Vacuum pump system

Abstract

A vacuum pumping system 100 comprises a plurality of vacuum pumps 102a-d, each pump having a respective speed or power threshold, and a plurality of motors 104a-d, each motor driving a respective pump

Description

FIELD OF THE INVENTION The present invention relates to a vacuum pump system comprising a plurality of vacuum pumps, and a method of controlling the vacuum pump system. BACKGROUND Vacuum pump systems are used in numerous industrial, scientific, and commercial applications to create vacuum or low-pressure environments. Examples of fields in which vacuum pumps may be employed include semiconductor manufacturing, chemical processing, and pharmaceuticals. Vacuum pump systems comprising multiple vacuum pumps arranged in series and / or in parallel are known. SUMMARY OF THE INVENTION In a first aspect, there is provided vacuum pump system comprising: a plurality of vacuum pumps, each vacuum pump being associated with a respective speed threshold or a respective power threshold; a plurality of motors, each motor being arranged to drive a respective one of the plurality of vacuum pumps; and one or more controllers configured to control the motors of the plurality of vacuum pumps such that either: sequentially, the speeds of the vacuum pumps are increased above their respective speed thresholds and subsequently decreased below their respective speed thresholds; or sequentially, the powers supplied to the vacuum pumps by the motors are increased above their respective power thresholds and subsequently decreased below their respective power thresholds. The one or more controllers may be configured to iteratively: control the motor of a selected vacuum pump of the plurality of vacuum pumps such that a speed of that vacuum pump is increased above its respective speed threshold or a power supplied to that vacuum pump is increased above its respective power threshold; responsive to one or more criteria being met: select a new vacuum pump; control the motor of the newly selected vacuum pump such that a speed of the newly selected vacuum pump is increased above its respective speed threshold or a power supplied to the newly selected vacuum pump is increased above its respective power threshold; and control the motor of the previously selected vacuum pump such that the speed of the previously selected vacuum pump is decreased below its respective speed threshold or the power supplied to the previously selected vacuum pump is decreased below its respective power threshold. The one or more controllers may be configured to determine, for each vacuum pump, an accumulated overload, wherein the accumulated overload for a vacuum pump is determined based on one or more parameters selected from the group of parameters consisting of: a cumulative time that the speed of the vacuum pump is above the respective speed threshold of that vacuum pump; a cumulative time that the power supplied to the vacuum pump is above the respective power threshold of that vacuum pump; a time that the speed of the vacuum pump is above the respective speed threshold of that vacuum pump without dropping below that respective speed threshold; a time that the power supplied to the vacuum pump is above the respective power threshold of that vacuum pump without dropping below that respective power threshold; P*t, where P is the power supplied to the motor of the vacuum pump and t is time that the vacuum pump is above its respective speed threshold or power threshold, e.g. without dropping below its respective speed or power threshold; l2t, where I is a current supplied to the motor of the vacuum pump and t is time that the vacuum pump is above its respective speed threshold or power threshold, e.g. without dropping below its respective speed or power threshold; a number of cycles of a drive of the motor of the vacuum pump for which the vacuum pump is above its respective speed threshold or power threshold; a temperature of a part of the vacuum pump; and a temperature of a part of the motor of the vacuum pump. The accumulated overload for a vacuum pump may be P*t. The one or more controllers may be configured to control the motors of the plurality of vacuum pumps such that, in a time period, each of the vacuum pumps experiences an approximately equal accumulated overload. The one or more controllers may be configured such that only those vacuum pumps of the plurality of vacuum pumps that satisfy one or more predefined criteria are controlled such that their speeds are increased above their respective speed thresholds and subsequently decreased below their respective speed thresholds or the powers supplied thereto are increased above their respective power thresholds and subsequently decreased below their respective power thresholds. The one or more predefined criteria to be satisfied by a vacuum pump may include one or more criteria selected from the group of criteria consisting of: a criterion that the vacuum pump is switched on; a criterion that a pressure within the vacuum pump is greater than or equal to a predefined pressure setpoint for that vacuum pump; a criterion that a speed of the vacuum pump is between a respective minimum speed threshold for the vacuum pump and the respective speed threshold for the vacuum pump; a criterion that a power supplied to the vacuum pump is between a respective minimum power threshold for the vacuum pump and the respective power threshold for the vacuum pump; a criterion that a temperature of the vacuum pump is at or below a temperature threshold; a criterion that a temperature of the motor of the vacuum pump is at or below a temperature threshold; a criterion that a current supplied to the variable speed motor of the vacuum pump is at or below a threshold current; a criterion that the speed of the vacuum pump has been below its respective speed threshold for greater than or equal to a predefined time threshold; and a criterion that the power supplied to the vacuum pump has been below its respective power threshold for greater than or equal to a predefined time threshold. The one or more controllers may comprise a proportional integral, PI, controller. The speed threshold of a vacuum pump may be a maximum continuous speed of the vacuum pump. The power threshold of a vacuum pump may be a maximum power that can be supplied to the vacuum pump to provide continuous operation of the vacuum pump. In a further aspect, there is provided a method for controlling a vacuum pump system, the vacuum pump system comprising a plurality of vacuum pumps and a plurality of motors, each vacuum pump being associated with a respective speed threshold or a respective power threshold, each motor arranged to drive a respective vacuum pump. The method comprises controlling, by one or more controllers, the motors of the plurality of vacuum pumps such that either: sequentially, the speeds of the vacuum pumps are increased above their respective speed thresholds and subsequently decreased below their respective speed thresholds; or sequentially, the powers supplied to the vacuum pumps by the motors are increased above their respective power thresholds and subsequently decreased below their respective power thresholds. In a further aspect, there is provided a vacuum pump system comprising: a plurality of vacuum pumps, each vacuum pump being associated with a respective speed threshold or a respective power threshold; a plurality of motors, each motor being arranged to drive a respective one of the plurality of vacuum pumps; and one or more controllers configured to control the motors of the plurality of vacuum pumps such that either: sequentially, the speeds of the vacuum pumps are decreased below their respective speed thresholds and subsequently increased above their respective speed thresholds; or sequentially, the powers supplied to the vacuum pumps by the motors are decreased below their respective power thresholds and subsequently increased above their respective power thresholds. The speed threshold of a vacuum pump may be a minimum non-zero speed at which that vacuum pump can be continuously operated. The power threshold of a vacuum pump may be a minimum non-zero power that can be supplied to the vacuum pump to provide continuous operation of the vacuum pump. The one or more controllers may be configured to iteratively perform the steps of: control the motor of a selected vacuum pump of the plurality of vacuum pumps such that a speed of the selected vacuum pump is decreased below its respective speed threshold or a power supplied to that vacuum pump is decreased below its respective power threshold; and responsive to one or more criteria being met: select a new vacuum pump; control the motor of the newly selected vacuum pump such that a speed of the newly selected vacuum pump is decreased below its respective speed threshold or a power supplied to the newly selected vacuum pump is decreased below its respective power threshold; and control the motor of the previously selected vacuum pump such that the speed of the previously selected vacuum pump is increased above its respective speed threshold or a power supplied to the previously selected vacuum pump is increased above its respective power threshold. The one or more controllers may be configured to determine, for each vacuum pump, an accumulated load, wherein the accumulated load for a vacuum pump is determined based on one or more parameters selected from the group of parameters consisting of: a cumulative time that the vacuum pump is below its respective nominal minimum speed; a cumulative time that the power supplied to the vacuum pump is below the respective power threshold of that vacuum pump; a time that the vacuum pump is below its respective speed threshold without exceeding its respective speed threshold; a time that the power supplied to the vacuum pump is below the respective power threshold of that vacuum pump without exceeding that respective power threshold; l2t, where I is a current supplied to the motor of the vacuum pump and t is time that the vacuum pump is below its respective speed threshold or power threshold, e.g. without exceeding its respective speed or power threshold; a number of cycles of a drive of the motor of the vacuum pump for which the vacuum pump is below its respective speed threshold or power threshold; and a temperature of a part, such as a bearing, e.g. an outer bearing, or gear, of the vacuum pump. The one or more controllers may be configured to control the motors of the plurality of vacuum pumps such that, in a time period, each of the vacuum pumps experiences an approximately equal accumulated load. In a further aspect, there is provided a method for controlling a vacuum pump system, the vacuum pump system comprising a plurality of vacuum pumps and a plurality of motors, each vacuum pump being associated with a respective speed threshold or a respective power threshold, each motor arranged to drive a respective vacuum pump. The method comprises controlling, by one or more controllers, the motors of the plurality of vacuum pumps such that either: sequentially, the speeds of the vacuum pumps are decreased below their respective speed thresholds and subsequently increased above their respective speed thresholds; or sequentially, the powers supplied to the vacuum pumps by the motors are decreased below their respective power thresholds and subsequently increased above their respective power thresholds. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic illustration (not to scale) of a vacuum pump system; Figure 2 is a schematic illustration of a graph illustrating operation of a vacuum pump; Figure 3 is a process flow chart showing certain steps of a method of controlling the vacuum pump system; Figure 4 is a schematic illustration of a graph illustrating operation of a plurality of vacuum pumps; Figure 5 is a process flow chart showing certain steps of a further method of controlling the vacuum pump system; and Figure 6 is a schematic illustration of a further vacuum pump system. DETAILED DESCRIPTION Figure 1 is a schematic illustration (not to scale) of an embodiment of a vacuum pump system 100. ln this embodiment, the vacuum pump system 100 comprises a plurality of vacuum pumps 102a-d, a plurality of motors 104a-d, a plurality of variable speed drives (VSD) 106a-d, and a controller 108. In this embodiment, the vacuum pump system 100 comprises four vacuum pumps 102a-d, namely a first vacuum pump 102a, a second vacuum pump 102b, a third vacuum pump 102c, and a fourth vacuum pump 102d. In this embodiment, the vacuum pump system 100 comprises four vacuum motors 104a-d, namely a first motor 104a, a second motor 104b, a third motor 104c, and a fourth motor 104d. In this embodiment, the vacuum pump system 100 comprises four VSDs 106a-d, namely a first VSD 106a, a second VSD 106b, a third VSD 106c, and a fourth VSD 106d. Although the vacuum pump system 100 of this embodiment comprises four of each of vacuum pumps 102a-d, motors 104a-d, and VSDs 106a-d, it will be appreciated by those skilled in the art that, in practice, the vacuum pump system 100 may comprise a different number of vacuum pumps 102a-d, motors 104a-d, and VSDs 106a-d, such as more than four of each. The vacuum pumps 102a-d may be any appropriate type of vacuum pumps, such rotary vane pumps, diaphragm pumps, turbomolecular pumps, or liquid ring pumps. In some embodiments, all of the vacuum pumps 102a-d may be the same type of vacuum pump, and indeed may be substantially identical to one another. In other embodiments, one or more of the vacuum pumps 102a-d may be a different type of vacuum pump from one or more of the other vacuum pumps. In this embodiment, the vacuum pumps 102a-d are coupled to a facility (not shown in Figure 1), such as one or more semiconductor fabrication chambers, by one or more vacuum lines. The vacuum pumps 102a-d are operable to create a vacuum or low-pressure environment within the facility. The vacuum pumps 102a-d may be connected together in parallel or in series to create the vacuum or low-pressure environment. The motors 104a-d are arranged to drive the vacuum pumps 102a-d. The motors 104a-d are variable-speed motors. The motors 104a-d are electric motors. Each motor 104a-d is physically connected to a respective vacuum pump 102a-d, for example either directly or through a coupling or belt-drive system, to drive that respective vacuum pump 102a-d. The VSDs 106a-d (which may also be referred to as inverters, variable frequency drives (VFD), or adjustable speed drives (ASD)) control the speeds of the motors 104a-d. More specifically, the VSDs 106a-d control the frequency and voltage of electrical power supplied to the motors 104a-d, thereby allowing control of the speeds of the motors 104a-d and, consequently, the speeds of the vacuum pumps 102a-d. Each VSD 106a-d is electrically connected to an input electric power supply (not shown in Figure 1). Each VSD 106a-d is further electrically connected, e.g. wired, to a respective motor 104a-d to provide and regulate electrical power to that motor 104a-d. In this embodiment, the controller 108 is a proportional integral, PI, controller. The controller 108 is coupled, for example via a wired or wireless connection, to each of the VSDs 106a-d. The controller 108 is configured to communicate with the VSDs 106a-d, to control the VSDs 106a-d to adjust the speeds of the motors 104a-d. The controller 108 may control the VSDs 106a-d, and thus the motors 104a-d and vacuum pumps 102a-d, independently of one another. In some embodiments, the controller 108 may comprise a humanmachine interface for receiving a human user input. In some embodiments, the controller 108 may monitor one or more system parameters, e.g., based on measurements received from one or more sensors within the vacuum pump system 100. Examples of such system parameters include, but are not limited to, a vacuum level, a motor speed, a temperature, and / or a pressure. Apparatus, including the controller 108, for implementing the above arrangement, and performing the method steps described below, may be provided by configuring or adapting any suitable apparatus, for example one or more computers or other processing apparatus or processors, and / or providing additional modules. The apparatus may comprise a computer, a network of computers, or one or more processors, for implementing instructions and using data, including instructions and data in the form of a computer program or plurality of computer programs stored in or on a machine-readable storage medium such as computer memory, a computer disk, ROM, PROM etc., or any combination of these or other storage media. In this embodiment, each of the vacuum pumps 102a-d is associated with a respective first speed threshold. In some embodiments, for example embodiments in which the vacuum pumps 102a-d are the same type of vacuum pump or are substantially identical to each other, the first speed thresholds of the vacuum pumps 102a-d may be equal. In this embodiment, each of the vacuum pumps 102a-d is associated with a respective second speed threshold. In some embodiments, for example embodiments in which the vacuum pumps 102a-d are the same type of vacuum pump or are substantially identical to each other, the second speed thresholds of the vacuum pumps 102a-d may be equal. In this embodiment, the first speed threshold of each of the vacuum pumps 102a-d is greater than the second speed threshold of that the vacuum pump 102a-d. The first speed threshold of a vacuum pump 102a-d may be a maximum continuous speed for that pump, i.e. a maximum speed that the pump may be operated at continuously that will not lead to an overload condition of the pump at any time. The first speed threshold for a vacuum pump 102a-d may be a rated speed value for the pump which may be determined, e.g. empirically, by a user or manufacturer of the pump. The second speed thresholds of the vacuum pumps 102a-d may be nonzero speeds. The second speed thresholds of a vacuum pump 102a-d may be a minimum continuous speed for that pump, i.e. a minimum non-zero speed that the pump may be operated at continuously that will allow the pump lubrication to work correctly. The second speed threshold for a vacuum pump 102a-d may be a rated speed value for the pump which may be determined, e.g. empirically, by a user or manufacturer of the pump. The maximum continuous speeds of the vacuum pumps 102a-d and the minimum continuous speeds may be upper and lower bounds, respectively, of respective nominal speed ranges for the vacuum pumps 102a-d. These nominal speed ranges may be ranges of vacuum pump speeds that the vacuum pumps 102a-d are designed to operate within to achieve preferred (and typically optimal) pump performance, efficiency, and longevity. The maximum continuous speed of a vacuum pump may be the maximum speed of the nominal speed range or specification for the vacuum pump. The maximum continuous speed of a vacuum pump may be provided or specified by a manufacturer of the vacuum pump and / or specified in technical documentation for the vacuum pump. The maximum continuous speed of a vacuum pump may be determined via testing and / or modelling, and / or may be based on one or more factors, such as pump design, pump type, motor design, motor type, pump size, pump capacity, pumping efficiency, and / or pump performance. The maximum continuous speed may be based on extensive testing, engineering considerations, and the vacuum pump's intended application. The minimum continuous (non-zero) speed of a vacuum pump is the minimum speed within the nominal speed range or specification for the vacuum pump. The minimum continuous speed (non-zero) speed of a vacuum pump may be the minimum speed at which the lubrication of the vacuum pump works correctly. The minimum continuous speed of a vacuum pump may be provided or specified by a manufacturer of the vacuum pump and / or specified in technical documentation for the vacuum pump. The minimum continuous speed of a vacuum pump may be determined via testing and / or modelling, and / or may be based on one or more factors, such as pump design, pump type, motor design, motor type, pump size, pump capacity, pumping efficiency, and / or pump performance. The minimum continuous speed may be based on extensive testing, engineering considerations, and the vacuum pump's intended application. The nominal speed range for a vacuum pump is the expected, recommended, or specified range within which the vacuum pump is intended to operate optimally or effectively according to its design and intended use. The nominal speed range may be provided or specified by a manufacturer of the vacuum pump and / or specified in technical documentation for the vacuum pump. The nominal speed range of a vacuum pump may be determined via testing and / or modelling, and / or may be based on one or more factors, such as pump design, pump type, motor design, motor type, pump size, pump capacity, pumping efficiency, and / or pump performance. The nominal speed range may be based on extensive testing, engineering considerations, and the vacuum pump's intended application. The first and second speed thresholds can be expressed in rotations per minute (RPM), which determines the rotational speed of a shaft of the vacuum pump. The first and second speed thresholds may be considered to be thresholds for the vacuum pump speed beyond which it is typically preferred the vacuum pump speed does not go, in normal / preferred operation. In this embodiment, each of the vacuum pumps 102a-d is associated with a respective first power threshold. In some embodiments, for example embodiments in which the vacuum pumps 102a-d are the same type of vacuum pump or are substantially identical to each other, the first power thresholds of the vacuum pumps 102a-d may be equal. In this embodiment, each of the vacuum pumps 102a-d is associated with a respective second power threshold. In some embodiments, for example embodiments in which the vacuum pumps 102a-d are the same type of vacuum pump or are substantially identical to each other, the second power thresholds of the vacuum pumps 102a-d may be equal. In this embodiment, the first power threshold of each of the vacuum pumps 102a-d is greater than the second power threshold of that the vacuum pump 102a-d. The first power threshold of a vacuum pump 102a-d may be a maximum continuous power for that pump, i.e. a maximum power (or torque) that the pump may be provided with continuously that will not lead to an overload condition of the pump at any time. The first power threshold for a vacuum pump 102a-d may be a rated power for the pump which may be determined, e.g. empirically, by a user or manufacturer of the pump. The second power thresholds of the vacuum pumps 102a-d may be nonzero power values. The second power threshold of a vacuum pump 102a-d may be a minimum continuous power for that pump, i.e. a minimum power that the pump may be operated with continuously that will allow the pump lubrication to operate correctly. The second power threshold for a vacuum pump 102a-d may be a rated power for the pump which may be determined, e.g. empirically, by a user or manufacturer of the pump. The maximum continuous powers of the vacuum pumps 102a-d and the minimum continuous powers may be upper and lower bounds, respectively, of respective nominal power ranges for the vacuum pumps 102a-d. These nominal power ranges may be ranges of vacuum pump powers that the vacuum pumps 102a-d are designed to operate within to achieve preferred (and typically optimal) pump performance, efficiency, and longevity. The maximum continuous power of a vacuum pump may be the maximum power of the nominal power range or specification for the vacuum pump. The maximum continuous power of a vacuum pump may be provided or specified by a manufacturer of the vacuum pump and / or specified in technical documentation for the vacuum pump. The maximum continuous power of a vacuum pump may be determined via testing and / or modelling, and / or may be based on one or more factors, such as pump design, pump type, motor design, motor type, pump size, pump capacity, pumping efficiency, and / or pump performance. The maximum continuous power may be based on extensive testing, engineering considerations, and the vacuum pump's intended application. The minimum continuous (non-zero) power of a vacuum pump is the minimum power within the nominal range or specification for the vacuum pump. The minimum continuous power of a vacuum pump may be provided or specified by a manufacturer of the vacuum pump and / or specified in technical documentation for the vacuum pump. The minimum continuous power of a vacuum pump may be determined via testing and / or modelling, and / or may be based on one or more factors, such as pump design, pump type, motor design, motor type, pump size, pump capacity, pumping efficiency, and / or pump performance. The minimum continuous power may be based on extensive testing, engineering considerations, and the vacuum pump's intended application. The nominal power range for a vacuum pump is the expected, recommended, or specified power range for the vacuum pump to enable the pump to operate optimally or effectively according to its design and intended use. The nominal power range may be provided or specified by a manufacturer of the vacuum pump and / or specified in technical documentation for the vacuum pump. The nominal power range of a vacuum pump may be determined via testing and / or modelling, and / or may be based on one or more factors, such as pump design, pump type, motor design, motor type, pump size, pump capacity, pumping efficiency, and / or pump performance. The nominal power range may be based on extensive testing, engineering considerations, and the vacuum pump's intended application. While the nominal speed and power ranges for a vacuum pump defines the expected or standard operational speeds and powers of the vacuum pump, respectively, vacuum pumps may be capable of operating outside one or both of these nominal ranges without causing immediate failure and / or significant detriment to the vacuum pump. However, running a vacuum pump significantly outside its nominal speed or power range for a prolonged time could result in decreased efficiency, increased wear, reduced lifespan, and / or potential malfunction. Operating a vacuum pump 102a-d at a speed above its first speed threshold or with a power above its first power threshold is herein referred to as “overloading” the vacuum pump. In some embodiments described herein, the overloading of a VSD-equipped vacuum pump may comprise deliberately increasing the current drawn by the VSD / inverter above an upper threshold level for a short duration of time. This can be done by increasing the motor speed or increasing the load on the motor. Preferably, the duration is short (e.g., less than 120 s) to avoid damage to the pump, motor, and / or VSD, and is typically followed by a time of lower load to allow for thermal recovery of the apparatus. In some embodiments described herein, operating a vacuum pump 102a-d at a speed below its second speed threshold or below its second power threshold may comprise deliberately decreasing the current drawn by the VSD / inverter below a lower threshold level for a short duration of time. This can be done by decreasing the motor speed or decreasing the load on the motor. Preferably, the duration is short (e.g. less than 120 s) to damage to the pump, motor, and / or VSD, and is typically followed by a time of higher load or the switching off of the vacuum pump. Figure 2 is a schematic illustration of a graph 200 that illustrates a way in which a vacuum pump 102a-d may be operated outside its nominal speed and / or power range, and specifically above its first speed threshold and / or first power threshold. The graph 200 comprises an x-axis 202 representative of time in seconds, and a y-axis 204 indictive of I2 (i.e., current squared) in A2. The current, I, is that supplied to the motor 104a-d by the VSD 106a-d to drive the vacuum pump 102a-d. The graph 200 indicates an l2-threshold 206. In this example, the I2-threshold 206 is equal to 400 A2. The l2-threshold 206 corresponds to a current that, when supplied to a motor 104a-d, would cause that motor 104a-d to drive the corresponding vacuum pump 12a-d at its first speed threshold and / or with its first power threshold. Thus, I2 values above the l2-threshold 206 may correspond to vacuum pump speeds above the first speed threshold of the vacuum pump and / or powers above the first power threshold, whereas I2 values below the I2-threshold 206 may correspond to vacuum pump speeds below the first speed threshold of the vacuum pump and / or powers below the first power threshold. The graph 200 comprises a plot 208 that shows a value of the square of the current, I, supplied to a motor 104a-d and used to drive a vacuum pump 102a-d over time, t. As can be seen from the graph 200, in this example operation, during a first time period 210, approximately between t = 0 s and t = 5 s, the motor 104a-d of the vacuum pump 102a-d is supplied with a current I that corresponds to an I2 value that exceeds the l2-threshold 206. Thus, during the first time period 210, the vacuum pump 102a-d is operated such that its speed exceeds its first speed threshold and / or the power exceeds the first power threshold, i.e., it is overloaded. During a second time period 212, approximately between t = 5 s and t = 12 s, the motor 104a-d of the vacuum pump 102a-d is supplied with a current I that corresponds to an I2 value that is below the l2-threshold 206. Thus, during the second time period 212, the vacuum pump 102a-d is operated such that its speed is below its first speed threshold and / or its power is below its first power threshold. During a third time period 214, approximately between t = 12 s and t = 15 s, the motor 104a-d of the vacuum pump 102a-d is supplied with a current I that corresponds to an I2 value that exceeds the l2-threshold 206. Thus, during the third time period 210, the vacuum pump 102a-d is operated such that its speed exceeds its first speed threshold and / or the power exceeds the first power threshold, i.e., it is overloaded. During a fourth time period 216, after approximately and t = 15 s, the motor 104a-d of the vacuum pump 102a-d is supplied with a current I that corresponds to an I2 value that is below the l2-threshold 206. Thus, during the fourth time period 216, the vacuum pump 102a-d is operated such that its speed is below its first speed threshold and / or its power is below its first power threshold. In this example, the vacuum pump 102a-d is overloaded during the first time period 210 before decreasing its speed and / or power in the second time period 212 below its first speed / power threshold, thereby allowing excess generated heat to dissipate from the vacuum pump 102a-d and / or the motor 104a-d and / or the VSD 106a-d. After the second time period 212, the vacuum pump 102a-d is again overloaded during the third time period 214 before decreasing its speed and / or power in the fourth time period 216 below its first speed / power threshold, thereby again allowing excess generated heat to dissipate from the vacuum pump 102a-d and / or the motor 104a-d and / or the VSD 106a-d. Operating a vacuum pump in this way, i.e. overloading the pump for short periods of time (e.g. up to 120 s) before reducing the pump speed to below the nominal maximum speed, advantageously tends to allow for increased pump performance while managing or limiting heat buildup so as to avoid damage to the pump, motor, and / or VSD. What will now be described is an embodiment of a method of controlling operation of the plurality of vacuum pumps 102a-d in the vacuum pump system 100 in which the vacuum pumps 102a-d are overloaded. Figure 3 is a process flow chart showing certain steps of a method 300 of controlling the plurality of vacuum pumps 102a-d such that, sequentially, the speeds of the vacuum pumps are increased above their respective first speed thresholds and subsequently decreased below their respective first speed thresholds. It will be appreciated by those skilled in the art that the method of Figure 3 could be implemented to control the plurality of vacuum pumps 102a-d such that, sequentially, the powers supplied to the vacuum pumps by the motors are increased above their respective first power thresholds and subsequently decreased below their respective first power thresholds. It should be noted that certain of the process steps depicted in the flowchart of Figure 3 and described below may be omitted or such process steps may be performed in differing order to that presented below and shown in Figure 3. Furthermore, although all the process steps have, for convenience and ease of understanding, been depicted as discrete temporally-sequential steps, nevertheless some of the process steps may in fact be performed simultaneously or at least overlapping to some extent temporally. For example, it is noted that at least steps s308 and s310, described below, may be performed in parallel at least to some extent, but have been represented as separate steps in the below account and Figure 3 for the sake of representing the overall process in a simple flowchart form. As described above, the vacuum pump system 100 comprises a plurality of vacuum pumps 102a-d and a plurality of variable speed motors 104a-d. Each vacuum pump 102a-d has a respective first speed threshold. Each variable speed motor 104a-d is arranged to drive a respective vacuum pump 102a-d. In this embodiment, the method 300 comprises controlling, by the controller 108 the variable speed motors 104a-d, via the VSDs 106a-d, such that, sequentially, the speeds of the vacuum pumps 102a-b are increased above their respective first speed thresholds and subsequently decreased below their respective first speed thresholds. In this embodiment, the method 300 comprises the following steps. At step s302, the controller 108 selects the first vacuum pump 102a to be overloaded. The selection of a vacuum pump for overloading may be based on one or more selection criteria. For example, in some embodiments, a vacuum pump 12a-d may only be selected for overloading if it satisfies one or more predefined selection criteria. The one or more predefined selection criteria may include, for example, one or more of the following selection criteria: a criterion that the vacuum pump is switched on; a criterion that a pressure within the vacuum pump is greater than or equal to a predefined pressure setpoint for that vacuum pump; a criterion that a speed of the vacuum pump is between the respective second speed threshold for the vacuum pump and the respective first speed threshold for the vacuum pump; a criterion that a power supplied to the vacuum pump is between the respective second power threshold for the vacuum pump and the respective power threshold for the vacuum pump; a criterion that a motor speed of the variable speed motor of the vacuum pump is between a respective nominal minimum motor speed and a respective nominal maximum motor speed; a criterion that a temperature of the vacuum pump is at or below a temperature threshold; a criterion that a temperature of the variable speed motor of the vacuum pump is at or below a temperature threshold; a criterion that a current supplied to the variable speed motor of the vacuum pump is at or below a threshold current; a criterion that the speed of the vacuum pump has been below its respective first speed threshold for greater than or equal to a predefined time threshold; a criterion that the power supplied to the vacuum pump has been below its respective power threshold for greater than or equal to a predefined time threshold; a criterion that the motor speed of the variable speed motor of the vacuum pump has been below its respective nominal maximum motor speed for greater than or equal to a predefined time threshold; a criterion that a total amount of overload time for the vacuum pump is below an overload threshold amount; a criterion that a DC-bus peak to peak voltage is below a threshold DC-bus peak to peak voltage level; a criterion that an inlet pressure and / or of the vacuum pump is / are each below a respective threshold level ( which may be based on water evaporation conditions); and one or more criteria dependent on operating conditions, such as ambient temperature, supply voltage, pump load, etc. At step s304, the controller 108 controls the first motor 104a of the selected first vacuum pump 102a, via the first VSD 106a, such that a speed of the first vacuum pump 102a is increased above the respective nominal maximum speed of the first vacuum pump 102a. At step s306, the controller 108 selects a new vacuum pump to be overloaded (i.e., a vacuum pump other than the first vacuum pump 102a), e.g. based on the selection criteria. In this example, the controller 108 may select the second vacuum pump 102b to be overloaded. At step s308, the controller 108 controls the motor 104b of the newly selected (i.e., second) vacuum pump 102b, via the corresponding VSD 106b, such that a speed of the newly selected vacuum pump 102b is increased above the respective nominal maximum speed of the second vacuum pump 102b. At step s310, the controller 108 controls the motor 104a of the previously selected (i.e., first) vacuum pump 102a, via the first VSD 106a, such that a speed of the previously selected vacuum pump 102a is decreased below the respective nominal maximum speed of the first vacuum pump 102a. The decreasing of the speed of the previously selected vacuum pump may be performed based on one or more of the selection criteria described above in relation to at step s302. For example, decreasing the speed of a vacuum pump may be delayed or avoided depending on operating conditions, such as ambient temperature, supply voltage, pump load, etc. For example, relatively low ambient temperatures may allow a pump to cool down faster allowing for its overloading to be performed for longer periods at a time. Any or all of steps s306, s308, and s310 may be performed responsive to one or more criteria being met. The one or more criteria may include, for example, one or more of the following criteria: a criterion that the vacuum pump currently being overloaded has experienced a threshold amount of “accumulated overload” (which is described in more detail later below); a criterion that the vacuum pump currently being overloaded has been overloaded for a threshold amount of time (e.g., either cumulatively or for the current instance of overloading); a criterion that a number of cycles of the vacuum pump currently being overloaded (or its motor or VSD) is equal to or greater than a threshold value; and / or a criterion that a temperature of the vacuum pump currently being overloaded (or its motor or VSD) is equal to or greater than a threshold value. This advantageously tends to provide that the overloading of a vacuum pump is limited before the speed of that vacuum pump is reduced and the thermal recovery of the vacuum pump is enabled. After step s312, the controller 108 determines whether overloading of the vacuum pumps 102a-d is to be continued. If, at step s312, the controller 108 determines that the overloading of the vacuum pumps 102a-d is to be continued, the method 300 returns to step s306, whereat the controller 108 selects a new vacuum pump to be overloaded. Steps s308-s312 are then performed again, with step s308 being performed for the newly selected vacuum pump, and step s310 being performed for the previously selected vacuum pump. In some examples, the third and fourth vacuum pumps 102c,d may be sequentially selected in successive cycles of steps s306-s312. Each performance of the steps s306-s310 may be responsive to the above-mentioned one or more criteria being met. However, if, at step s312, the controller 108 determines that the overloading of the vacuum pumps 102a-d is to be discontinued, the method 300 proceeds to step s314. At step s314, the controller 108 controls the motor 104a-d of the currently selected vacuum pump 102a-d, via the relevant VSD 106a-d, such that a speed of that vacuum pump 102a is decreased below its respective first speed threshold. Thus, a method 300 of controlling the plurality of vacuum pumps 102a-d is provided. Figure 4 is a schematic illustration of a graph 400 that illustrates a way in which the vacuum pumps 102a-d may be operated outside of their nominal ranges of speeds, and specifically above their first speed threshold. The graph 400 of Figure 4 may correspond to a performance of the method 300 of Figure 3. The graph 400 comprises an x-axis 402 representative of time in seconds, and a y-axis 404 indictive of I2 (i.e., current squared) in A2. The current, I, is that supplied to the motors 104a-d by the VSDs 106a-d to drive the vacuum pumps 102a-d. The graph 400 indicates an l2-threshold 406. In this example, the I2-threshold 406 is equal to 400 A2. The l2-threshold 406 corresponds to a current that, when supplied to a motor 104a-d, would cause that motor 104a-d to drive the corresponding vacuum pump 12a-d at its first speed threshold. In this example, the l2-threshold 406 is assumed to be equal for all the vacuum pumps 102a-d. However, it will be appreciated by those skilled in the art that, in practice, one or more of the vacuum pumps 102a-d may have or may correspond to a different l2-threshold than one or more of the other vacuum pumps 102a-d. The graph 400 comprises a first plot 408 that shows the square of the current, I, supplied to the first motor 104a and used to drive the first vacuum pump 102a over time, t. The graph 400 further comprises a second plot 410 that shows the square of the current, I, supplied to the second motor 104b and used to drive the second vacuum pump 102b over time, t. The graph 400 further comprises a third plot 412 that shows the square of the current, I, supplied to the third motor 104c and used to drive the third vacuum pump 102c over time, t. The graph 400 further comprises a fourth plot 414 that shows the square of the current, I, supplied to the fourth motor 104d and used to drive the fourth vacuum pump 102d over time, t. As can be seen from the graph 400, in this example operation, the controller 108 controls the motors 104a-d such that, in sequence, the speeds of the vacuum pumps 102a-b are increased above their respective first speed thresholds and subsequently decreased below their respective first speed thresholds. More specifically, during a first time period 421, the first vacuum pump 102a is overloaded. At the start of the first time period 421, the speed of the first vacuum pump 102a is increased above its first speed threshold and, at the end of the first time period 421, the speed of the first vacuum pump 102a is reduced to below its first speed threshold. During a second time period 422, the second vacuum pump 102b is overloaded. At the start of the second time period 422, the speed of the second vacuum pump 102b is increased above its first speed threshold and, at the end of the second time period 422, the speed of the second vacuum pump 102b is reduced to below its first speed threshold. During a third time period 423, the third vacuum pump 102c is overloaded. At the start of the third time period 423, the speed of the third vacuum pump 102c is increased above its first speed threshold and, at the end of the third time period 423, the speed of the third vacuum pump 102c is reduced to below its first speed threshold. During a fourth time period 424, the fourth vacuum pump 102d is overloaded. At the start of the fourth time period 424, the speed of the fourth vacuum pump 102d is increased above its first speed threshold and, at the end of the fourth time period 424, the speed of the fourth vacuum pump 102d is reduced to below its first speed threshold. After the fourth time period, the overloading of the vacuum pumps 102a-d may end, or one or more vacuum pumps 102a-d may be overloaded again. For example, the sequential overloading of the vacuum pumps 102a-d may be performed again, adopting the same or a different sequence of pumps. In this example, the first, second, third, and fourth time periods 421-424 overlap each other at least to some extent. More specifically, the end of the first time period 421 overlaps with the beginning of the second time period 424, the end of the second time period 422 overlaps with the beginning of the third time period 423, and the end of the third time period 423 overlaps with the beginning of the fourth time period 424. In this way, the vacuum pumps 102a-d may be controlled to provide substantially continuous overloaded pumping performance. This tends to be provided without any individual vacuum pump 102a-d being overloaded excessively. In this embodiment, the controller 108 is configured to determine, for each vacuum pump 102, a so-called “accumulated overload”. The accumulated overload for a vacuum pump 102a-b may be determined based on one or more parameters selected from the group of parameters consisting of: a cumulative time that the vacuum pump 102a-d is above its respective first speed / power threshold (i.e., a cumulative time that the vacuum pump is above its respective first speed / power threshold across all or multiple instances of overloading that vacuum pump); a time that the vacuum pump 102a-d is above its respective first speed / power threshold without dropping below its respective first speed / power threshold (i.e., a time that the vacuum pump is above its respective first speed / power threshold for the present instance of overloading); P*t, where P is the power supplied to the motor of the vacuum pump and t is time that the vacuum pump is above its respective first speed / power threshold, for example without dropping below its respective first speed / power threshold (e.g., either cumulatively over all or multiple overloading instances, or for the present overloading instance only); an l2t value where I is a current supplied to the variable speed motor 104a-d of the vacuum pump 102a-d and t is time that the vacuum pump is above its respective first speed / power threshold (e.g., either cumulatively over all or multiple overloading instances, or for the present overloading instance only); a number of cycles of a variable speed drive 106a-d of the variable speed motor 104a-d of the vacuum pump 102a-d for which the vacuum pump 102a-d is above its respective first speed / power threshold; a temperature of a part of the vacuum pump 102a-d (e.g. of a gear or bearing); and a temperature of a part of the motor 104a-d of the vacuum pump 102a-d. Preferably, the accumulated overload for a vacuum pump 102a-d is equal to P*t or l2t. Preferably, the controller 108 is configured to control the variable speed motors 104a-d such that, in a time period, each of the vacuum pumps 102a-d experiences an approximately equal (e.g., + / -10%) accumulated overload. As mentioned above, the vacuum pumps 102a-d may be capable of operating outside their nominal ranges to some extent without causing immediate failure or significant detriment. Figures 2 to 4 describe embodiments in which the vacuum pumps may be overloaded, i.e. operated so as to exceed their nominal maximum speeds, without excessive overloading on any individual pump. However, in other embodiments, the vacuum pumps may be operated in such a way that the speed of the vacuum pumps are below their respective second speed thresholds or the powers supplied to the vacuum pumps are below their second power thresholds, without excessive wear being experienced by any individual pump. What will now be described is a further embodiment of a method of controlling operation of the plurality of vacuum pumps 102a-d in the vacuum pump system 100 in which the vacuum pump 102a-d are operated in such a way. Figure 5 is a process flow chart showing certain steps of a method 500 of controlling the plurality of vacuum pumps 102a-d. It should be noted that certain of the process steps depicted in the flowchart of Figure 5 and described below may be omitted or such process steps may be performed in differing order to that presented below and shown in Figure 5. Furthermore, although all the process steps have, for convenience and ease of understanding, been depicted as discrete temporally-sequential steps, nevertheless some of the process steps may in fact be performed simultaneously or at least overlapping to some extent temporally. For example, it is noted that at least steps s508 and s510, described below, may be performed in parallel at least to some extent, but have been represented as separate steps in the below account and Figure 5 for the sake of representing the overall process in a simple flowchart form. In this embodiment, the method 500 comprises controlling, by the controller 108 the variable speed motor 104a-d, via the VSDs 106a-d, such that, sequentially, the speeds of the vacuum pumps 102a-b are decreased below their respective second speed thresholds and subsequently increased above their respective second speed thresholds. It will be appreciated by those skilled in the art that the method of Figure 5 could be implemented to control the plurality of vacuum pumps 102a-d such that, sequentially, the powers supplied to the vacuum pumps by the motors are decreased below their respective second power thresholds and subsequently increased above their respective second power thresholds. In this embodiment, the method 500 comprises the following steps. At step s502, the controller 108 selects the first vacuum pump 102a for speed reduction. The selection of a vacuum pump for underloading may be based on one or more selection criteria. For example, in some embodiments, a vacuum pump 12a-d may only be selected for underloading if it satisfies one or more predefined selection criteria. The one or more predefined selection criteria may include, for example, one or more of the following selection criteria: a criterion that the vacuum pump is switched on; a criterion that a pressure within the vacuum pump is greater than or equal to a predefined pressure setpoint for that vacuum pump; a criterion that a speed of the vacuum pump is between the second speed threshold for the vacuum pump and the respective first speed threshold for the vacuum pump; a criterion that a power supplied to the vacuum pump is between the second power threshold for the vacuum pump and the respective first power threshold for the vacuum pump; a criterion that a motor speed of the variable speed motor of the vacuum pump is between a respective nominal minimum motor speed and a respective nominal maximum motor speed; a criterion that a temperature of the vacuum pump is at or below a temperature threshold; a criterion that a temperature of the variable speed motor of the vacuum pump is at or below a temperature threshold; a criterion that a current supplied to the variable speed motor of the vacuum pump is at or below a threshold current; a criterion that the speed of the vacuum pump has been above its respective second speed threshold for greater than or equal to a predefined time threshold; a criterion that the power supplied to the vacuum pump has been above its respective second power threshold for greater than or equal to a predefined time threshold; a criterion that the motor speed of the variable speed motor of the vacuum pump has been above its respective nominal minimum motor speed for greater than or equal to a predefined time threshold; a criterion that a total amount of overload time for the vacuum pump is below an overload threshold amount; a criterion that a DC-bus peak to peak voltage is below a threshold DC-bus peak to peak voltage level; a criterion that an inlet pressure and / or of the vacuum pump is / are each below a respective threshold level (which may be based on water evaporation conditions); and one or more criteria dependent on operating conditions, such as ambient temperature, supply voltage, pump load, lubrication supply, etc. At step s504, the controller 108 controls the first motor 104a of the selected first vacuum pump 102a, via the first VSD 106a, such that a speed of the first vacuum pump 102a is decreased below the respective second speed threshold of the first vacuum pump 102a. At step s506, the controller 108 selects a new vacuum pump to have its speed reduced (i.e., a vacuum pump other than the first vacuum pump 102a), e.g. based on the selection criteria. In this example, the controller 108 selects the second vacuum pump 102b to be underloaded. At step s508, the controller 108 controls the second motor 104b of the newly selected (i.e. second vacuum pump 102b, via the second VSD 106b, such that a speed of the second vacuum pump 102b is decreased below the respective second speed threshold of the second vacuum pump 102b. At step s510, the controller 108 controls the first motor 104a of the previously selected first vacuum pump 102a, via the first VSD 106a, such that a speed of the first vacuum pump 102a is increased above the respective second speed threshold of the first vacuum pump 102a. Any or all of steps s506, s508, and s510 may be performed responsive to one or more criteria being met. The one or more criteria may include, for example, one or more of the following criteria: a criterion that the vacuum pump currently being underloaded has experienced a threshold amount of “accumulated load” (which is described in more detail later below); a criterion that the vacuum pump currently being operated below its second speed threshold has been so for a threshold amount of time (e.g., either cumulatively or for the current instance of reducing speed below the second speed threshold); a criterion that a number of cycles of the vacuum pump currently being operated below its second speed threshold (or its motor or VSD) is equal to or greater than a threshold value; and / or a criterion that a temperature of the vacuum pump currently being operated below its second speed threshold (or its motor or VSD) is equal to or greater than a threshold value. This advantageously tends to provide that the operating of a vacuum pump below its second speed threshold is limited before the speed of that vacuum pump is increased and the thermal recovery of the vacuum pump is enabled. After step s512, the controller 108 determines whether sequential speed reduction of the vacuum pumps 102a-d is to be continued. If, at step s512, the controller 108 determines that the sequential speed reduction of the vacuum pumps 102a-d is to be continued, the method 500 returns to step s506, whereat the controller 108 selects a new vacuum pump to have its speed reduced. Steps s508-s512 are then performed again, with step s508 being performed for the newly selected vacuum pump, and step s510 being performed for the previously selected vacuum pump. In some examples, the third and fourth vacuum pumps 102c,d may be sequentially selected in successive cycles of steps s506-s512. Each performance of the steps s056-s510 may be responsive to the above-mentioned one or more criteria being met. However, if, at step s512, the controller 108 determines that the sequential speed reduction of the vacuum pumps 102a-d is to be discontinued, the method 500 proceeds to step s514. At step s514, the controller 108 controls the motor 104a-d of the currently selected vacuum pump 102a-d, via the relevant VSD 106a-d, such that a speed of that vacuum pump 102a is increased above its respective second speed threshold. Thus, a further method 500 of controlling the plurality of vacuum pumps 102a-d is provided. In an analogous way to that described in more detail earlier above with respect to Figure 4 and the case of overloading the vacuum pumps 102a-d, the method 500 of Figure 5 may be implemented in such a way that the vacuum pumps 102a-d may be controlled to provide substantially continuous operation below a level of the second speed thresholds. This tends to be provided without any individual vacuum pump 102a-d experiencing excessive detriment. In this embodiment, the controller 108 is configured to determine, for each vacuum pump 102, a so-called “accumulated load”. The accumulated load for a vacuum pump 102a-b may be determined based on one or more parameters selected from the group of parameters consisting of: a cumulative time that the vacuum pump 102a-d is below its respective second speed / power threshold (i.e., a cumulative time that the vacuum pump is below its respective second speed / power threshold across all or multiple instances of underloading that vacuum pump); a time that the vacuum pump 102a-d is below its respective second speed / power threshold without increasing above its respective second speed / power threshold (i.e., a time that the vacuum pump is below its respective second speed / power threshold for the present instance of underloading); an l2t value where I is a current supplied to the variable speed motor 104a-d of the vacuum pump 102a-d and t is time that the vacuum pump is below its respective second speed / power threshold (e.g., either cumulatively over all or multiple underloading instances, or for the present underloading instance only); P*t, where P is the power supplied to the motor of the vacuum pump and t is time that the vacuum pump is below its respective second speed / power threshold, for example without exceeding its respective second speed / power threshold (e.g., either cumulatively over all or multiple underloading instances, or for the present underloading instance only); a number of cycles of a variable speed drive 106a-d of the variable speed motor 104a-d of the vacuum pump 102a-d for which the vacuum pump 102a-d is below its respective second speed threshold; a temperature of a part of the vacuum pump 102a-d (e.g. of a gear or bearing); and a temperature of a part of the motor 104a-d of the vacuum pump 102a-d. Preferably, the accumulated underload for a vacuum pump 102a-d is equal to P*t or l2t. Preferably, the controller 108 is configured to control the variable speed motors 104a-d such that, in a time period, each of the vacuum pumps 102a-d experiences an approximately equal (e.g. + / -10%) accumulated load. The above-described apparatus and methods provide an intelligent control algorithm for a vacuum pump system with multiple pumps each equipped with a separate VSD. The control algorithm may rotate overload / underload over the different VSDs, thus modifying (e.g., boosting) pump performance more continuously. The above-described apparatus and methods tend to provide for more flow, the reduction in duration or elimination of pumpdown cycles, and the faster reaction of the vacuum pump system to pressure changes. In the above embodiments, overload and / or underload may be evaluated with an l2t-algorithm. Such an algorithm tends to account for heat dissipation being a limiting factor for both the motor and inverter. Heat dissipation in the motor stator tends to be mostly proportional to the I2. If the continuous sum of I2 over time is below a limit, the heat dissipation may be considered acceptable. The above-described methods and apparatus tend to control the overload / underloading of individual vacuum pumps, rotate or cycle overloading / underloading between available vacuum pumps, and decide when the rotating overload / underload algorithm should be implemented. In some embodiments, the overloading / underloading of a vacuum pump is for brief period of time, e.g. ranging from a few seconds up to 120 seconds. Running continuously at this increased speed / power in the same operating conditions may tend or cause inverter problems such as failures, such as overcurrent or overload. In some embodiments, a vacuum pump may be overloaded / underloaded for a longer (but limited) period of time, such as an hour or more. Overloading / underloading a vacuum pump continuously in the same operating conditions for longer may tend harm the pump. In the above-described embodiments, a timer may be set to define a maximum allowed overload / underload time. In the above-described embodiments, a respective input or setting may enable / disable overload / underload of each vacuum pump. The above-described methods and apparatus tend calculate and may output, for each vacuum pump, the accumulated overload (e.g., P*t, l2t) and / or the total amount of overload cycles per VSD. This may be useful in component lifing and / or determining maintenance schedules for components of the vacuum pump system. This may also be used to evaluate whether a vacuum pump has had sufficient time to thermally recover before being overloaded / underloaded again. The above-described methods and apparatus tend to provide for an equal amount of overload / underload distribution over the vacuum pumps so as not to put more strain on any individual vacuum pump, motor, or VSD. In the above embodiments, vacuum pumps that are already running (e.g. within their nominal speed ranges) may be prioritised for overloading / underloading, as they tend to be able to react faster to an overload / underload command. In the above embodiments, the controller may automatically decide when the rotating overload / underload algorithm should be activated. For central vacuum systems where, for example, a multi-pump controller (MPC) is present (such as that described in more detail above with reference to Figure 1), it may be desirable to achieve a constant process pressure. The vacuum pumps may be controlled to change their speeds (and flow) to have the process pressure as close as possible to a pressure setpoint while load changes are present. In some such systems, one vacuum pump may be considered to be the “leader” while all other pumps are “followers”. The followers may be run at speed references that can change, for example, every 5-10 seconds, optimized to energy consumption or maintenance requirements of the whole system. The leader may be PI controlled to react fast to pressure changes and fine tune the process pressure. Overloading the leader pump can be beneficial to give the follower pumps more time to react. For instance, if a follower pump needs to be started up, this might take a few seconds leading to an undesired process pressure increase. Overloading the leader pump could lead to more stable pressure control. Some vacuum pump systems have multiple, e.g. 2 to 6, pumps on one platform. Giving one of the pumps the possibility to overload and rotating that overloading around, tends to allow for continuously boosting the maximum pump flow by a significant amount. Preferably, the controller decides how much overload / underload on one pump is acceptable. Preferably, if for example vacuum pumps are simultaneously being overloaded and underloaded, the total flow of all pumps should be higher than in the situation where all pumps are running at their maximum continuous speeds. Otherwise, excess strain may be on certain pumps without a benefit in higher total flow. In the above embodiments, the vacuum pump system comprises multiple VSD-equipped vacuum pumps controlled by a single, central MPC. However, in other embodiments, the vacuum pump system has a different configuration of components or architecture. By way of example, Figure 6 is a schematic illustration of a further embodiment of a vacuum pump system, hereinafter referred to as the “further vacuum pump system” 600. In this embodiment, the further vacuum pump system 600 comprises a plurality of vacuum pumps 602a-d, a plurality of motors 604a-d, a plurality of VSDs 606a-d, and plurality of controllers 608a-d. The vacuum pumps 602a-d, the motors 604a-d, and the VSDs 606a-d may be arranged and connected together in the same way as those of the vacuum pump system 100 described in more detail earlier above with reference to Figure 1 In this embodiment, one or more of the controllers 608a-d is a PI controller. Each controller 608a-d is coupled, for example via a wired or wireless connection, to a respective one of the VSDs 606a-d. Each controller 608a-d is configured to communicate with a respective VSD 606a-d, to control that VSD 606a-d and thereby adjust the speed of the motors 604a-d coupled thereto. The controllers 608a-d may control the VSDs 606a-d, and thus the motors 604a-d and vacuum pumps 602a-d, independently of one another. The controllers 608a-d may be operatively coupled together so as to allow them to communicate with each other, thereby allowing for coordinated control of 5 the vacuum pumps 602a-d of the type described in more detail above. In some embodiments, an additional, central controller may be included, which may communicate with the controllers 608a-d to enable the coordinated control of the vacuum pumps 602a-d. In some embodiments, one or more of the controllers 608a-d may 10 comprise a human-machine interfaces for receiving a human user input. In some embodiments, one or more of the controllers 608a-d may monitor one or more system parameters, e.g. based on measurements received from one or more sensors within the further vacuum pump system 600. Examples of such system parameters include, but are not limited to, a vacuum level, a motor speed, a 15 temperature, and / or a pressure. Reference numeral list 100 - vacuum pump system 102a-d - vacuum pumps 104a-d - motors 106a-d - variable speed drives 108 - controller 200 - graph 202 - x-axis 204 - y-axis 206 - l2-threshold 208 - plot 210 - first time period 212 - second time period 214 - third time period 216 - fourth time period 300 - method s302 - s314 - method steps 400 - graph 402 - x-axis 404 - y-axis 406 - l2-threshold 408 - first plot 410 - second plot 412 - third plot 414 - fourth plot 421 - first time period 4222 - second time period 423 - third time period 424 - fourth time period 5 500 - method s502 - s514 - method steps 600 - further vacuum pump system 602a-d - vacuum pumps 604a-d - motors 10 606a-d - variable speed drives 608a-d - controllers

Claims

1. A vacuum pump system comprising:a plurality of vacuum pumps, each vacuum pump being associated with a respective speed threshold or a respective power threshold;a plurality of motors, each motor being arranged to drive a respective one of the plurality of vacuum pumps; andone or more controllers configured to control the motors of the plurality of vacuum pumps such that either:sequentially, the speeds of the vacuum pumps are increased above their respective speed thresholds and subsequently decreased below their respective speed thresholds; orsequentially, the powers supplied to the vacuum pumps by the motors are increased above their respective power thresholds and subsequently decreased below their respective power thresholds.

2. The vacuum pump system of claim 1, wherein the one or more controllers are configured to iteratively:control the motor of a selected vacuum pump of the plurality of vacuum pumps such that a speed of that vacuum pump is increased above its respective speed threshold or a power supplied to that vacuum pump is increased above its respective power threshold;responsive to one or more criteria being met:select a new vacuum pump;control the motor of the newly selected vacuum pump such that a speed of the newly selected vacuum pump is increased above its respective speed threshold or a power supplied to the newly selected vacuum pump is increased above its respective power threshold; andcontrol the motor of the previously selected vacuum pump such that the speed of the previously selected vacuum pump is decreased below itsrespective speed threshold or the power supplied to the previously selected vacuum pump is decreased below its respective power threshold.

3. The vacuum pump system of any preceding claim, wherein the one or more controllers are configured to determine, for each vacuum pump, an accumulated overload, wherein the accumulated overload for a vacuum pump is determined based on one or more parameters selected from the group of parameters consisting of:a cumulative time that the speed of the vacuum pump is above the respective speed threshold of that vacuum pump;a cumulative time that the power supplied to the vacuum pump is above the respective power threshold of that vacuum pump;a time that the speed of the vacuum pump is above the respective speed threshold of that vacuum pump without dropping below that respective speed threshold;a time that the power supplied to the vacuum pump is above the respective power threshold of that vacuum pump without dropping below that respective power threshold;P*t, where P is the power supplied to the motor of the vacuum pump and t is time that the vacuum pump is above its respective speed threshold or power threshold;l2t, where I is a current supplied to the motor of the vacuum pump and t is time that the vacuum pump is above its respective speed threshold or power threshold;a number of cycles of a drive of the motor of the vacuum pump for which the vacuum pump is above its respective speed threshold or power threshold;a temperature of a part of the vacuum pump; anda temperature of a part of the motor of the vacuum pump.

4. The vacuum pump system of claim 3, wherein the accumulated overload for a vacuum pump is P*t.

5. The vacuum pump system of claim 3 or 4, wherein the one or more controllers are configured to control the motors of the plurality of vacuum pumps such that, in a time period, each of the vacuum pumps experiences an approximately equal accumulated overload.

6. The vacuum pump system of any preceding claim, wherein:the one or more controllers are configured such that only those vacuum pumps of the plurality of vacuum pumps that satisfy one or more predefined criteria are controlled such that their speeds are increased above their respective speed thresholds and subsequently decreased below their respective speed thresholds or the powers supplied thereto are increased above their respective power thresholds and subsequently decreased below their respective power thresholds; andthe one or more predefined criteria to be satisfied by a vacuum pump include one or more criteria selected from the group of criteria consisting of:a criterion that the vacuum pump is switched on;a criterion that a pressure within the vacuum pump is greater than or equal to a predefined pressure setpoint for that vacuum pump;a criterion that a speed of the vacuum pump is between a respective minimum speed threshold for the vacuum pump and the respective speed threshold for the vacuum pump;a criterion that a power supplied to the vacuum pump is between a respective minimum power threshold for the vacuum pump and the respective power threshold for the vacuum pump;a criterion that a temperature of the vacuum pump is at or below a temperature threshold;a criterion that a temperature of the motor of the vacuum pump is at or below a temperature threshold;a criterion that a current supplied to the variable speed motor of the vacuum pump is at or below a threshold current;a criterion that the speed of the vacuum pump has been below its respective speed threshold for greater than or equal to a predefined time threshold; anda criterion that the power supplied to the vacuum pump has been below its respective power threshold for greater than or equal to a predefined time threshold.

7. The vacuum pump system of any preceding claim, wherein the one ormore controllers comprises a proportional integral, PI, controller.

8. The vacuum pump system any preceding claim, wherein:the speed threshold of a vacuum pump is a maximum continuous speed of the vacuum pump; and / orthe power threshold of a vacuum pump is a maximum power that can be supplied to the vacuum pump to provide continuous operation of the vacuum pump.

9. A method for controlling a vacuum pump system, the vacuum pump system comprising a plurality of vacuum pumps and a plurality of motors, each vacuum pump being associated with a respective speed threshold ora respective power threshold, each motor arranged to drive a respective vacuum pump, the method comprising:controlling, by one or more controllers, the motors of the plurality of vacuum pumps such that either:sequentially, the speeds of the vacuum pumps are increased above their respective speed thresholds and subsequently decreased below their respective speed thresholds; orsequentially, the powers supplied to the vacuum pumps by the motors are increased above their respective power thresholds and subsequently decreased below their respective power thresholds.

10. A vacuum pump system comprising:a plurality of vacuum pumps, each vacuum pump being associated with a respective speed threshold or a respective power threshold;a plurality of motors, each motor being arranged to drive a respective one of the plurality of vacuum pumps; andone or more controllers configured to control the motors of the plurality of vacuum pumps such that either:sequentially, the speeds of the vacuum pumps are decreased below their respective speed thresholds and subsequently increased above their respective speed thresholds; orsequentially, the powers supplied to the vacuum pumps by the motors are decreased below their respective power thresholds and subsequently increased above their respective power thresholds.

11. The vacuum pump system of claim 10, wherein:the speed threshold of a vacuum pump is a minimum non-zero speed at which that vacuum pump can be continuously operated; and / orthe power threshold of a vacuum pump is a minimum non-zero power that can be supplied to the vacuum pump to provide continuous operation of the vacuum pump.

12. The vacuum pump system of claim 10 or 11, wherein the one or more controllers are configured to iteratively:control the motor of a selected vacuum pump of the plurality of vacuum pumps such that a speed of the selected vacuum pump is decreased below its respective speed threshold or a power supplied to that vacuum pump is decreased below its respective power threshold; andresponsive to one or more criteria being met:select a new vacuum pump;control the motor of the newly selected vacuum pump such that a speed of the newly selected vacuum pump is decreased below its respective speed threshold or a power supplied to the newly selected vacuum pump is decreased below its respective power threshold; andcontrol the motor of the previously selected vacuum pump such that the speed of the previously selected vacuum pump is increased above its respective speed threshold or a power supplied to the previously selected vacuum pump is increased above its respective power threshold.

13. The vacuum pump system of any of claims 10 to 12, wherein the one or more controllers are configured to determine, for each vacuum pump, an accumulated load, wherein the accumulated load for a vacuum pump is determined based on one or more parameters selected from the group of parameters consisting of:a cumulative time that the vacuum pump is below its respective nominal minimum speed;a cumulative time that the power supplied to the vacuum pump is below the respective power threshold of that vacuum pump;a time that the vacuum pump is below its respective speed threshold without exceeding its respective speed threshold;a time that the power supplied to the vacuum pump is below the respective power threshold of that vacuum pump without exceeding that respective power threshold;l2t, where I is a current supplied to the motor of the vacuum pump and t is time that the vacuum pump is below its respective speed threshold or power threshold;a number of cycles of a drive of the motor of the vacuum pump for which the vacuum pump is below its respective speed threshold or power threshold; anda temperature of a part of the vacuum pump.

14. The vacuum pump system of claim 13, wherein the one or more controllers are configured to control the motors of the plurality of vacuum pumps such that, in a time period, each of the vacuum pumps experiences an approximately equal accumulated load.

15. A method for controlling a vacuum pump system, the vacuum pump system comprising a plurality of vacuum pumps and a plurality of motors, each vacuum pump being associated with a respective speed threshold ora respective power threshold, each motor arranged to drive a respective vacuum pump, the method comprising:controlling, by one or more controllers, the motors of the plurality of vacuum pumps such that either:sequentially, the speeds of the vacuum pumps are decreased below their respective speed thresholds and subsequently increased above their respective speed thresholds; orsequentially, the powers supplied to the vacuum pumps by the motors are decreased below their respective power thresholds and subsequently increased above their respective power thresholds.

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