Abstract:
This invention relates to improvements in electric power systems for aircraft and, more particularly, to such systems which provide both starter and generator functions. The present invention has particular applicability to starter generators for helicopters having a body with an end bell, a commutator head assembly and a fan cover assembly, and wherein the starter generator does not have an end bell that permits air to exhaust through said end bell. The starter generator of the present invention has a commutator head assembly having a frame with a top end and a base end, the top end being adapted to fit within the fan cover assembly and be retained therein by a suitable means, the commutator head assembly having with a plurality of slots that are designed to exhaust air flow passing over one or more brushes in the starter generator.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to improvements in electric power systems for aircraft and, more particularly, to such systems which provide both starter and generator functions. The present invention has particular applicability to starter generators for helicopters. 
     Typically, in airborne electrical power generation systems, it is desirable to have a single system which provides both the starter and generator functions. The weight savings on an aircraft can be substantial when a dedicated starter is eliminated. For this reason, electrical power generating systems which are capable of providing engine start functions can provide both cost and weight savings. 
     The starter-generator commonly utilizes a machine that combines an induction motor with a synchronous generator. In this machine, a rotor-shaped stator is positioned inside a squirrel-cage induction rotor which has an array of magnets attached on the outer diameter thereof. The compound/dual machine operates as a starter by using the induction rotor to accelerate the permanent magnet rotor up to some low synchronous speed, where ac power can be applied to the outside stator of the synchronous-generator so as to lock-in the permanent magnet rotor synchronously with the rotating field created in the armature (stator) of the synchronous-generator. 
     Frequently, the weakest link in the design of a starter-generator is the brushes. The carbon brushes utilized in starter-generators wear rapidly when brush temperatures exceed 450 degrees F. During startup, the starter generator brush temperatures can exceed 800 to 1000 degrees F. or higher, thus significantly exceeding the 450 degree F. threshold for rapid wear. The life of the starter-generator is frequently governed by the life of the brushes. With the development of higher horsepower turbine engines the starter-generators utilized to start such engines have been experiencing rapid brush and armature commutator wear. Besides the cost of overhauling the starter generator after relatively short hours of use the rapid wearing of the brushes causes a build up of carbon dust inside the starter generator, in the engine compartment and outside on the cowling as well. 
     OBJECTS OF THE INVENTION 
     There is a particular need for improvements to a particular type of starter generator which is frequently used in helicopters. One common type of starter generator is a type sold by Aircraft Parts Corp. (APC) and also by Lucas and possibly by others. These starter generators are usually 150 amp generators and are characterized, because of the aircraft they are designed to be used on, as having a base that prevents the flow of air from exiting directly out the drive end of the starter generator. An example of this type of starter generator is shown in FIGS. 1 and 2. Because of their design, these generators have high maintenance and low hours between overhaul of the generator. These generators typically run hot and dirty, i.e., they generate high amounts of carbon dust due to the rapid wear of the brushes and this dust proliferates fouling the aircraft engine cowling and compartments. In addition, these generators also produce copper dust from wear to the armature commutator. This dust is a good conductor and can potentially create shorts in the starter-generator. Because of the rapid wear of the brushes due to the carbon itself and the high operating temperatures of the starter generators, the generators must be repaired and overhauled frequently at considerable cost and downtime. 
     Accordingly, it is an object of the present invention to provide improvements to starter generators that are designed such that air does not flow directly from the drive end of the starter generator. 
     It is an object of the present invention to provide an improved starter generator that requires low maintenance and has long brush life. 
     It is a another object of the invention to provide an improved starter generator housing that permits the starter generator to better withstand higher temperatures thereby improving brush life. 
     Another object of the present invention is to provide an improved brush spring design that reduces armature commutator and brush wear during operation of the starter generator. 
     A further object of the present invention is to provide a new brush composition and design that has provided longer brush life under diverse operating conditions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a prior art starter generator of the type manufactured by APC and Lucas. These generators are sold, for example, under the designations APC 150SG Series and Lucas (LSI)23032 Series and others. 
     FIG. 1A is an end view of the starter generator of FIG.  1 . 
     FIG. 2 is side view of an another prior art starter generator of the type manufactured by APC. These generators are sold under the designation, for example, APC 150SG122Q and others. 
     FIG. 2A is an end view of the starter generator of FIG.  2 . 
     FIG. 3 is a side view of the starter generator of FIG. 1 with the improvements of the present invention. 
     FIG. 3A is an air-in end view of the starter generator of FIG.  3 . 
     FIG. 4 is a side view of the starter generator of FIG. 1 with the improvements of the present invention. 
     FIG. 4A is an end view of the starter generator of FIG.  4 . 
     FIG. 5 is an end view of the fan cover assembly. 
     FIG. 6 is a side view of the fan cover assembly of FIG.  5 . 
     FIG. 7 is an end view of the drive end of an end bell of the starter generator of the present invention. 
     FIG. 7 a  is a cut away view of the end bell of FIG. 5 taken along section A—A. 
     FIG. 7 b  is a side view of the end bell of FIG. 5 taken along section B—B. 
     FIG. 8 is an end view of an alternate embodiment of an end bell of the starter generator of the present invention. 
     FIG. 8 a  is a cut away view of the end bell of FIG. 8 taken along section A—A. 
     FIG. 8 b  is a side view of the end bell of FIG. 6 taken along section B—B. 
     FIG. 9 is a side view of the commutator head assembly of the starter generator of the present invention. 
     FIG. 9 a  is partial blown up view of the detail of area A of the commutator head of FIG.  9 . 
     FIG. 9 b  is a view of the commutator head of FIG. 9 taken along B—B. 
     FIG. 10 shows the brush spring of the present invention. 
     FIG. 11 shows the brush of the present invention. 
     FIG. 11 a  shows a side view of the brush of the present invention. 
     FIG. 11 b  shows the brush of FIG. 11 taken along A—A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIGS. 3 and 4 there is a starter generator  10  having a body  11  with an end bell  12 , a commutator head assembly  13  and a fan cover assembly  14 . Attached to the body  11  is a block  15  for making the appropriate electrical connections. The fan cover assembly  14  has a cylindrical body  16  with an inwardly extending flange or lip  17  about the circumference of the body at one end  18  of the body  16 . The fan cover assembly  14  has an air inlet screen  19  to protect the fan and provide ventilation to the generator. The screen is typically connected to the fan cover assembly  14  by spot welding or other suitable means at a plurality of locations  20  along lip  17  of the fan cover. At the opposite end of the body (FIG.  6 ), there is a shoulder portion  21  and skirt  22 . The shoulder portion  21  and  22  are adapted to receive a portion of the commutator head assembly  13 . 
     As shown in more detail in FIGS. 9 a ,  9   b  and  9   c , commutator head assembly  13  has a frame  23  with a top end  24  and a base end  25 . The top end  24  is adapted to fit within the fan cover assembly  14  and be retained therein by a suitable means such as screws that pass through screw holes  26  and  27  in the fan cover assembly  14  and commutator head assembly  13  respectively. The commutator head assembly has a side wall  28  at the base end  25  of the assembly. The side wall  28  has a upper portion  29  and a bottom portion  30 . In the bottom portion  30  of the side wall are provided are a plurality of slots  31  that are designed and placed so that air flow past the brushes in brush holder  39  and exits the starter generator. The presence of the channels is a particular advantage with respect to the inside brush in the starter generator as it typically runs considerably hotter than the outside brush and wears out more frequently as a result. Although the starter generators of the prior art FIGS. 1 and 2 have air outlet areas  75  these air outlet areas are placed in a position in the prior art starter generators to inefficiently exhaust the air past the brushes to reduce their operating temperature. 
     The presence of the slots in their position in the present invention provides significant increase in brush life. Whereas prior art brushes had a life of only 100 to 300 hours, it is not uncommon for the starter generators of the present invention to have brushes with a useful life of 1500 hours or more. In addition, because of the high temperatures in the prior designs, lengthening the brushes to add additional brush wear surface did not add significantly to the life span of the brushes. However, with the present invention, lengthening the brush from a length of 1.060 inch to 1.210 inch, a mere {fraction (5/32)} inch, increases the useful life of the brush by at least an additional 400 hours over and above the increase of over 1100 hours that the improvements of the present invention provide. 
     As seen in the FIGS. 3,  4  and  9 , the slots are generally oval in shape although other shapes are possible. The slots have a pair of generally parallel sides  33  and  34  which are joined together by U shaped end walls  35  and  36 . By effeciently directing the air flow past the brushes, the high temperatures encountered by the brushes is considerably reduced thus decreasing wear and significantly extending brush life. The slots  31  are pass through the side wall  28  and because of the curvature of the commutator head assembly the opening on the outer edge of the sidewall is wider than the opening on the inner edge. The slots have a length at the outside of the side wall in the range of approximately 1.75 to 2.00 inches and a height in the range of approximately 0.200 to 0.250 inch. The slots have a length at the inside of the side wall in the range of approximately 1.50 to 1.75 inch and a height in the range of approximately 0.200 to 0.250. The length of each of the slots  31  should be as long as possible without unduly reducing the strength of support areas  37  which are adapted to receive a retaining means in orifice  38  for retaining the commutator head assembly in position with body  11 . 
     The commutator head typically has a brush holder  39  for receiving and retaining the brushes in position and a plurality of brush springs  40  which provide the brushes with the proper tension (see FIGS.  9 , 10 ). The shoulder portion  21  and skirt  22  of the fan cover assembly  14  are adapted to receive a part of the upper portion  29  of the side wall of the commutator head assembly  13 . However, the fan cover assembly should not extend over the opening of the slots  31 . The position of the slots and their shape and size are such that they exhaust air entering the fan cover away from the brushes. Exhausting hot air reduces heat deterioration and wear of the armature/commutator and the brushes during operation. 
     At the end  41  of the body  11  opposite the commutator head assembly  13  is an end bell  12 . As shown in FIGS. 7 and 8 the end bell is generally disk shaped. The end bell of FIG. 7 is provided with a plurality of orifices  43  which provide clearance for bolts on the aircraft. In another end bell design, as shown in FIG. 8, the end bell has orifices added thereto to provide an additional area for air coming into from the fan cover to exit the area of the brushes. A second series of orifices  42  are used to attach the end bell to the body in both FIGS. 7 and 8. As seen in FIGS. 7 a  and  8   a  the end bell has a bearing liner  44 . The bearing liner  44  is connected via a web  45  to side wall  46 . Extending upwardly from sidewall  46  is support  47 . The length of the support and sidewall is to be minimized so that the maximum air flow can be achieved through the orifices  57  in the body  11 . The length of the cut out between the supports  47  should be approximately 1.75 to 2.00 inch. As seen in FIGS. 5B and 6B, support  47  extends up from the sidewall and has an upper surface  48  that is generally parallel to the bottom of the base. The side edges  49  and  50  of the support  47  may be perpendicular to the upper surface  48  or have a curvature as shown in FIGS. 7B and 8B. The support has a length of approximately 0.5″ as measured along the upper surface  48 . The height of the support is about 0.135″ to 0.155″. 
     Inside the commutator head  13  as noted above, are a plurality of brush springs  40 . The brush springs  40  have the general shape depicted in FIG.  10 . i.e.,a circular spring with a plurality of windings  48 , In the prior art starter generators, the brush spring tension was high and the brushes were forced against the commutator. For example, the prior art brush spring had a tension force of 45-55 ounces or more. It has been found that the selection of the tension for the spring is critical for superior brush life and reduced armature commutator wear. Proper tension on the spring results in longer brush life. The tension is measured by holding the spring in the position where the solid lines are and then rotating the tension bar  49  through a maximum of 165°. The tension force should be as low as possible and may preferably be in the range of 20 to 28 ounces or lower. Preferably the brush spring should be made from a corrosion resistant material such as a stainless steel. 
     FIG. 11 depicts a typical brush arrangement of the present invention. The brush has a terminal  51  that connects to the brush holder  39 . Extending from the terminal  51  are a pair of shunts  52  and  53  with usually about seven strands. The strands are preferably sheathed in a silicone fiberglass sheath  54 . The brush  55  is usually provided with a mark or groove  56  that is used to measure wear. 
     The brush is preferably comprised of the following composition: 
     Boron &lt;0.002 preferably &lt;0.001 
     Iron &lt;0.015 preferably 0.01 
     Copper &lt;0.002 preferably &lt;0.001 
     Molybdenum &lt;0.002 preferably &lt;0.001 
     Magnesium &lt;0.02 preferably &lt;0.01 
     Silicon &lt;0.015 preferably 0.01 
     Silver &lt;0.60 preferably 0.56 
     Aluminum &lt;0.002 preferably &lt;0.001 
     Calcium &lt;0.15 preferably &lt;0.10 
     Phosphorous &lt;0.02 preferably &lt;0.01 
     Sulfur &lt;0.02 preferably &lt;0.01 
     Potassium &lt;0.02 preferably &lt;0.01 
     Nickel &lt;0.002 preferably &lt;0.001 
     Cobalt &lt;0.002 preferably &lt;0.001 
     Manganese &lt;0.002 preferably &lt;0.001 
     Titanium &lt;0.002 preferably &lt;0.001 
     Vanadium &lt;0.002 preferably &lt;0.001 
     Zinc &lt;0.02 preferably &lt;0.01 
     Lead &lt;0.02 preferably &lt;0.01 
     Lithium &lt;2.0 preferably &lt;1.0 most preferably about 0.50 to 0.60 
     Lithium carbonate &lt;5.0 preferably &lt;4 most preferably about 3.00 to 4.00 
     The remainder is carbon. 
     The brush length is usually about 1.06 long, however, because of the reduced spring tension and lower operating temperatures of the design of the present invention a brush length of 1.210 may be used. However, it will be appreciated that longer and shorter brush lengths may be used. 
     EXAMPLE 1 
     A starter generator made in accordance with the present invention was installed on a helicopter engine. At the same time a competing starter generator was installed on a similar helicopter. Both helicopters were in service for 100 hours of typical use. The engine compartment of the helicopter using the competing product had to be washed down frequently to remove carbon dust from the generator and engine compartment. The helicopter employing the present invention did not have to be so cleaned. After the 100 hours of use transpired the starter generators were inspected. The competing product was covered with carbon dust. The starter generator of the present invention was not. The brushes of both starter generators were examined after 100 hours of use. The competing starter generator had significant brush wear and was estimated to have only approximately 200 additional hours of wear remaining. The starter generator of the present invention had minimal wear and was estimated to have approximately 1700 hour of additional wear remaining based on the current rate of wear. 
     EXAMPLE 2 
     Table 1 depicts the brushes that are used in specific starter generators. The tension in the brush spring was obtained directly from the OEM Overhaul Manuals. The area of the brush that makes contact with the commutator was calculated and indicated in the table as “Brush Size per sq. in.” Based on the tolerance of the brush spring tension and the area of the brush the low, average and high value of the pressure on the brush from the brush spring can be calculated. The average brush spring pressure on the starter generators of the present invention is 119.65 oz/sq. in. The average brush spring pressure on the prior art starter generators is 199 oz/sq. in. causing OEM brushes to typically last a maximum of approximately 300 hours between brush changes. Brush spring tension is determined by the following formula: 
     
       
         Stress×Area=Force 
       
     
     119.65 oz/sq. in×0.201 sq. in.=24.05 oz average 
     Using Table 1 the tension for the brush spring can be calculated to match the desired tension in a known brush spring. If the size of the brush is known i.e., N1829-1 Brush=0.201 sq. in. ,then the tension on the brush spring is: 
     119.65 oz/sq. in.×0.201 sq. in.=24.05 oz.