Patent Publication Number: US-2003224888-A1

Title: Two speed drive systems and methods

Description:
TECHNICAL FIELD  
       [0001] This application relates generally to two speed drive systems and methods. More specifically, this application relates to two-speed drive systems for electric machines used in conjunction with internal combustion engines.  
       BACKGROUND  
       [0002] Internal combustion engines currently include two machines performing separate functions. The first machine is a starter machine for providing a starting torque to the crankshaft of the engine in order to facilitate the initiation of the combustion process of the engine. The second machine is a generator or alternator machine for receiving an input torque from the engine in order to generate an electrical output for meeting the electrical loads of the vehicle, as well as to charge the vehicle&#39;s battery.  
       [0003] The concept of using only one electric machine to do both functions, namely starting and generating, adds efficiency, and reduces assembly time. Additionally, the system allows a quiet stop-start capability. Here, the engine is allowed to stop and restart in slow moving (e.g., stop-and-go) traffic scenarios. This quiet stop-start capability provides both fuel saving and pollution reducing benefits. Efficiency is also increased because such starting and generating electric machines use less space, weigh less, eliminate the cost of one of the machines, and reduce assembly time. However, starting and generating electric machines give rise to issues that affect the cost, complexity, and reliability of such systems.  
       [0004] For example, starting and generating electric machines are often driven by a single belt or chain coupling the electric machine to the engine crankshaft. When a single speed belt or chain is used, the torque input changes between the starting and the generating functions. More specifically, during starting the electric machine applies an engine cranking torque to rotate the engine crankshaft. Once the engine has been started, a generating torque is applied by the crankshaft to the electric machine. Accordingly, a tensioning problem occurs in the belt or chain drive. Namely, the belt or chain drive has its tight side and slack side reverse depending on which torque is being applied. Accordingly, bi-directional tensioning devices are often required, which add to the cost, complexity, and decreased reliability of starting and generating electric machines.  
       [0005] Additionally, there is a gap between the torque and speed required to start an engine and the torque and speed required to generate electricity. High starting torques are due to compression stroke pressures, friction, cold lubrication, and the like. If a starter cannot rotate the crankshaft, a locked rotor condition exists on the starter rotor. This causes large current drains and excessive heat on the starting circuit with possible risk of damage and a no-start condition. A typical starter motor has a geared ratio of approximately 12 to 1, and has robust winding and heavy construction to avoid the risk of such locked rotor conditions. In this way, temporary overloads are accommodated.  
       [0006] A starter-generator unit typically solves this problem by use of large components, high pulley ratios, and/or combinations thereof. A starter-generator with about a 3 to 1 ratio is suitable for use in smaller engines with lower starting torques such as lawn mowers, golf carts, and the like. In order to provide a higher starting torque, a large crankshaft pulley combined with a small starter generator pulley can be used to provide a ratio of about 10 to 1. This larger ratio would increase the available starting torque from the starter generator. However, this larger ratio cannot be used during the generation of electricity. This is because during automotive applications, the generation of electricity a high engine speed, such as 6,000 revolutions per minute (rpm), exists. The high engine speed (6,000 rpm) combined with the large ratio (10:1) would create a generator speed of about 60,000 rpm. Such high generator speeds may cause damage to one or more components of the starter-generator (e.g., the rotor). A limit of about 18,000 rpm is common for most automotive generators.  
       [0007] One possible solution to the differing torque and speed requirements in starter-generator units is the use of a transmission having a different ratio for each of the starting function and the generating function. Such a transmission would require a gearbox and a switching clutch, which would add to the cost, complexity, and decreased reliability of starting and generating system.  
       [0008] Thus, a starter-generator that eliminates the aforementioned and other problems is desired.  
       SUMMARY  
       [0009] An electric machine for a vehicle comprising a shaft, a first one-way-clutch between the shaft and a driving member; and a second one-way-clutch between the shaft and a driven member is provided. The shaft is rotatably received within the electric machine. The first one-way-clutch transmits a first torque from the shaft to the driving member, but does not transmit a second torque from the driving member to the shaft. The second one-way-clutch transmits the second torque from the driven member to the shaft, but does not transmit the first torque from the shaft to the driven member.  
       [0010] A method for transmitting torques between a first shaft and a second shaft is provided. The method comprises applying a first torque to a driving member of the first shaft, the driving member of the first shaft being coupled to a driven member of the second shaft such that the first torque causes the second shaft to be rotated by the driven member; applying a second torque to a driving member of the second shaft, the driving member of the second shaft being coupled to a driven member of the first shaft such that the second torque causes the first shaft to be rotated by the driven member; preventing the first torque from rotating the driven member of the first shaft via a first one-way-clutch; and preventing the second torque from rotating the driving member of the first shaft via a second one-way-clutch.  
       [0011] A two-speed drive system for operatively connecting a first machine and a second machine is provided. The two-speed drive system comprises a first assembly, and a second assembly. The first assembly has a first driving member and a first driven member on a first shaft of the first machine. The first shaft receives a first torque from the first machine. The second assembly has a second driving member and a second driven member on a second shaft of the second machine. The second shaft receives a second torque from the second machine. A first coupling means engages the first driving member and the second driven member. A second coupling means engages the second driving member and the first driven member. A means for transmitting torque from the first shaft to the second shaft transmits the first torque from the first shaft to the first driving member without being transmitted to the first driven member. A means for transmitting torque from the second shaft to the first shaft transmits the second torque from the second shaft to the second driving member without being transmitted to the second driven member.  
       [0012] The above-described and other features are appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0013]FIG. 1 is a perspective view of an exemplary embodiment of a two-speed drive system;  
     [0014]FIG. 2 is a top view of the two-speed drive system of FIG. 1;  
     [0015] FIGS.  3 - 5  are top views of alternate exemplary embodiments of two-speed drive systems;  
     [0016]FIG. 6 is a perspective view of an alternate exemplary embodiment of a two-speed drive system;  
     [0017]FIG. 7 is a block diagram of a circuit for a starting and generating electric machine;  
     [0018]FIG. 8 is a first step of a control sequence of the circuit of FIG. 7;  
     [0019]FIG. 9 is a second step of a control sequence of the circuit of FIG. 7;  
     [0020]FIG. 10 is a third step of a control sequence of the circuit of FIG. 7; and  
     [0021]FIG. 11 is a fourth step of a control sequence of the circuit of FIG. 7. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0022] Referring now to FIGS. 1 and 2, a two-speed drive system  10  is illustrated. Two-speed drive system  10  operatively connects an electric machine  12  and an internal combustion engine  14 . Electric machine  12  is adapted to provide a starting torque for starting engine  14 , as well as generating electricity by receiving a generating torque from the started engine. Two-speed drive system  10  is a simple and inexpensive means for connecting the torques of electric machine  12  and engine  14 . It should be recognized that two-speed drive system  10  is capable of use in applications other than operatively connecting electric machine  12  and engine  14 , namely in applications where a two-speed drive system operatively connects two machines each capable of driving one another.  
     [0023] Electric machine  12  includes a pulley assembly  16  connected to a shaft  17 . Pulley assembly  16  comprises a driving pulley  28 , a driven pulley  30 , and a one-way clutch  32  positioned between shaft  17  and the driven pulley  30 .  
     [0024] Engine  14  includes a pulley assembly  18  connected to a crankshaft  19 . Pulley assembly  18  comprises a driving pulley  34 , a driven pulley  36 , and a one-way clutch  38  positioned between crankshaft  19  and the driven pulley  36 .  
     [0025] One-way clutches  32  and  38  are configured to only transmit torque to or from a shaft. Such one-way clutches are also known as overrunning clutches, sprag type clutches, roller clutches, and the like. For example, such one-way clutches are described in U.S. Pat. Nos. 5,183,139, 5,279,399, 5,676,226, and 5,718,314, the contents of which are incorporated herein by reference thereto.  
     [0026] A starting belt  22  is configured to frictionally engage the driving pulley  28  and the driven pulley  36  when the electric machines is mounted in close proximity to the engine. A generating belt  20  is configured to frictionally engage driving pulley  34  and driven pulley  30 .  
     [0027] During a starting operation, the electric machine  12  applies a clockwise starting torque  24  to shaft  17 . The driving pulley  28  is fixedly engaged to the shaft  17  by means such as, but not limited to, welding, a key and keyway, bolts, and the like. The driving pulley  28  converts the starting torque  24  from shaft  17  to a driving force on the starting belt  22  to drive the starting belt in a clockwise direction. The starting belt  22  frictionally engages the driving pulley  28  and the driven pulley  36 . Accordingly, the starting belt  22  transfers the driving force of the belt  22  to driven pulley  36  of pulley assembly  18 . Driven pulley  36  converts the driving force to a torque on crankshaft  19  through one-way clutch  38 .  
     [0028] Specifically, the one-way clutch  38  is adapted to transmit torques from the driven pulley  36  to the crankshaft  19 . However, and as described below in more detail, the one-way clutch  38  is not adapted to transmit torques from the crankshaft  19  to the driven pulley  36  once the engine  14  has been started. In this manner, the application of starting torque  24  by shaft  17  drives the crankshaft  19 , which in turn facilitates the starting of the engine  14 .  
     [0029] In addition, the driven pulley  30  is coupled to shaft  17  by way of one-way clutch  32 . One-way clutch  32  is not adapted to transmit torques from the shaft  17  to the driven pulley  30 , but it is adapted to transmit torque from the driven pulley  30  to the shaft  17  as described below in more detail (e.g., during a generating operation of electric machine  12 ). Thus and during the starting operation, the starting torque  24  applied to shaft  17  by the electric machine  12  is not transmitted to the driven pulley  30 . In contrast, during the starting operation the driven pulley  30  “spins” or “free wheels” on shaft  17  when the starting torque  24  is applied by the shaft.  
     [0030] Once the engine  14  has been started, the engine  14  applies a clockwise generating torque  26  to crankshaft  19 . The driving pulley  34  is fixedly engaged to the crankshaft  19  by for example, a key and keyway. The driving pulley  34  converts the generating torque  26  from the crankshaft  19  to a driving force on the generating belt  20  to drive the generating belt in a clockwise direction. The generating belt  20  frictionally engages the driving pulley  34  and the driven pulley  30 . Accordingly, the generating belt  20  transfers the driving force of the belt  20  to the driven pulley  30 . The driven pulley  30  converts the driving force to a torque on shaft  17  through one-way clutch  32 .  
     [0031] Specifically, the one-way clutch  32  is adapted to transmit torques from the driven pulley  30  to the shaft  17 . However and as provided above, the one-way clutch  32  is not adapted to transmit torques from the shaft  17  to the driven pulley  30  during the starting of the engine  14 . In this manner, the application of generating torque  26  by crankshaft  19  drives the shaft  17 , which in turn facilitates the generation of electricity by the electric machine  12 .  
     [0032] In addition, the driven pulley  36  is coupled to crankshaft  19  by way of one-way clutch  38 . One-way clutch  38  is not adapted to transmit torques from the crankshaft  19  to the driven pulley  36 , but it is adapted to transmit torque from the driven pulley  28  to the crankshaft  19 . Accordingly, while the engine  14  is being started or has been started the generating torque  26  applied to the crankshaft  19  is not transmitted to the driven pulley  36 . Once the engine has been started (e.g., during a generating operation) the driven pulley  36  “spins” or “free wheels” on crankshaft  19  when the generating torque  26  is applied by the crankshaft.  
     [0033] Accordingly, the two-speed drive system  10  is configured to transmit only one of the starting torque  24  and the generating torque  26  at a time. In other words, one-way clutch  32  is configured to “freely spin” or “free wheel” driven pulley  30  on shaft  17  when the electric machine  12  applies the starting torque  24  to the shaft, but is configured to transmit the torque generated from crankshaft  19  to shaft  17 . Conversely, one-way clutch  38  is configured to “freely spin” or “free wheel” driven pulley  36  on the crankshaft  19  when the engine  14  applies generating torque  26  to the crankshaft, but is configured to transmit the torque generated from shaft  17  to crankshaft  19 .  
     [0034] Since the two-speed drive system  10  transmits the torque from the driving pulleys  28  or  34  to only one belt  20  or  22  at a time, the belts require tensioning only on one side. Specifically, each belt has a tight side (e.g., a pulled side) and a slack side (e.g., a low tension side), and tensioning is only needed on the slack side of each belt. In the embodiment of FIG. 1, the generating belt  20  has a slack side  21 , while starting belt  22  has a slack side  23 . Here, the starting torque  24  from the driving pulley  28  results in the slack side  23 . Similarly, the generating torque  26  from the driving pulley  34  results in the slack side  21 .  
     [0035] In a first exemplary embodiment, tensioning is provided to the slack side of the belts  20  and  22  by way of a combination of a tensioning pulley and adjustably mounting the electric machine  12 .  
     [0036] The electric machine  12  is adjustable to provide tension to the slack side  21  of the generating belt  20 . Here, the electric machine is adjusted during installation until the belt  20  is taut or has been provided with the prescribed tension. At that point, the electric machine  12  is secured in the adjusted position by, for example, securement to the engine  14 .  
     [0037] A tensioning pulley  25  is positioned on the starting belt  22  to provide tension to the slack side  23 . Here, the tensioning pulley  25  is operatively engaged with either the inner or the outer surface of the starting belt  22  to provide tension to the slack side  23  of the belt. The tensioning pulley  25  is rotatable mounted on a shaft or structure (not shown) such that tension is provided to the slack side  23  as the starting belt  22  is rotated.  
     [0038] In a first exemplary embodiment, the tensioning pulley  25  is constantly biased into operative engagement with the starting belt  22 . In an alternate exemplary embodiment, the tensioning pulley  25  is adjustable to ensure that the belt  22  is taut or has been provided with the prescribed tension. Here, the tensioning pulley  25  is secured in position once the belt  22  is taut or has been provided with the prescribed tension.  
     [0039] Thus, the electric machine  12  provides tension to the slack side  21  of the generating belt  20  by way of its adjustable mounting, while the tensioning pulley  25  provides tension to the slack side  23  of the starting belt  22 .  
     [0040] It should be recognized that tension is provided by way of example as being provided to the generating belt  20  by adjusting the electric machine  12  and to the starting belt  22  by the tensioning pulley  25 . Of course, it is contemplated for the tensioning pulley  25  to provide tension to the generating belt  20  and the adjustable mounting of the electric machine  12  to provide tension to the starting belt  22 .  
     [0041] Alternately and as described with respect to FIG. 6, it is contemplated for a tensioning pulley to be provided at each of the belts  20  and  22  to provide the prescribed tension.  
     [0042] It should also be recognized that pulley assembly  16  is described above by way of example only as including one-way clutch  32 , and pulley assembly  18  is described above by way of example only as including one-way clutch  38 . Of course, any location for one-way clutches  32  and  38  that ensures transmission of only one of the torques  24  and  26  at a time are contemplated. For example, in FIG. 3 pulley assembly  16  is illustrated as comprising both one-way clutches  32  and  38 . Specifically in FIG. 3, the one-way clutch  32  is positioned between the shaft  17  and the driven pulley  30 , and the one-way clutch  38  is positioned between the shaft  17  and the driving pulley  28 . In this manner, the driving pulley  28  is driven by the shaft  17 , while the driven pulley  30  drives the shaft  17 .  
     [0043] Alternately, in FIG. 4 pulley assembly  18  is illustrated as comprising both one-way clutches  32  and  38 . Here, the one-way clutch  32  is positioned between the crankshaft  19  and the driving pulley  34 , and the one-way clutch  38  is positioned between the crankshaft  19  and the driven pulley  36 . In this manner, the driving pulley  34  is driven by the crankshaft  19 , while the driven pulley  36  drives the crankshaft  19 .  
     [0044] In FIG. 5, pulley assembly  18  comprises one-way-clutch  32 , while pulley assembly  16  comprises one-way-clutch  38 . Here, the one-way clutch  32  is positioned between the crankshaft  19  and the driving pulley  34 , and the one-way clutch  38  is positioned between the shaft  17  and the driving pulley  28 . In this manner, the driving pulley  28  is driven by the shaft  17 , while the driving pulley  34  is driven by the crankshaft  19 .  
     [0045] In all embodiments, one-way-clutches  32  and  38  are configured such that two-speed drive system  10  transmits the torques generated by either the starting torque or the generating torque at a time.  
     [0046] As described above, there is a gap between the torque and speed required to start engine  14  and the torque and speed required to generate electricity from electric machine  12 . More specifically, the starting torque  24  is generally higher than the generating torque  26  because the starting torque must overcome the engine compression to start the engine, while the generating torque must only rotate a rotor within the electric machine  12 . Two-speed drive system  10  mitigates the differing torque and speed requirements through the use separate belts  20  and  22  for starting and generating. Hence, starting belt  22  allows driving pulley  28  of pulley assembly  16  and driven pulley  36  of pulley assembly  18  to provide a starting ratio, while generating belt  20  allows driving pulley  34  of pulley assembly  18  and driven pulley  30  of pulley assembly  16  to provide a generating ratio.  
     [0047] For example and in accordance with an exemplary embodiment, the starting ratio between pulley assembly  16  and pulley assembly  18  is between about 3:1 and 10:1, while the generating ratio between pulley assembly  18  and pulley assembly  16  is between about 2:1 and 3:1. In the example where the starting ratio between pulley assembly  16  and pulley assembly  18  is 6:1, for every six revolutions of the shaft  17 , the crankshaft  19  rotates one time. Similarly, in the example where the generating ratio between pulley assembly  16  and pulley assembly  18  is 3:1, for every three revolutions of the shaft  17 , the crankshaft  19  rotates one time.  
     [0048] Thus, two-speed drive system  10  provides a means for providing tension to belts  20  and  22 , while also providing for separate starting and generating ratios. Of course, alternate starting and generating ratios greater than or less than the aforementioned ratios are contemplated in accordance with the present disclosure.  
     [0049] Referring now to FIG. 6, an alternate embodiment of two-speed drive system  10  is illustrated. Here, engine  14  further includes one or more accessory pulleys, namely pulley  40  and pulley  42 . The pulleys  40  and  42  are frictionally engaged to the generating belt  20 , which now takes on the further task of an accessory belt. In this manner, the accessory pulleys  40  and  42  are driven by the crankshaft  19  to drive other vehicle accessory systems, including but not limited to engine cooling systems, air conditioning systems, power steering systems, and the like. For example, the pulley  40  is illustrated for driving a water pump (not shown) for an engine cooling system (not shown) of engine  14 , while the pulley  42  is illustrated driving a power steering pump  44 .  
     [0050] Occasionally a gap exists between the torque and speed required for operation of electric machine  12  and the torque and speed required for operation of accessories coupled to the pulleys  40  and  42 . In this instance, the pulleys  40  and  42  are configured to provide ratios sufficient to compensate for any torque and speed differences.  
     [0051] In the embodiment of FIG. 6, tensioning is provided to the slack side  21  of belt  20  by a first tensioning pulley  46 , and tensioning is provided to the slack side  23  of belt  22  by a second tensioning pulley  48 . The tensioning pulleys  46  and  48  are operatively engaged with the belts  20  and  22 , respectively, to provide tension to the slack side of the belts. Namely and as described above with respect to FIG. 1, the tensioning pulleys  46  and  48  can be constantly biased into operative engagement with the inside or outside surface of the belts, and/or can be adjustable to ensure that the belts are taut. The tensioning pulleys  46  and  48  are rotatable mounted on a shaft or structure (not shown) such that tension is provided to the slack sides  21  and  23 , respectively, as the belts  20  and  22  are rotated.  
     [0052] In the embodiments of FIGS.  1 - 6 , the two-speed drive system  10  is described by way of example only as a belt-driven system. However, it should be recognized that the two-speed drive system  10  can be used with other coupling means, for example, chain-driven systems, gear-driven systems, combinations thereof, and the like. In a chain-driven system, pulley assemblies ( 16  and  18 ) are sprocket assemblies, and belts ( 20  and  22 ) are chains. In a gear-driven system, pulley assemblies ( 16  and  18 ) are gear assemblies, and belts ( 20  and  22 ) are eliminated. In the gear-driven system, the gear assemblies may be directly meshed to one another, or alternately intermediate gears may be used to mesh the gear assemblies to one another.  
     [0053] The starting and generating operation of an electric machine capable of providing a motor torque to a shaft, as well as generating an electrical current in response to receiving another motor torque from the shaft is described by way of example with respect to a circuit  50  illustrated in FIGS. 7 through 11. The electric machine is a three-phase rotary machine including a rectifier bridge  52  and a unit  54  for controlling the rectifier bridge. The electric machine includes a coil-carrying rotor  56  constituting the primary magnetic circuit associated with two rings and with two brushes that convey excitation current (of the order of a few amps); and a stator  58  carrying a plurality of coils constituting the secondary magnetic circuit, connected in star or delta configuration in the common case of a three-phase structure and acting, during generating operation, to deliver converted electrical power to the rectifier bridge  52  (several tens of amps at a voltage of the same order as the battery voltage).  
     [0054] Bridge  52  is connected to the various phases of the stator  58  and is connected between ground and a power supply terminal of a battery  60 . Bridge  52  includes a plurality of diodes  62  forming a rectifier bridge, and a plurality of switches  64 , such as transistors. Switches  64  are connected in parallel with respective diodes  62  and control the various phases (e.g., starting and generating) of the electric machine.  
     [0055] During a starting function, diodes  62  act as freewheel diodes, whereas in a generating function, the diodes act as a rectifier bridge. Switches  64  are advantageously MOSFET type transistors. Switches  64  include a diode between drain and source. Consequently, the switches  64  enable bridge  52  to be implemented using transistor components only that then act both as switches and as freewheel diodes.  
     [0056] The starting function of the electric machine is achieved by imposing DC on the primary magnetic circuit rotor  56  and by delivering signals that are phase-shifted by 120 degrees to the phases of stator  58 , which signals are ideally sinewave signals, but may optionally be squarewave signals or trapezoidal wave signals. Referring now to FIGS.  8 - 11 , an example of a control sequence for switches  64  is illustrated. The sequence is made up of squarewave signals issued by the control unit. The signals A, B, and C shown in these Figures are control signals for those of switches  64  in bridge  52  which are connected to ground. The signals A′, B′, and C′ which control the other transistors, i.e. those connected to battery  60 , are signals that are inverted relative to the signals A, B, and C, without overlapping them. This is shown in FIG. 11 where the signal C′ is drawn for controlling the transistor connected to the transistor controlled by the signal C. With this kind of control, the rotor performs one full revolution while each of the phases goes through a number of periods equal to the number of pairs of poles of the rotor (e.g. eight).  
     [0057] This starting operation is used for example to generate a starting torque to start an engine, thereby making it possible to eliminate the starter and the associated drive ring, and also the power cabling generally associated with the starter motor. To enable the engine to be started in this way, the control signals for switches  64  are advantageously variable frequency signals, at a frequency that is regulated to be increasing by unit  54 , so as to avoid any slip of the rotor relative to the rotating magnetic field created by the stator. For example, frequency regulation may be provided by unit  54  in such a manner so as to guarantee that the alternator has a speed profile that enables an engine to be started.  
     [0058] Control unit  54  includes a means for recognizing a code signal that authorizes engine starting. This signal is transmitted to the unit  54  by a code transmitter means inside the vehicle. The unit  54  switches on the transistors  64  in a manner suitable for starting the engine only if it receives the code signal. Consequently, the control unit  54  and the code transmitter means, which transmit the unlocking signal to the unit, constitute a system for immobilizing the engine.  
     [0059] After the engine is started, unit  54  controls the transistors  64  to operate in a generating mode. Unit  54  controls switches  64  so that all of them are open circuit across the terminals of all of the diodes. The bridge  52  then reverts to being a rectifier bridge. In another possible embodiment, the transistors  64  are controlled to short-circuit the conductive diodes. They are caused to be open circuit only across the terminals of non-conductive diodes. Thus, a current no longer passes through the conductive diodes, such that the short circuits made in this way serve to reduce losses. To synchronize control of transistors  64  relative to the switching from the conductive state to the nonconductive state of diodes  62 , unit  54  is connected to a means for detecting when the diodes pass from one state to another. By way of example, these means may be constituted by a sensor, such as a Hall effect sensor, for measuring the angular position of the rotor relative to the stator. Such a sensor may also be used for determining the speed of the rotor, e.g. by counting pulses in a given time window, so as to enable the unit to detect that the engine has started and thus switch from operating in starting mode to operating in generating mode.  
     [0060] Control unit  54  also includes a means for allowing a stop-start capability of the engine  14 . Here, the engine  14  is allowed, for example, to stop and restart in slow moving (e.g., stop-and-go) traffic scenarios.  
     [0061] Also, means  66  are provided for regulating voltage so as to maintain the voltage of battery  60  at a suitable level. Provision is also made for a switch  68 , e.g. another MOSFET type switch, whose ON or OFF state is controlled by the control unit. Switch  68  is designed to short circuit the regulator in starting mode so that the secondary magnetic circuit  58  is then directly excited by battery  60 .  
     [0062] It should be recognized that the operation of the electric machine has been described above by way of example with respect to FIGS.  7 - 11 . Of course, and as applications require, alternate means of operating an electric machine to apply a starting torque, and receive a generating torque are contemplated for use with the drive system of the present disclosure.  
     [0063] While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.