Patent Abstract:
A motor is driven with variable frequency AC power converted from DC power, and an AC output power from the motor is converted to DC. In the conversion, an electromotive force regenerated by the motor is consumed by a plurality of resistors. When the consumption of the electromotive force is impeded in any of systems including any of the plurality of resistors, the electromotive force is consumed by the systems including the remaining resistors. Even if a system including any of the plurality of resistors fails, the electromotive force can by consumed by the other systems including the remaining resistors, thus providing a reliable rheostatic brake.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present invention is related to U.S. patent application Ser. No. 11/297,490 entitled “Electric Drives System for Vehicle, Electric Control System for Vehicle, Electric Drive Method for Vehicle”, filed on Dec. 9, 2005. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to an AC drive apparatus, a vehicle control apparatus, a power conversion method, and a vehicle control method. 
   With the advance of power electronics, a vehicle drive system, for example, has increasingly employed an AC motor instead of a DC motor. In such a system which employs an AC motor, electric energy is supplied to the motor for use as motive energy upon startup of the system, and a so-called rheostatic brake is employed for forcing the motor to operate as a generator upon braking such that electric energy generated thereby is consumed by a resistor to produce a braking force. Such a system is shown, For example, in JP-A-6-46505. 
   SUMMARY OF THE INVENTION 
   However, the foregoing system cannot produce the braking force if the rheostatic brake fails. While a mechanical brake is often provided together with an electric brake to enable the production of a braking force by applying a friction force to an axil, the rheostatic brake tends to produce a larger braking force and requires less maintenance operations than the mechanical brake, so that the mechanical brake is used only during low-speed operations or for a final stop. Therefore, the rheostatic brake has been requested to improve the reliability. It is an object of the present invention to provide an AC drive apparatus which has a reliable rheostatic brake, a vehicle control apparatus, a power conversion method, and a vehicle control method. 
   To achieve the above object, an AC drive apparatus of the present invention includes a plurality of electric brakes each including a resistor for consuming an electromotive force regenerated by a motor, and a switch for connecting the resistor. If the consumption of the electromotive force is impeded in one of electric brakes including any of the plurality of resistors, the electromotive force Is consumed by electric brakes including the remaining resistors. 
   More specifically, the AC drive apparatus has a plurality of rheostatic brakes, each brake is connected to DC section through a switch, and the DC section is configured between a rectifier of which power is from a prime mover and the bidirectional converter of which power is from AC motor. 
   According to the present invention, the AC drive apparatus realized thereby comprises reliable electric brakes More specifically, an electromotive force regenerated as DC power through the bidirectional converter is consumed by a plurality of rheostatic brakes to produce a braking force. Also, when some of the plurality of rheostatic brakes fails, the failed rheostatic brake is disconnected, such that a vehicle operator can still continue a vehicle decelerating operation, or can keep enough time to transit to a mechanical bake because an entire braking force is not lost, though the braking force becomes lower than when all the rheostatic brakes are sound. Generally, in recent years, a forced cooling scheme based on an electric fan Is employed in the rheostatic brake with the intention of improving the utilization factor and reducing the size and weight. For this reason, opportunities of failure are increasing, so that the present invention is also effective as means which responds to the need for both the reduction in size and weight and the improvement in reliability. 
   Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block circuit diagram illustrating a method of implementing an AC drive system which comprises a prime mover as a power source, and a plurality of rheostatic brake circuits (First Embodiment); 
       FIG. 2  is a graph showing an exemplary AC voltage control characteristic; 
       FIG. 3  is a graph showing an exemplary control characteristic of a DC voltage versus switch on/off duty; 
       FIG. 4  is a block circuit diagram illustrating a method of implementing an AC drive system which comprises a trolley as a power source, and a plurality of rheostatic brake circuits (Second Embodiment); 
       FIG. 5  is a block circuit diagram illustrating a method of implementing an AC drive system which comprises a prime mover and a trolley as power sources, and a plurality of rheostatic brake circuits (Third Embodiment); and 
       FIGS. 6A ,  6 B,  6 C are diagrams illustrating a cooling unit in detail. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   In the following, embodiments of the present invention will be described with reference to the accompanying drawings in the order of a first to a third embodiment. 
   First Embodiment 
   An AC drive system of the present invention comprises a plurality of rheostatic brakes connected in parallel to achieve the object of avoiding a loss of an overall braking force due to a failure of a rheostatic brake.  FIG. 1  illustrates the AC drive system according to a first embodiment of the present Invention. An AC generator  2  (or called the “main generator  2 ,” which is also applied to the following description) driven by a prime mover  1  feeds an AC output to a rectifier  3 . The rectifier  3  outputs a DC voltage which includes ripples reduced by a smoothing capacitor  4 . For setting the DC voltage to a required value which Is calculated by a controller  7 , a field regulator  17  controls an exciting current of the generator  2 . This DC voltage is fed to a motor  9  through a bidirectional converter  8  which converts the DC voltage to an AC voltage at art arbitrary frequency. Output values of an output current detector  14  associated with the bidirectional converter  8  and a speed detector  16  are fetched into a controller  7  which determines a switching state of the bidirectional converter  8  together with a motor output torque setting unit  5  and a voltage across a smoothing capacitor, and outputs a switching pulse to the bidirectional converter  8 . With these operations, a vehicle equipped with the AC drive system conducts acceleration/deceleration control. 
   When the vehicle is accelerated, a torque is determined based on an accelerator unit  5  trodden state and output values of the current detector  14  associated with the bidirectional converter  8  and the speed detector  16 . To provide the determined torque to the motor, the controller  7  controls the rotation speed of the prime mover  1  by using the values of the output current detector  14 , speed detector  16  and DC voltage detector  15  in a relationship as shown in  FIG. 2 . Likewise, to determine a switching state of the bidirectional converter  8 , the output values of the output current detector  14 , the speed detector  16 , the torque setting unit  5  and the voltage across the smoothing capacitor are supplied to the controller  7 . Since the motor  9  generates an increasingly larger torque as the bidirectional converter  8  supplies an AC current at a higher frequency, the vehicle is accelerated. 
   When the vehicle is decelerated, the motor  9  enters a regenerative mode to convert motive energy of the vehicle to AC electric energy. This AC electric energy is converted to DC power by the bidirectional converter  8 . In this event, a DC voltage outputted by the bidirectional converter  8  is controlled to a higher value than a DC voltage which is outputted by the generator  2  through the rectifier  3 . The controller  7  determines a time for which a resistor a ( 12   a ) of an electric brake a ( 10   a ) and a resistor b ( 12   b ) of an electric brake b ( 10   b ) are connected to a DC section, ie., a duty in accordance with the values of the DC voltage detector  15  and a braking force setting unit  6 , as well as a difference between the two values. Switches  11   a ,  11   b  for connecting the resistors a ( 12   a , b ( 12   b ) to the DC section, which comprise semiconductor devices or the like, start an on/off operation when the DC voltage value exceeds a set value a (2,000 volts by way of example), for example, as shown in  FIG. 3 , and repeat the on/off operation such that the DC voltage value falls within a set value b (3,000 volts by way of example). In this event, in a range of 100 to 2,000 volts, the switches  11   a ,  11   b  remain off. 
   Further, the on/off duty is increased as the DC voltage value is increased, and the on/off duty is set to 100% when the DC voltage value reaches a set value b, for example, as shown in  FIG. 3  (i.e., the switches  11   a,    11   b  remain on). 
   The controller  7  detects currents passing through the resistors a, b by current detectors  18   a  and  18   b , and calculates the power consumed by the resistors a, b such that the electric energy generated by the motor  9  is entirely consumed by the resistors a, b. With these operations, the vehicle equipped with the AC drive system is decelerated. 
   If one of the rheostatic brakes  10   a ,  10   b  fails during the decelerating operation, the electric energy generated by the motor  9  cannot be entirely consumed. Thus, if the same electric energy as that before the failure was continuously regenerated to the DC section, the DC voltage would continue to rise and eventually exceed the withstanding voltage level of each device, so that, for preventing this inconvenience, the regenerated energy must be limited. Since the regenerated energy is limited by the capacity of a sound rheostatic brake, a conventional vehicle equipped only with a single rheostatic brake results in a temporary loss of an entire braking force However, the AC drive system according to the present invention illustrated in  FIG. 1  comprises a plurality of rheostatic brakes, so that even it the electric brake  10   a  (also called the “rheostatic brake  10   a ”) fails, the switch a is made inoperative, and the second electric brake  10   b  (also called the “rheostatic brake  10   b ”) can continue the braking operation though the braking force is reduced to one-half as much as when the whole system is sound. 
   Referring now to  FIGS. 6A ,  6 B,  6 C, a detailed description will be given of the structure of a cooling unit composed of the resistor a ( 12   a ) and a cooling fan a ( 13   a ) (an area surrounded by a one-dot chain line within the electric brake  10   a ), and a cooling unit composed of the resistor b ( 12   b ) and a cooling fan b ( 13   b ) (an area surrounded by a one-dot chain line within the electric brake  10   b ). Since the former cooling unit is substantially identical in configuration to the latter cooling unit, the following description will focus on the cooling unit composed of the resistor a ( 12   a ) and cooling fan a ( 13   a ) in  FIG. 1 . 
   As illustrated in  FIG. 6A , a housing comprises a cylinder housing and a square pillar housing connected thereto, both of which are made of metal (preferably, a steel plate). The cooling fan a ( 13   a ) is stored in the cylindrical housing. The cooling fan a ( 13   a ) rotates to feed cooling air in a direction indicated by arrows in the figure. The cooling air passes through the square pillar housing, flowing as indicated by the right-hand arrow in the figure, and is emitted from the square pillar housing by an air blasting action of the cooling fan a ( 13   a ). 
   As illustrated in  FIG. 6B , resistor elements  12   a - 1 - 12   a - 7 , which make up the resistor a ( 12   a ), are arranged side by side within the square pillar housing in the air blasting direction. The cooling air fed by the cooling fan a ( 13   a ) sequentially cools down the resistor elements  12   a - 1 - 12   a - 7 . Each of the resistor elements  12   a - 1 - 12   a - 7  is composed of an upper metal plate and a lower metal plate which are connected by four metal plates for example. Electric couplers are attached to both ends of the upper metal plate. 
   The foregoing electric couplers are electrically connected to each other to make up an electric circuit as illustrated in  FIG. 6C . 
   Here, as illustrated in  FIG. 6A , a temperature sensor  22   a  is mounted halfway in the cylindrical housing in the air passing direction. The temperature sensor  22   a  detects the temperature within the cylindrical housing to send a temperature signal to the controller  7 . Also, a pressure sensor  23   a  is mounted near the rear end of the cylindrical housing in the air passing direction. The pressure sensor  23   a  detects the pressure to find the fan working soundly near the exit of the cylindrical housing to send a pressure signal to the controller  7 , so as to increase reliability of the electric brake. 
   The controller  7  monitors the temperature signal and pressure signal, and determines an abnormal temperature or an abnormal pressure if one (or both) of these signals reaches a predetermined value or higher to bring the switches  11   a,    11   b  into a disconnected state. Alternatively, the controller  7  controls the switches  11   a,    11   b  such that they remain off for a longer time in their on/off operations. 
   Second Embodiment 
   An AC drive system according to a second embodiment comprises a trolley which is substituted for the components of the first embodiment for connecting the AC output of the AC generator  2  driven by the prime mover  1  to the rectifier  3  to supply DC power in the first embodiment. In a system which comprises a trolley  20  that does not have a capacity large enough to absorb regenerated power, a vehicle itself must consume the regenerated power. Therefore, when a braking force is required, the trolley is disconnected from the AC drive system by a trolley connector  19  upon detection of a positive output from the braking force setting unit  6 , in order to prevent the power from flowing from the trolley to the resistors  11   a,    11   b  of the rheostatic brakes  10   a,    10   b.  In this way, the AC rive system of the second embodiment can perform similar operations to those of the first embodiment. 
   Third Embodiment 
   An AC drive system according to a third embodiment comprises both means for connecting the AC output of the AC generator  2  driven by the prime mover  1  to the rectifier  3  to supply DC power in the first embodiment, and means for supplying DC power by a trolley  20  in the second embodiment. A special vehicle system such as an electric truck may be provided with the power through a trolley  20 , but must operate even in a place where the trolley is not installed. Thus, the AC drive system continues to operate while switching the power source by a trolley/motor switching unit  21  between the power generated by the prime mover  1  and the power supplied through the trolley  2 . In this way, the AC drive system of the third embodiment can perform similar operations to those of the first and second embodiments. 
   The configurations of the first to third embodiments can also be applied to an electric propeller ship which employs a grid resistor for a speed restraining operation. 
   It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.

Technology Classification (CPC): 8