Abstract:
An electrical machine includes one or more multiple-phase electrical power windings and an auxiliary winding. The auxiliary winding when closed generates electromagnetic fields which partially cancel fields generated during the operation of the electrical machine. The result is reduced audible noise from the electrical machine.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electrical machines and more particularly to noise reduction designs for electrical machines. 
     2. Description of the Related Art 
     In the design of electrical machines, reducing audible noise generated by operation of the machine can be important. One field in which noise reduction is particularly important is in the design of alternators for motor vehicles. 
     An important operating point at which it is very important for an alternator operate quietly is at engine idle. There, because the vehicle is otherwise relatively quiet due to low engine speed and the lack of wind and road noise, alternator noise can be particularly annoying to the occupants of the vehicle. 
     The audible noise produced by operation of an automotive claw pole alternator is partly electromagnetic in origin. In particular, a third-harmonic field can produce significant noise. In a common 12-pole alternator, this third harmonic field is a 36-pole field. The third harmonic field is due to the interaction of the magnetomotive force produced by the stator currents with the permanence of the air gap between the rotor and the stator. 
     Electrical machine designs which can reduce the audible noise generated by electromagnetic fields in the machine will provide significant advantages over the prior art. Particularly in the automotive environment, the advantage will be in the form of increased customer satisfaction. 
     SUMMARY OF THE INVENTION 
     The present invention provides an electrical machine comprising a rotor and a stator disposed in opposition to the rotor. The stator further comprises one or more multiple-phase electrical power windings coupled to receive or provide electrical power, and an auxiliary electrical winding. The rotor has “n” magnetic poles, and the auxiliary electrical winding is an “m•n”-pole winding, where “m” and “n” are integers. 
     The present invention further provides a method for controlling an electrical machine having a rotor and a stator, the stator further having one or more multiple-phase power windings and an auxiliary winding. The method comprises electrically closing the auxiliary winding under some operating conditions and electrically opening the auxiliary winding under other operating conditions. The operating conditions governing the opening and closing of the auxiliary winding can include rotational speed of the rotor of the electrical machine. 
     Electrical machines and control thereof according to the present invention can reduce the audible noise generated by the machine. In doing so, the present invention provides advantages over the prior art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded view showing a rotor and a stator of an electrical machine, in this case a claw pole alternator. 
     FIG. 2 is an electrical schematic showing the power windings and rectifier of an electrical machine such as the alternator of FIG.  1 . 
     FIG. 3 is a winding diagram for an auxiliary stator winding  40  according to one embodiment of the present invention. 
     FIG. 4 illustrates an electrical switch included for switchably opening and closing auxiliary stator winding  40  of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Refer first to FIGS. 1 and 2. An electrical machine according to this embodiment of the present invention includes a rotor  20  and a stator  22 . Those skilled in the art will recognize rotor  20  as the rotor from a Lundell or “claw pole” alternator. In this case, rotor  20  has  12  magnetic poles. In this embodiment of the present invention, the alternator is a three-phase machine, with stator  22  having a three-phase winding comprising phase windings  24 ,  26  and  28 . Alternating current electrical power is generated in windings  24 ,  26  and  28  when rotor  20  rotates. Bridge rectifier  30  rectifies the alternating current power generated in windings  24 ,  26  and  28 , providing the rectified power to output terminals  32  and  34  of the electrical machine. Neutral connection  36  of windings  24 ,  26  and  28  is also connected to rectifier  30 , in order to provide enhanced power output at high speed and high output current. Under these operating conditions, a third harmonic flux induces a third harmonic voltage in windings  24 ,  26  and  28 . This voltage is of sufficient magnitude to cause the diodes coupled to neutral point  36  to conduct, thus providing enhanced electrical output. 
     Refer additionally to FIG.  3 . Stator  22  further includes an auxiliary electrical winding  40 , wound in the slots of stator  22  as shown. In this embodiment of the present invention, auxiliary winding  40  is a 36 -pole winding (that is, it consecutively occupies each of the  36  slots in stator  22 ). Ends  42  and  44  of auxiliary winding  40  are preferably short-circuited. 
     The 36 -pole third harmonic field generated by operation of the electrical machine induces a voltage in auxiliary winding  40 . Because ends  42  and  44  of auxiliary winding  40  are short-circuited, the induced voltage causes a current to flow in auxiliary winding  40 . This current generates a field which opposes and partially cancels the third harmonic flux, reducing the noise associated with third harmonic flux. 
     Refer additionally now to FIG.  4 . One disadvantage of the use of auxiliary winding  40  is that the added output due to the neutral connection  36  of stator windings  24 ,  26  and  28  being connected to rectifier  30  is substantially lost, due to the reduction of the third harmonic flux by auxiliary winding  40 . However, a switching device such as transistor  50  can be provided to control the short-circuiting of auxiliary winding  40 . Thus, near idle (where the concern about alternator noise is the greatest and where noise associated with third harmonic flux can be a particularly large contributor), transistor  50  can be closed. This will short-circuit auxiliary winding  40  to provide the noise-reduction benefits described above. At higher engine speeds, transistor  50  can be opened, resuming the benefit of having neutral connection  36  coupled to rectifier  30 . Suitable control circuitry  52 , such as a microprocessor-based controller having access to engine RPM data, can be used to control transistor  50 . Selection of transistor  50  can be facilitated by winding auxiliary winding  40  with multiple turns. Thus, less current will be required to provide its noise-reducing effect, and a lower-current-capacity transistor  50  can be selected. 
     Auxiliary winding  40  can be manufactured in various ways. First, it can be simply wound of wire in a conventional manner. It may also manufactured by providing electrically-conductive slot liners made of stamped metal such as copper. The slot liners can be inserted into the slots of stator  22  to line the bottom and/or sides of the slots. The slot liners are then electrically coupled together to create a one-turn auxiliary winding according to the winding diagram of FIG.  3 . Stator phase windings  24 ,  26  and  28  are then wound into the slots of stator  22  in the conventional manner. Also, stator phase windings  24 ,  26  and  28  may be wound first, with conductive wedges then placed on top of the phase windings. The wedges can be electrically interconnected to create an auxiliary winding according to FIG.  3 . 
     Laboratory testing has demonstrated that an auxiliary winding  40  according to the present invention can provide very substantial reductions in audible noise caused by third harmonic flux. 
     Various other modifications and variations will no doubt occur to those skilled in the arts to which this invention pertains. For example, the invention described herein is, of course, applicable to motors as well as generators. Such variations which generally rely on the teachings through which this disclosure has advanced the art are properly considered within the scope of this invention. This disclosure should thus be considered illustrative, not limiting; the scope of the invention is instead defined by the following claims.