Abstract:
A method and a circuit for controlling an ac machine comprises controlling a full bridge network of commutation switches which are connected between a multiphase voltage source and the phase windings to switch the phase windings between a parallel connection and a series connection while providing commutation discharge paths for electrical current resulting from inductance in the phase windings. This provides extra torque for starting a vehicle from lower battery current.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The benefits of priority based on U.S. Provisional Appl. No. 60/885,939 filed Jan. 22, 2007, and U.S. Provisional Appl. No. 60/913,363 filed Apr. 23, 2007 are claimed herein. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    This invention was made with United States Government support under Contract No. DE-AC05-000R22725 between the United States Department of Energy and UT-Battelle, LLC. The United States Government has certain rights in this invention. 
     
    
     TECHNICAL FIELD 
       [0003]    The field of the invention relates generally to ac motor drives for hybrid vehicles and more particularly to starting sequences involving a battery. 
       DESCRIPTION OF THE BACKGROUND ART 
       [0004]    The following data was taken from the nameplate of a commercially available hybrid vehicle battery and from testing the battery. The battery power rating was 21 kW, 201.6V, 6.5 Amp-hrs. The nominal current was 21000 w/201.6V=104 A. The battery was rated at 104 A for 6.5 Amp-hrs./104 Amps=224 secs. However, testing showed that producing the maximum specified motor torque of 400 Nm would require 350 dc amperes of current. The fully charged battery under this condition can only lasted for 6.5 Amp-hr/350 A=66 secs. 
         [0005]    The battery consideration is particularly important for the plug-in electric and hybrid vehicles for reducing the current requirement during starting and for accelerating in traffic. It is therefore an object of the invention to improve starting torque while reducing starting current requirements. 
       SUMMARY OF THE INVENTION 
       [0006]    For a hybrid motor vehicle, the change of the connections of electrical motor windings in each phase from a parallel connection (three-phase delta configuration) to a series connection (3-phase Y configuration) to provide more turns at low speed can produce the same torque with a lower current requirement. This can reduce the demand on the battery capacity, because during starting and initial acceleration the battery may be the only source available. If other sources are available, such as a super capacitor and field enhancement, the capacity of the battery can be further reduced with a connection-change switch. In the present invention, this is carried out by utilizing a switching device as a connection change switching device to be added to an inverter to switch the phase windings to a series connection when starting or accelerating and back to invention also provides commutation paths for dissipating energy stored in the inductive motor windings. 
         [0007]    In a further aspect of the invention the commutation paths are provided by SCRs with a low continuously operating current rating of, for example, 25 Amps, but a high pulse rating of, again for example, 350 Amps, to withstand starting and operating transients. The ratio between pulse and continuous current ratings depends on the specific SCRs. 
         [0008]    The advantages of the present invention include: very low cost, benign failure mode, high efficiency with over 99% efficiency, small size, long life expectancy, and easy cooling. 
         [0009]    Other objects and advantages of the invention will be apparent to those of ordinary skill in the art from the description of the preferred embodiments which follows. In the description reference is made to the accompanying drawings, which form a part hereof, and which illustrate examples of the invention. Such examples, however, are not exhaustive of the various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1   a  is an electrical schematic of a set of three-phase motor windings with switches arranged for a series connection of the phase windings; 
           [0011]      FIG. 1   b  is an electrical schematic of a set of three-phase motor windings with switches arranged for a parallel connection of the phase windings; 
           [0012]      FIG. 2  is an electrical schematic diagram of a dual inverter for changing between Y and delta connections to a three phase motor. 
           [0013]      FIGS. 3-6  are fragmentary schematic diagrams of a connection change device for changing the switching between series and parallel connections without commutation paths for discharging inductive currents; 
           [0014]      FIG. 7-10  are fragmentary schematic diagrams of a connection change device for changing the switching between series and parallel connections with commutation paths for discharging inductive currents according to the present invention; 
           [0015]      FIG. 11  is a three-phase inverter with a plurality of SCRs for providing commutation paths for discharging inductive currents caused by changing connections of the motor windings; 
           [0016]      FIGS. 12   a - 12   d  are schematics a connection change device; and 
           [0017]      FIG. 13  is a graph of power output vs. speed for a series connection and a parallel connection of the motor windings. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1   a  shows the windings W for a typical three-phase ac machine M connected to a three-phase voltage source S for providing phase voltages Va, Vb and Vc through a plurality of winding terminals. The winding terminals are labeled with A 1 , B 1 , C 1 , X 1 , Y 1 , Z 1 , A 2 , B 2 , C 2 , X 2 , Y 2 , and Z 2 . Eight on-off switches SW 1 -SW 8  are connected between the winding terminals and the phase voltages to provide a connection change device as discussed below. For this configuration in which the windings for each phase A, B or C, are in series, the switch between terminals A 1  and A 2  is open. The switch between terminal X 1  and A 2  is closed to connect the phase windings for phase A in series. Similarly, the switches between terminals B 1  and B 2  and between terminals C 1  and C 2  are open. The switches between terminals Y 1  and B 2  and Z 1  and C 2  are closed to connect the respective phase windings for phases B and C in series. The switch between terminals X 1  and Y 1  is open, as is the switch between terminals Y 1  and Z 1 . 
         [0019]    In  FIG. 1   b , the motor M has the eight commutation switches SW 1 -SW 8  arranged for a parallel connection of phase windings W in the motor M. The switch SW 1  between terminals A 1  and A 2  is now closed and the switch SW 2  between terminal X 1  and A 2  is open to connect the phase windings for phase A in parallel. Similarly, the switches SW 3  and SW 4  between terminals B 1  and B 2  and between terminals C 1  and C 2  are closed. The switches SW 5  and SW 6  between terminals Y 1  and B 2  and Z 1  and C 2  are open to connect the respective phase windings for phases B and C in parallel. The switch SW 7  between terminal X 1  and Y 1  is closed, as is the switch SW 8  between terminals Y 1  and Z 1 . 
         [0020]      FIG. 2  shows a dual inverter  11  of the prior art having two bridges  11   a ,  11   b  of power electronics switches ESW 1 -ESW 12  such as insulated gate bipolar transistors (IGBTs) or the silicon controlled rectifiers (SCRs) with anti-reversing diodes D 1 -D 12  for applying the phase signals to the motor windings W. To apply this technology to provide a change in the connection of the motor windings W between Y and delta for an effective turn change ratio of 1.73 would require at least twelve high power switches. The turn change ratio is less than 2.0 effective turn change ratio of the series and parallel connection change; furthermore at the present time the cost for the electronics switches is relatively expensive. 
         [0021]      FIGS. 3-6  show connection change switching of the eight switches seen in  FIGS. 1   a  and  1   b  in a system according to existing technology. A connection change device  20  would have a stationary ring-shaped outer portion  21  and a rotatable rotor  22 . The stationary outer portion  21  would receive phase voltages Va, Vb and Vc at respective contacts  23   a,    23   b  and  23   c.  Terminals X 1 , Y 1  and Z 1  would also connect to contacts  24   a,    24   b  and  24   c  on the outer portion  21 . Terminals A 2 , B 2  and C 2  would connect to contacts  25   a,    25   b  and  25   c  on the rotor  22 . The rotor  22  would be rotated part of a turn to switch the connections of the motor windings and returned to return the switching to the original configuration. 
         [0022]      FIG. 3  shows the position of the connection change device  20  with a series connection of the motor phase windings. Contact  25   a  for terminal A 2  contacts contact  24   a  for terminal X 1 , corresponding to the closing of the switch SW 2  in  FIG. 1   a . Contact  25   b  for terminal B 2  contacts contact  24   b  for contact Y 1  and contact  24   c  for terminal C 2  contacts contact  24   c  for terminal Z 1  also corresponding to the switches SW 7 , SW 8  that are closed in  FIG. 1   a . Three other contacts  26   a,    26   b  and  26   c  are connected together to represent the Y-connection neutral point. 
         [0023]      FIG. 4  illustrates the position of the connection change device  20  with a parallel connection of motor windings. Contact  25   a  for terminal A 2  contacts contact  23   a  for phase voltage Va; contact  25   b  for terminal B 2  contacts contact  23   b  for phase voltage Vb and contact  25   c  for terminal C 2  contacts contact  23   c  for phase voltage Vc. Contacts X 1 , Y 1  and Z 1  are connected together by contacts  26   a,    26   b  and  26   c  and this corresponds to the closing of switches SW 7  and SW 8  between contacts X 1 , Y 1  and Z 1  as shown in  FIG. 1   b.    
         [0024]      FIGS. 5 and 6  show the switching from the position in  FIG. 4  (parallel) to the position in  FIG. 3  (series) for starting a vehicle, for example. For example, as the contact  25   a  for terminal A 2  slides away from contact  23   a  for terminal Va in  FIG. 5  and toward contact  24   a  for terminal X 1  in  FIG. 6 , there is a brush-type commutation action and sparking can occur. This is particularly true where there is no commutation path for absorbing energy from the motor windings. The same is true for the contacts  25   b  and  25   c  as they break contact with contacts  23   b  and  23   c  for phase voltages Vb and Vc. The contacts  26   a,    26   b  and  26   c  also slide away from contacts X 1 , Y 1  and Z 1  representing the opening of the switches SW 7  and SW 8  in  FIGS. 1   a  and  1   b.    
         [0025]      FIGS. 7 and 8  show a connection-change device  30  of the present invention having a stationary portion  31  and a movable portion  32 . This device has the contacts described for the embodiment of  FIGS. 3-6 , but adds commutating contacts  27   a,    27   b  and  27   c  to the stationary portion  31 . These contacts  27   a,    27   b  and  27   c  are each connected to the DC+ and DC− of a commutation path, for discharging inductive current from the motor windings. The movable portion  32  also has contacts  28   a,    28   b  and  28   c  for connection to the DC+ and DC− of a commutation path, for discharging inductive current from the motor windings. 
         [0026]    The position of the contacts shown in  FIG. 7  corresponds to the series connection illustrated in  FIG. 3 . It can be understood clearly by comparing the labels shown in these two figures. The position of the contacts in  FIG. 8  corresponds to the parallel connection in  FIG. 4 . 
         [0027]      FIGS. 9 and 10  show a commutation during the connection change from parallel phase windings to series phase windings for starting a motor vehicle. The series connection provides more turns and more starting torque for less starting current, which is an object of the invention. As seen in  FIG. 9 , the contacts  25   a,    25   b  and  25   c  connected to terminals A 2 , B 2 , and C 2  start to contact their corresponding commutation brushes  27   a,    27   b  and  27   c,  before breaking contacts with contacts  25   a,    25   b  and  25   c  which are connected to the phase voltage terminals Va, Vb and Vc. The commutation brushes  28   a,    28   b  and  28   c  contact the contacts for terminals X 1 , Y 1 , and Z 1  brushes before contacts  26   a,    26   b  and  26   c  break contact to open the corresponding switches. In  FIG. 10 , it can be seen that contacts for terminals X 1  and A 2  contact the commutation discharge contacts  27   a  and  28   a  as this switch SW 2  is closed between the two motor windings of phase A. Thus, the commutation contacts  27   a - 27   c  and  28   a - 28   c  and their commutation paths provide for sparkless opening and closing of the switches SW 1 -SW 8 . 
         [0028]      FIG. 11  shows an embodiment of the present invention where the commutation paths shown in  FIGS. 7-10  are provided by commutation module  45  comprising a double bridge of twelve (12) small current, high pulse rating SCRs  48  that are connected to the terminals in the connection change device  30  as described previously. The connection change device  30  is connected between the multiphase voltage source and the phase windings, and has a set of contacts as described in relation to  FIGS. 3-10  for conducting load currents of a magnitude conducted in the windings of the ac machine. The SCRs have a low continuously operating current rating of, for example, up to and including approximately 25 Amps, but a high pulse rating of approximately 350 Amps or greater to withstand starting and operating transients. In  FIG. 11 , a conventional 3-phase inverter  14  supplies phase voltages Va, Vb and Vc to the connection change device  20  represented by the switches connected between pairs of the terminals A 1 , B 1 , C 1 , X 1 , Y 1 , Z 1 , A 2 , B 2 , C 2 , X 2 , Y 2 , and Z 2 . 
         [0029]      FIGS. 12   a  to  12   d  show how the timing of the SCRs in the module  45  are coordinated with the operation of the connection change device  20 , which may be rotary as shown in  FIGS. 3-10  or a linear device as shown in  FIGS. 12   a  to  12   d . As shown in  FIG. 12   a , for the parallel connection of the windings the triggering signal contacts are not touching any stationary contacts, no SCR triggering signal is produced at this position.  FIG. 12   b  shows during the process of changing connection from parallel ( FIG. 12   a ) towards series connection. when triggering-signal- 2  contact is touching X 1  right before X 1  is leaving the conductor that is connected to the neutral, a triggering signal that is the potential difference between X 1  and A 2  is produced. The triggering signals are applied to all four of the SCRs shown. Triggering signal  1  is not used during this parallel to series change of connections.  FIG. 12   c  shows that, in a series connection position, no triggering signal is produced.  FIG. 12   d  shows that when X 1  and A 2  connections are about to open, Va starts touching triggering signal  1  contact to produce a triggering signal. Triggering signal  2  is not used during this series to parallel change. Other means such as electronic sensors based on the described requirements can be used. 
         [0030]      FIG. 13  shows a graph of power output vs. speed for the series connection and the parallel connection. It can be seen that the series connection curve  50  provides relatively higher power than the parallel connection curve  51  at low speed, but at some point levels off and the parallel connection provides more torque and power at higher speeds. 
         [0031]    This has been a description of a preferred embodiment of the invention at the time of filing this patent application, and it will be apparent to those of ordinary skill in the art that certain details while part of an illustrative example, may be varied in other embodiments without departing from the spirit and scope of the invention, which is defined by the following claims.