WO2019166883A1

WO2019166883A1 - Method for operating a vacuum pumping system and vacuum pumping system suitable for implementing such method

Info

Publication number: WO2019166883A1
Application number: PCT/IB2019/050130
Authority: WO
Inventors: Andrea BERTALLOT; Andrea Lepore; Roberto Carboneri; Giuseppe De Palma
Original assignee: Agilent Technologies, Inc. A Delaware Corporation
Priority date: 2018-02-28
Filing date: 2019-01-08
Publication date: 2019-09-06
Also published as: IT201800003152A1

Abstract

The present invention relates to a method for operating a vacuum pumping system comprising a vacuum pump, a motor for driving said vacuum pump and an oil tank connected to said vacuum pump for providing oil as coolant and/or lubricating fluid. More particularly, the present invention relates to a method for operating such a vacuum pumping system at cold start. According to the invention, in case of "cold start", before starting the pump, the windings of the motor stator are energized while the motor rotor is kept stationary (200) and the power supplied to the windings of the motor stator is used for heating the motor stator (300). The heat can be transferred from the motor stator to the oil (400) and the temperature of the oil can be raised before the vacuum pump is started. As a result, when the motor rotor starts to rotate and to drive in rotation the pump rotor, the viscosity of the oil has decreased and the required torque has been reduced. The present invention also relates to a vacuum pumping system (50) which is particularly suitable for implementing the aforesaid method.

Description

METHOD FOR OPERATING A VACUUM PUMPING SYSTEM AND VACUUM PUMPING SYSTEM SUITABLE FOR IMPLEMENTING SUCH METHOD

Technical Field

The present invention relates to a method for operating a vacuum pumping system comprising a vacuum pump, a motor for driving said vacuum pump and an oil tank connected to said vacuum pump for providing oil as coolant and/or lubricating fluid.

More particularly, the present invention relates to operating such a vacuum pumping system at cold start.

The present invention also relates to a vacuum pumping system which is particularly suitable for implementing the aforesaid method.

Background Art

Vacuum pumps are used to achieve vacuum conditions, i.e. for evacuating a chamber (so-called “vacuum chamber”) for establishing sub-atmospheric pressure conditions in said chamber. Many different kinds of known vacuum pumps – having different structures and operating principles – are known and each time a specific vacuum pump can be selected according to the needs of a specific application, namely according to the degree of vacuum that is to be attained in the corresponding vacuum chamber.

In general, a vacuum pump comprises a pump housing, in which one or more pump inlet(s) and one or more pump outlet(s) are provided, and pumping elements, arranged in said pump housing and configured for pumping a gas from said pump inlet(s) to said pump outlet(s): by connecting the pump inlet(s) to the vacuum chamber, the vacuum pump allows the gas in the vacuum chamber to be evacuated, thus creating vacuum conditions in said chamber.

More specifically, several different kinds of vacuum pumps are known in which the pumping elements comprise a stationary stator and a rotatable rotor, which cooperate with each other for pumping the gas from the pump inlet(s) to the pump outlet (s). In such vacuum pumps, the rotor is generally mounted to a rotating shaft which is driven by a motor, namely by an electric motor.

Even more specifically, vacuum pumping systems are known in which the vacuum pump is connected to an oil tank, whereby oil can be transferred from the oil tank to the vacuum pump for acting as coolant and lubricating fluid.

By way of example, a vacuum pumping system of this kind is schematically shown in Figures 1 and 2.

In the example shown in Figures 1 and 2 the vacuum pumping system 150 comprises a rotary vane vacuum pump 110; rotary vane vacuum pumps are generally used to attain low vacuum conditions, i.e. in a pressure range from atmospheric pressure down to about 10^-1 Pa.

As shown in Figs. 1 and 2, a conventional rotary vane vacuum pump 110 generally comprises an outer housing 112, receiving a pump body 114 within which a stator surrounding and defining a cylindrical pumping chamber 116 is defined. The pumping chamber 116 accommodates a cylindrical rotor 118, which is eccentrically located with respect to the axis of the pumping chamber 116; one or more radially movable radial vanes 120 (two in the example shown in Fig. 2) are mounted on said rotor 118 and kept against the wall of the pumping chamber 116 by means of springs 122.

During operation of the vacuum pump 110, gas is sucked from a vacuum chamber through an inlet port 124 of the pump and passes, through a suction duct 126, into the pumping chamber 116, where it is pushed and thus compressed by vanes 120, and then it is exhausted through an exhaust duct 128 ending at a corresponding outlet port 130.

A proper amount of oil is introduced from an oil tank (not shown) into the outer casing 112 for acting as coolant and lubricating fluid. In the example shown in Figure 2, for instance, the inner casing 114 is immersed in an oil bath 132.

In order to drive the rotor 118 of the vacuum pump, the vacuum pumping system 150 further comprises a motor 140 and the pump rotor 118 is mounted to a rotation shaft which is driven by said motor.

The motor 140 generally is an electric motor comprising a stationary stator and a rotating rotor cooperating with each other, as well as an output shaft connected to the motor rotor, which is either connected to or integral with the rotation shaft of the pump rotor for driving said pump rotor in rotation.

A vacuum pumping system as shown in Figs. 1 and 2 is disclosed, for instance, in EP 1 591 663 by the same Applicant.

A major drawback of the vacuum pumping systems of the kind disclosed above is that at cold start (i.e. when the vacuum pump is not running yet and the oil temperature in the oil tank is below a given value) a high torque is required for drive the pump rotor in rotation.

This is mainly due to oil viscosity, which is strongly dependent on the temperature and is very high at low temperature.

It is therefore evident that such drawback is especially severe in applications in which ambient temperature is low.

In order to provide the torque required at cold start, the motor of the vacuum pumping system has to be oversized with respect to the requirements of the vacuum pumping system running at nominal speed.

This negatively affects the manufacturing costs of the vacuum pumping system as well as its overall size, which represents a severe drawback during shipping of the vacuum pumping system and installation thereof, especially in those applications in which little room is available.

It is therefore an object of the present invention to overcome the above-mentioned drawback of prior art, by providing a method for operating a vacuum pumping system allowing to avoid the need for an oversized motor.

It is a further object of the present invention to provide a vacuum pumping system which is particularly suitable for implementing such method.

The above and other objects are achieved by means of a method for operating a vacuum pumping system and a vacuum pumping system as claimed in the appended claims.

Disclosure of Invention

According to embodiments of the invention, a method for operating a vacuum pumping system comprising a vacuum pump, including a pump stator and a pump rotor cooperating with each other for pumping a gas, an electric motor, including a motor stator and a motor rotor and configured to drive said pump rotor in rotation, and an oil tank, connected to said vacuum pump for providing oil acting as coolant and/or lubricating fluid, is provided.

Before starting the pump (i.e. before the rotor of the pump starts to rotate), the method according to embodiments of the invention provides for supplying power to the motor stator while the motor rotor is kept stationary (i.e. the motor rotor does not rotate) and the power supplied to the motor stator is used for heating the motor stator.

According to embodiments of the inventions, before starting the pump, provides for checking if a first predetermined condition is met. For instance, this predetermined condition can be related to the temperature of the oil in the oil tank, which has to be higher than a predetermined minimum threshold.

If the aforesaid predetermined threshold is not met, power can be supplied to said motor stator while said motor rotor is kept stationary, the power supplied to said motor stator can be used for heating said motor stator and this heat can be transferred from said motor stator to said oil, thus increasing oil temperature and correspondingly reducing oil viscosity.

According to embodiments of the inventions, the motor rotor is kept stationary while supplying power to the motor stator until a second predetermined condition is met. For instance, this second predetermined condition can be related to the oil temperature in the oil tank, to the temperature of the motor stator or to any other relevant parameter.

Once, the aforesaid second predetermined condition is met, the motor rotor starts to rotate.

According to a preferred embodiment of the invention, the motor stator comprises windings and the power supplied to the motor stator energizes the windings of the motor stator.

The heat can be transferred from the motor stator to the oil and the temperature of the oil can be raised before the vacuum pump is started.

Advantageously, when the motor rotor starts to rotate and to drive in rotation the pump rotor, the viscosity of the oil has decreased and the required torque has been reduced.

This advantage is especially important in those applications in which ambient temperature is low.

Since an oversized motor with respect to the requirements of the vacuum pumping system running at nominal speed is no longer required for cold start of the vacuum pumping system, a motor designed for driving the vacuum pump at nominal conditions can be employed.

Since an oversized motor with respect to the requirements of the vacuum pumping system running at nominal speed is no longer required, the overall size of the vacuum pumping system can be reduced, which represents a remarkable advantage during shipping of the vacuum pumping system and installation thereof, especially in those applications in which little room is available.

The manufacturing costs of the vacuum pumping system can also be reduced.

Additional advantages can be achieved by embodiments the invention.

A reduction of the starting ramp time for bringing the pump rotor form the cold start to the nominal speed can be achieved.

Provision of any external heating elements for heating the oil in the oil tank as well as of the related control electronics can be avoided, which involves a reduction in the manufacturing costs of the vacuum pumping system, as well as in its complexity.

The heating of the windings of the motor stator can be measured and controlled by using suitable algorithms, preferably without the need for specific, additional electronics.

According to a preferred embodiment of the invention, while the vacuum pump is not running the pressure in the chamber to be evacuated can be measured by measuring variations in the electric resistance of the windings of the motor stator (or other windings provided in the vacuum pumping system), thus preferably avoiding the need for dedicated pressure gauges.

The measurement of the vacuum level through the variations in the electric resistance of the windings of the motor stator can be obtained by using suitable algorithms, preferably without the need for specific, additional electronics.

Although the heat can be transferred from the motor stator to the oil by any thermal transmission phenomenon, according to a preferred embodiment of the invention, the heat is transferred from the motor stator to the oil by thermal conduction.

Although the method according to the invention can be implemented in a wide range of vacuum pumping system with different constructions and designs, the present invention also provides embodiments of a vacuum pumping system which is particularly suitable for implementing the method according to the invention.

In embodiments of the vacuum pumping system according to the invention, one or more thermal conductors connecting the motor stator to the oil tank are provided.

In a preferred embodiment of the vacuum pumping system according to the invention, the aforesaid thermal conductors can comprise one or more pipes made of a material having a high thermal conductivity and connecting the motor stator to the oil tank and to the vacuum pump.

According to the aforesaid preferred embodiment, at cold start of the vacuum pump, said pipes are used for transferring heat from the motor stator to the oil in the oil tank. Conversely, during normal operation of the vacuum pump, said pipes can be used for transferring oil from the oil tank to the pumping chamber of the vacuum pump, where the oil act as a coolant and lubricating fluid.

In a particularly preferred embodiment of the vacuum pumping system according to the invention, the motor stator is received within the pumping chamber of the vacuum pump which houses the pump stator and the pump rotor and the aforesaid thermal conductors comprise one or more pipes made of a material having a high thermal conductivity and connecting the motor stator to the oil tank.

According to the aforesaid particularly preferred embodiment of the vacuum pumping system according to the invention, the pump rotor is preferably made as a hollow body comprising a cavity and the motor is received inside the pump rotor, the motor rotor being fastened to or integral with the inner surface of the cavity provided in the pump rotor and the motor stator being located inside said cavity.

The aforesaid particularly preferred embodiment of the vacuum pumping system according to the invention provides for a very compact arrangement, allowing to optimize heat transfer between the motor stator and the oil tank.

In embodiments of the vacuum pumping system according to the invention, said vacuum pumping system will be provided with a control unit programmed to carry out the method according to any embodiments of the invention.

The invention can be implemented in any vacuum pumping system comprising a vacuum pump, including a pump stator and a pump rotor cooperating with each other for pumping a gas, an electric motor, including a motor stator and a motor rotor and configured to drive said pump rotor in rotation, and an oil tank, connected to said vacuum pump for providing oil acting as coolant and/or lubricating fluid, is provided.

More particularly, the invention can be implemented in a vacuum pumping system comprising a rotary vane vacuum pump.

Brief Description of Drawings

Further features and advantages of the present invention will become more evident from the detailed description of a preferred embodiment of the invention, given by way of non-limiting example, with reference to the accompanying drawings, in which:

- Fig. 1 is a schematic perspective view of a vacuum pumping system according to prior art;

- Fig. 2 is a schematic cross-sectional view of the vacuum pump of the vacuum pumping system of Fig. 1;

- Fig. 3 is a block diagram schematically illustrating the main steps of the method according to the present invention;

- Fig. 4 is a schematic longitudinal sectional view of a vacuum pumping system which do not form part of the present invention and which is particularly suitable for implementing the method according to the invention;

- Fig. 5 is a schematic longitudinal sectional view of a vacuum pumping system according to a preferred embodiment of the present invention, which is particularly suitable for implementing the method according to the invention;

- Fig. 6 is a schematic cross-sectional view of the vacuum pumping system of Fig. 5.

Best Mode for Carrying Out the Invention

In the following, a preferred embodiment of the invention will be described in detail with reference by way of non-limiting example to a vacuum pumping system comprising a rotary vane vacuum pump. In any case, it is to be noted that the present invention could also be applied to vacuum pumping systems including a different kind of vacuum pump.

Fig. 3 depicts a method for operating a vacuum pumping system comprising a vacuum pump, such as a rotary vane vacuum pump, which comprises a pump housing, in which a pump inlet and a pump outlet are provided and which receives a stationary pump stator and a rotatable pump rotor cooperating with each other for pumping a gas from the pump inlet to the pump outlet, an electric motor, which comprises a motor stator and a motor rotor cooperating with each other for driving in rotation said pump rotor, and an oil tank, which is connected to the vacuum pump so that oil can be provided to the vacuum pump for acting as coolant and lubricating fluid.

When such vacuum pumping system has to be started and the oil temperature in the oil tank is below a given value, we talk about the so-called “cold start” of the vacuum pumping system. For instance, when the vacuum pumping system has to be started and the oil temperature in the oil tank is at ambient temperature, we talk about “cold start” of the vacuum pumping system. The “cold start” of the vacuum pumping system requires a remarkably high torque due to the viscosity of oil at low temperature.

A first condition is to be met (step 100) for starting the pump without adopting special measures. For instance, the temperature of oil in the oil tank has to be equal to or higher than a predetermined threshold.

If such first condition is not met, before starting the pump the method according to embodiments of the invention provides for a step in which the windings of the motor stator are energized by supplying power to the motor stator while the motor rotor (and therefore the pump rotor) is kept stationary (step 200).

During this step 200, the power supplied to the motor stator is not used for driving in rotation the motor rotor; instead, it is dissipated as heat, thus heating the motor stator and leading to an increase of its temperature (step 300).

This heat can be transferred from the motor stator to the oil, thus achieving an increase of the oil temperature and a corresponding reduction of its viscosity (step 400).

Supplying power to the motor stator windings while keeping the motor rotor stationary(step 200) is continued until a second, preset condition is met (step 500).

This second condition can be chosen, for instance, among the following ones:

the duration of the aforesaid step 200 reaches a predetermined value;
the temperature of the motor stator reaches a predetermined value;
the temperature of oil in the oil tank reaches a predetermined value.

A combination of the aforesaid or other conditions can also be envisaged.

Once the aforesaid second condition is met, rotation of the motor rotor is started so that the vacuum pump is started, the pump rotor being driven in rotation by the motor rotor (step 600).

Afterwards, the rotational speed of the pump rotor is progressively increased from zero to the nominal value (step 700).

Thanks to embodiments of the invention, in case of “cold start” when the pump rotor begins to rotate the oil temperature is higher than its initial temperature (e.g. the ambient temperature). As the oil viscosity strongly depends on its temperature, this means the oil viscosity when the pump rotor begins to rotate can be significantly lower than the oil viscosity at its initial temperature, so that the torque required for starting to drive the pump rotor in rotation can be significantly reduced.

Accordingly, for the same technical specifications of the driven vacuum pump, a motor which is e.g. smaller, lighter and/or cheaper with respect to prior art arrangements can be employed.

Advantageously, the technical specifications of the motor can be selected according to the nominal operation conditions of the vacuum pump, with no need for an oversized motor at cold start.

Advantageously, a compact, light and cheap vacuum pumping system can be obtained.

This is further enhanced by embodiments of the invention in which no additional, dedicated device for heating the oil in the oil tank has to be provided.

Advantageously, thanks to embodiments of the invention, the time needed for increasing the rotational speed of the pump rotor from zero to the nominal value, i.e. the so-called “starting ramp time” (step 700) can be reduced with respect to prior art solutions.

Although in embodiments of the invention the heat can be transferred from the motor stator to the oil tank (step 400) by any thermal transmission phenomenon, including radiation, according to a preferred embodiment of the invention the heat is transferred from the motor stator to the oil tank by thermal conduction, thus effectively increasing the oil temperature without subjecting the windings of the motor stator to excessive temperatures and thermal loads.

The method according to embodiments of the invention can be implemented in any vacuum pumping system comprising a vacuum pump, an electric motor for driving said vacuum pump and an oil tank connected to said vacuum pump for providing oil acting as coolant and lubricating fluid, including existing vacuum pumping systems. Preferably, the vacuum pumping system will be provided with a control unit programmed to carry out the method according to any embodiments of the invention.

The method according to embodiments of the invention is most effectively implemented in vacuum pumping systems having a construction which allows an effective heat transfer from the motor stator to the oil.

For instance, the method according to the invention could be effectively implemented in a vacuum pumping system designed according to the teachings of US 2014/0363319 by the same Applicant, as schematically shown in Fig. 4.

The vacuum pumping system 250 includes a vacuum pump 210 comprising an outer housing 212, in which pump inlet 224 and a pump outlet 230 are defined and inside which a pump stator 214 and a pump rotor 218 cooperating with each other for pumping a gas from the pump inlet to the pump outlet are arranged.

The outer casing 212 of the pump is filled with such an amount of oil that the pump stator 214 is immersed in an oil bath acting as cooling and lubricating fluid.

The vacuum pumping system is further provided with an electric motor 240, which comprises a motor stator 242 and a motor rotor 244, cooperating with each other for driving in rotation the pump rotor 218.

The motor 240 is immersed in an oil tank 232 which is in fluid communication with the outer housing 212 of the vacuum pump 210.

It is evident that, as the motor is directly immersed in the oil tank, in a vacuum pumping system designed according to the teachings of US 2014/0363319 an effective heat transfer from the motor stator to the oil can be achieved.

The present invention further provides for a vacuum pumping system which is particularly suitable for implementing the method according to the invention, a non-limiting exemplary embodiment of which will be described in the following with reference to Figs. 5 - 6.

The vacuum pumping system according to the invention comprises:

- a vacuum pump, such as a rotary vane vacuum pump, which comprises a pump housing, in which a pump inlet and a pump outlet are provided and which receives a stationary pump stator and a rotatable pump rotor cooperating with each other for pumping a gas from the pump inlet to the pump outlet;

- an oil tank, which is connected to said vacuum pump by means of an oil circuit comprising one or more pipes for providing oil from said oil tank to said vacuum pump;

- an electric motor, which comprises a motor stator and a motor rotor cooperating with each other for driving in rotation the pump rotor,

wherein one or more thermal conductors are provided connecting said motor stator to said oil tank.

Thanks to this arrangement, heat can be transferred from the motor stator to the oil in the oil tank by conduction.

In a preferred embodiment of the invention, said thermal conductors are pipes of the oil circuit connecting the oil tank to the vacuum pump.

In other words, said oil circuit comprises one or more pipes made of a material having a high thermal conductivity and passing through the motor stator.

Thanks to this arrangement, not only said pipes can be used for transferring heat from the motor stator to the oil tank at cold start of the pump, but they can also be exploited for cooling the vacuum pump during steady operation, as, oil can be sucked from the oil tank through said pipes and into the vacuum pump during operation of the vacuum pump.

A particularly preferred embodiment of a vacuum pumping system according to the invention is shown in Figs. 5 – 6.

With reference to Figs. 5 – 6, the vacuum pumping system 50 includes a rotary vane pump 10, which comprises a pump housing 12, in which a pump inlet 24 and a pump outlet 30 are provided and which receives a stationary pump stator 14 which surrounds and defines a cylindrical pumping chamber 16, which is connection with the pump inlet 24 and the pump outlet 30. The pumping chamber 16 accommodates a rotatable cylindrical rotor 18, which is eccentrically located with respect to the axis of said cylindrical pumping chamber. One or more radially movable radial vanes 20 (three in the example shown in Figs. 5 – 6) are mounted on said pump rotor 18 and are kept against the wall of the pumping chamber 16, either by means of corresponding springs or by the centrifugal force.

When the vacuum pump is running, gas is sucked from a vacuum chamber (not shown) to be evacuated through the pump inlet 24 of the pump and passes through an inlet duct 26 into the pumping chamber 16 where it is pushed and thus compressed by the vanes 20, and then it is exhausted through an exhaust duct 28 ending at the pump outlet 30.

The vacuum pumping system 50 further comprises an oil an oil tank 32 connected to the vacuum pump 10, whereby a proper amount of oil can be introduced from the oil tank 32 into the vacuum pump 10, so that the pump housing 12 is immersed in an oil bath, which acts as coolant and lubricating fluid.

The vacuum pumping system 50 further comprises a motor 40 for driving in rotation the pump rotor 18.

According to embodiments of the invention, the motor 40 is located in the pumping chamber 16 of the vacuum pump 10. More specifically, the motor 40 is an electric motor comprising a stationary stator 42 and a rotating rotor 44 cooperating with each other, and the motor stator 42 and the motor rotor 44 are located in the pumping chamber 16 of the vacuum pump 10.

To this purpose, the pump rotor 18 is made, at least in part, as a hollow body, so that a cavity 22 is defined within the body of said pump rotor and the motor 40 is received within said cavity 22.

More particularly, a cylindrical cavity 22 is defined in the cylindrical pump rotor 18, which cavity is parallel to and concentric with the body of said pump rotor, and the motor 40 is received within said cylindrical cavity 22.

In the shown embodiment, the cavity 22 extends over the whole axial length of the pump rotor 18, so that said pump rotor has the overall shape of a hollow cylinder. However, in alternative embodiments, the cavity 22 could extend over a portion only of the axial length of the pump rotor 18.

In the shown embodiment, the motor is a permanent magnet motor and the motor rotor comprises a plurality of permanent magnets 46 which are fixed to the inner surface of the cavity 22 of the pump rotor 18.

As the permanent magnets 46 of the motor rotor are fixed to the inner surface of the cavity 22 of the pump rotor 18, the motor rotor 44 and the pump rotor 18 together form a single rotor unit.

In the shown embodiment, these permanents magnets are shaped as slightly curved, rectangular slabs 46, arranged substantially parallel to the longitudinal axis of the pump rotor 18 and extending over a substantial portion of the axial length of the cavity 22, said slabs 46 being equally spaced along the inner wall of the cavity 22 in the circumferential direction.

Said slabs 46 preferably are even in number and they are preferably arranged so that the polarity of each slab is opposite to the polarity of the adjacent slabs.

It will be evident to the person skilled in the art that the motor rotor 44 could also be made with a different shape. For instance, such motor rotor could be made as a cylindrical sleeve fitted into the cavity 22 of the pump rotor 18. Furthermore, the motor rotor could be made integral with the inner surface of the cavity 22 of the pump rotor. Even in these alternative embodiments, the motor rotor 44 and the pump rotor 18 together form a single rotor unit.

The motor stator 42 is located inside the cavity 22 of the pump rotor 18 is fastened to or integral with the pump housing 12 and/or the pump stator 14. Said motor stator comprises a body made of ferromagnetic material (such as, ferrite, SMC materials and the like), having substantially the same axial length as the permanent magnets 46 and provided with a plurality of radial arms 48 carrying respective windings (not shown).

In the shown embodiment, the motor stator is made as a generally cylindrical body arranged parallel to and concentric with the cylindrical cavity 22. In other word, the air gap between the motor stator 42 and the motor rotor 44 has a constant width along the circumference of said motor stator and rotor 42, 44. Accordingly, in the shown embodiment, the motor rotor 44 and the pump rotor 18 are concentrically driven with respect to the longitudinal axis of said motor stator (i.e. to the longitudinal axis of the cavity 22).

However, in alternative embodiments of the invention, it is possible that the motor stator is made as a cylindrical body arranged parallel to the cylindrical cavity 22 but in an eccentric position with respect to the longitudinal axis of said cavity. In other word, the air gap between the motor stator 42 and the motor rotor 44 has a width at each point along the circumference of said motor stator and rotor 42, 44 which is variable over time. Accordingly, in such embodiments, the motor rotor 44 and the pump rotor 18 would be eccentrically driven with respect to the longitudinal axis of said motor stator (i.e. to the longitudinal axis of the cavity 22) and the axis of the motor rotor 44 (and of the pump rotor 18) moves following a circular or elliptical trajectory.

Thanks to the cooperation of the motor stator 42 and the motor rotor 44, during rotation of the pump rotor 18, said pump rotor 18 is magnetically suspended without contact inside the pumping chamber 16, which involves a remarkable reduction of the noise generated by the vacuum pump as well as of the vibrations generated by the vacuum pump, thus increasing the working life and reliability of the vacuum pumping system.

The vacuum pump 10 is closed at both its axial ends and the pump rotor 18 can be provided, at both its axial ends, with bushings (not shown), interposed between said pump rotor and the pump housing 12, which in turn is provided with seats for receiving said bushings. Due to the fact that the pump rotor 18 is suspended during operation of the pump, there is no contact on the bushings and such absence of contact advantageously involves a reduction in the power absorbed by the pump.

According to embodiments of the invention, the motor stator 42 is provided with one or more longitudinal through-hole(s) 51 (only one, centrally arranged through-hole in the example shown in Figs. 5 - 6) accommodating respective thermal conductor(s) 52 connecting the motor stator 42 to the oil tank 32.

More particularly, in the embodiment shown in Figs. 5 – 6, such thermal conductor is in the form of a pipe 52 made of a material with a high thermal conductivity, which extends through the motor stator 42 and projects into the adjacent oil tank 32, ending with a mouth 54 which is arranged so as to be always below the level of oil in the oil tank 32 during operation of the vacuum pumping system 50.

Thanks to such arrangement, not only the pipe 52 can be used for transferring heat from the motor stator 42 to the oil tank 32 at cold start of the pump, but it can also be exploited for cooling the vacuum pump during steady operation.

In fact, during operation of the vacuum pump, oil is sucked from the oil tank 32 through the pipe 52 and into the vacuum pump 10; to this purpose, the pipe 52 is provided with radial orifices 56 at both axial ends of the motor stator 42.

This arrangement turns out to be particularly effective for cooling the vacuum pump, as the oil is introduced in the vacuum pump close to the longitudinal axis of the pump itself.

It will be evident that, even if a similar arrangement could also be provided in a vacuum pumping system having a conventional structure with separate pump module and motor module, as disclosed in Figs. 1 – 2, the preferred embodiment shown in Figs. 5 – 6 is particularly advantageous for the purposes of the invention.

In fact, thanks to its extremely compact design, the vacuum pump 10, the motor 40 and the oil tank 32 are very close to each other, which allows to minimize heat losses and optimize heat transfer.

It is evident that the above disclosure has been given by way of non-limiting example and that several variants and modifications within the reach of the person skilled in the art are possible, without departing from the scope of the invention as defined by the appended claims.

For instance, although in the description of preferred embodiments of the invention reference has been made to a vacuum pumping system including a rotary vane vacuum pump, the invention could also be implemented in vacuum pumping systems including a different kind of vacuum pump.

Claims

Method for operating a vacuum pumping system, which vacuum pumping system comprises a vacuum pump, comprising a pump housing, in which a pump inlet and a pump outlet are provided and which receives a stationary pump stator and a rotatable pump rotor cooperating with each other for pumping a gas from said pump inlet to said pump outlet, an electric motor, which comprises a motor stator and a motor rotor cooperating with each other for driving in rotation said pump rotor, and an oil tank, which is connected to the vacuum pump for providing oil to said vacuum pump, characterized in that, before starting the pump, it comprises:

- checking if a predetermined condition is met (100; 500) and, in case said predetermined condition is not met:

- supplying power to said motor stator while said motor rotor is kept stationary (200);

- using said power supplied to said motor stator for heating said motor stator (300);

- transferring heat from said motor stator to said oil, thus incre
asing oil temperature and correspondingly reducing oil viscosity (400).
Method according to claim 1, wherein supplying power to said motor stator while said motor rotor is kept stationary (200) is continued until the duration of said step reaches a predetermined value.
Method according to claim 1 or 2, wherein supplying power to said motor stator while said motor rotor is kept stationary (200) is continued until the temperature of said motor stator reaches a predetermined value.
Method according to claim 1 or 2 or 3, wherein supplying power to said motor stator while said motor rotor is kept stationary (200) is continued until the temperature of said oil reaches a predetermined value.
Method according to any of the preceding claims, wherein heat is transferred from said motor stator to said oil (400) by thermal conduction.
Vacuum pumping system (50) comprising:

- a vacuum pump (10), comprising a pump housing (12), in which a pump inlet (24) and a pump outlet (30) are provided and which receives a stationary pump stator (14) and a rotatable pump rotor (18) cooperating with each other for pumping a gas from said pump inlet to said pump outlet;

- an oil tank (32), which is connected to said vacuum pump (10) by means of an oil circuit comprising one or more pipes for providing oil from said oil tank to said vacuum pump;

- an electric motor (40), which comprises a motor stator (42) and a motor rotor (44) cooperating with each other for driving in rotation said pump rotor, characterized in that it comprises one or more thermal conductors (52) connecting said motor stator (42) to said oil tank (32).
Vacuum pumping system (50) according to claim 6, wherein said one or more thermal conductor is /are pipe(s) (52) of said oil circuit connecting said oil tank to said vacuum pump.
Vacuum pumping system (50) according to claim 6 or 7, wherein said pump rotor (18) is at least partially made as a hollow body, whereby a cavity (22) is defined within the body of said pump rotor, the motor stator (42) and the motor rotor (44) are received in said cavity (22) and said motor stator (42) is provided with one or more longitudinal through-holes accommodating respective thermal conductors (52) connecting said motor stator (42) to said oil tank (32).
Vacuum pumping system (50) according to claim 8, wherein said one or more thermal conductors are in the form of pipe(s) (52) extending through said motor stator (42) and project into said oil tank (32), ending with a mouth (54) which is arranged below the oil level in said oil tank (32).
Vacuum pumping system (50) according to claim 9, wherein said one or more pipes (52) are provided with radial orifices (56) at both axial ends of said motor stator (42).