Abstract:
A miniature PMDC motor has a wound rotor  15  comprising a rotor core  16  and a commutator  17  mounted on a shaft  14.  A cooling fan  20  is directly mounted onto the commutator  17  by use of a mechanical snap-fit type connection. Projections  29  on the commutator mate with recesses in the fan  20  to prevent relative rotational movement and detents including resilient fingers  23  and projection surfaces  34  hold the fan axially.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to electric motors and in particular, to a miniature electric motor having an internal cooling fan.  
           [0002]    Miniature electric motors with internal cooling fans are well known with the fan providing a source of cooling air flow through the motor so that the motor can run at a higher current rating or power than it could be otherwise.  
           [0003]    In a miniature electric motor, and in particular, miniature PMDC motors with wound rotors, the fans are attached directly to the armature core, as shown in GB 2189944 for example or they may be mounted directly onto the commutator about the terminals or tangs. Commutator mounted fans are made of more expensive thermoset plastic to withstand the high temperatures of the commutator which can be especially high under stall conditions when the rotor is locked, resulting in no cooling air flow from the fan.  
           [0004]    The armature core fan can be made of cheaper thermoplastic as it is not in direct contact with the commutator and thus, does not have to withstand such high temperatures.  
           [0005]    In a PMDC motor with a wound rotor in which the rotor windings are connected to the commutator by way of Insulation Displacing Terminals (IDT) as shown for example, in GB 2128818, lead wires are strung across recesses in a crown or terminal portion of a base of the commutator into which at least the terminals of the commutator segments are inserted. The terminal portion, however, provides a big impediment to cooling air flow across the commutator surface generated by a fan fixed to the rotor core due to the physical size and location of the terminal portion.  
           [0006]    Thus, there is a need for an internal cooling fan arrangement within a miniature electric motor having a commutator with insulation displacing terminals which has or can readily receive/support an internal cooling fan for the motor.  
         SUMMARY OF THE INVENTION  
         [0007]    Accordingly the present invention provides an electric motor having a wound rotor and a stator, the rotor having a rotor core mounted on a shaft, a commutator mounted on the shaft adjacent one end of the rotor core and rotor windings wound around the rotor core and connected to terminals of the commutator, and a fan for generating a flow of cooling air, wherein the commutator has a base and a plurality of commutator segments fixed to the base, each segment having a brush contact portion and a terminal and the base having a support portion supporting the brush contact portion of the segments and a terminal portion supporting the terminals and wherein the fan has an integral inner collar from which a plurality of fan blades extend, the collar being fitted to the terminal portion.  
           [0008]    Preferably, the collar is fixed to the terminal portion by complementary formations, including snap-fit detents. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    Two preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:  
         [0010]    [0010]FIG. 1 is a side view of a motor according to a first embodiment of the present invention;  
         [0011]    [0011]FIG. 2 is an elevation view of a rotor of the motor of FIG. 1 with an exploded fan;  
         [0012]    [0012]FIG. 3 is an end view of the rotor of FIG. 2 with the fan fitted;  
         [0013]    [0013]FIG. 4 is a sectional view of a portion of the rotor of FIG. 3;  
         [0014]    [0014]FIG. 5 is an enlarged sectional view of a part of FIG. 4; and  
         [0015]    [0015]FIG. 6 is an exploded pictorial view of the fan and a terminal portion of the rotor of FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    [0016]FIG. 1 shows, in side view, a small PMDC motor  10  representing the preferred embodiment of the invention. The motor has a deep drawn metal rear housing  11  supporting a permanent magnet stator. The open end of the housing is closed by an end cap  12  which supports motor terminals  13  and brushes. The housing  11  has ventilation apertures  19 . A wound rotor or armature cooperates with the stator. The motor has a shaft  14  journalled in bearings fitted to the end cap  12  and closed end of the housing.  
         [0017]    The rotor  15  is shown in FIG. 2. The rotor  15  comprises rotor core  16  mounted on the shaft  14 . A commutator  17  is mounted on the shaft  14  at one end of the core  16  adjacent the end cap  12  so that the brushes make sliding contact with the commutator  17 . Windings  18  are wound around poles of the rotor core  16  and electrically terminated on the commutator  17 . A fan  20  is fitted to the commutator  17 .  
         [0018]    As shown in FIG. 2, the fan  20  is fitted to the commutator  17  after the rotor  15  has been wound. This allows the rotor to be wound and balanced without interference by the fan  20 . The fan  20  is pressed onto the commutator  17 , in particular on to the part which supports the commutator terminals  21 , known as the terminal portion  22 , and is held in place by snap fit fingers  23 .  
         [0019]    An end view of the commutator  17  and fan  20  assembly is shown in FIG. 3. FIG. 4 is a cross-section of the assembly viewed along section line IV-IV of FIG. 3 and FIG. 5 shows in detail a simple snap fit finger arrangement as indicated by circle V in FIG. 4.  
         [0020]    The commutator  17  is a two part, mechanical connection or insulation displacing type commutator. The support is in two parts, a cylindrical sleeve  24  supporting the commutator segments  25  and the terminal portion  22  which supports lead wires from the windings  18  as well as receiving the sleeve  24  and the terminals  21  of the commutator segments  25 . The terminals  21  are insulation displacing terminals which have an axial slot forming two arms which straddle and grip respective lead wires to make electrical connection therewith as the two parts are pressed together.  
         [0021]    An exploded perspective view of the fan  20  and the terminal portion  22  in shown in FIG. 6. The terminal portion  22  has a plurality of housings  26  which receive the terminals and support the lead wires as the terminals are pressed into the housings  26  and onto the lead wires. The fan  20  is of the centrifugal type and has an inner annular ring or collar  27  from which fan blades  28  extend radially outwardly. The blades  28  are shown as flat blades designed for use in either direction of rotation.  
         [0022]    The collar  27  is keyed to the terminal portion  22  by projections  29  on the housing which engage slots  30  in the collar  27  of the fan. This stops the fan  20  rotating about the terminal portion  22  and limits axial movement of the fan  20  towards the rotor core  16 . Larger cut-outs in the collar  27  snugly accepts the housings giving additional rotational integrity. Resiliently deformable fingers  23  extend axially from the slots  30 . The distal end of the fingers has a tapered head  32  with a square shoulder  33 .  
         [0023]    The shoulder  33  engages a corresponding abutment  34  on a small projection  35  on the housings  26  when the fan is pressed into position to prevent removal or axial movement of the fan  20  with regard to the commutator housing  26  in a direction away from the rotor core. The projection may have an axially outer surface which is tapered to assist the tapered head of the finger  23  to resiliently deflect the finger over the projection  35  during assembly. An enlarged detail view is shown in FIG. 5.  
         [0024]    The fan blades  28  are shown with a chamfer on their outer edge. This chamfer is to avoid components or structures within the motor, such as posts for holding the brushes or if on the other side, it may be to avoid the stator magnets.  
       ADVANTAGE OF THE INVENTION  
       [0025]    The use of a snap fit fan attached to the commutator allows the rotor to be completely wound and balanced and the commutator to be dressed before the fan is added to the rotor. Although this is usual, the snap fit fan, being a molded part can be balanced by design and requires no other parts to fix the fan to the commutator which may otherwise affect the balance of the rotor. Prior art fans which were pressed onto the rotor core winding slots still required glue to ensure reliable attachment. The glue is a source of unbalance as its location and volume varies with each application. GB 2189944 also disclosed a snap fit fan but this requires a separate mounting plate to be fixed to the rotor core before the rotor is wound. This is an additional part and it does complicate the winding procedures.  
         [0026]    The embodiments described above are given by way of example only and various modifications will be apparent to persons skilled in the art without departing from the scope of the invention as defined in the appended claims.  
         [0027]    While a cylindrical mechanical connection type commutator with a two part base has been used to demonstrate the invention, application of the invention is not so limited and can be applied to commutators with a one part base and with any type of contact material including copper, silver and carbon.  
         [0028]    The cooling fan is directly mounted on a radially outer portion of the commutator base which supports the commutator terminals. The mounting is mechanical without the use of adhesive or glue. The assembly process is simple, reliable and robust.