Patent Publication Number: US-2023154655-A1

Title: Air cooled resistor

Description:
RELATED APPLICATIONS 
     The present application claims priority to European Patent Application No. 21208830.6, filed on Nov. 17, 2021, and entitled “AIR COOLED RESISTOR,” which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to an air cooled resistor arrangement. The air cooled resistor is advantageously incorporated into a vehicle and configured to dissipate electric power generated by an electric traction motor during braking. The present invention also relates to a vehicle comprising an electric traction motor for propelling the vehicle, as well as the air cooled resistor. Although the invention will mainly be directed to a vehicle in the form of a truck using an electric traction motor propelling the vehicle, the invention may also be applicable for other types of vehicles at least partially propelled by an electric traction motor, such as e.g. an electric vehicle, a hybrid vehicle comprising an electric machine as well as an internal combustion engine for propulsion. 
     BACKGROUND 
     The propulsion systems of vehicles are continuously developed to meet the demands from the market. A particular aspect relates to the emission of environmentally harmful exhaust gas. Therefore, vehicles propelled by electric machines and/or electric machine receiving electric power from hydrogen fuel cells have been increasingly popular, in particular for trucks and other heavy duty vehicles. 
     In comparison to a vehicle propelled solely by an internal combustion engine (ICE), a vehicle propelled by an electric machine conventionally struggles with obtaining the desired functionality of auxiliary braking. For an ICE operated vehicle, the auxiliary braking can be achieved by means of a retarder, etc. However, for an electric vehicle, the auxiliary braking functionality can be a dimensioning factor for the cooling system since the cooling capacity of e.g. a fuel cell electric vehicle (FCEV) as well as a battery electric vehicle (BEV) is a limiting factor. The reason is that for such type of vehicles, the auxiliary braking places a lot of energy in the cooling system. 
     There is thus a desire to provide a means for improving the dissipation of electric power when e.g. the vehicle battery is fully charged, i.e. when the so-called state-of-charge level is above a predetermined threshold limit. 
     SUMMARY 
     It is thus an object of the present invention to at least partially overcome the above described deficiencies. 
     According to a first aspect, there is provided an air cooled resistor arrangement, comprising a first elongated tube member comprising a first open inlet portion and a first open outlet portion, the first elongated tube member extending from the first open inlet portion to the first open outlet portion, a second elongated tube member comprising a second open inlet portion and a second open outlet portion, the second elongated tube member extending from the second open inlet portion to the second open outlet portion, an electrically conductive resistor element comprising an electric resistive material, the resistor element being connectable to a source of electric power, wherein the resistor element is arranged on a surface of the first elongated tube member, wherein the first elongated tube member is housed inside the second elongated tube member, the first elongated tube member and the second elongated tube member being spaced apart from each other continuously along the extension of the first elongated tube member by an air gap perpendicular to the extension of the first elongated tube member such that a flow of air entering the air cooled resistor arrangement is directed through the first elongated tube member and the air gap, and heated by the resistor element before being exhausted through the first and second open outlet portions. 
     The wording “tube member” should, if not explicitly referred to as otherwise, be construed as an elongated member which is open at its axial ends. The elongated member can have any suitable cross-sectional shape, such as e.g. a circular shape, the shape of a cylindric cylinder, an oval shape, a rectangular shape, etc. 
     Further, the electric resistive material, which may also be referred to as an electric resistance material, should be construed as a material which can resist the conduction of electric current, i.e. it has the ability to resist electric power. When receiving electric power, heat is generated in the electric resistive material. The electric resistive material e.g. be manufactured from ceramic materials, metal, metal alloys, etc. As is well known, the electrical resistivity of a material is different depending on the specific type of material used. The specific type of material used for the present invention is thus dependent on e.g. the application of use and the availability of such material. To put it differently, the skilled person can use the type of material that suits the application of use best. 
     The present invention is based on the insight that the use of a first and a second elongated tube member enables for an improved heat transfer of the air directed through the resistor arrangement, which air is configured to dissipate the electric power received by the electrically conductive resistor element from the source of electric power. In particular, by means of the present invention, air is flowing on each side of the electrically conductive resistor element which improves the heat transfer. Moreover, the air gap between the first and second elongated tube members provides an electrical insulation between the electrically conductive resistor element and the ambient environment outside the second elongated tube member. Also, the elongated tube members can be advantageously implemented to guide the heated air to a desired position of the vehicle. 
     Preferably, the cross-sectional flow area perpendicular to the flow direction along the first and second elongated tube members is small in comparison to the length of the longitudinal extension. Hereby, the air is able to flow through the first elongated tube member and the air gap at a relatively high air flow velocity. 
     In order to even further increase the heat transfer, the air cooled resistor arrangement may comprise turbulators. The turbulators may be provided inside the first elongated tube member and/or in the air gap formed between the first and second elongated tube members. Such turbulators may be formed by e.g. a radial protrusion arranged on the surface of the first and/or second elongated tube members. The protrusion hereby generates a turbulence of the flow of air, which may further increase the heat transfer. 
     According to an example embodiment, the second elongated tube member may be closed in a direction perpendicular to the elongation of the second elongated tube member between the second inlet portion and the second outlet portion. Hence, second elongated tube member is only open at the second inlet portion and the second outlet portion. Hereby, ambient air is prevented from reaching the air gap between the first and second elongated tube members. To put it differently, the second elongated tube member can hereby prevent the heat from the electrically conductive resistor element to reach the ambient environment. An advantage is thus that the second elongated tube member can form a heat shield to its ambient environment. This is particularly advantageous as the air cooled resistor can be positioned in the vicinity of components which are less heat resistant. 
     In a similar vein, and according to an example embodiment, the first elongated tube member may be closed in a direction perpendicular to the elongation of the first elongated tube member between the first inlet portion and the first outlet portion. 
     According to an example embodiment, the resistor element may extend between the first open inlet portion and the first open outlet portion. Hereby, the air directed through the first elongated tube member and the air gap between the first and second elongated tube members is heated throughout the entire travel along the elongated tube members. 
     According to an example embodiment, the air cooled resistor arrangement may further comprise at least one connecting element, the at least one connecting element connecting the first and second elongated tube members to each other. Hereby, the electrically conductive resistor element is kept in its desired position relative to the second elongated tube member. In yet further detail, the at least one connecting element advantageously ensures that a desired distance is provided between the first and second elongated tube members. Hence, the at least one connecting element provides for a desired air gap between the first and second elongated tube members. 
     According to an example embodiment, the at least one connecting element may be formed by an electrically insulating material. Hereby, the risk of accidentally electrifying the second elongated tube member is reduced. 
     According to an example embodiment, the first elongated tube member may comprise a heat conductive structure, the heat conductive structure is arranged on an inner wall portion of the first elongated tube member and extends in a direction perpendicular to the extension of the first elongated tube member and away from the second elongated tube member. The heat conductive structure thus extends towards a centre geometric axis of the first elongated tube member. An advantage is that the heat conductive structure is heated by the resistor element receiving electric power received from the source of electric power, thereby further improving the heat transfer through first and second elongated tube members. 
     According to an example embodiment, the heat conductive structure may extend in a direction parallel with the extension of the first elongated tube member. According to an example embodiment, the heat conductive structure may extend from the first open inlet portion to the first open outlet portion. An advantage is that the heat transfer is even further improved since the air will be heated by a larger area of material. The heat conductive structure may be arranged in different forms and shapes. According to an example embodiment, the heat conductive structure may be arranged in a honeycomb pattern. According to another example embodiment, the heat conductive structure may be formed by a plurality of taper shaped elements arranged on the inner wall portion of the first elongated tube member. 
     According to an example embodiment, the second elongated tube member may have a circular cross section. A circular cross section is particularly advantageous as it can sustain an air flow of relatively high pressure. The air cooled resistor arrangement can thus, for example, be arranged downstream an air compressor generating a flow of high pressurized air directed into the air cooled resistor. 
     According to an example embodiment, the first elongated tube member may have one of a circular cross section or a rectangular cross section. 
     According to an example embodiment, the resistor element may be formed by at least one resistor winding arranged around the surface of the first elongated tube member. Hereby, the first elongated tube member can be uniformly heated by electric power received through the resistor winding. Thus, a uniform heat distribution is provided. 
     According to a second aspect, there is provided a braking system for a vehicle, comprising an electric traction motor configured to propel the vehicle and to controllably regenerate electric power during regenerative braking of the vehicle, an electric machine comprising an output shaft, an air blower connected to the output shaft of the electric machine, the air blower being operable by the electric machine by rotation of the output shaft, wherein the air blower is arranged in an air conduit, an air cooled resistor arrangement according to any one of the example embodiments described above in relation to the first aspect, and a source of electric power electrically connected to the electric machine and to the electrically conductive resistor element of the air cooled resistor, the source of electric power comprising an electric storage system configured to receive and supply electric power, wherein the electric machine and the air cooled resistor arrangement are operated by electric power received from the electric power system, the electric power system being further electrically connected to the electric traction motor and configured to receive electric power during regenerative braking. 
     According to a third aspect, there is provided a vehicle, comprising an electric traction motor configured to propel the vehicle, a source of electric power comprising an electric storage system, wherein the source of electric power is electrically connected to the electric traction motor, and an air cooled resistor arrangement according to any one of the example embodiments described above in relation to the first aspect, wherein the electrically conductive resistor element is electrically connected to the source of electric power for dissipating electric power generated by the electric traction motor during braking. 
     The electric traction motor is thus arranged in a regenerative braking mode. It should however be observed that the regenerative braking mode should not necessarily be referred to as a mode where electric power is supplied to a vehicle battery. As is evident from the above, the electric power generated during regenerative braking can be supplied to the air cooled resistor, whereby the air cooled resistor dissipates the electric power generated by the electric traction motor. This may be particularly applicable when e.g. the state-of-charge level of the battery is above a predetermined threshold limit. 
     Effects and features of the second and third aspects are largely analogous to those described above in relation to the first aspect. 
     Further features of, and advantages will become apparent when studying the appended claims and the following description. The skilled person will realize that different features may be combined to create embodiments other than those described in the following, without departing from the scope of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above, as well as additional objects, features, and advantages, will be better understood through the following illustrative and non-limiting detailed description of exemplary embodiments, wherein: 
         FIG.  1    is a lateral side view illustrating an example embodiment of a vehicle in the form of a truck; 
         FIG.  2    is a schematic illustration of a braking system according to an example embodiment, 
         FIG.  3   a    is a perspective view of an air cooled resistor arrangement according to an example embodiment, 
         FIG.  3   b    is a detailed schematic illustration of the air cooled resistor arranged in  FIG.  3   a    according to an example embodiment, 
         FIG.  4    is a cross-sectional view of the air cooled resistor arrangement according to another example embodiment, and 
         FIGS.  5   a - 5   b    are perspective view of an alternative cross-section for the first elongated tube member of the air cooled resistor arrangement according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness. Like reference character refer to like elements throughout the description. 
     With particular reference to  FIG.  1   , there is depicted a vehicle  10  in the form of a truck. The vehicle comprises a traction motor  101  for propelling the wheels of the vehicle. In  FIG.  1   , the truck is depicted as being front wheel driven but is should be readily understood that the invention is equally applicable for a rear wheel driven truck, or a four wheel driven truck, etc. The traction motor  101  is in the example embodiment an electric traction motor  101  in the form of an electric machine, which is arranged to receive electric power from a source of electric power ( 104  in  FIG.  2   ), which may be e.g. an electric power system and/or a fuel cell system. The vehicle  10  also comprises a control unit  114  for controlling various operations as will also be described in further detail below, and a braking system (not shown in detail in  FIG.  1   ) operable to perform an auxiliary braking action for the vehicle  10 . 
     The control unit  114  may include a microprocessor, microcontroller, programmable digital signal processor or another programmable device. The control unit may also, or instead, include an application specific integrated circuit, a programmable gate array or programmable array logic, a programmable logic device, or a digital signal processor. Where the control unit  114  includes a programmable device such as the microprocessor, microcontroller or programmable digital signal processor mentioned above, the processor may further include computer executable code that controls operation of the programmable device. 
     In order to describe the braking system  100  in further detail, reference is made to  FIG.  2    which is a schematic illustration of a braking system according to an example embodiment. As can be seen in  FIG.  2   , the braking system  100  comprises an electric traction motor  101 , in  FIG.  2    illustrated as a pair of electric traction motors  101 . The braking system  100  further comprises an electric power system  104  which is connected to the electric traction motor(s)  101  for supply of electric power to the electric traction motor(s)  101  when the electric traction motor(s)  101  is/are propelling vehicle  10 , and to receive electric power from the electric traction motor(s)  101  when the electric traction motor(s)  101  operates in a regenerative braking mode. Thus, the braking system  100  can be referred to as an auxiliary braking system  100 . 
     The source of electric power  104  further comprises an electric storage system  160 . The electric storage system  160  is preferably arranged in the form of a vehicle battery and will in the following be referred to as a battery  162 . The battery  162  is configured to receive electric power generated by the electric traction motor(s)  101  when the electric traction motor(s)  101  operates in the regenerative braking mode. The battery  162  is also arranged to supply electric power to the electric traction motor(s)  101  when the electric traction motor(s)  101  propel the vehicle  10 . Although not depicted in  FIG.  2   , the source of electric power  104  may comprise various components, such as traction inverters, brake inverters, a junction box, etc. 
     The above described control unit  114  is connected to the source of electric power  104 . The control unit  114  comprises control circuitry for controlling operation of the electric power system. The control unit  114  thus receives data from the source of electric power  104 , such as e.g. a state-of-(SOC) of the battery  162 , etc, and transmits control signals to the source of electric power  104 . As will be evident from the below disclosure, the control signals from the control unit  114  to the source of electric power  104  may, for example, comprise instructions to which device the source of electric power  104  should supply electric power during regenerative braking. 
     The braking system  100  further comprises an electric machine  102  connected to the source of electric power  104 . The electric machine  102  is thus operated by receiving electric power from the source of electric power  104 . The electric machine  102  is thus arranged as an electric power consumer. The braking system  100  also comprises an air blower  106 . The air blower  106  is preferably an air compressor  106  and will in the following be referred to as such. The air compressor  106  is arranged in an air conduit  111  and configured to receive air  113 . The received air  113  is pressurized by the air compressor  106  and supplied further through the air conduit  111  downstream the air compressor  106 . The air compressor  106  is connected to, and operable by, the electric machine  102 . As illustrated in  FIG.  2   , the air compressor  106  is mechanically connected to the electric machine  102  by an output shaft  107  of the electric machine  102 . In further detail, the air compressor  106  is operated by rotation of the output shaft  107 , which rotation is generated by operating the electric machine  102 . 
     According to the non-limiting exemplified embodiment in  FIG.  2   , the braking system  100  further comprises a flow restriction arrangement  103  in the air conduit  111 . The flow restriction arrangement  103  is arranged in downstream fluid communication with the air compressor  106  and configured to increase the pressure level of the flow of air exhausted by the air compressor  106 . The braking system  100  also comprises an air cooled resistor arrangement  200  in the air conduit  111 . 
     The air cooled resistor arrangement  200  is arranged in the air conduit  111  in downstream fluid communication with the air compressor  106 . The air cooled resistor arrangement  200  is also electrically connected to, and operable by, the source of electric power  104 . In particular, the air cooled resistor arrangement  200  is electrically connected to the source of electric power  104  by means of electric wire cabling  202  where, as can be seen in e.g.  FIG.  3   a   , an electrically conductive resistor element of the air cooled resistor arrangement  200  comprises supply connections for connecting to the electric wire cabling  202 . Thus, also the air cooled resistor arrangement  200  is arranged as an electric power consumer. When the air cooled resistor arrangement  200  receives electric power from the source of electric power  104 , the pressurized air  113  from the air compressor is heated by the air cooled resistor arrangement  200 , which is described in further detail below with reference to  FIGS.  3   a - 5   b   . The pressurized and heated air is thereafter directed towards the ambient environment or other components in need of thermal management. The air from the air cooled resistor arrangement  200  is preferably directed into a muffler  150  of the braking system  100 . The muffler  150  reduces noise and can also provide a pressure drop of the air. 
     Although not depicted in  FIG.  2   , it should be readily understood that the control unit  114  can be connected to other components in addition to the connection to the source of electric power  104 . For example, the control unit  114  may be connected to the electric traction motor(s)  101 , the battery  162 , the electric machine  102 , the air cooled resistor arrangement  200 , as well as connected to an upper layer vehicle control system (not shown). 
     During operation of the braking system  100 , i.e. when the electric traction motor  101  operates as generators to control the vehicle speed, i.e. the vehicle  10  operates in the regenerative braking mode, electric power is transmitted from the electric traction motor  101  to the source of electric power  104 . If the battery  162  is not able to receive all, or parts of the electric power generated by the electric traction motor  101 , for example because of the current electric charging capacity, i.e. the level of electric power the battery is able to receive until being fully charged, has been reached, the excess electric power should preferably be dissipated. In the present case, the source of electric power  104  is controlled to supply electric power to the electric machine  102 . The electric machine  102  is hereby, by the received electric power from the electric power system  104 , rotating the output shaft  107  to operate the air compressor  107 . The air compressor  107  in turn pressurize air  117  and supply the pressurized air further through the air conduit  111 . 
     Accordingly, the control circuitry of the control unit  114  determines a level of electric power dissipation for the source of electric power  104 , i.e. a level of electric power that should be dissipated since it is not suitable to supply such power to the battery  162 . The level of electric power dissipation is hence a difference between the level of electric power generated during the regenerative braking and the current electric charging capacity of the battery  162 . If the electric machine  102  is able to handle, i.e. receive and be operated by, electric power corresponding to the level of electric power dissipation, all excess electric power, i.e. the generated power not being supplied to the battery  162  for charging, is supplied to the electric machine  102 . 
     However, there may be situations where the electric machine  102  is unable to receive a sufficient amount of electric power, or there is a desire to split the electric power between components of the braking system. For example, electric machine  102  may have a motor dissipation threshold. In further detail, the motor dissipation threshold is a maximum capacity of how much electric power the electric machine  102  can receive. Another limiting factor could be a temperature level of the air compressor  106 , as well as a temperature level of the electric machine  102 , e.g. at high ambient temperature conditions. If the electric machine  102  receives too much electric power, the rotational speed of the output shaft  107  is at a risk of being too high, or the temperature level of the air compressor  106  could be too high. 
     As such, the control circuitry of the control unit  114  may hereby control the source of electric power  104  to supply electric power also to the air cooled resistor arrangement  200 . The source of electric power  104  may be controlled to supply electric power also to the air cooled resistor arrangement  200  for other reasons than the electric power level being higher than the motor dissipation threshold, for example to simply reduce the rotational speed of the output shaft  107  to reduce the operation of the air compressor  106 , i.e. the speed of the air compressor  106 . The split of electric power supply to the electric machine  102  and the air cooled resistor arrangement  200  can also, for example, be controlled to provide a desired brake performance, a low outlet temperature and/or to reduce wear of components of the braking system  100 , etc. In particular, the temperature level of the air cooled resistor arrangement  200  may be used as an input parameter when determining how much electric power to supply to the electric machine  102 . 
     In order to describe the air cooled resistor arrangement in further detail, reference is now made to  FIG.  3   a   .  FIG.  3   a    is a perspective view of an air cooled resistor arrangement according to an example embodiment. As can be seen, the air cooled resistor arrangement  200  comprises a first elongated tube member  204 , which can also be referred to as an inner tube. In particular, the first elongated tube member  204  extends in an axial direction  208  from a first open inlet portion  203  to a first open outlet portion  205 . The air cooled resistor arrangement  200  also comprises a second elongated tube member  206 , which can be referred to as an outer tube. The second elongated tube member  206  extends in the axial direction from a second open inlet portion  207  to a second outlet portion  209 . Thus, and as can be seen in  FIG.  3   a   , the first elongated tube member  204  is housed by the second elongated tube member  206  in the axial direction  208  of the air cooled resistor arrangement  200  and the first  203  and second  207  open end portions allow the air  113  to enter the first  204  and second  206  elongated tube members. The air  113  is travelling through the first  204  and second  206  elongated tube members and is exhausted through the first  205  and second  209  open outlet portions. 
     In yet further detail, the first  204  and second  206  elongated tube members are spaced apart from each other in the radial direction along the axial direction  208  such that an air gap  210  is formed between the first  204  and second  206  elongated tube members. Each of the first  204  and second  206  elongated tube members has an inner surface and an outer surface. The inner surface  211  of the first elongated tube member  204  is facing an axially extending geometric centre axis. The outer surface  212  of the first elongated tube member  204  and the inner surface  214  of the second elongated tube member  206  are facing each other, while the outer surface  216  of the second elongated tube member  206  is facing away from the first elongated tube member  204 . A diameter of the outer surface  212  of the first elongated tube member  204  is thus smaller compared to a diameter of the inner surface  214  of the second elongated tube member  206  to form the air gap  210  therebetween. 
     The air cooled resistor  200  further comprises an electrically conductive resistor element  220 . The electrically conductive resistor element  220  is electrically connected to the source of electric power ( 104  in  FIG.  2   ). In particular, supply connections  222 ,  222 ′ of the electrically conductive resistor element  220  is connected to the electric wire cabling  202  for electrically connecting the electrically conductive resistor element  220  to the source of electric power  104 . 
     The electrically conductive resistor element  220  comprises, or is formed by, an electric resistive material and is configured resist the conduction of electric current received from the source of electric power  104 . As can be seen in  FIG.  3   a   , the electrically conductive resistor element  220  is arranged on the outer surface  212  of the first elongated tube member  204 . Although not depicted in  FIG.  3   a   , the electrically conductive resistor element  220  could be provided with an electrical isolation around the windings. When electric power is received by the electrically conductive resistor element  220  from the source of electric power  104 , the electrically conductive resistor element  220  is heated. Since the electrically conductive resistor element  220  is arranged on the first elongated tube member  204 , also the first elongated tube member  204  is heated. 
     As described above, air  113  is supplied into the first  203  and second  207  open end portions. The air  113  is thus flowing through the air cooled resistor arrangement  200 , through the first elongated tube member  204  as well as through the air gap  210  formed between the first  204  and second  206  elongated tube members. The air  113  will hereby be heated by the heat generated by the electrically conductive resistor element  220  when travelling between the open inlet portions  203 ,  207  and the open outlet portions  205 ,  209 . 
     Moreover, the first  204  and second  206  elongated tube members are in  FIG.  3   a    exemplified as having a circular cross section. Also, the first  204  and second  206  elongated tube members are arranged as solid structures along the axial direction, i.e. they are only open at the first and second open inlet portions as well as at the first and second open outlet portions. To put it differently, the first  204  and second  206  elongated tube members are closed in a direction which is perpendicular to the axial direction  208  between the open inlet and outlet portions. 
     As exemplified in  FIG.  3   a   , the electrically conductive resistor element  220  is arranged on the surface of the first elongated tube member  204  along the entire surface from the first open inlet portion  203  to the first open outlet portion  205 . As is also exemplified in  FIG.  3   a   , the electrically conductive resistor element  220  is arranged as a resistor winding around the surface of the first elongated tube member  204  and extends along the elongation of the first elongated tube member  204 . To put it differently, the resistor winding is arranged in the form of a coil spring enclosing the first elongated tube member  204 . Although  FIG.  3   a    illustrates the electrically conductive resistor element  220  as arranged on the outer surface  212  of the first elongated tube member  204 , the electrically conductive resistor element  220  can also be integrated in the surface of the first elongated tube member  204 . 
     As is also exemplified in  FIG.  3   a   , the air cooled resistor arrangement  200  comprises a heat conductive structure  230 . The heat conductive structure  230  is arranged on the first elongated tube member  204 , preferably on the inner surface, e.g. on an inner wall portion of the first elongated tube member  204 . As exemplified in  FIG.  3   a   , the heat conductive structure  230  is arranged in a honeycomb pattern  232  and extends radially in a direction away from the second elongated tube member  206 , i.e. towards the axially extending geometric centre axis. Preferably, the heat conductive structure  230  also extends in a direction parallel to the extension of the first elongated tube member  204 , i.e. in the axial direction  208 . According to an example, the conductive structure  230  extends along the first elongated tube member  204  form the first open inlet portion  203  to the first open outlet portion  205 . 
     Turning now to  FIG.  3   b    which is a detailed schematic illustration of the air cooled resistor arranged in  FIG.  3   a    according to an example embodiment. As can be seen in  FIG.  3   b   , the air cooled resistor arrangement  200  also comprises at least one, in  FIG.  3   b    illustrated as two, connecting elements  302 . The connecting element  302  is arranged between the first elongated tube member  204  and the second elongated tube member  206 . In particular, the connecting element  302  is arranged between outer surface  202  of the first elongated tube member  204  and the inner surface  214  of the second elongated tube member  206 . The connecting element  302  is thus connected to the outer surface  202  of the first elongated tube member  204  and to the inner surface  214  of the second elongated tube member  206 . 
     The connecting element  302  is in  FIG.  3   b    depicted as positioned at the open inlet portions. However, the connecting element  302  may be positioned elsewhere along the elongation of the first  204  and second  206  tube members. Also, a plurality of connecting elements  302  may be provided along the elongation of the first  204  and second  206  tube members. Such a plurality of connecting elements  302  are preferably distributed along the elongation at even intervals. 
     Moreover, the connecting element  302  is preferably formed by an electrically insulating material to reduce the risk of accidentally transmit electric power to the second elongated tube member  206 . 
     In order to describe the air cooled resistor arrangement according to further example embodiments, reference is made to  FIGS.  4 - 5     b . The above described connecting elements  302  are omitted from the description and illustration of  FIGS.  4 - 5     b  but should be construed as an alternative implementation also for these example embodiments. 
     Starting with  FIG.  4    which is a cross-sectional view of the air cooled resistor arrangement  200  according to another example embodiment. In a similar vein as the air cooled resistor  200  described above in relation to  FIGS.  3   a - 3   b   , the air cooled resistor  200  depicted in  FIG.  4    comprises a first  204  and second  206  elongated tube member arranged to form the air gap  210  therebetween. 
     In the embodiment depicted in  FIG.  4   , the electrically conductive resistor element  220  is wrapped around the outer surface  212  of the first elongated tube member  204 . Further, the air cooled resistor arrangement  200  comprises a heat conductive structure  230  arranged on the inner surface  211  of the first elongated tube member  204 . The heat conductive structure  230  extends radially inwards from the inner surface  211  of the first elongated tube member  204 . In a similar vein as described above, the heat conductive structure  230  may also extend along the axial direction  208  of the first elongated tube member  204 . 
     As exemplified in  FIG.  4   , the heat conductive structure  230  is formed by a plurality of taper shaped elements  234  arranged on the inner wall portion of the first elongated tube member. The heat conductive structure  230  is in  FIG.  4    also formed by a plurality of radially extending heat structures  234  having a first end  235  and a second end  236 , where each of the first  234  and second  235  ends are connected to the inner surface  211  of the first elongated tube member  204 . The flow of air  113  is thus flowing through the first elongated tube member  204  and heated by the inner surface  211  of the first elongated tube member  204  as well as heated by the heat conductive structure  230 . 
     Turning now to  FIGS.  5   a - 5   b   , which are perspective views of an alternative cross-section of the first elongated tube member  204  according to example embodiment. As is illustrated in  FIGS.  5   a - 5   b   , the cross-section of the first elongated tube member  204  is rectangular compared to the circular cross-section illustrated in  FIGS.  3   a   - 4 . The electrically conductive resistor element  220  is arranged on the outer surface  212  of the first elongated tube member  204 . In a similar vein as described above, an air gap  210  is formed between the first  204  and second  206  elongated tube members. 
     As depicted in the exemplified embodiment of  FIG.  5   b   , the air cooled resistor arrangement  200  may also comprise a heat conductive structure  230  arranged on the inner surface  211  of the first elongated tube member  204 . In the exemplified embodiment of  FIG.  5   b   , the heat conductive structure  230  is arranged in a wave-shaped pattern. 
     It is to be understood that the present disclosure is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims.