Abstract:
A synchronous motor system in which recovery due to a failed field excitation supply is achieved automatically without resort to, or by minimizing the length of time in asynchronous operation of the motor. A sensing system is deployed to provide a sensed signal that is caused by a failed field excitation supply to vary from a reference value by a predetermined amount. A control unit responds to detected failure by automatically changing a field winding system of the motor over from the failed field excitation supply to a standby field excitation supply. The standby supply can be shared by several synchronous motors.

Description:
BACKGROUND OF INVENTION 
   The present disclosure relates to synchronous motors. More particularly, the present disclosure relates to recovery from the loss of a field excitation unit for the motor. 
   When a synchronous motor loses its field excitation supply, it is either shut down or is operated in an asynchronous mode for the time that it takes to repair the field excitation supply. However, extended operation of the synchronous motor in an asynchronous mode can result in overheating, and requires an oversized motor and power system. Moreover, when operating in an asynchronous mode, a synchronous motor can normally only drive a reduced load vis-a-vis a full load when operating in a synchronous mode. 
   Accordingly, there is a need to solve the problem of how to recover from the loss of a field excitation supply of a synchronous motor so as to restore the motor to full load synchronous operation while minimizing the time running asynchronously or without resorting to asynchronous operation. 
   BRIEF DESCRIPTION OF THE INVENTION 
   A technical effect is a solution of the aforementioned problem that upon detection of a failed normal field supply unit, automatically changes a field winding system of a synchronous motor over to a standby field supply unit, thereby freeing the failed unit for servicing without interruption of the full load operation, or minimizing the time at reduced load, of the motor. 
   In a system embodiment, a synchronous motor system comprises at least one synchronous motor that includes a field winding system, a normal field supply unit, a standby field supply unit and a control unit. The control unit normally connects the normal field supply unit in circuit with the field winding system and in response to a failure of the normal field supply unit automatically changes over the field winding system from the normal field supply unit to the standby field supply unit. 
   In a method embodiment, the method recovers from a loss of a field supply unit of a synchronous motor by connecting the normal field supply unit in circuit with a field winding system of the synchronous motor. A failure of the normal field supply unit is detected and in response to the detected failure, the field winding system is automatically changed over from the normal field supply unit to a standby field supply unit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The sole FIGURE of the drawing is a block diagram of a synchronous motor system. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to the FIGURE, a synchronous motor system is illustrated by way of reference numeral  20 . Synchronous motor system  20  includes a plurality of synchronous motors  30  and  50 . Although two synchronous motors  30  and  50  are shown, it will be apparent to those skilled in the art that synchronous motor system  20  may include more or less than two synchronous motors. 
   Synchronous motor  30  includes a rotor  32 , a stator  34 , a field winding system  36 , and a motor start/run device  80 . Synchronous motor  50  includes a rotor  52 , a stator  54 , a field winding system  56 , and a motor start/run device  82 . 
   Synchronous motor system  20  also includes a control unit  22 , an incoming power supply  24  and an incoming power supply  26 , a normal field supply unit  40 , a normal field supply unit  60  and a standby field supply unit  70 . Although power supply  24  is shown in the diagram as a single source, it may be more than one to provide individual power supplies to the normal and standby field supply units  40 ,  60  and  70 . Similarly, although shown in the FIGURE as a single supply, power supply  26  may be more than one supply to provide individual power supplies to synchronous motor stators  34  and  54 . Power supply  24  is a relatively low voltage. Power supply  24  includes a switch  25  that, when closed, connects power supply  24  to normal and standby field supply units  40 ,  60  and  70  for use as a field excitation voltage. Power supply  26  includes a switch  27  that, when closed, connects power supply  26  to an electrical power source that provides a relatively high voltage to motor start/run devices  80  and  82  for use as a motor stator voltage supply. By way of example, for a large motor application in North America, the relatively high voltage may be 13.8 kilovolts and the relatively low voltage may be 460 volts. Switches  25  and  27  may by operable manually or may be operated automatically as part of an overall control sequence. 
   Normal field supply unit  40  and normal field supply unit  60  are connected in circuit with synchronous motors  30  and  50 , respectively, and with power supply  24 . Thus, normal field supply unit  40  includes an input switch  42  mechanism, a normal field supply  44 , a current sensor  45 , and an output switch mechanism  46 . When both input switch  42  mechanism and output switch mechanism  46  are closed, a field excitation voltage is applied to field winding system  36  of synchronous motor  30 . Normal field supply unit  60  includes an input switch  62  mechanism, a normal field supply  64 , a current sensor  65 , and an output switch mechanism  66 . When both input switch  62  mechanism and output switch mechanism  66  are closed, a field excitation voltage is applied to field winding system  56  of synchronous motor  50 . 
   Synchronous motor system  20  further includes a standby field supply unit  70  that is shared by synchronous motors  30  and  50 . Standby field supply unit  70  is connected in circuit with power supply unit  24  and with both field winding system  36  of synchronous motor  30  and field winding system  56  of synchronous motor  50 . Standby field supply unit  70  includes an input switch  72  mechanism, a standby field supply  74 , a current sensor  75 , an output switch mechanism  76  and an output switch mechanism  78 . It will be apparent to those skilled in the art that the input switch mechanisms  42 ,  62  and  72  and the output switch mechanisms  46 ,  66 ,  76  and  78  may be any suitable switching mechanism, such as, circuit breakers, contactors, power relays, motorized switches and the like. 
   Motor start/run device  80  and motor start/run device  82  are connected in circuit with power supply  26  and in circuit with the windings of motor stators  34  and  54  of synchronous motors  30  and  50 , respectively. Motor start/run devices  80  and  82  may be simple switching devices that start and run synchronous motors  30  and  50  by applying rated voltage to stators  34  and  54  to accelerate motors  30  and  50  to synchronizing speed, at which point the motor field is applied to pull motors  30  and  50  into synchronism. Alternatively, motor start/run devices  80  and  82  may be assisted starting systems, such as reactor-assist, autotransformer-assist, capacitor-assist, capacitor-reactor-assist, electronically-controlled starter, or similar scheme. These schemes are designed to accelerate the synchronous motor to synchronizing speed (and subsequent pull-in to synchronous operation by applying the motor field), while minimizing the impact of starting the motor on the power system  26 . Alternatively, motor start/run devices  80  and  82  may be variable-voltage or variable-speed drive systems, having the ability to start and run the motor at variable speed. 
   Control unit  22  has connections to output switch mechanisms  46 ,  66 ,  76  and  78  to control the opening and closing thereof. Control unit  22  also has connections to input switch mechanisms  42 ,  62  and  72 , to control the opening and closing thereof, where desirable to achieve correct functioning of the operational strategy. Control unit  22  also has connections to normal field supply  44 , normal field supply  64  and standby field supply  74  to properly co-ordinate the functioning of these devices during operation and changeover. 
   In a normal operating mode, normal field supply units  40  and  60  are operable to provide voltage to field winding systems  36  and  56  of synchronous motors  30  and  50 , respectively. To this end, control unit  22  controls output switch mechanisms  46  and  66  to be closed and output switch mechanisms  76  and  78  to be open. Thus, normal field supply units  40  and  60  are connected to synchronous motors  30  and  50 , but standby field supply unit  70  is not connected to either synchronous motor  30  or  50 . 
   Current sensors  45 ,  65  and  75  of field supplies  40 ,  60  and  70  sense the respective field load currents and provide these sensed currents to control unit  22 . If normal field supply unit  40  should fail or have faulty operation, the load current of synchronous motor  30  field system will deviate from a reference value. If this deviation exceeds a predetermined amount, a failure will be detected. In response to the detected failure, control unit  22  automatically changes field winding system  36  from normal field supply unit  40  over to standby field supply unit  70 . To accomplish this, control unit  22  opens output switch mechanism  46  of normal field supply unit  40  and closes output switch mechanism  76  of standby field supply unit  70 . In this scenario, synchronous motor  30  is now receiving field voltage from standby field supply unit  70  and synchronous motor  50  continues to receive field voltage from normal field supply unit  60 . Faulty normal field supply unit  40  is disconnected from synchronous motor  30  and, therefore, is available for servicing. 
   On the other hand, if normal field supply unit  60  should fail or have faulty operation, the load current of synchronous motor  50  field system will deviate from a reference value. If this deviation exceeds a predetermined amount, a failure will be detected. In response to the detected failure, control unit  22  automatically changes field winding system  56  from normal field supply unit  60  over to standby field supply unit  70 . To accomplish this, control unit  22  opens output switch  66  of normal field supply unit  60  and closes output switch  78  of standby field supply unit  70 . In this scenario, synchronous motor  50  is now receiving field voltage from standby field supply unit  70  and synchronous motor  30  continues to receive field voltage from normal field supply unit  40 . Faulty normal field supply unit  60  is disconnected from synchronous motor  50  and, therefore, is available for servicing. 
   The reference value may be determined as a percentage or absolute value of the desired field current. Desired field current is derived from the motor field regulating system that is a part of control unit  22 . In the case of a DC-fed exciter, such as would typically be found on a constant-speed motor application, where the motor is synchronized to a fixed-frequency power supply, the reference value would be compared with the DC line current in the motor field system. Alternatively, in the case of an ac-ed excitation system, such as would typically be found on a variable-speed motor drive application, the reference value would be compared with the average of the individual phase currents to the motor field system. 
   It will be apparent to those skilled in the art that although field winding current is sensed in the illustrated exemplary embodiment, other electrical parameters could be sensed, as well, provided they vary in response to a failure of the normal field supply unit. 
   In either of the aforementioned scenarios, the motor with the unfaulted field power supply will continue to operate with full load rating being available. The motor having the faulted field power supply may continue to operate at rated power, or may temporarily run asynchronously until the load torque has been reduced to allow re-synchronization to take place. The load reduction initiation and re-synchronization process is normally carried out under the automatic control of the control unit  22 . In the event that both normal field power supply units fail simultaneously, and when only one standby unit is installed, one motor would be shutdown, and the standby unit switched to the other motor to enable its continued operation. 
   It should be apparent to those skilled in the art that current sensors  45 ,  65  and  75  can be packaged in the field supply units, installed in the synchronous motors  30  and  50  or can be separate units. It will be further apparent to those skilled in the art that standby field supply unit  70  can be shared by more than two synchronous motors. It will be further apparent that in some synchronous motor system embodiments, more than one standby field supply unit may be used and shared by two or more of the motors. 
   It should also be apparent to those skilled in the art that the disclosure described herein applies to synchronous motors, whether started full-voltage (direct-on-line), by an assisted of ‘soft’ starter, or when run at variable speed from a drive system. 
   It should also be apparent to those skilled in the art that the disclosure described herein applies to different types of field supply units. For example, the field supply units may be either brushless DC excitation or slipring DC excitation for fixed speed operation. Alternatively and by way of example, the field supply units may be brushless AC excitation for variable speed operation or slipring DC excitation for variable speed operation. 
   It should also be apparent to those skilled in the art that the disclosure described herein applies to various power supplies, e.g., one power supply to all of the field supply units, separate power supplies to each of the field supply units, one power supply to all motor stators or separate power supplies to each of the motor stators. 
   It should also be noted that the terms “first”, “second”, “third”, “upper”, “lower”, and the like may be used herein to modify various elements. These modifiers do not imply a spatial, sequential, or hierarchical order to the modified elements unless specifically stated. 
   While the present invention has been described with reference to one or more exemplary embodiments, 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 present invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the scope thereof. Therefore, it is intended that the present invention not be limited to the particular embodiment(s) 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.