Abstract:
The present invention relates to a self-propelled surface milling machine, preferably in the form of an asphalt-milling machine, snow-milling machine or Surface Miner, having a mill roll which is drivable about a rotational axis, and a mill roll drive comprising an electrical motor which is accommodated within the mill roll, wherein stator and rotor of the electrical motor are accommodated within a dust and air-tightly sealed motor housing. It will be suggested to associate a cooling device comprising a closed circuit liquid cooling system to the electrical motor located within the milling roll body. Due to the high heat capacity of a suitable coolant, such as oil or mixed water and glycol small volume flows and hence small conduit cross sections will be sufficient. On the other hand, any incorporation of dust into the mill-roll drive as well as any formation of dust by discharge air can be avoided by the closed form of the circuit liquid cooling system.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a self-propelled surface milling machine, preferably in the form of an asphalt-milling machine, snow-milling machine or Surface Miner, having a mill roll which is drivable about a rotational axis, and a mill roll drive comprising at least one electrical motor which is accommodated within the mill roll, wherein stator and rotor of the electrical motor are accommodated within a dust and air-tightly sealed motor housing. 
         [0002]    Surface milling machines are continually self-propelled working machines which, with the aid of a rotating roll, crush a layer of asphalt or soil or the like by milling and which commonly continuously proceed with the aid of caterpillars to force the roll into the milling goods. In doing this said roll forms the main operational unit requiring high energy and thus is in need of an appropriate drive. In this regard DE 10 2007 007 996 B4 suggests a diesel-electric drive, wherein the milling roll of a Surface Miner is driven by means of an electrical motor provided with energy by a generator. The references WO 03/058031 A1, DE 10 2008 008 260 A1, DE 10 2007 044 090 A1, DE 10 2007 028 812 B4, DE 199 41 800 C2, DE 199 41 799 C2 or DE 20 2007 002 403 U1 disclose further embodiments as well, wherein instead of electromotoric drives hydraulic drives are also utilized in part which are fed with hydraulic power by an hydraulic pump driven by a diesel engine. 
         [0003]    From DE 10 2007 007 996 B4 a Surface Miner having an internal electrical motor drive for the mill roll is known. Here two squirrel cage motors each having an associated planetary gear are accommodated within the mill roll body, such that the mill roll drives are safely protected against external influences and damage by e. g. stones. In order to protect both gear and electrical motor against dust, adjacent front sides of the motor-gear-unit which is seated in a tubular frame part are sealed with pot-shaped housing parts which are brought into dust-tight connection with the support frame by a plug washer. 
         [0004]    Having such encapsulated electric drives within the mill roll thermal problems will arise because heat generated in the gear and in the motor will not sufficiently be discharged. 
         [0005]    Generally electrical motors are cooled by surface cooling or natural draft cooling having forced-air- or self-ventilation. However, these known methods of cooling do not apply to mobile surface milling machines such as Surface Miners, asphalt-milling machines and the like due to reasons of dust incorporation which may be caused by a mill roll operating in or on the soil. Furthermore, the drive, while in use, might also partially submerse into water, so an embodiment wherein the motor is sealed will be preferred. Moreover, depending on the soil at the operational site, heavy formation and resuspension of dust may be caused by strong flow of air that exits the motor, which might not be acceptable in most applications. 
         [0006]    It has thus been considered, to provide aspiration of cooling air through a snorkel type of device on a highest point of the machine, as in this location low formation of dust and hence low incorporation of dust into the motor may be achieved. However, the problem of dust formation by the exiting cooling air is still to be solved. Avoiding dust formation as far as possible can be achieved by a hermetically sealed motor wherein even exiting cooling air will be captured in a conduit and will be fed into an outlet at an elevated location on top of the machinery. Nevertheless a residual amount of dust will remain in the motor since dust formation is substantial with surface milling machines and aspirating power may not be increased arbitrarily. 
         [0007]    Operating with a hermetically sealed motor wherein the air is circulated in a closed circuit air system and is cooled by means of a heat exchanger having an air inlet and outlet on top has been taken into account. Herein, however, a problem will arise in that the large amounts of air required for this will require very large conduit cross sections down to the mill-roll drive and backwards which hardly can be accommodated in limited location and accordingly are difficult to protect against mechanical damage. 
       SUMMARY OF THE INVENTION 
       [0008]    Accordingly, the object of the present invention is to create an improved mobile surface milling machine of the type mentioned above, which circumvents prior art disadvantages and will suitably be elaborated by the invention. Especially a reduction in thermal stress of the mill roll drive shall be achieved without further increase of dust load. 
         [0009]    According to the invention the object will be accomplished by a self-propelled surface milling machine according to the description herein. Preferred embodiments will be the object of the description herein. 
         [0010]    It will thus be suggested, to associate a cooling device to the electrical motor of the mill roll rive which is located within the mill roll body, said cooling device having a closed circuit liquid cooling system. Due to high heat capacity of a suitable coolant, such as oil or mixed water and glycol, small volume flows and hence small conduit cross sections will be sufficient. On the other hand, any incorporation of dust into the mill-roll drive as well as any formation of dust by discharge air can be avoided by the closed form of the circuit liquid cooling system. 
         [0011]    Basically heat discharge from the liquid coolant may be done by various methods. In one preferred embodiment the circuit liquid cooling system has an heat exchanger which is arranged outside the mill roll, for cooling of the coolant, the heat exchanger being connected to a section of the circuit liquid cooling system associated to the electrical motor by coolant conduits extending out of the front side of the mill roll and which preferably may further proceed along or within the support frame for supporting of the roller body. Said heat exchanger may basically be arranged within the mill roll as well, but outside the motor housing, in order to transfer heat from the coolant to the surroundings. However, being arranged outside the mill roll the oil cooler or the heat exchanger will be stroked closer by surrounding air. Said heat exchanger may advantageously be arranged in a location in the machine well above the mill roll, in order to avoid clogging of the heat exchanger by dust. For localization of the heat exchanger various positions may be taken into account. 
         [0012]    For circulation of coolant a pump is provided which in one suitable embodiment of the invention may be arranged on one shaft end of the drive shaft of the electrical motor, the shaft end facing the external side of the mill roll. By this the pump is easy to access for maintenance. Suitably the pump is located outside the sealed motor housing, so there is no need to open the latter for maintenance. 
         [0013]    Alternatively or additionally said pump may as well be utilised for circulation of the gear lubricant which lubricates a gear connected to the electrical motor. Even in this case said pump may suitably be arranged on the shaft end of the electrical motor outside the sealed motor housing, the shaft end being located beyond the mill roll. Said pump, the dual or multiple pumps may suitably be driven by the drive shaft of the electrical motor, respectively. However, alternatively or additionally, a separate drive motor may be employed for said pump as well. 
         [0014]    In one advantageous embodiment of the invention a brake may be arranged on said shaft end of the electrical motor, the shaft end being located beyond the mill roll, which brake suitably acts on the drive shaft of the electrical motor and which accordingly also makes use of the transmission of the gear connected to the electrical motors for braking action, and consequently a smaller sized brake may be employed. Simultaneously the brake becomes easier to access and thus maintenance will be facilitated as well. In case that pump and brake will simultaneously be provided the brake, in one advantageous embodiment of the invention, may suitably be located between pump and electrical motor; thereby brake as well as pump is suitably arranged coaxially to the drive shaft of the electrical motor. 
         [0015]    Basically the coolant of the circuit liquid cooling system may be circulated within the electrical motor by various methods. For example in one embodiment of the invention jacket cooling of the stator sheet or direct cooling of the stator winding may be provided, for example by a separation cylinder for the rotor. The coolant may also be led through a cylindrical liquid chamber formed by the housing or a cooling coil which may be integral with the housing or incorporated into the stator sheet package. In order to accomplish cooling of the rotor as well the liquid may be led through the rotor via a rotational passage. 
         [0016]    Advantageously the cooling device for the electrical motor also provides cooling of the winding heads. Especially the cooling device in the internal space of the sealed motor housing may have a closed circuit liquid cooling system including forced circulation wherein the circuit liquid cooling system mentioned above has a heat exchanger which is stroked by the cooling air of the closed circuit liquid cooling system for cooling of the cooling air. Especially said cooling air may hereby be led across the winding heads for cooling. The cooling air will then be depleted of heat by heat exchange with the circuit liquid cooling system which in turn will discharge the heat to the surroundings. 
         [0017]    Said forced circulation of the cooling air within the motor housing may advantageously be effected by at least one ventilation wheel which may be located on top of the motor shaft for co-rotation with the latter. Advantageously two of these ventilation wheels may be located on top of the rotor shaft which advantageously may be formed in the form of a radial fan. 
         [0018]    In one embodiment of the invention the closed circuit air cooling system in the interior of the sealed motor housing will specifically be led across the winding heads. Therefore air passage means and/or guide means may be provided in the respective winding head space in order to guide the cooling air across the winding heads as well as through the cooling coils being exposed in the winding head spaces of the circuit liquid cooling system. By cooling down circulating internal air directly in or at the winding head space efficient cooling of the winding heads may be achieved without sacrificing a compact design. Incorporation of the cooling coils of the circuit liquid cooling system into the winding head is not required. 
         [0019]    Basically said cooling air passage means and/or guide means may have various forms. In one embodiment of the invention they are provided such that cooling air at the neck of the winding head, i. e. at the transition between winding head and stator sheets, passes through the winding head and circulates around the winding head, whereby the air flow passing through the winding head will pass between the external side of the winding head and housing, circulating around the front side of the winding head to the internal side of the winding head or will pass backwardly around the winding head. 
         [0020]    Especially the cooling air passage means and/or guide means may comprise preferably slot-shaped passage recesses in the winding head which are located on the neck of the winding head which are distributed across the circumference of the winding head. These passage recesses in the winding head may be achieved by various means keeping the winding cords on the neck of the winding head apart or spreading them apart. For example sleeve-shaped spacers may be provided between the cord strands exiting the stator sheets. In one embodiment of the invention other spacers preferably in the form of loops or ribbons may also be provided which will bundle the winding cords and which will keep the desired slot-shaped passage recesses clear. 
         [0021]    In addition or as an alternative to the passage recesses mentioned above, extending radially through the winding head, cooling air recesses extending through the winding head approximately axially in longitudinal direction may also be provided. In case that radial passage recesses described above are provided, these advantageously are in communication with said axial cooling air recesses. Thereby, improved cooling may as well be accomplished in the front area of the winding head. 
         [0022]    In one embodiment of the invention the air passage means and/or guide means for the cooling air define a multitude of flow paths passing annularly around the winding heads, each one passing through said passage recesses and annularly around a respective segment of the winding head in which a respective passage recess is provided. Said flow paths each pass radially through a passage recess, followed by axially passing between the winding head and the machine housing and then along the winding head, followed by radially passing around a section of the winding head located on the front side and axially on an internal side of the winding head back to the passage recess; optionally flow direction may also be oriented in reverse direction. 
         [0023]    The cooling coils may basically be located in various places within the winding head space; however, they are advantageously positioned in a section with strong cooling air circulation. In one advantageous embodiment of the invention the cooling coils may be located at the front sides of the winding heads. Thereby, high transfer of heat from the cooling air into the cooling coils can be accomplished while at the same time a compact design will be achieved. 
         [0024]    In order to achieve cooling of the rotor cooling air may also be led into the rotor. In order to achieve this, it will especially be provided that the winding head spaces, while being circuit closed air spaces, i. e. which are not in communication with the outer side of the machine, are, however, connected to each other via at least one air duct extending axially through the rotor. Advantageously, four or more axial cooling air recesses may pass through the rotor through which the two winding head spaces including circulating cooling air can communicate with each other. 
         [0025]    To achieve this, in one advantageous embodiment of the invention, the cooling air is led through the rotor in countercurrent. Said air passage means and/or guide means advantageously comprise a countercurrent device leading cooling air through the cooling air recesses in the rotor in the opposite direction. While a first set of cooling air recesses passes cooling air from a left side winding head space to a winding head space on the right side a second set of cooling air recesses in the rotor functions to pass cooling air in the opposite direction from the right side winding head space into the winding head space on the left side. 
         [0026]    Said countercurrent device may comprise attachment plates mounted directly on the rotor which are provided with vent holes each being in flow communication with at least one air duct in the rotor, the attachment plates located on adjacent front sides of the rotor being rotationally offset against each other in such that said vent holes of one attachment disc are in communication with a first set of air ducts of the rotor and the vent holes of the other attachment disc are in communication with a second set of air ducts of the rotor. Said attachment plates may advantageously constitute part of said ventilation wheels and possess paddle-shaped air conveying means. Advantageously said ventilation wheels may exhibit radial deviation means taking the form of the attachment plates extending through the passage recesses into the winding head, while on the other hand the vent holes pass by the ventilation part and constitute inlet ducts each being in communication with at least one cooling air duct in the rotor. 
         [0027]    In this context rotor ducts may be provided which on one side are connected to the internal side of the winding head through vent openings in the attachment disc and will lead into the inner circumferential side of the radial paddles of the respective attachment disc on the external side. 
         [0028]    Specifically if the motor is frequently operated at rpm significantly below its nominal rotation speed the conveying effect of such ventilation wheels driven by the motor shaft is no longer sufficient to cool the rotor. In one advantageous embodiment of the invention at least one ventilation wheel that can be driven by a fan motor independently from the rotor shaft may also be provided alternatively or in addition to said ventilation wheels located on the rotor shaft. 
         [0029]    Alternatively or additionally, in an embodiment of the invention, instead of an air- or liquid-cooled asynchronous squirrel-cage motor, a motor requiring no or almost no cooling of the rotor may be used. Especially, the electrical motor may be designed as a synchronous motor with a permanent magnet rotor. In such a permanent magnet type synchronous motor, comprising permanent magnets in the rotor instead of rotor bars, there are almost no rotor losses, so that no intensive cooling of the rotor is required. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    The invention is exemplified in more detail by way of examples of preferred embodiments and the accompanying drawings wherein: 
           [0031]      FIG. 1 : is a schematic representation of a self-propelled surface milling machine in the form of a Surface Miner, which may, however, according to one advantageous embodiment of the invention, be as well designed as an asphalt milling machine, 
           [0032]      FIG. 2 : is a schematic longitudinal section across the mill roll of the surface milling machine of  FIG. 1 , showing two mill roll drive units accommodated in the interior of the mill roll, each designed as an electrical motor with coupled planetary gear, 
           [0033]      FIG. 3 : is a longitudinal section across one of the electrical motors of  FIG. 2 , showing the closed circuit air system within the sealed motor housing, the cooling air being led in countercurrent direction through axial cooling air recesses from one winding head space to the winding head space on the opposite side and backwards, 
           [0034]      FIG. 4 : is a longitudinal section across one of the electrical motors of  FIG. 2  according to another embodiment of the invention, wherein a radial fan is provided on the shaft beyond the support plate of the motor, and 
           [0035]      FIG. 5 : is a longitudinal section across one of the electrical motors of  FIG. 2  according to another advantageous embodiment of the invention, wherein the electrical motor is designed as a synchronous motor with a permanent magnet rotor and the cooling air circuit is designed to provide cooling of the winding heads and is led in countercurrent direction through recesses in the rotor from one winding head space to the winding head space on the opposite side and backwards. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0036]      FIG. 1  shows a self-propelled surface milling machine such as a Surface Miner or asphalt milling machine, the main working unit thereof being a mill roll  2  which is rotationally drivable about a horizontal axle, the circumference of the former being equipped with milling tools suitable to crush a soil or asphalt layer in a milling action. Thereby, the surface milling machine  1  is continuously advanced by means of caterpillars  3  so that said mill roll  2  experiences continuous feed motion. Machine body  4  for which said caterpillars  3  provide mobile support on the ground and support of said mill roll  2  furthermore comprises conveying means for eliminating milled material. The milled material derived from the mill roll will then be transferred to an receiving conveyor  5  passing the milled material to a loading conveyor  6  for transfer of crushed material, for example, to a truck. Said receiving and loading conveyors  5  and  6  may, for instance, be designed as conveyor belt systems. 
         [0037]    According to  FIG. 2 , said mill roll  2  may be driven by electrical motors  20  which are connected to mill roll  2  by means of a gear designed as a planetary gear  8  and which optionally may be located inside the mill roll. Mill roll drives  7  each consisting of an electrical motor  20  and a planetary gear  8  at the same time serve for supporting of mill roll body  9 . As shown in  FIG. 2 , the two mill roll drives  7  are located on the right and left side inside the mill roll body  9  so that they preferably do not protrude from the front side of mill roll body  9 . The electrical motor  20  of each mill roll drive  7  is rigidly fixed to a support frame part  33  via its motor housing  21  and a support housing part  40 , the support frame part  33  engaging into the front side of the mill roll body  9  and being connected to the machine body  4  of the surface milling machine  1 . Alternatively, the motor housing  21  may constitute part of the support housing. A second support housing part  34  is in turn pivotally mounted, thereby applying two pivotal mounting points as far apart from each other as possible, and rigidly formed in axial and radial directions. In the embodiment shown in  FIG. 2  a conically shaped tilted fixed support  35  as well as a spaced radial support  36  are employed, cf.  FIG. 2 . 
         [0038]    Advantageously said gear  8  is designed as a planetary gear which may be designed as a multiple stage gear to permit realisation of a suitably large transmission stage within a small installation space. 
         [0039]    As a power supply for electrical motors  20  a generator which is driven by a combustion engine, for example in the form of a diesel engine is advantageously provided. 
         [0040]    Advantageously the electrical motors  20  may be fed from the generator optionally through a frequency converter or directly, i. e. in the absence of frequency converter or with the frequency converter being bridged, respectively. Actually a jumper will form a bypass of the supply conduit circumventing the frequency converter, said jumper being switchable by means of a switching element, for example designed as a circuit breaker, so that the motor can optionally be supplied either through the frequency converter or by circumventing the same. 
         [0041]    Instead of using multiple electrical motors  20  for driving the main working unit  2  only one electrical motor may be provided. In the embodiment shown two electrical motors  20  are provided each being in drive connection with mill roll  2 . 
         [0042]    The electrical machine  20  shown in  FIG. 3  comprises a shaft  19  with a rotor  12 , which both are pivotally mounted on support plates constituting part of a machine housing  21  and/or which frontally seal a jacket  22  surrounding stator  13  of the machine  20 . Said jacket  22  is provided with a jacket cooling through which coolant is circulated in a liquid cooling circuit  23 . Said jacket is attached gap-free, flush and/or flatly to the stator sheets in order to properly achieve heat transfer from stator  13  into the cooling jacket  22 . 
         [0043]    In addition to said liquid cooling circuit  23  cooling device  24  of the electrical machine  20  comprises an air cooling  25  for cooling of the winding heads  11  protruding into the winding head spaces  26 , which are defined by housing  21  or more precisely jacket  22  and support plates on both sides of stator  13  and rotor  12 . As shown in  FIG. 3 , stator  13  comprises a winding  14  which is partially embedded into the stator sheets of the stator  13  and beyond said stator sheets will form basket-shaped winding heads  11  from both sides. 
         [0044]    For cooling of said winding heads  11  an internal circulation of cooling air will be generated by ventilation wheels  16 , i. e. no surrounding air will be passed through the machine or over the winding heads  11 , respectively, but an internal circuit air cooling system will be generated for cooling said winding heads  11 . In order to withdraw heat from cooling air as shown in  FIG. 3  cooling coils  15  are provided in the winding head spaces  26  through which coolant is circulated. The circuit liquid cooling system extending through said cooling coils  15  may principally be separated from circuit liquid cooling system  23  of the jacket cooling  22 . However, connection of the cooling coils  15  to the circuit liquid cooling system  23  of the jacket cooling may advantageously be provided, which, depending on the thermal load of each of the machine parts, will apply parallel or serial connection of the cooling coils  15  to the jacket cooling  22  and the circuit liquid cooling system  23  feeding the latter. 
         [0045]    In order to achieve effective cooling of circulating cooling air said cooling coils  15  are advantageously provided with ribs on their external surface, for instance in the form of multiple axial ribs on each cooling coil to increase size of the heat transfer surface of cooling coils. 
         [0046]    In the embodiment shown in  FIG. 3  cooling coils  15  are essentially located on the front side of the winding heads  11  in a gap provided between the front side of said winding heads  11  and the support plates, said cooling coils  15  essentially extending annularly about the axle of shaft  19 . 
         [0047]    In the embodiment according to  FIG. 3  ventilation wheels  16  effecting air circulation are located directly on said shaft  19  and are driven by the latter. Advantageously, said ventilation wheels  16  are accommodated in the internal space  26  of the basket-shaped winding heads  11 . In the embodiment shown the ventilation wheels  16  are provided with radially operating paddles such that air is forced radially into the annular interstice limited by winding heads  11  from the inside and by jacket  22  from the outside, cf.  FIG. 3 . 
         [0048]    As is shown  FIGS. 3 and 4  winding heads  11  are provided with radial passage recesses  37  at their necks, i. e. in the transitional area towards the stator sheets, allowing the cooling air to pass through winding heads  11 . 
         [0049]    Said passage recesses  37  constitute part of air passage means and guide means effecting annular air circulation around basket-shaped winding heads  11 , as exemplified by flow arrows in  FIG. 3 . The cooling air forced towards the neck of the respective winding head  11  by ventilation wheels  16  passes through said passage recesses  37  and is then led along winding head  11  onto its outside surface, passing between winding head  11  and jacket  22  to the front side of the respective winding head  11  and around this front side back to the internal side of winding head  11 . Meanwhile the cooling air strikes over cooling coils  15  at the front side of winding head  11  so that heat previously transferred to the cooling air by the winding of winding head  11  will be withdrawn. 
         [0050]    The cooling air guidance further comprises air ducts  38  across rotor  12  from one winding head space  26  to the other winding head space on the opposite side and backwards. 
         [0051]    This cooling air guidance is effected by ventilation wheels  16  which are designed as attachment plates or compression plates respectively and which closely fit to the front side of the rotor  12  and are located on shaft  19 . The ventilation wheels essentially consist of a radially cantilevered flange to which suitable air conveyance means, for instance in the form of conveyor blades or conveyor paddles, respectively, are attached, and which is provided with vent holes which are located across the circumference and which are in communication with axial cooling air recesses or air ducts  38 , respectively, in rotor  12 , said air ducts axially passing through said rotor  12  and each one exits said rotor  12  on the front side thereof. The number of air ducts  38  in rotor  12  is twice of that of the vent holes in the attachment plates so that each of the attachment plates will be in communication with every other one of said air ducts  38  in rotor  12  by means of its vent holes. The two attachment plates are rotationally offset against each other such that a first set of air ducts  38  in rotor  12  is in communication with the left internal space of the winding head  11  via the vent holes while a second set of air ducts  38  of the rotor  12  is in communication with the internal space of the winding head  11  on the right side via the vent holes located in the other attachment disc so that cooling air circulation as exemplified by the flow arrows in  FIG. 3  is achieved. 
         [0052]    Cooling air circulation is embodied as follows: The fan of the ventilation wheels  16 , operating radially, forces cooling air through the passage recesses  37  provided at the necks of the winding heads  11  to the outside of the winding heads  11 . The cooling air forced through passage recesses  37  then circulates around winding heads  11  similarly to air guidance shown in  FIG. 3 , striking the external side between respective winding head  11  and jacket  22 , followed by surrounding the front side of winding head  11  and passing through cooling coils  15 , then reaching the internal side of winding heads  11 . From this point cooling air is forced into the vent holes of the respective attachment disc which in this respect constitute inlet ducts for air ducts  38  of the rotor  12 . The cooling air then passes via said cooling air ducts  38  through rotor  12 , reaching the fan of ventilation wheel  16  of the attachment plate provided on the external side of the rotor. At this point, the cooling air will accordingly pass through and around winding head  11  and then return across rotor  12  in countercurrent direction so that by said two ventilation wheels  16  a countercurrent cooling air flow is generated in rotor  12 . 
         [0053]    The electrical machine shown in  FIG. 4  is essentially designed according to the machine shown  FIG. 3 , the difference essentially being in that flow of internal air is generated by ventilation wheel  31  mounted on the shaft beyond the support plate and forces internal air into air ducts  38  of the rotor behind cooling coil  15  on the right side of  FIG. 4 , said support plate being provided with cooling air outlets and inlets, so cooling air can be circulated across the external surface of said support plate. For this purpose a bowl-shaped housing cap through which a closed circuit liquid cooling system is provided, is located on said outer side of the support plate. During standstill or at low rpm extensive cooling of the electrical machine  20  may be achieved by means of a fan motor. In this case the fan motor will drive an additional ventilation wheel mounted on the fan motor which in turn is mounted on the external side of the support plate. 
         [0054]    In the embodiment according to  FIG. 5  the electrical motor is designed as a synchronous motor with a permanent magnet rotor wherein the rotor does not contain bars but permanent magnets. This results in that almost no rotor losses occur so that the motor does not require extensive rotor cooling. As shown in  FIG. 5  the circuit liquid cooling system  23  may possess a jacket cooling section for cooling of jacket  22  and thus may furthermore comprise cooling coils  15  in winding head spaces  26  for cooling of the cooling air therein. 
         [0055]    The permanent magnet motor  20  comprises a rotor  12  equipped with permanent magnets  18  mounted on shaft  19  and having a stator  13  which is cooled by said liquid jacket cooling, which can be combined with an external heat exchanger serially, in parallel, or with both of them. The ventilation wheels  16  mounted on the shaft  19  will initiate the internal air flow within the respective winding head spaces  26 . The air passes through winding  14  as well as cooling coils  15 , which preferably consist of ribbed coils within the respective winding head space  26 , thereby forming a closed circuit. 
         [0056]    As shown in  FIGS. 3 to 5 , a pump  27  is advantageously located at the end of drive shaft  19  of the electrical motor  20  facing the external surface of mill roll body  9  of mill roll  2  which may serve for the circulation of the coolant of the circuit liquid cooling system  23  and/or for the circulation of lubricant for the planet gear  8  coupled with the electrical motor  20 . If oil is used as a coolant the oil may possibly be pumped through the electrical motor in order to provide cooling thereof as well as through the gear in order to provide lubrication as well as cooling thereof. Alternatively, however, the pump may also comprise two separate pump units, wherein one of which will circulate the coolant while the other one will circulate the lubricant for the gear. 
         [0057]    Said pump  27  is advantageously driven by drive shaft  19  of the electrical motor  20 . 
         [0058]    As shown in  FIG. 2 , in addition to pump  27  a brake  28  may also be provided at said shaft end. Optionally additional parts such as a rotation sensor may be provided. By arranging of pump  27  and brake  28  outside the motor housing  21  on the shaft end located on the outside of the mill roll of the electrical motor  20  ease of accessibility of said components is provided, thus further increasing availability of the machine. This easy-to-maintain construction also has the advantage in that brake  28 , even if it is exclusively designed as a holding brake, can be used as an emergency stop, although it will thermally be overloaded during this. This is due to the fact that its easy accessibility allows rapid repair. Furthermore, due to arranging the pump  27  at the end of the shaft of the electrical motor  20  no further additional power supply, for instance by means of wires, will be required.