Patent Abstract:
An improved drum motor, preferably electronically commutated, features a unitary stationary central shaft ( 18 ) supporting a stationary inner stator ( 52 ), and an external rotor ( 49 ), including permanent magnets ( 50 ), secured to an inner surface of a generally cylindrical rotatable casing part ( 18 ). This permits variable speed operation while avoiding any need for an internal gear linkage. Safety is improved by making sealing plates ( 76, 78 ) at respective axial ends of the casing ( 14 ) stationary, and providing an annular peripheral seal ( 90 ) around each sealing plate. Respective rolling bearings ( 24, 44 ) near each end of the casing ( 14 ) facilitate rotation of the external rotor ( 49 ) relative to the stator ( 52 ), and a clamping arrangement ( 30 ) minimizes noise from the bearings.

Full Description:
FIELD OF THE INVENTION 
   The present invention relates generally to an external rotor motor having a stationary bearing shaft, and more particularly to a drum motor. 
   BACKGROUND 
   German Utility Model DE 296 23 889 U1, JOERISSEN, discloses a so-called drum motor that is used in a variety of industries, for example to drive conveyor belts. In that document, the drum tube is secured at both ends to a respective cover, that therefore rotates together with the drum tube, and is driven via a gear linkage by a motor in the interior of the drum tube. To permit better cleaning, a cap made of stainless steel is adhesively bonded onto each cover. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide an improved motor of the general type just described. 
   According to the invention, this object is achieved by replacing the gear drive with a stationary central stator which cooperates with a permanent magnet external rotor on an inner surface of a casing part rotatable relative to the stator. 
   It is thereby possible to drive the casing part directly by means of the external rotor motor that is used, thus resulting in a simple design and eliminating the need to use a gear linkage. Electronic commutation proves very advantageous in this context, because it permits not only drive operation at high rotation speeds but also drive operation at very low rotation speeds—thus eliminating the need for an adjustable gear linkage—and because, in such a motor, the rotation speed is easily modifiable, e.g. by modifying the operating voltage. 
   A further advantageous feature of the invention is to make the axial end covers of the drum tube stationary, and to provide an annular seal with respect to the adjacent drum tube ends. A closure member of this kind is joined to the stationary support part, i.e. does not rotate, thus reducing the risk of injury at this point and also simplifying and facilitating cleaning, e.g. in the food industry or the pharmaceutical industry. 

   
     BRIEF FIGURE DESCRIPTION 
     Further details and advantageous features of the invention are evident from the exemplary embodiment described below, which is in no way to be understood as a limitation of the invention. 
       FIG. 1  is a longitudinal section through a preferred embodiment of an electronically commutated external rotor motor according to the invention; and 
       FIG. 2  is an enlargement of detail A of  FIG. 1 , showing the relative positions of stationary stator and external rotor. 
   

   DETAILED DESCRIPTION 
   In the description that follows, terms such as “left” and “right” refer to the respective figure of the drawings. 
     FIG. 1  shows a so-called “drum motor”  10  that is driven directly by an electronically commutated external rotor motor  12  and preferably is adapted to drive conveyor belts. It has an external tubular casing part  14  made of ferromagnetic material, preferably steel, that can be of slightly convex configuration on its outer side  16 . 
   External rotor motor  10  has a stationary support part  18  that, because of its appearance, is often referred to informally as a “shaft.” This shaft  18  is stationary during operation, i.e. does not rotate. 
   This stationary shaft  18  has a cylindrical segment  20 , of greater diameter, on which is mounted inner race  22  of a ball bearing  24 , whose outer race  26  is arranged displaceably inside a cylindrical inner surface  28  of casing part  14  and is acted upon, toward the right, by a compression spring  30  whose left end is braced against a prong ring  32  or other abutment. A prong ring has, on its external periphery, one or more prongs which, upon assembly, dig into the cylindrical inner surface  28  of casing part  14 , the result being that prong ring  32  constitutes an abutment for compression spring  30 , so that the latter can clamp ball bearing  24 , which contributes to noise reduction. 
   Shaft  18  furthermore has a cylindrical segment  36  of smaller diameter, on which is mounted inner race  38  of a ball bearing  40 , whose outer race  42  is arranged on a cylindrical portion  44  of casing part  14  and is secured there on the left by a shoulder  46  and on the right by a snap ring  48 . The two ball bearings  24 ,  40  therefore have different sizes, and they support casing part  14  rotatably on shaft  18 . 
   Mounted in the cylindrical inner recess  28  of casing part  14  are permanent magnets  54  of external rotor motor  12 , which define an external rotor  49 . This is then magnet arrangement  50  of the motor part, which extends to the left from a shoulder  51  and coacts with an internal stator  52  whose lamination stack is pressed onto shaft  18 , which preferably is likewise made of ferromagnetic material and thus forms part of the magnetic circuit of internal stator  52 . Shaft  18  is equipped with a shoulder  56  that defines the location of the lamination stack. 
   Adjoining permanent-magnet arrangement  50  to the left is a nonmagnetic spacer ring  58 , made e.g. of brass, and this is followed to the left by a magnet ring  60  that serves to control one or more galvanomagnetic sensors  62 , e.g. to control Hall generators (not depicted). The function of sensors  62  is to sense the rotational position of casing part  14  relative to stationary axis  18 , which must occur very precisely, especially when motor  12  is running slowly and a rotation speed control system is being used. 
   Magnets  50 ,  60  are preferably magnetized in the radial direction. Magnet arrangement  50  can be implemented with, for example, four poles, and magnet ring  60  preferably has a greater number of poles, so that the rotational position can be sensed as accurately as possible. 
   Sensor  62  is mounted on a circuit board  66 , which in turn is mounted on shaft  18  and carries electronic components of the electronically commutated external rotor motor  12 , and extends approximately perpendicular to rotation axis  67  of casing part  14 . For passage of a connection to circuit board  66 , shaft  18  has an axial bore  68  and a radial bore  70  intersecting it. The winding of motor  12  is indicated at  72 . 
   Two sealing plates  76 ,  78  are provided to seal the interior of drum motor  10 . These are of identical configuration, so a description of right sealing plate  78  will suffice. The latter has, on its radially inner side, a portion  80  that can deflect radially outward and is equipped with an inwardly projecting catch ridge  82  that, in the assembled state, engages into an annular groove  84  of shaft  18  that is approximately complementary to it. 
   Shaft  18  is formed, in a region to the left of sealing plate  76 , with a frusto-conical segment  86  to facilitate assembly of sealing plate  76 , and with a frusto-conical segment  88  to facilitate assembly of sealing plate  78 . This makes it easier to splay, and slide on, sealing plates  76 ,  78  during final assembly. It is very advantageous that sealing plates  76 ,  78  do not rotate; this decreases the risk of injury to the user, and simplifies cleaning of drum motor  10 . Sealing plates  76 ,  78  can be made of metal or a suitable plastic. 
   On its outer side, sealing plate  78  is equipped with two sealing elements  90 , e.g. two sealing lips, a radial packing ring, or the like. The inner surface of casing part  14 , located opposite sealing elements  90 , is ground and polished. To facilitate assembly, hollow frusto-conical segments  92  are provided on the inner side of casing part  14 , adjacent the sealing plates. 
   With the invention, in contrast to drum motors having an internal gear linkage, shaft  18  can be continuous, thus imparting particularly high stability to drum motor  10 . Electronically commutated motor  12  does not have a rotatable shaft. The continuous stationary shaft  18  means that two rolling bearings  24 ,  40  are sufficient. Since casing part  14  is integral with external rotor motor  12 , rather than a separate element, the weight of drum motor  10  is correspondingly reduced. 
   Assembly 
   Motor magnets  54 , spacer  58 , and magnet ring  60  are adhesively bonded into casing part  14 , optionally with spot-grinding, and then magnetized in a suitable apparatus. Rolling bearing  40  is also installed in recess  44  and secured with snap ring  48 . 
   The stator lamination stack is pressed onto shaft  18 , and circuit board  66  is mounted on shaft  18 . Ball bearing  24  is then pressed onto shaft  18  at the desired location. 
   After these preparatory actions, shaft  18 , along with the parts installed on it, is inserted with its insertion end (i.e. right end  94  in this case) into the prepared casing part  14  from the left. Insertion is facilitated by the fact that outer race  26  of left ball bearing  24  is axially displaceable in recess  28 , to allow it to be axially clamped by spring  30 . 
   In the process, segment  36  of shaft  18  is pressed into inner race  38  of rolling bearing  40 , and sensor  62  is slid into the interior of control magnet  60 . An important advantage of the invention is that the control electronics (on circuit board  66 ) are integrated into motor  10 . 
   External connection of motor  10  is accomplished through transverse bore  70  and longitudinal bore  68 . To simplify assembly, an electrical plug connector (not depicted) can be provided at the transition from transverse bore  70  to longitudinal bore  68 . 
   Depending on the application, one or more Hall generators or a resolver, a GMR (Giant Magneto Resistor) sensor, an MR sensor, etc. can be used as sensor  62 . Sensing of the rotor position using the so-called “sensorless” principle is also not excluded in the context of the invention. 
   Spring  30  is then introduced and is placed under load and secured by prong ring  32  or another securing element. Lastly, sealing plates  76 ,  78  are installed. Assembly is thus very simple and time-saving. Shaft  18  can optionally be put together from several parts, but a one-piece construction is preferred. The use of a large-diameter shaft, and bearings with small radial dimensions, yields the advantage that very good heat transfer out from the stator lamination stack  52  via shaft  18  is possible. 
   An air gap  80 , shown in the enlargement in  FIG. 2 , is located between rotor magnets  50  and lamination stack  54 . 
   Many variants and modifications are of course possible within the scope of the present invention. Although motor  12  is shown as an external rotor motor having a permanent magnet rotor  50 , in other embodiments the rotor can nevertheless also be implemented as a short-circuit rotor (having a short-circuit winding), a synchronous motor, a reluctance motor (having a magnetically soft rotor), etc. Since a collectorless motor allows very different rotation speeds to be set without great difficulty, the structure shown is particularly preferred for low rotation speed applications. 
   Rotor magnets  50  may have a trapezoidal or sinusoidal magnetization depending on the motor principle used, a trapezoidal magnetization being preferred for rotor magnets  50 , and a sinusoidal magnetization being preferred for sensor magnets  60 .

Technology Classification (CPC): 7