Abstract:
An engine driven generator ( 17 ) is provided with first and second stage fans ( 41,42 ) at one end thereof. The two-stage fan system draws in ambient air and forces it through the generator windings ( 38 ) and rotor-stator air gap ( 37,39 ) for the purpose of improving generator cooling. Stationary guide vanes ( 49 ) are disposed between the two fans ( 41,42 ) for the purpose of de-swirling the flow therebetween. The fans are of the centrifugal type and are preferably backward curved.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to transport refrigeration systems and, more particularly, to the cooling of an engine driven electric generator therefor. 
         [0002]    Generally, transport refrigeration systems such as those used on truck trailers, have employed belt driven and/or mechanically linked, shaft driven compressor units and/or belt driven, or otherwise mechanically linked, fan powering systems. That is, the unit engine, such as a diesel engine, is mechanically connected to drive both the compressor unit(s) and the unit refrigeration systems fans and other components. 
         [0003]    A significant improvement to such an arrangement was a so called “all electric” transport refrigeration system as described in U.S. Pat. No. 6,223,546, assigned to the assignee of the present invention. In such a unit, a generator is mechanically connected to the system engine and is used to generate the electrical power needed to drive the compressor and the various refrigeration system fans and other components. 
         [0004]    Heretofore, such an engine driven generator has been cooled by a single stage centrifugal fan disposed on one end thereof and adapted to circulate air over the stator and armature components for the cooling thereof. 
         [0005]    Relatively low engine speeds are desired in trailer refrigeration systems in order to attain low fuel burn rates. This limitation on shaft speed results in relatively low pressure rise capability from a single stage fan. Yet, high pressure rise is needed to drive flow through the generator windings and rotor-stator air gap. System space constraints make large cooling fans or push-pull fan options impractical. 
         [0006]    Augmentation of a cooling fan by the use of a second fan in cooling rotating machinery as shown in U.S. Pat. Nos. 6,700,235 and 7,129,604. However, the fans are mounted at opposite ends of the generator such that one of the fans is blowing cooler air across the rotating machinery, while the other fan is drawing warmer air away from the rotating machinery. This is disadvantageous since the warmer air is less dense and therefore results in reduced air mass flow rate. 
       SUMMARY OF THE INVENTION 
       [0007]    Briefly, in accordance with one aspect of the invention, a two stage centrifugal fan is drivingly connected to one end of the rotor of an engine driven generator so as to provide relatively high pressure air at relatively modest fan tip speeds to cool the stator and rotor of the generator. 
         [0008]    By another aspect of the invention, stationary guide vanes are placed between the two stages to control the flow from one stage to the next. 
         [0009]    By yet another aspect of the invention, the fans are of the centrifugal type. 
         [0010]    By yet another aspect of the invention, the centrifugal fans are backward curved. 
         [0011]    In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the spirit and scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is an exploded perspective view of an engine driven generator in accordance with the prior art. 
           [0013]      FIG. 2  is a longitudinal sectional view of an engine driven generator in accordance with the present invention. 
           [0014]      FIG. 3  is an exploded view of the two-stage cooling fan in accordance with the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0015]      FIG. 1  depicts a unitary engine driven generator which is cooled by the circulation of air in accordance with the prior art. The generator  17  includes an outer stator assembly  18  that is fixedly attached to the bell housing  19  of a suitable prime mover such as a diesel engine  21 . A rotor assembly  22  is affixed directly to the engine flywheel  23  to create a continuous drive connection between the engine drive shaft, the engine flywheel and the rotor assembly  22  of the generator  17 . A cover  24  and a generator cooling fan  26  have been removed to show the details of the rotor assembly  22 . 
         [0016]    The outer stator assembly  18  includes a core section which may be fabricated from ferrous laminations or powdered metal. A main winding that provides primary power to the refrigeration system and an auxiliary winding that is electrically connected to the battery charging device are disposed in slots in the stator core in the conventional manner. Attachment of the stator assembly  18  to the bell housing  19  is accomplished by use of a series of elongated threaded fasteners  27  passing through mating openings  28  in the stator core. The fasteners  27  in turn pass through axially aligned openings  29  provided in an adaptor plate  31  and thence into, axially aligned threaded openings  32  in the bell housing  19 . 
         [0017]    The rotor assembly  22  includes a steel rotor hub  33  having a plurality of axial openings therethrough which are adapted to receive a plurality of elongated threaded fasteners  34  therethrough. The threaded fasteners  34  are adapted to be received in axially aligned threaded openings  36  in the engine flywheel  23  to provide the integral connection between the rotor assembly  22  and the engine flywheel  23  and drive shaft. 
         [0018]    In operation, the diesel engine  21  drives the flywheel  23  and the rotor assembly  22  with its associated rotor magnets, thereby inducing in the stator windings, synchronous voltages in a manner well familiar to those skilled in the art of synchronous generator design. Other details of the prior art system are shown and described in U.S. Pat. No. 6,223,564, assigned to the assignee of the present invention and incorporated herein by reference. 
         [0019]    It should be understood that while some cooling of the generator  17  occurs by radiation from the outer stator assembly  18 , the primary cooling function is accomplished by the fan  26  which is attached to and driven by the rotor assembly  22  to bring air in through the cover  24  and circulate it through both the stator windings and the rotor. While the stator windings develop a significant amount of heat and require the cooling air to prevent them from reaching excessive temperatures and being damaged thereby, it is also necessary to cool the rotor in order to prevent the permanent magnets from demagnetizing. In this regard, the applicants have found that it is difficult to force air through the rotor because of the high airflow resistance imposed by the tight rotor-stator air-gap and other restrictive flow-path elements. The applicants have found this to be further complicated by limitations on the rotational speed of the engine-generator-fan system. 
         [0020]    Referring now to  FIGS. 2 and 3 , a generator  17  is shown to include a prime mover  21  drivingly connected to a rotor assembly  22 , with a flywheel  23  mechanically connected therebetween. An outer stator assembly or casing  18  and an inner stator assembly  37  are stationary and are supported in a cantilevered manner by way of the bell housing  19 . 
         [0021]    The rotor assembly  22  includes an armature  38  mounted on a shaft  39  for rotation by the prime mover  21  within the inert stator assembly  37  to thereby generate electricity. The armature  38  and its shaft  39  are also mounted in a cantilevered fashion so as to not include any bearings. 
         [0022]    A pair of centrifugal first and second stage fans  41  and  42  are mounted, in series, to the shaft  39  so as to be rotated at the same rotational speed as the armature  38 . This connection is made by way of a hub  43 . That is, the hub  43  forms the central body portion of the second stage fan  42  and includes a female portion  44  which is connected to the drive shaft  39  as shown, and a male portion  46  on which the hub  47  of the first stage fan  41  is mounted. Other fan-shaft arrangements may be envisioned for the system such as double-stepped or double-tapered shafts to accommodate the two-stage assembly. 
         [0023]    Mounted between the two fans  41  and  42  are a plurality of radially extending guide vanes  48  which are mounted by the associated second stage casing and inlet assembly  49  to the support structure  50  and the outer stator assembly  18 . The guide vanes  48  are stationary and act to remove the swirl from the air flowing inwardly from the first stage fan  41  before it arrives to the second stage fan  42 . The guide vanes are also known as stator vanes. An inlet casing  51  with an opening  52  is attached to the support structure  49  and acts to form an isolating chamber  53  around the first and second stage fans  41  and  42  and their associated guide vanes  48 . 
         [0024]    In operation, the first and second stage fans  41  and  42  are rotated at a constant speed by way of the shaft  39  such that ambient air is drawn into the opening  52  and passed through the first stage fan  41  which raises the pressure to an intermediate level. The air then passes through the guide vanes  48 , which act to remove the swirl and orient the flow in the radial direction. The flow then passes through the second stage fan  42 , which acts to increase the pressure to a higher pressure, with the higher pressure air then passing through the various openings in the inner stator assembly  37  and the armature  38  to provide cooling thereto. The air then passes out of the generator at a series of openings  50  around the generator periphery. The first and second stage fans  41  and  42  are preferably of the backward curved type, but may be of any form while remaining in the scope of the present invention. 
         [0025]    While certain preferred embodiments of the present invention have been disclosed in detail, it is to be understood that various modifications in its structure may be adopted without departing from the spirit and scope of the following claims.