Patent Publication Number: US-2005140230-A1

Title: Electric motor and vehicle powered thereby

Description:
FIELD OF THE INVENTION  
      THIS INVENTION relates to electric motors and to vehicles powered by such motors.  
     BACKGROUND TO THE INVENTION  
      The mechanical output power of any motor is given by: 
 
P mech =T.w  1 
 
 Where 
          P=power,     T=mechanical torque at the drive shaft, in Nm,     w=rotational speed, in radians per second, of the drive shaft. 
 
 The electromagnetic power of a direct current (d.c.) motor, in general, takes the form 
 
 P   em =K.D.L.I.B.w  2 
 
 Where 
    K=a constant which takes winding factors etc, into account but is not a function of size for a particular motor construction.     D=outer diameter of armature.     L=active length of armature.     I=armature current.     B=magnetic flux density of field coils (or permanent magnets) in the air gap.     w=rotational speed, in radians per second, of the drive shaft.        

      Power losses will be ignored since this is not a detailed analysis of the motor but serves to illustrate the concept behind the invention. Then, from equations 1 and 2 we can determine the torque; 
 
T=K.D.L.I.B  3 
 
 From equation 3 it can be seen that for a high torque motor it is necessary to increase one or more of the parameters, diameter (D), length (L), current (I), or magnetic flux density (B). Magnetic flux density B has a maximum practical limit determined by the magnetic material used and is not a function of geometry. If D or L is increased, the size of the motor increases. Further as the current I is increased the efficiency of the motor eventually drops dramatically since resistive losses are proportional to I 2 . Hence for a particular power rating, the power density and efficiency, and therefore the size, of the motor are determined by the torque and speed requirements. If speed and torque can be selected then, from the equations, it can be seen that a high rotational speed with low torque gives a much smaller motor for the same power rating. 
 
      Normally conventional motors operate at speeds in the region of 3000 rpm. One approach to obtain a smaller, more efficient motor of the same power rating, is to design the motor to run, say, at 12000 rpm, resulting in an equivalent decrease in torque and hence in D and L. However, a higher speed does not suit most practical applications. The obvious solution to this problem is to use a gear box to reduce the speed and increase the torque of the output shaft to practical levels. Although this solution increases size and cost, there are many applications where this solution is suitable. The solution becomes limiting as power ratings go up. This is for mechanical reasons such as centrifugal forces on the rotor become excessive at the high speeds, rotor bearings come under increased strain and windage losses become unacceptable.  
      The present invention seeks to a provide a high power density motor which allows for increased rotor speed without restricting the choice of drive shaft speed and torque.  
     BRIEF DESCRIPTION OF THE INVENTION  
      According to one aspect of the present invention there is provided an electric motor construction which comprises at least two rotors including rotor shafts, there being a power output shaft and step down power transmission means connecting said rotor shafts to the output shaft.  
      In one form the electric motor construction comprises a number of rotor/stator combinations, said rotor/stator combinations being arranged in an array about said output shaft and there being means for connecting said combinations to one another.  
      In another form the electric motor construction includes at least two rotors and a single stator, the stator having cylindrical cavities therein for receiving the rotors. In this form there can be a single stator having at least two rotor cavities, said stator having a central bore in which said output shaft is mounted, said rotor cavities spaced from one another around said output shaft. Preferably said stator has four rotor cavities, the rotor cavities being equally spaced apart around said output shaft.  
      Bearings can be provided in said bore, said power output shaft turning in said bearings.  
      Outer races of rotor bearings can be fast in rotation with the stator, said rotors turning in said rotor bearings.  
      In a preferred form said step-down power transmission means comprises a main gear carried by said output shaft and a pinion carried by each rotor, the pinions being in mesh with said gear wheel.  
      Said rotors can be squirrel cage rotors having bars in which current is induced when current flows in the stator windings.  
      Said rotors can be in the form of permanent magnets.  
      The electric motor construction can further include cooling channels which pass through the or each stator, and means for causing cooling air to flow through said channels.  
      Said means for causing cool air to flow can be impellers driven by the rotors. A specific construction includes an impeller for blowing air into a cooling channel and an air guide for directing air emerging from that channel back into a further channel.  
      A further impeller can be provided for drawing air out of said further channel.  
      According to another aspect of the present invention there is provided, in combination, a vehicle road wheel comprising a rotatable rim and a non-rotating axle, the rim rotating with respect to the axle when the wheel is revolving, and an electric motor combination as defined above, said stator being fast with said axle and said output shaft being connected to said rim so that the rim is driven by said output shaft.  
      According to a further aspect of the present invention there is provided in combination, a vehicle road wheel comprising a rotatable rim and a non-rotating axle, the rim rotating with respect to the axle when the wheel is revolving, and an electric motor combination in which said step-down power transmission means comprises a main gear carried by said output shaft and a pinion carried by each rotor, the pinions being in mesh with said gear wheel, said stator being fast with said axle and said main gear and power output shaft being connected to said rim.  
      According to a still further aspect of the present invention there is provided a vehicle road wheel comprising a non-rotatable axle, a rotatable power output shaft, said power output shaft being hollow and said axle being co-axially within the power output shaft, there being bearings between said axle and said shaft so that the power output shaft can rotate on the axle, a stator encircling said shaft, the stator having stator cavities, rotors in said cavities, each rotor being carried by a rotor shaft, bearings between said stator and said rotor shafts so that the rotors can rotate within their cavities, a pinion on each rotor shaft and a main gear co-axial with and fast in rotation with said power output shaft, said pinions meshing with said main gear.  
      In this form the vehicle wheel can include a wheel rim comprising a cylindrical portion onto which a tyre can be fitted and a plate through which wheel studs project, the wheel studs being carried by said power output shaft.  
      To provide for braking, the vehicle road wheel can include a brake shoe in a recess in the stator and hydraulic means for urging the brake shoe against a part of the motor that rotates when the motor is running.  
      In one form said shoe is in a recess in an end face of the stator and is moved axially of the motor to apply the brake. In another form said shoe is in the circumference of the stator and is moved radially outwardly into contact with a rotating part of the wheel to apply the brake. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:— 
       FIGS. 1 and 2  are a schematic front elevation and a section respectively of a first embodiment of an electric motor in accordance with the present invention;  
       FIGS. 3 and 4  are a schematic end elevation and a schematic side elevation respectively of a rotor of the electric motor of  FIGS. 1 and 2 ;  
       FIGS. 5 and 6  are schematic end and side elevations respectively of a stator of the electric motor of  FIGS. 1 and 2 ;  
       FIGS. 7 and 8  are a schematic front elevation and a diagrammatic axial section respectively of a single rotor and an associated stator, illustrating an electronic commutator arrangement;  
       FIG. 9  is a schematic section of a single rotor and an associated stator of the electric motor of  FIGS. 1 and 2 , illustrating a motor cooling and bearing lubrication system;  
       FIGS. 10 and 11  are a schematic front elevation and a schematic axial section of a further embodiment of a motor in accordance with the present invention;  
       FIG. 12  is a pictorial view of a stator for receiving multiple rotors;  
       FIG. 13  is a pictorial view of a squirrel cage rotor;  
       FIGS. 14 and 15  are a diagrammatic front elevation and a diagrammatic plan view illustrating an air cooling system for an electric motor;  
       FIG. 16  is an axial section through an electric motor fitted to a vehicle wheel;  
       FIGS. 17 and 18  are schematic representations of vehicles fitted with electric motors;  
       FIGS. 19 and 20  are views similar to those of  FIGS. 14 and 15  and illustrate a mechanical brake; and  
       FIGS. 21 and 22  illustrate a further mechanical brake. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
      Referring firstly to  FIGS. 1 and 2 , an electric motor in accordance with the present invention is generally designated  10 . The motor  10  comprises four rotors  12  and four stators  14 . Each rotor  12  includes a drive shaft  16  mounted in bearings  18 . A pinion gear  20  is mounted on each shaft  16 .  
      The gears  20  mesh with a main gear  22  which is connected to a main drive shaft  24 . The main drive shaft  24  is mounted in main shaft bearings  26 . In use, all four rotors  12  are energised to drive the main gear  22  and, consequently, the main drive shaft  24 .  
      The rotor  12  includes two permanent magnets  28  and  30  ( FIGS. 3 and 4 ) having a high magnetic flux density. The magnets  28  and  30  are mounted on opposite sides of a core portion  32 . The core portion  32  is mounted on the rotor drive shaft  16 . Thus, each rotor  12  only has two poles exposed on its surface, a north pole N from magnet  28  and a south pole S from magnet  30  as shown in  FIGS. 3 and 4 . The rotor&#39;s surfaces are smooth to reduce windage losses.  
      Applicants have found that whilst more than one magnetic pole pair per rotor  12  can be used this does not lead to improved performance of the motor  10 . Multiple pole pairs per rotor  12  require a more complicated construction and increase the complexity of the armature windings of the stators.  
      The windings  34  (see  FIGS. 5 and 6 ) are grouped into two separate phases aa′ and bb′. Windings a and a′ form a continuous coil such that the current flows in one direction through a and returns in the opposite direction through a′. Similarly, windings b and b′ form a continuous coil such that the current flows in one direction through b and returns in the opposite direction through b′. The windings  34  are therefore grouped in four quadrants  36 ,  28 ,  40  and  42  and with four windings  34  per quadrant, such that quadrants  36 ,  40  comprise phase aa′ and quadrants  38 ,  42  comprise phase bb′. The two phases aa′ and bb′ are thus positioned 90° apart from each other in mechanical angle. The phase currents when switched through aa′ and bb′, in use, are in addition switched separately at 90° apart in electrical time phase angle with respect to one another. The direction of the rotor  12  is determined by which phase is leading. The stator  14  is of laminated construction (see  FIG. 6 ) which serves to reduce eddy current losses.  
      Each of the four phase windings aa′ of each of the stator  14  are connected in series and the start of the first aa′ winding and the end of the fourth aa′ winding are connected to two power terminals (not shown) for connection to a power source (not shown). Similarly, each of the four phase windings bb′ of each of the rotors  12  are connected in series and the start of the first bb′ winding and the end of the fourth bb′ winding are connected to two power terminals (not shown) for connection to a power source (not shown). Therefore, there are two terminals per phase, resulting in a total of four terminals.  
      The switching of the currents through the armature windings  34  is synchronised to the rotational position of the rotors  12 . In order to achieve this, one end surface  46  (see  FIG. 7 ) of each rotor  12  is painted in two alternating, contrasting colours in four equal quadrants, preferably black  48  and white  50 .  
      An optical sensor  52  is embedded in one of the stators  14  and faces the end surface  46  of the rotor  12  as shown in  FIG. 8 . The optical sensor  52  may be positioned in any one of four mechanical positions with respect to the stator windings  34  of  FIG. 5 : 
          i) between adjacent single windings a and b′;     ii) between adjacent single windings a and b;     iii) between adjacent single windings b and a′; or     iv) between adjacent single windings a′ and b′. 
 
 In addition, the magnetic North-South axis of the rotor  12  is positioned midway in the white section  50  as indicated in  FIG. 7  so that the magnets  28 ,  30  are located entirely within the white section  50 . Alternatively, the North-South axis can be positioned perpendicular to the axis shown in  FIG. 7  so that the magnets  28 ,  30  are located entirely within the black section  48 . The optical sensor  52 , together with power switching transistors (not shown), forms an electronic commutator for the electrical motor  10 . Only one sensor  52  is necessary since all four rotors  12  are mechanically linked by way of the pinions  20  and the drive gear  22  and are all held in the correct position by the gear teeth. 
       

      For motors with higher power ratings, cooling system as shown in  FIG. 9  may be used. The cooling system  64  comprises an air-cooled heat exchanger  66  and cooling fluid passages  68  located within the stator  14 . The fluid passages  68  also lead to the rotor bearings  18 . A centrifugal pump  70  is mounted on the rotor drive shaft  16 . The pump  70  pumps the coolant, which is preferably oil, from the heat exchanger  66 , in the direction A through the fluid passages  68  and back to the heat exchanger  66  in direction B. The coolant can thus provide lubrication for the bearings  18  as well as the cooling function as described.  
      Each rotor  12  of the motor  10  has its own pump  70  in this embodiment but a single pump  70  may be provided.  
      The four rotors  12  and their associated stators  14  may be constructed as four separate motors, each individually mounted about an axially extending tube  54  as shown in  FIGS. 10 and 11 . The tube  54  contains the main drive shaft  24  and its supporting bearings  26 .  
      Alternatively, the four stators  14  may be constructed as one unit such as is shown at  56  in  FIG. 12 . Cover plates (not shown) may, in this configuration, be used as mountings for the bearings  26  of the main drive shaft  24  and the bearings  18  of the rotors  12 .  
      The motor disclosed in  FIGS. 3 and 4  has a rotor  12  using permanent magnets  28 ,  30 . In  FIG. 13  there is disclosed a rotor  58  which comprises rotor conductor bars  60  and end conductor rings  62  forming a squirrel cage such as is used in an induction motor. Alternating current flowing in the stator windings (not shown in  FIG. 13  but similar to those shown in  FIG. 5 ) induces current in the bars  60  resulting in the production of torque which rotates the rotor  58 . Four such units as shown in  FIG. 13  can be used as the rotors  12  in the motor  88  shown in  FIG. 16 . The windings  34  are carried by the stator  14 .  
      Cooling fluid or heat sink devices (not shown) may be used for cooling purposes. In  FIGS. 14 and 15  a stator  72  is shown which has four cylinders  74  for receiving rotors the shafts of which are designated  76 . There is a central bore  78  for a shaft (not shown) which carries the gear  22  (not shown), and a plurality of channels  80 . 1 ,  80 . 2 .  
      To induce airflow through the channels  80 . 1 ,  80 . 2 , the shafts  76  have impellers  82 . 1 ,  82 . 2  etc fitted to them. Air flow guides are fitted over the impellers  82 . 1 ,  82 . 2  etc. Only the guide  84  over the impeller  82 . 1  is shown. Air is drawn in by the impellers  82 . 1 ,  82 . 2  etc and blown into first sets of channels  80 . 1 .  
      The air emerging from the sets of channels  80 . 1  is guided by guides  86  into second sets of channels  80 . 2 . Airflow is shown by the arrows in  FIGS. 14 and 15 .  
      In  FIG. 16  an electric motor, generally designated  88 , is shown fitted to a wheel  90  by mounting bolts  92  which are screwed into the vehicle stub axle and mounting bracket assembly  94 . The wheel  90  includes a wheel rim  96  which receives the motor  88 . The main drive shaft  24  is hollow and turns on bearings  98  to allow the shaft  24  to rotate freely on the stub axle  100 . The main gear  22  is immovably fixed to the main drive shaft  24 . The wheel rim  96  is drivingly fixed to the main drive shaft  24  by way of four mounting bolts  102 . The main drive shaft  24  is held in place, with the wheel bearings  98 , on the vehicle stub axle  100  and mounting bracket assembly  94  by way of a single lock nut  104 .  
      A dust cover  106  and oil seals  108 ,  110  protect the gear  22  and the pinion gears  20  from the ingress of dust and water. The dust cover  106  also serves as an oil reservoir to hold lubricating oil for the gear  22  and pinion gears  20 .  
      Modification of existing conventional vehicles to incorporate the motor  88  of  FIG. 16  is achieved by stripping and removing the conventional wheel hub assemblies down to the bare stub axle and mounting the motor  88 , including the hollow main drive shaft  24 , directly thereon.  
      In  FIG. 17 a  vehicle  112  is shown schematically. The vehicle  112  includes an internal combustion engine  114 . The rear wheels  116  of the vehicle  112  are fitted with electric motors  88 . The motors  88  are supplied with power from a battery pack  118  via separate power supply modules  120  and  122 . The power supply modules  120  and  122  control the magnitude and direction of the current. If required, the modules  120  and  122  can also change the direction of current flow between the motors  88  and the battery pack  118 . Thus, the motors  88  can supply a driving force to the vehicle  112  or they can serve as generators to charge the battery pack  118 . In this way, the motors  88  may also supply a regenerative braking force to the vehicle  112  while charging the battery pack  118 .  
      Feedback transducers  124  and  126  from a brake pedal (not shown) and an accelerator pedal (not shown) respectively as well as a transducer  128  for determining the position of a gear selection lever  130  of the vehicle  112  are provided. The transducers  124 ,  126  and  128  are all connected to a microprocessor  132  which is used to control the operation of the modules  120  and  122 .  
      An indicator panel  134  is provided inside the vehicle  112 . A lever  136  is used to engage the motors  88  in either a forward or reverse direction. The indicator panel  134  can also include a voice command system (not shown) to allow for easier control of the system by the driver of the vehicle  112 .  
      The microprocessor  132  also controls a starter motor  138  so as automatically to start the internal combustion engine  114  when it is necessary to switch from electric power to petrol power. A second microprocessor (not shown) may be provided to monitor the operation of the microprocessor  132 . If the microprocessor  132  fails, then the second microprocessor can be used to operate the system.  
      A gearbox and clutch  140  is provided to connect the engine  114  to the rear wheels  116 , or to the front wheels, when required.  
      In  FIG. 18 , the vehicle  112  does not have a gearbox  140  but has a generator  142  which can be of the same construction as the motors  88 . The generator  142  is driven by the internal combustion engine  114  and supplies electricity directly to the motors  88 . In this embodiment, the battery pack  118  is much smaller than that shown in  FIG. 16  and is only required for standby power and/or surge demand purposes. A charge regulator  144 , which is connected to the microprocessor  132 , is provided to regulate the rate of charge of the battery pack  118 . In this configuration the generator  142  drives the vehicle  112  continuously via the motors  88  and a conventional drive train for a petrol or diesel engine is not required.  
       FIGS. 19 and 20  illustrate one way of incorporating a mechanical brake into an integrated wheel and motor such as is shown in the rear wheels  116  of  FIG. 17 . It will be understood that mechanical braking is in addition to the braking effect obtained by using the motor “in reverse” as a generator. The mechanical brake is incorporated into the motor without increasing the overall dimensions thereof.  
      A brake pad  146  is fitted into a recess  148  provided therefor in an end face of the stator  150 . Behind the brake pad  146  there is at least one cylinder  152  (three in the illustrated embodiment) in which there are pistons  154  and piston rods  156 . The rods  156  bear on the back face of the pad  146  and urge it against the gear  22 . The gear  22  is not shown in  FIGS. 19 and 20 . The cylinders  152  are connected to an hydraulic circuit (not shown) connected to a master cylinder (not shown) operated by a brake pedal (not shown).  
      In the embodiment of  FIGS. 21 and 22  brake pads  158  are mounted in recesses  160  provided therefor in the periphery of the stator  162 . Cylinders  164  extend radially and, at their inner ends, join axially extending passages  166  which are connected into the hydraulic brake circuit.