Abstract:
An electric motor, e.g., a linear motor, includes a cooling system, which may permit the dissipation of heat from the primary part of the electric motor in a simple but effective manner. The iron core and the coils may be surrounded by a potting compound that stabilizes one of the coils and the electrical leads of the latter so that channels are formed in the gaps of the iron core. A coolant, e.g., air, flows from a first flow chamber through the channels into a second flow chamber. The direct contact with the iron core, potting compound, and optionally with the regions of the coils that are not filled with compound produces effective cooling action, without having to forego advantages of stability provided by an encapsulated primary part.

Description:
FIELD OF THE INVENTION 
   The present invention relates to an electric motor having a cooling system. The present invention may be used for rotary motors, and it may be used, e.g., for linear motors. 
   BACKGROUND INFORMATION 
   For many applications, in which linear movements must ultimately be made, a linear motor as a direct drive may provide advantages over a conventional rotary drive. Since no mechanical elements, such as ball spindles or toothed belts, are required for converting a rotational movement into a linear movement, the possible conveying speed and the positioning accuracy of such a direct drive may not be unnecessarily limited. In this context, speed and force may be controlled over a wide range. Problems with reversing play do not occur during the positioning. In the case of linear direct drives, a comparatively low amount of wear occurs with rapid and frequent changes of direction, so that the service life increases in comparison with rotary drives, and, in the process, the positioning accuracy may not decrease with the operating time. 
   These features make linear motors interesting for, e.g., pick-and-place applications. In this case, e.g., individual electronic circuits (chips) of a diced silicon wafer must be received and inserted into a housing. Since the chips are very small, several tens of thousands of chips may fit on a wafer of normal size. On one hand, a gripping arm must be able to work very rapidly (several chips per second), and, on the other hand, it must be positioned in a very exact manner, in order to avoid damaging the chips while receiving them, and to be able to position them with the accuracy necessary for further processing. 
   Waste heat, which causes the operating temperature to increase, is formed in the coils of the primary part of an electric motor. Since temperatures that are too high may lead to the destruction of the motor, one may not often dispense with a cooling system. 
   In European Patent No. 0 793 870, a synchronous motor (linear or rotary) having a cooling system is described, whose primary part also contains cooling tubes in the gaps of an iron ore, next to the coils. Coolant flows through the cooling tubes, in order to remove heat from the motor. Primary parts of this kind may be encapsulated with a synthetic resin, in order to ensure that the coils, the leads for the coils, and the cooling tubes are securely supported. The use of cooling coils means an additional expenditure and consequently increases the cost of a linear motor to a considerable extent. 
   In, for example, FIG. 8 of U.S. Pat. No. 4,839,545, is shown an iron core, whose plates have separate cooling channels. However, these cooling channels can be blocked by synthetic resin, when they are situated close to the coils (in order to ensure effective cooling). In order to completely encapsulate the coils, synthetic resin must initially be introduced beyond the fill level actually desired, since the synthetic resin shrinks in the further manufacturing process. In this context, residues of the synthetic resin can remain in the cooling channels and block them. 
   It was already proposed that the encapsulation of the primary part be eliminated, in order to attain simple cooling. U.S. Pat. No. 5,751,077 describes a primary part of a linear motor in a sealed housing, whose coils are in direct contact with a coolant, since encapsulation is completely dispensed with. The housing of the primary part forms, together with the iron core, two flow chambers, between which the coolant flows past the coils and consequently provides for their cooling. However, due to the forces acting on the electric lines in such a linear motor, the failure to encapsulate and, thus, fix the coils, and in particular the electrical leads for the cells, in position does not produce a stable set-up. 
   SUMMARY 
   It is an aspect of the present invention to provide an electric motor having a cooling system for the primary part. The cooling system may allow effective heat dissipation, may be simple to produce, and may ensure sufficient stability of the primary part. 
   In the primary part of an electric motor, the iron core, which supports the coils and has teeth and gaps, may be only partially encapsulated in its housing. At least one region of the gaps of the iron core may remain unencapsulated. These unencapsulated regions of the gaps form channels and connect flow chambers on both sides of the iron core. A coolant may flow from a first flow chamber through the unencapsulated regions into the second flow chamber, and in so doing, it may dissipate heat, due to the direct contact with the iron core and the potting compound that may be as thermally conductive as possible. It may be possible to introduce the potting compound so that a part of the coils also comes in direct contact with the coolant, which may achieve particularly effective heat dissipation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a side view of a linear motor according to an exemplary embodiment of the present invention. 
       FIG. 2  illustrates a plan view of a primary part according to an exemplary embodiment of the present invention. 
       FIG. 3  illustrates a gap according to an exemplary embodiment of the present invention. 
       FIG. 4  illustrates a side view of an exemplary embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a linear motor having primary part  1  and secondary part  14 . Secondary part  14  may be made of, for example, a support having permanent magnets arranged in rows. Primary part  1  is made up of a housing  6  having an iron core  2 , which includes teeth  5  and gaps  4  on the side facing secondary part  14 . Iron core  2  is usually made up of punched and coated sheet-metal parts, in order to prevent eddy-current losses in iron core  2 . Situated in gaps  4  are coils  3 , which generate magnetic fields necessary for pushing the linear motor along, when they are sufficiently energized. Coils  4  of primary part  1  are supplied with current via an electrical terminal  11  on housing  6 . Therefore, electrical leads  15  are placed inside housing  6 , between electrical terminal  11  and individual coils  3 . In an exemplary embodiment, coils  3  are prefabricated coils, which are slipped over teeth  5  and partially fill up gaps  4 . As described in European Published Patent Application No. 0 793 870 discussed above, each coil  3  may wrap around only one tooth  5 . It may be provided for only every second tooth  5  to be wound, as shown in  FIG. 2 . However, the cooling system for a linear motor described in the following may also be used for other types of winding, for example, when coil  3  wraps around several teeth  5 , and/or coils  3  partially overlap each other. 
   The ohmic losses in coils  3  produce waste heat, which heats primary part  1 . If the temperature in the primary part increases too sharply, then the motor may be destroyed, since the insulation between the individual winding wires of coils  3  may be damaged (typically at or above approximately 130° C.). Thus, the heating of the motor limits the maximum current and, therefore, the maximum motor power and acceleration. 
   In addition to the removal of the heat, there may be a further problem in fixing coils  3  and their electrical leads  15  in position in a suitable manner. Since forces act on current-carrying conductors in magnetic fields, the current-carrying conductors are subject to a considerable mechanical loading. If the design of primary part  1  allows coils  3  or their electrical leads  15  to move, then they may be damaged due to wear (ruptured cable, short circuit). In addition, the quality of the motor control suffers, since the currents specified for the motor are not only converted into a (possibly desired) thrust, but also into (possibly unwanted) movements within the primary part. These problems particularly involve electrical leads  15  for coils  3 . 
   According to an example embodiment of the present invention, a portion of primary part  1  may be provided with a potting compound  13 . Synthetic resins having a high thermal conductivity and a high dielectric strength may be used here. 
   Coils  3  do not completely fill up gaps  4  between teeth  5 , but leave open a portion of gap  4  on its end facing away from secondary part  14 , as in, for example, the exemplary embodiment described with the aid of  FIGS. 1 and 2 . Potting compound  13  may be introduced up to a level H of the side of primary part  1  facing secondary part  14 , so that coils  3  and their electrical leads  15  are completely covered and, therefore, fixed in position. However, level H is selected so that channels  9   a  are formed in gaps  4 , where the volume of gaps  4  is not occupied by a coil  3 . 
   As illustrated in  FIG. 2 , gaps  9   a  connect a first flow chamber  7  to a second flow chamber  8 , which are otherwise separated from each other by iron core  2 . Depending on the design, lateral channels  9   b  may also be formed, when iron core  2  does not fill up housing  6  over its entire length. 
   If a suitable coolant is forced through one or more connections  10   a  into first flow chamber  7 , then this coolant will flow through channels  9   a,b  into second flow chamber  8  and leave it again through one or more connections  10   b.  In this context, a heat exchange occurs with all of the contact surfaces, for example, with iron core  2  and potting compound  13 . Thus, in comparison with a non-cooled linear motor, effective cooling of primary part  1  is achieved without additional cooling tubes being necessary. In this context, coils  3  and their electrical leads  15  are fixed in position by potting compound  13 . 
   Connections  10   a,b  may be positioned arbitrarily on housing  6 , and therefore, in an optimized manner for the specific application. Thus, it is possible to position connections  10   a,b  on the side of housing  6 , as is drawn, but it is also possible to feed or discharge the coolant via one or more connections  10   a,b,  through the upper side of housing  6 . In each instance, no changes may be necessary in the interior of housing  6 , since separate cooling tubes may be dispensed with. 
   Since it may be less important to fix coils  13  themselves in position than to fix electrical leads  15  in position, one may also reduce encapsulation level H. This results in a portion of coils  3  no longer being encapsulated and therefore being in direct contact with the coolant: The exposed regions of coils  3  in the two flow chambers  7 ,  8  and the surfaces of coils  3 , which, together with iron core  2 , form channels  9   a . If one selects a lower encapsulation level H, as described, so that parts of coils  3  are exposed, then care may be taken that at least electrical leads  15  of coils  3  are fixed in position by the potting compound. For this purpose, it may be provided that at least 20% of the coils be encapsulated. 
   In electric motors, it may be customary for unencapsulated coils to at least be impregnated with a thin layer of synthetic resin, in order to prevent problems with moisture. 
   This measure may also be advisable for any unencapsulated regions of coils  3 . 
   In this example embodiment a cover of housing  6  for secondary part  14  may be dispensed with. Potting compound  13  may seal tight housing  6 . 
   An exemplary design of gaps  4  is described in light of  FIG. 3 . In order to facilitate the positioning of prefabricated coils  3  into gaps  4 , projections  12 , which constitute a limit stop for coils  3  in their nominal position, are situated near the closed end of gaps  4 . This may ensure that coils  3  also leave open the region necessary for forming channels  9   a.    
   The cross-sectional area of gaps  4  reduced by the projection may increase again in the direction of the closed end of gaps  4 , in order to obtain sufficiently sized channels  9   a . The exact alignment of coils  3  may enable encapsulation height H to be set more precisely, when complete coverage of all coils  3  is desired. In addition, the aforementioned risk of a blockade formed by remaining potting compound  13  may not be present in the case of channels  9   a  according to  FIG. 1  or  FIG. 3 , although channels  9   a  may be very close to coils  3 . If the volume of potting compound  13  is reduced by the curing process, potting compound  13  may withdraw unhindered through gaps  4  in the direction of coils  3 . 
   In an exemplary embodiment, it may be possible to introduce potting compound  13  so that channels  9   a  are situated on the side of coils  3  facing secondary part  14 . Housing  6  may then be sealed by a cover (for instance, a thin plate or plastic) on the side facing secondary part  14 . 
   In this example embodiment, coils  3  may still leave open regions of gaps  4 , which face secondary part  14  and remain free of potting compound  13 . This, in turn, forms channels  9   a , which interconnect first and second flow chambers  7 ,  8 . 
   In this manner, it is possible to not cover a part of coils  3  (now, facing secondary part  14 ) with potting compound  13  and to impregnate this part, if necessary. 
   An exemplary embodiment, which is represented in  FIG. 4  and is made up a combination of the two first ones, may be more expensive to manufacture. Potting compound  13  is introduced so that only coils  3  are completely or partially covered. Regions of gaps  4  remain free of potting compound  13  on the side of coils  3  facing secondary part  14 , and on the side of the coils facing away from secondary part  14 , so that channels  9   a  interconnect first and second flow chambers  7 ,  8  on both sides of the coils. If coils  3  are not completely encapsulated, then the exposed regions of coils  3  may be impregnated. At least electrical leads  15  of coils  3  may be held by potting compound  13 , for which an encapsulation level H of approximately 20% of the coil height may be considered to be a lower limiting value for useful stabilization. 
   While the first described exemplary embodiment having channels  9   a  on the side of coils  3  facing away from secondary part  14  may provide a certain heat shielding of primary part  1  in the direction of the side facing away from secondary part  14 , the cooling in the second exemplary embodiment (channels  9   a  on the side of coils  3  facing secondary part  14 ) may prevent the heat input into secondary part  14  from being too large. This may substantially prevent demagnetization of the permanent magnets of secondary part  14 . The third exemplary embodiment connects the cooling possibilities of the two first exemplary embodiments to each other. 
   Air may be used as a coolant, since it is already available in many applications. In certain applications, the air need not specifically be removed after it flows out of connection  10   b . However, the present invention may theoretically be used with other gaseous coolants (e.g., inert gas) or liquid coolants (e.g., water, oil). 
   The present invention may be suited for linear motors, since, in this case, fill level H may easily be adjusted by controlling the volume of potting compound used. If one considers rotary motors to be curved linear motors, it becomes clear that the foregoing may also applicable to rotary motors without any further changes. Using suitable methods, one must only see to it that potting compound  13  leaves channels  9   a  open in gaps  4 . This may be accomplished, for example, by encapsulating individual, approximately linear sections of iron core  2  step by step, or by subsequently removing potting compound  13  from a motor, which is completely encapsulated, as usual, and has closed gaps  4 . The latter may also be an option for the production of linear motors, in particular for the third exemplary embodiment having channels  9   a  on both sides of coils  3 .