a probability of the rotor colliding with the stator greater than about 5%, more preferably a probability of the rotor colliding with the stator greater than about 10%, more preferably a probability of the rotor colliding with the stator greater than about 20%, more preferably a probability of the rotor colliding with the stator greater than about 30%, more preferably a probability of the rotor colliding with the stator greater than about 40%, more preferably a probability of the rotor colliding with the stator greater than about 50%, more preferably a probability of the rotor colliding with the stator greater than about 60%, more preferably a probability of the rotor colliding with the stator greater than about 70%, more preferably a probability of the rotor colliding with the stator greater than about 80%, and more preferably a probability of the rotor colliding with the stator greater than about 90%. Most preferably, a probability of the rotor colliding with the stator greater than about 99% when coasting down from that particular RPM.

[0038] "A rotor colliding with a stator" may exclude a rotor contacting particulate matter deposited on a stator.

[0039] For purposes of the present invention, "substantially no probability of the rotor colliding with the stator" may be defined as less than about 0.1%, and more preferably less than about 0.01%, of the rotor colliding with the stator during coastdown from that particular RPM. Preferably, the rotor will not collide with the stator during the life of the vacuum pump or during repair.

[0040] Due to the potentially serious consequences associated with a collision between the rotor and the stator, one skilled in the art will appreciate that when coasting down the rotation of the rotor from an intermediate RPM, it may be determined that there is a "relatively high probability that the rotor will collide with the stator," as opposed to "substantially no probability that the rotor will collide with the stator." Collisions between the rotor and the stator can damage the vacuum pump and may also damage the process tool to which the vacuum pump is connected. Thus, prior art vacuum pumps operate such that there is substantially no probability that the rotor will collide with the stator. Even a 1% probability of a collision occurring when coasting down the rotor from an operating RPM to a stop would be considered unacceptable. Of course, a higher probability of a collision occurring would be even less acceptable. The present invention allows for the rotor to rotate at operating RPMs above a threshold RPM that would have been avoided by prior art methods. The method of the present invention also allows for the clearance between the rotor and the stator to be reduced to a magnitude that results in a relatively high probability of a collision occurring between the rotor and the stator when the rotor is coasted down from an operating RPM to a stop.

[0041] Additionally, "relatively high probability of the rotor colliding with the stator" can be broadly defined to include variations in the probability of a collision occurring as a function of how much the operating RPM exceeds the threshold RPM: the greater the amount by which the operating RPM exceeds the threshold RPM, the greater the probability of a collision occurring between the rotor and stator if the rotor is allowed to coast down from the operating RPM to a stop.

[0042] For purposes of the present invention, thermal steady state may be defined as the temperature of the vacuum pump when the rotor rotation is held at a particular rotational speed (RPM) until the rotor temperature remains substantially constant, e.g., for at least one minute, varying by less than about ±0.1°C.

[0043] The vacuum pump may be a dry vacuum pump. Preferably, the dry vacuum pump is a Roots pump, more preferably a multi-stage Roots pump. For example, the vacuum pump may be an nXRi dry multi-stage Roots pump or an nXLi dry multi-stage Roots pump manufactured by Edwards Vacuum.

[0044] The pumping chamber may house one or more rotors and / or stators. Preferably, the pumping chamber may be defined by an outer wall that substantially surrounds the one or more rotors and / or stators.

[0045] The vacuum pump may comprise a plurality of rotors. Preferably, one or more rotors are arranged on one or more rotor shafts. Each rotor may comprise a multi-lobe piston arranged for rotation with the rotor shaft. Preferably, each rotor comprises a 2-, 3-, 4-, or 5-lobe piston. Preferably, each rotor of the vacuum pump has substantially the same dimensions.

[0046] Preferably, the rotors of the vacuum pump may be arranged in rotor stages. Each rotor stage may comprise a first rotor disposed on a first rotor shaft and a corresponding second rotor disposed on a second rotor shaft. The first and second rotor shafts may be substantially parallel. In use, the first and second rotor shafts may be arranged to rotate in opposite directions. The rotational paths of the lobes of the first and second rotors overlap without the first or second rotors touching.

[0047] Typically, the outer wall of the pump chamber may comprise an inlet through which fluid may enter the pump chamber. The outer wall of the pump chamber may further comprise an outlet through which fluid may exit the pump chamber. Typically, the pump inlet may be at or towards a first end of the pump chamber and / or the pump outlet may be at or towards a second end of the pump chamber. Typically, if a pump is used, the pump inlet may be connected to an exhaust chamber and / or a further vacuum pump.

[0048] When a pump is used, the pumping direction can be defined as the direction in which the majority of the fluid flowing through the vacuum pump can flow. Typically, the outlet is located substantially downstream from the inlet in the pumping direction.

[0049] Without deviating from the above, the pump inlet may be configured such that, in use, fluid enters the main chamber in a direction substantially perpendicular to the pump direction. Additionally or alternatively, the pump outlet may be configured such that, in use, fluid exits the main chamber in a direction substantially perpendicular to the pump direction. Advantageously, this may allow for miniaturization of the pump.

[0050] Typically, the vacuum pump may further comprise a motor, typically an electric motor, arranged in use to drive in rotation one or more of the rotors. Typically the motor is located external to the pump chamber. A motor may be coupled to each rotor shaft. Alternatively, each rotor shaft may be coupled to a separate motor.

[0051] Preferably, the pump may comprise a plurality of rotor stages. The plurality of rotor stages may be arranged along the length of the first and / or second rotor shaft. The rotor stages may be separated by partition walls (i.e. stators). The partition walls may comprise connecting ducts allowing fluid connection between adjacent rotor stages.

[0052] Typically, a pump may include from about 1 to about 10 rotor stages, preferably from about 2 to about 8 rotor stages. The number of rotor stages may depend on the type of pump.

[0053] Typically, the rotor(s) are metallic, for example made from iron or an alloy thereof.

[0054] Typically, the stator(s) are made of metal, for example aluminum or an aluminum alloy.

[0055] One or more of the stators may be in the form of a partition, preferably each stator may be a partition, typically located between each rotor of adjacent rotor stages.

[0056] During operation of the pump, the operating RPM of the rotor(s) may be from about 5000 RPM to about 16000 RPM, preferably from about 6000 RPM to about 16000 RPM, and more preferably from about 12000 RPM to about 15000 RPM.

[0057] Typically, the intermediate RPM is less than or equal to a threshold RPM. Preferably, the intermediate RPM is about half the rotational speed of the operating RPM, preferably about 25% to about 75% of the operating RPM, and more preferably about 40% to about 60% of the operating RPM.

[0058] Typically, the vacuum pump includes a temperature sensor configured to measure a temperature of the vacuum pump. The method may further include measuring the temperature of the vacuum pump via the temperature sensor during the residence time and determining whether the temperature is equal to or less than a threshold temperature. Typically, the temperature of the vacuum pump refers to the temperature of a rotor of the vacuum pump.

[0059] Typically the temperature sensor is configured to measure the temperature of the rotor directly, but may also be configured to measure the temperature of a component of the vacuum pump from which the temperature of the rotor can be inferred, such as the stator and / or rotor shaft. Preferably the temperature sensor is configured to measure the temperature of the stator.

[0060] When the pump is in operation, the temperature of the pump (e.g., rotor) is about 50° C. to about 150° C., preferably about 70° C. to about 120° C. This may be referred to as the operating temperature. The operating temperature is generally at steady state. One skilled in the art will appreciate that the temperature of a vacuum pump in operation will depend on factors such as the ambient temperature, the rotational speed of the rotor, etc.

[0061] An operating temperature is the temperature of the pump (e.g., rotor or stator) when the rotor is rotating at an operating RPM at thermal steady state. Preferably, when the temperature of the pump is at an operating temperature, the rotor cannot coast down from an operating RPM to a stop without a relatively high probability of the rotor colliding with the stator. For the avoidance of doubt, a relatively high probability of the rotor colliding with the stator is as defined herein above.

[0062] Typically, the threshold temperature is about 10° C. to about 70° C. lower than the operating temperature, and preferably about 20° C. to about 50° C. lower.

[0063] In an embodiment, the pump may include a plurality of temperature sensors. Each temperature sensor may be configured to measure the temperature of a different component of the vacuum pump. Preferably, each temperature sensor may be configured to measure the temperature of a different stage of the vacuum pump. More preferably, each temperature sensor may be configured to measure the temperature of a stator of a different stage of the vacuum pump.

[0064] Additionally or alternatively, the pump may include a temperature sensor configured to measure the temperature of the motor or a component thereof.

[0065] Typically, the temperature sensor may be a thermistor.

[0066] Advantageously, measuring the temperature of the vacuum pump (rotor) via a temperature sensor can improve the efficiency of the method since the pump can begin to coast down to a stop as soon as it falls below a threshold temperature.

[0067] In an embodiment, the temperature of the vacuum pump (rotor) can be measured via a temperature sensor at predetermined time intervals during the residence time. Preferably, the predetermined time interval is every second or less. Preferably, the predetermined time interval is less than about 0.5 seconds, more preferably less than about 0.1 seconds. Alternatively, the temperature of the vacuum pump (rotor) can be measured substantially continuously.

[0068] Advantageously, by measuring the temperature of the vacuum pump via a temperature sensor at predetermined time intervals or substantially continuously, the time until the temperature of the vacuum pump falls below a threshold temperature and the rotor coasts down to a stop in rotation can be reduced, which can further improve the efficiency of the method.

[0069] The method may further include initiating a coast down of rotation of the rotor from the intermediate RPM to a stop only when the temperature of the vacuum pump as measured by the temperature sensor is below a threshold temperature.

[0070] Advantageously, by initiating the coast-down of the rotor from an intermediate RPM to a stop only once the temperature of the vacuum pump is below a threshold temperature, it is possible to ensure that there is substantially no probability of the rotor colliding with the stator and to provide a time and energy efficient coast-down process. Furthermore, such a method ensures that there is substantially no probability of the rotor colliding with the stator when stopped, regardless of the operating RPM at which the rotor was rotating during use. Thus, such a method provides flexibility to adapt to the operating conditions of the pump.

[0071] Advantageously, by measuring the temperature of the vacuum pump, preferably via the rotor or the stator, most preferably via the stator, it is possible to more accurately determine whether a rotor-stator collision is avoided, since it is mainly the difference in thermal expansion of the rotor and the stator that causes the collision when stopping the rotation.

[0072] Alternatively, the rotor can rotate at an intermediate RPM for a predetermined dwell time. The predetermined dwell time can be selected depending on the particular pump configuration. Preferably, the predetermined dwell time can be determined to ensure that the dwell time is sufficient for the temperature of the vacuum pump to be below the threshold temperature by the end of the predetermined dwell time.

[0073] This method can be performed automatically when the vacuum pump is turned off, so the user may not be aware that the process is being used, and this may be applied to other methods according to the invention as well.

[0074] One skilled in the art will appreciate that the threshold temperature and / or threshold RPM and / or the predetermined dwell time may be determined by finite element analysis and / or experimentation.

[0075] Rotating the rotor at an intermediate RPM for a predetermined dwell time can advantageously simplify the method. Additionally, operating at a predetermined dwell time does not require a temperature sensor, potentially reducing the cost of the vacuum pump.

[0076] Typically, the predetermined residence time can be from about 0 seconds to about 600 seconds, preferably from about 30 seconds to about 480 seconds.

[0077] Typically, the vacuum pump is equipped with a controller. The method of coasting the rotor from an operating RPM to a stop can be initiated by a single operator input to the controller. Preferably, once initiated, the controller can automatically execute the method of the present invention.

[0078] Typically, a controller may be connected to the motor. The controller may be configured to control the rotational speed of the rotor. Preferably, the controller may be connected to one or more temperature sensors of the vacuum pump.

[0079] The controller may be configured to control the rotational speed of the rotor in response to a signal generated by the temperature sensor(s). The controller may be configured to compare the temperature measured by the temperature sensor with a predetermined temperature value, e.g., a threshold temperature, and adjust the rotational speed of the rotor accordingly by sending a signal to the motor.

[0080] Preferably, once the method according to the invention has been initiated, the temperature sensor is capable of sending a signal to the controller indicative of the temperature of the vacuum pump. This signal may be sent substantially continuously or at predetermined time intervals.

[0081] Preferably, the controller is connected to a display device. The display device may comprise a screen.

[0082] Preferably, the controller may be configured to allow a user to input commands. The input of commands may be via a switch, a touch screen, or other means. Preferably, the method according to the invention may be initiated by a single user input to the controller, e.g. a switch. Advantageously, this allows the method to run fully automatically after an initial input by the user. This may reduce the possibility of user error and improve the efficiency of the method.

[0083] Typically, the vacuum pump comprises a cooling system configured to reduce a temperature of the vacuum pump (e.g., rotor and / or stator). The method may include activating the cooling system to reduce a temperature of the vacuum pump. Preferably, the method may further include initiating the cooling system or increasing the cooling performance of the cooling system when the rotor is rotating at an intermediate RPM.

[0084] During operation of the vacuum pump, the motor that drives the rotor to rotate generates heat, which can cause thermal expansion of the vacuum pump components.

[0085] The cooling system may comprise a cooling fan and / or a fluid cooling system. The fluid cooling system may be a water cooling system.

[0086] The cooling system may be configured to cool the motor during operation. Additionally or alternatively, the cooling system may be configured to reduce a temperature of one or more rotors of the vacuum pump.

[0087] In embodiments where the cooling system comprises a cooling fan, increasing the cooling rate of the cooling system can include increasing a rotational speed of the cooling fan. In embodiments where the cooling system comprises a fluid cooling system, increasing the cooling rate of the cooling system can include increasing a fluid flow rate.

[0088] Typically, as the rotational speed of the rotor is reduced during the method according to the invention, the cooling rate of the cooling system can be increased. Advantageously, the reduction in the temperature of the vacuum pump by the cooling system can reduce the residence time required for the temperature of the vacuum pump to drop below a threshold temperature. This can reduce the time required to achieve rotor rotation stop according to the method of the invention.

[0089] Typically the cooling system may be connected to a controller such that operation of the cooling system is controlled by the controller. Advantageously this may allow operation of the cooling system to be automated and require no user input other than initiating the method via the controller.

[0090] In an embodiment, the cooling effect of the cooling system can be increased while the rotor is rotating at a rotational speed greater than a threshold RPM, thereby reducing the temperature of the vacuum pump below the threshold temperature, which can cause the rotor to coast down until it stops rotating.

[0091] The cooling rate of the vacuum pump can be increased by reducing the rotational speed of the rotor to an intermediate RPM below the threshold RPM, preferably while the cooling system is operating. Thus, the method according to the invention allows for a faster and more energy efficient stopping of the rotation of the rotor(s) of the vacuum pump.

[0092] Typically, the threshold RPM is about 30% to about 70% of the maximum rotational speed of the rotor, preferably about 40% to about 50% of the maximum rotational speed of the rotor. For example, the threshold RPM can be about 5000 RPM to about 8000 RPM, preferably about 6000 RPM to about 7500 RPM. Those skilled in the art will appreciate that the threshold RPM may depend on the particular pump to which the method is applied and / or the particular application in which the pump is used. Furthermore, those skilled in the art will appreciate that the operating RPM can be less than or equal to the maximum rotational speed of the rotor.

[0093] In a further aspect, the present invention provides a method of stopping rotation of a rotor of a vacuum pump. The vacuum pump includes a pumping chamber including a rotor and a stator. The rotor is rotating at a thermal steady state at an operating RPM.

[0094] The method includes measuring a temperature of the vacuum pump (rotor) and determining from the temperature of the vacuum pump whether the rotor is rotating at an operating RPM greater than a threshold RPM. If the operating RPM is greater than the threshold RPM, the rotation of the rotor is stopped according to the method defined in the above aspect, otherwise the rotor is coasted down from the operating RPM to a stop without a dwell time at an intermediate RPM.

[0095] The threshold RPM is the maximum rotational speed at thermal steady state where the rotor can coast down to a stop with virtually no probability of the rotor colliding with the stator.

[0096] Advantageously, the method can ensure that the method according to the above embodiment is used only if the rotor is rotating at an operating RPM greater than the threshold RPM. If the rotor is rotating at an operating RPM less than the threshold RPM, the probability of the rotor colliding with the stator is substantially zero. Thus, methods as shown in the above embodiment are not required and the rotor rotation can be stopped without rotating the rotor at an intermediate RPM during the dwell time. Advantageously, this can ensure that the stopping of the rotor of the vacuum pump is performed as efficiently as possible. As mentioned above, a cooling system can be used to bring the temperature of the vacuum pump (e.g. the rotor) below the threshold temperature.

[0097] Preferably, the temperature of the vacuum pump (eg the rotor) is measured using one or more temperature sensors.

[0098] In a further aspect, the present invention provides a vacuum pump comprising a pumping chamber including a rotor, a stator, and a controller configured to control a rotational speed of the rotor.

[0099] The rotor and stator are positioned such that the distance between them ensures that if, during operation, the rotor rotates above a threshold RPM and coasts to a stop, there is a relatively high probability that the rotor will collide with the stator, the threshold RPM being the maximum rotational speed at thermal steady state at which the rotor can coast to a stop with substantially no probability of the rotor colliding with the stator.

[0100] During operation where the rotor is coasting down from an operating RPM greater than the threshold RPM, the controller is configured to reduce the rotational speed of the rotor to an intermediate RPM where there is substantially no probability of the rotor colliding with the stator, and to hold the rotational speed at the intermediate RPM for a dwell time sufficient for the vacuum pump to be at or below the threshold temperature, the intermediate RPM being less than or equal to the threshold RPM. After the dwell time, the controller is configured to coast down the rotation of the rotor until it stops spinning.

[0101] The threshold temperature is the thermal steady state temperature of the vacuum pump (eg, rotor) when the rotor is rotating at the threshold RPM.

[0102] The vacuum pump may further comprise a cooling system configured to reduce a temperature of the vacuum pump.

[0103] The cooling system may comprise a cooling fan and / or a fluid cooling system. The fluid cooling system may be a water cooling system. Preferably, the cooling system comprises a cooling fan.

[0104] The cooling system can be configured to cool the motor during operation. Advantageously, reducing the temperature of the vacuum pump using the cooling system can reduce the dwell time required for the temperature of the vacuum pump to drop below a threshold temperature. As a result, this can reduce the time required to achieve rotor stoppage.

[0105] Typically, the cooling system may be connected to a controller such that the controller controls the operation of the cooling system. Advantageously, this may allow the operation of the cooling fan to be automated and require no user input other than to initiate the spin down via the controller, e.g., pressing an "off" command.

[0106] Typically the pump is a multi-stage vacuum pump. Preferably the pump is a multi-stage Roots pump. For example the pump may be an nXLi dry multi-stage Roots pump or an nXRi dry multi-stage Roots pump manufactured by Edwards Vacuum.

[0107] In a further aspect, the present invention provides a vacuum pump including a pumping chamber including a rotor and a stator, and a controller configured to control the rotational speed of the rotor, the rotor and stator being positioned such that a minimum distance between them results in a relatively high probability of the rotor colliding with the stator if, during operation, the rotor rotates above a threshold RPM and coasts down to a stop.

[0108] The threshold RPM is the maximum rotational speed at thermal steady state that allows the rotor to coast down to a stop with substantially no probability of the rotor colliding with the stator. The pump can include a temperature sensor configured to measure a temperature of the vacuum pump. The controller can be connected to the temperature sensor.

[0109] During operation where the rotor is coasted down from an operating RPM greater than the threshold RPM, the controller is configured to reduce the rotational speed of the rotor at a rate such that there is substantially no probability of the rotor colliding with the stator. This can be accomplished by a temperature sensor providing a temperature signal to the controller at predetermined time intervals or substantially continuously.

[0110] For the avoidance of doubt, "relatively high probability of the rotor colliding with the stator" and "substantially no probability of the rotor colliding with the stator" are as defined herein.

[0111] The controller may compare the temperature signal received from the temperature sensor to a predetermined rotor speed vs. pump threshold temperature. The maximum rotor speed vs. temperature threshold may determine a maximum allowable thermal steady state pump temperature at each rotor speed at which the rotor speed can coast down to a stop with substantially no probability of the rotor colliding with the stator. Preferably, if the measured temperature of the vacuum pump exceeds the predetermined rotor speed vs. pump temperature threshold, the controller will slow the rate of reduction in rotor rotation.

[0112] For the avoidance of doubt, features of the aspects and embodiments described herein may be combined and still fall within the scope of the present invention.

[0113] Preferred features of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0114] [Figure 1a] 1 shows a schematic of the operation of a prior art pump; [Figure 1b] 1 shows a schematic of the operation of a prior art pump; [Figure 1c] 1 shows a schematic of the operation of a prior art pump; [Figure 1d] 1 shows a schematic of the operation of a prior art pump; [Figure 2a] 2 shows a schematic diagram of the operation of a pump according to the invention; [Figure 2b] 2 shows a schematic diagram of the operation of a pump according to the invention; [Figure 2c] 2 shows a schematic diagram of the operation of a pump according to the invention; [Figure 2d] 2 shows a schematic diagram of the operation of a pump according to the invention; [Diagram 3] 3 shows a flow chart of a method of operating a pump according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0115] Figures 1(a) to 1(d) show schematics of a pump according to the prior art, each showing a cross-section of a rotor stage (1) of a vacuum pump.

[0116] The rotor stage (1) comprises a rotor (2) mounted on a rotor shaft (3). The rotor shaft (3) is substantially parallel to a pump direction (A). The pump direction (A) defines the direction of flow of bulk fluid during use of the pump. The pump direction (A) may define a direction between a pump inlet (not shown) and a pump outlet (not shown). The rotor stage (1) further comprises a pair of stators (4, 5). The pair of stators includes an upstream stator (4) and a downstream stator (5). The terms "upstream" and "downstream" define the relative position of each stator with respect to a particular rotor. Each of the upstream stator (4) and downstream stator (5) may be in the form of a stage partition wall.

[0117] Figure 1(a) shows the configuration of the rotor stage (1) when the pump is off, i.e. when the rotor (2) is not rotating relative to the pair of stators (4, 5). It shows the arrangement of the rotor (2) within the rotor stage (1) and the clearances within the pump when stationary.

[0118] Between the rotor (2) and the upstream stator (4), there is an upstream rotor clearance (x) that defines the minimum distance between the rotor (2) and the upstream stator (4). Between the rotor (2) and the downstream stator (5), there is a downstream rotor clearance (y) that defines the minimum distance between the rotor (2) and the downstream stator (5). In general, when the rotor (2) is not rotating relative to the pair of stators (4, 5), the upstream rotor clearance (x) is smaller than the downstream rotor clearance (y).

[0119] The pump is constructed with relatively small upstream rotor clearance (x) and downstream rotor clearance (y) while taking into account manufacturing tolerances of the components and ensuring that the rotor does not contact the stator, thereby reducing fluid leakage between the stages of the vacuum pump.

[0120] When the rotor (2) is stationary, the fluid pressures in the rotor stages are in equilibrium, i.e., there is no substantial difference in fluid pressure between the upstream end of the rotor stage (1) and the downstream end of the rotor stage (1). The upstream end of the rotor stage (1) may be defined as between the upstream stator (4) and the rotor (2). The downstream end of the rotor stage (1) may be defined as between the rotor (2) and the downstream stator (5).

[0121] Figure 1(b) shows the rotor stage when the pump is switched on, i.e. during the first 20 seconds of use. When the pump is switched on, the rotor (2) rotates about its axis of rotation (Z) relative to the stator (4, 5).

[0122] As the rotor (2) rotates, a pressure gradient is created between the upstream and downstream ends of the rotor stage (1). The fluid pressure may be lower at the upstream end of the rotor stage and higher at the downstream end of the rotor stage. This may therefore bias the position of the rotor (2) towards the upstream end of the rotor stage (1).

[0123] Furthermore, rotational force effects and settling of the bearings (not shown) in which the rotor shaft (3) is rotatably mounted contribute to displacing the rotor towards the upstream end of the rotor stage (1).

[0124] Displacement of the rotor (2) towards the upstream end of the rotor stage (1) may reduce the upstream clearance (x) and increase the downstream clearance (y) compared to FIG. 1(a).

[0125] This upstream displacement of the rotor is also a factor that can be taken into account when designing the pump to minimise the risk of the rotor (2) colliding with the stator (4).

[0126] Figure 1(c) shows the rotor stage (1) when the pump is operating and the temperature of the pump (rotor) rises to an operating temperature, for example 85°C, which is higher than the threshold temperature.

[0127] As the temperature of the vacuum pump increases, the rotor (2) and the stators (4, 5) experience differential thermal expansion since they are made of different materials, which effectively displaces the rotor (2) towards the downstream end of the rotor stage (1).

[0128] Displacement of the rotor (2) towards the downstream end of the rotor stage (1) may result in an increase in the upstream clearance (x) and a decrease in the downstream clearance (y) compared to FIG. 1(b).

[0129] Figure 1(d) shows the rotor stage (1) when the rotation of the rotor (2) is stopped and the temperature of the pump (rotor) exceeds 70°C. The temperature of the vacuum pump exceeds the threshold temperature.

[0130] When the rotor (2) stops rotating, the pressure gradient between the upstream and downstream ends of the rotor stage (1) decreases. This removes the biasing force of the rotor (2) towards the upstream end of the rotor stage (1), causing further displacement of the rotor (2) towards the downstream end of the rotor stage (1). As shown, this further displacement causes the rotor (2) to collide with the downstream stator (5). This collision occurs because the downstream clearance (y) becomes substantially zero, resulting in direct contact between the rotor (2) and the stator (5).

[0131] This collision can lead to damage to the rotor (2) and / or stator (5) along with machine downtime.

[0132] Figures 2(a) to 2(d) show a schematic of the operation of a pump according to the invention, in which a cross-sectional view of a rotor stage (6) of a pump according to the invention is shown.

[0133] Figures 2(a) and 2(b) are substantially the same as Figures 1(a) and 1(b) and the conditions and steps within the pump are the same so the description will not be repeated. The rotor stage (6) comprises a controller (10) configured to control the rotational speed of the rotor (7).

[0134] Figure 2(c) shows the rotor stage (6) when the pump is operating and the temperature of the pump rises above the ambient temperature (e.g. 20°C) to an operating temperature, e.g. 85°C. The operating temperature is above a threshold temperature. Because the rotor and stator are made of materials with different thermal expansion coefficients, their heating effectively displaces the rotor towards the downstream end of the rotor stage (6).

[0135] When the pump is turned off using the method according to the invention, the rotational speed of the rotor (7) is reduced from the operating RPM to an intermediate RPM, which is below the threshold RPM.

[0136] The rotor stage (6) further comprises a temperature sensor (11) configured to measure the temperature of the stator (8, 9) during operation. The temperature of the rotor can be inferred from the temperature of the stator.

[0137] The rotational speed of the rotor (7) is held at the intermediate RPM during the dwell time. During the dwell time, the temperature sensor (11) continuously measures the temperature of the stators (8, 9). The controller (10) can compare the temperature signal received from the temperature sensor (11) to a threshold temperature. If the temperature of the stators (8, 9) indicates that the rotor has dropped below the threshold temperature (e.g., below 70°C), the drive to the rotor (7) can be removed such that the rotor stops (e.g., coasts down).

[0138] Figure 2(d) shows the rotor stage (6) when the rotor (7) has stopped rotating. As shown, there is no collision between the rotor (7) and the stators (8, 9) while coasting down until the rotor (7) stops rotating.

[0139] Between FIG. 2(c) and FIG. 2(d), the downstream clearance (y) is slightly reduced, but not enough to cause a collision between the rotor (7) and the stator (9).

[0140] As the pump cools further, it will return to the configuration shown in FIG. 2(a) and will reach its original configuration at approximately ambient temperature (eg, 20° C.).

[0141] 3 shows a flow chart of a method of operating a pump according to the present invention. Initially, prior to initiation of a method of stopping rotation of a rotor of a vacuum pump, the rotor is rotating at an operating RPM (12). The operating RPM is greater than a threshold RPM such that the rotor cannot coast down from the operating RPM to a stop without a relatively high probability of the rotor colliding with the stator.

[0142] Next, the method includes rotating the rotor at an intermediate RPM (13). Rotating at the intermediate RPM provides substantially no chance of the rotor colliding with the stator. The intermediate RPM is below a threshold RPM. The threshold temperature is a thermal steady state temperature of the vacuum pump when the rotor is rotating at the threshold RPM. The rotor is rotated at the intermediate RPM for a dwell time sufficient for the vacuum pump to be below the threshold temperature.

[0143] The method may further include activating a cooling system (14). The cooling system may be configured to reduce a temperature of the vacuum pump. Preferably, the temperature sensor is configured to measure a temperature of the rotor or the stator.

[0144] The method may further include measuring the temperature of the vacuum pump via a temperature sensor during the residence time (15). The temperature may be measured via the temperature sensor at predetermined time intervals during the residence time. Preferably, the predetermined time intervals are every second or less.

[0145] The method includes coasting down the rotation of the rotor until it stops (16).

[0146] It should be understood that various modifications can be made to the illustrated embodiments without departing from the spirit and scope of the invention as defined by the appended claims as interpreted under patent law. [Explanation of symbols]

[0147] 1 rotor stage (prior art) 2 Rotor (Prior Art) 3 Rotor shaft (prior art) 4 Stator (prior art) 5 Stator (prior art) 6 rotor stages 7 Rotor 8 Stator 9 Stator 10 Controller 11 Temperature Sensor 12 Spin the rotor at operating RPM 13 Rotor spinning at medium RPM 14 Start the cooling system 15 Measure the temperature of the vacuum pump with a temperature sensor 16 Coast down the rotor rotation until it stops

Claims

1. 1. A method for stopping rotation of a rotor of a vacuum pump, comprising the steps of: The vacuum pump includes a pump chamber including a rotor and a stator; Prior to commencing the method, the rotor is rotating at an operating RPM; the operating RPM is greater than a threshold RPM such that the rotor cannot coast down from the operating RPM to a stop without a relatively high probability of the rotor colliding with the stator; The method comprises: a) rotating the rotor at an intermediate RPM with substantially no probability of the rotor colliding with the stator for a dwell time sufficient for the vacuum pump to be at or below a threshold temperature, the intermediate RPM being less than or equal to the threshold RPM; b) thereafter coasting down the rotation of the rotor until it stops; Including, the threshold RPM being a maximum rotational speed at thermal steady state that allows the rotor to coast to a stop with substantially no probability of the rotor colliding with the stator; The method of claim 1, wherein the threshold temperature is the temperature of the vacuum pump when the rotor is rotating at the threshold RPM in a thermal steady state.

2. 2. The method of claim 1, wherein the vacuum pump comprises a temperature sensor configured to measure a temperature of the vacuum pump, the method further comprising measuring a temperature of the vacuum pump via the temperature sensor during the dwell time and determining whether the temperature is equal to or less than the threshold temperature.

3. 3. The method of claim 2, further comprising measuring the temperature of the vacuum pump via the temperature sensor at predetermined time intervals during the residence time.

4. 3. The method of claim 2, further comprising initiating a coast-down of rotation of the rotor from the intermediate RPM to a stop only when the temperature of the vacuum pump as measured by the temperature sensor is below the threshold temperature.

5. The method of claim 2 , wherein the temperature sensor is configured to measure a temperature of the rotor or the stator.

6. 2. The method of claim 1, wherein during step (a), the rotor rotates at a medium RPM for a predetermined dwell time.

7. The method of claim 6 , wherein the predetermined residence time is 600 seconds or less.

8. The method of claim 1 , wherein the vacuum pump comprises a controller, and the method is initiated by a single user input to the controller.

9. 10. The method of claim 1, wherein the vacuum pump comprises a cooling system configured to reduce a temperature of the vacuum pump, the method comprising activating the cooling system to reduce a temperature of the vacuum pump.

10. The method of claim 1 , wherein the threshold RPM is between 5000 RPM and 8000 RPM.

11. 1. A method for stopping rotation of a rotor of a vacuum pump, comprising the steps of: the vacuum pump comprises a pumping chamber including a rotor and a stator, the rotor rotating and at a thermal steady state; The method comprises: a) measuring the temperature of the vacuum pump; b) determining whether the rotor is rotating at an operating RPM greater than a threshold RPM; c) if the rotor is rotating at an operating RPM greater than the threshold RPM, stopping rotation of the rotor by a method according to any one of claims 1 to 10; Including, The method, wherein the threshold RPM is a maximum rotational speed at thermal steady state, where the rotor can coast down to a stop with substantially no probability of colliding with the stator.

12. A vacuum pump comprising a pump chamber including a rotor and a stator, and a controller configured to control a rotational speed of the rotor, the rotor and the stator are positioned such that a minimum distance between them results in a relatively high probability that the rotor will collide with the stator if, during operation, the rotor rotates above a threshold RPM and coasts down to a stop; the threshold RPM being the maximum rotational speed at which the rotor can coast to a stop in a thermal steady state with substantially no probability of the rotor colliding with the stator; During operation when the rotational speed of the rotor is reduced from an operating RPM greater than the threshold RPM, the controller is configured to reduce the rotational speed of the rotor to an intermediate RPM at which there is substantially no probability of the rotor colliding with the stator, and to hold the rotational speed at the intermediate RPM for a dwell time sufficient for the vacuum pump to be at or below a threshold temperature, the intermediate RPM being less than or equal to the threshold RPM; The controller is configured to coast down rotation of the rotor after the dwell time until rotation stops, and the threshold temperature is a temperature of the vacuum pump when the rotor is rotating at the threshold RPM in a thermal steady state.

13. 13. The vacuum pump of claim 12, further comprising a temperature sensor configured to measure a temperature of the rotor and / or the stator.

14. 14. A vacuum pump according to claim 12 or 13, further comprising a cooling system configured to reduce a temperature of the rotor.

15. 14. A vacuum pump according to claim 12 or 13, wherein the pump is a multi-stage vacuum pump.

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