Abstract:
In a multi-phase motor drive that includes a bus capacitor, a multi-phase motor, a multi-phase inverter, multiple switches each having an on-state and an off-state, and multiple current sensors each being in series with respective phase winding, a method for checking the accuracy of circuit parameters of the motor drive, including using the switches to produce a first loop that includes the capacitor bank, a first phase winding, a first current sensor, a second phase winding, and a second current sensor, using the current sensors to determine a magnitude of current in the first and second phase windings, comparing a magnitude of current indicated by the first current sensor and the second current sensor, and determining a magnitude of a difference between the current in the first and second phase windings.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates generally to supplying electric power from a variety of sources, such as electric generators, solar cells, fuel cells and batteries, to various loads, such as electric appliances, motor drives and down-stream power converters. 
   2. Description of the Prior Art 
   Power electronics is an enabling technology which provides a means to regulate electric power supplied from a variety of sources to various loads. The power sources may include, but are not limited to, electric generators, solar cells, fuel cells and batteries. On the other hand, the loads may include, but are not limited to, electric appliances, motor drives and down-stream power converters. With foreseeable near-term and long-term global energy shortage, precise electric energy regulation using power electronics becomes indispensable. Examples include wind turbines, solar cells power-tracked by electronic converters, appliances operated by variable-speed electronic motor drives, hybrid electric vehicles, fuel-cell vehicles with high-power electronic converters and/or motor drives to maximize efficiencies. 
   To work under desired conditions, these power electronic converters and motor drives include a variety of sensors to monitor their operations. Accuracy and integrity of these sensors are therefore critical for proper operation and fault detection. The signals from these sensors are also useful for estimating the parameters of the systems, the power sources and the loads, which can be used to detect catastrophic failures or to monitor system aging. 
   There is a need in the industry for systems and methods to cross check the outputs of sensors and to estimate system parameters related to various systems including, but not limited to, power converters, home appliances, and vehicle electronic systems. 
   SUMMARY OF THE INVENTION 
   To address this need, this invention disclosure proposes methodologies to cross check the outputs of sensors and to estimate system parameters. These approaches can be used in many areas, including, but not limited to, power converters, home appliances, and on-vehicle electronic systems. 
   In a multi-phase motor drive that includes a bus capacitor, a multi-phase motor, a multi-phase inverter, multiple switches each having an on-state and an off-state, and multiple current sensors each monitoring the current on respective phase winding, a method for checking the accuracy and integrity of circuit parameters of the motor drive, including using the switches to produce a first loop that includes the capacitor bank, a first phase winding, a first current sensor, a second phase winding, and a second current sensor, using the current sensors to determine a magnitude of current in the first and second phase windings, comparing a magnitude of current indicated by the first current sensor and the second current sensor, and determining a magnitude of a difference between the current in the first and second phase windings. 
   The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 

   
     DESCRIPTION OF THE DRAWINGS 
     These and other advantages will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which: 
       FIG. 1  is a schematic diagram of a multi-phase motor drive; 
       FIG. 2  illustrates a loop that carries the same current through two of the current sensors in the motor drive of  FIG. 1 ; 
       FIG. 3  shows a current loop through the motor windings, a diode, and a switch in the motor drive of  FIG. 1 ; 
       FIG. 4  is a schematic diagram of two motor drives that share the same DC bus; 
       FIG. 5  illustrates loops carrying currents through the two motors of the motor drive of  FIG. 4 ; 
       FIG. 6  illustrates current loops through the motor windings, diodes, and switches in the motor drive of  FIG. 4 ; 
       FIG. 7  illustrates the motor drive of  FIG. 4  with the switches with higher circulating current turned off; 
       FIG. 8  is a schematic diagram of a multi-phase motor drive with a boost converter; 
       FIG. 9  illustrates two inverter switches used to discharge the bus capacitor of the motor drive of  FIG. 8 ; 
       FIG. 10  illustrates recharging the bus capacitor of the motor drive of  FIG. 8 ; 
       FIG. 11  is a schematic diagram of a hybrid electric traction inverter system without a boost converter; 
       FIG. 12  is a schematic diagram of a hybrid electric traction inverter system with a boost converter; 
       FIG. 13  illustrates current loops for checking the capacitance of a bus capacitor of the inverter system of  FIG. 11 ; 
       FIG. 14  shows the variation of bus voltage and current in the phase windings of the loop shown in  FIG. 13 ; 
       FIG. 15  shows current loops used to precharge the capacitors of the boost converter of  FIG. 12 ; 
       FIG. 16  shows current loop used to estimate inductance in the boost converter of  FIG. 12 ; and 
       FIG. 17  shows a loop used to estimate capacitances in the boost converter of  FIG. 12 ; 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a multi-phase motor drive  10 , typically includes a DC capacitor bank  12  C bus , a multi-phase inverter  14  with six switches  15 - 20 , six anti-parallel diodes  21 - 26 , and a multi-phase motor  28 . The power may be supplied by a battery pack  30  through contactors  32 . The drive system  10  includes current sensors  34 ,  35 ,  36  for signal monitoring and output control, each current sensor on each motor phase A, B, C, and a voltage sensor  38  on the DC bus  40 . 
     FIG. 2  shows one way to cross-check the outputs of the current sensors  34 ,  36  of  FIG. 1 . In this specific case, Phase-C upper switch  17  S c1  and the Phase-A lower switch  18  S a2  are turned on while switches  15 ,  16 ,  19 ,  20  are off. Current passes through Phases A and C. The current sensors  34  CS a  and  36  CS c  should sense the same current except with opposite polarities. In other words,
   i   loop   =i   c   =−i   a   (1) 
Meanwhile, the B phase current is zero:
 i b =0  (2) 
   Equation (1) can be used to cross-check the two current sensors  34  and  36 , and Equation (2) can be used to monitor the offset of the third current sensor  35 . Different switch combinations can be chosen to cross-check other current sensor pairs as well as offsets of a third current sensor. 
   Also, while switches  17  and  18  are ON, the loop current i loop  is governed by the following equation: 
                   v   ca     =         v   bus     -     v     c   ⁢           ⁢   1       -     v     a   ⁢           ⁢   2         =     L   ⁢       ⅆ     (     i   loop     )         ⅆ   t                   (   3   )               
wherein L is the inductance of the motor winding between Phase A and C, and the other parameters are as shown in  FIG. 2 .
 
   Equation (3) can also be expressed in an integration form: 
   
     
       
         
           
             
               
                 
                   
                     i 
                     loop 
                   
                   = 
                   
                     
                       
                         1 
                         L 
                       
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                                 v 
                                 bus 
                               
                               - 
                               
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                                   ⁢ 
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                     bus 
                   
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                     ⁢ 
                     
                         
                     
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                     and 
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                       V 
                       bus 
                     
                   
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                 ( 
                 4 
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                     v 
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                 5 
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                         v 
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                 6 
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   Equations (3)-(6) can be used to estimate the motor winding inductance, assuming the sensor signals i loop  and v bus  are accurate. 
   When the current is increased to a desired amplitude, either switch  17  S c1  or switch  18  S a2  may be off, but not both.  FIG. 3  shows with switch  17  S c1  off and switch  18  S a2  on that the motor current circulates through S a2  and diode  26  D c2 , and the loop current can be described by: 
   
     
       
         
           
             
               
                 
                   
                     v 
                     
                       c 
                       ⁢ 
                       
                           
                       
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                       t 
                     
                   
                 
               
             
             
               
                 ( 
                 7 
                 ) 
               
             
           
         
       
     
   
   Based on the estimated motor winding inductance L from Equations (3)-(6), the forward-voltage drop (v c2 -v a2 ) can be estimated by Equation (7). Similarly, the inductance of other motor winding pairs and the forward voltage drops of other semiconductor pairs can be estimated. 
   Equations (1)-(7) can be applied to determine the magnitude of the variables during system start-up, idling, or shut-down processes. 
     FIG. 4  shows a multi-phase motor drive  50  wherein the DC bus  40  is shared by multiple inverters  14  and  52 . Inverter  14  includes multiple current sensors  34 - 36 , diodes  21 - 26  and switches  15 - 20 . Inverter  52  includes multiple current sensors  54 ,  55 ,  56 , diodes  58 - 63  and switches  64 - 69 . Again, the current sensors associated with their respective inverter and motor can be cross-checked using the strategies described above with reference to  FIGS. 1-3 .  FIGS. 5-7  illustrate cross-checking the current sensors for inverters  14  and  52 . Preferably, the current sensors are cross-checked with the power source, i.e., battery pack  30 , disconnected. Then, the electric charge remaining in the bus capacitor  12  C bus  is the only energy source for the following process. 
     FIG. 5  shows that the two inverters  14 ,  52  are regulated to deliver approximately equal magnitudes of current to the two motors  28 ,  70  until the energy in capacitor  12  is exhausted. 
   When the dc bus voltage collapses to zero, the motor currents circulate through the windings of motors  28 ,  70 , diodes  21 ,  26 ,  58 ,  63  and switches  17 ,  18 ,  66 ,  69 , as shown in  FIG. 6 . However, even with good control, the motor currents are not equal; one will be larger than the other. 
     FIG. 7  assumes that motor  70  has the higher circulating current. To confirm, all switches  64 - 69  in inverter  52  are turned off. This forces the current through motor  70  to return to the dc bus  40  through the diodes  58  and  63 , as shown in  FIG. 7 . Since this current is larger than the current through inverter  14  and motor  28 , the dc bus voltage increases and becomes positive. However, if the current in motor  70  is only slightly higher than the current in motor  28 , the net dc bus current is very small and the dc bus voltage is increased very slowly. 
   With slightly positive dc bus voltage, the current in motor  70  will be reduced, while that of motor  28  will be increased. This forces these currents to become equal eventually. When these two currents are equal in amplitude momentarily, the net dc bus current is zero, as is the slope of the dc bus voltage. In other words, the dc bus voltage (a small positive value) and its slope become excellent indicators to determine when these two currents are exactly equal. When the conditions are met, the four current sensors  34 ,  36 ,  54 ,  56  associated in the process should provide the same signal amplitude. 
   A similar procedure can also be applied to cross-checking current sensors of a system  78  that includes a boost converter  80  and the inverter  14 , as illustrated in  FIG. 8 . The converter  80  includes switches  82 ,  84 , diodes  86 ,  88 , a current sensor  90 , capacitor  92 , and inductor  94 . In  FIG. 8 , current sensor  90  CS 1  of the DC/DC converter  80  can be cross-checked with the current sensors  34 - 36  CS a , CS b , or CS c  of inverter  14 . The procedure is described with reference to  FIGS. 9 and 10 . First, the DC bus  40  is disconnected from the power supply  30  and the charge in capacitors  92  C 1  and  12  C bus  is the remaining energy on the DC bus. 
   Then, two inverter switches  17  S c1  and  18  S a2  (in the illustrated example) are turned on in order to drain the energy in capacitors  92  C 1  and  12  C bus  through the windings of motor  28 . After the DC bus voltage is substantially or completely discharged, all switches, including those of converters  80  and inverters  14 , are turned off. Then, as shown in  FIG. 10 , the system  78  conducts motor inductive energy back to the DC bus  40  through the diodes  21  and  26  (D c2  and D a1 , respectively). This boosts the voltage on bus capacitor  12  C bus  toward a high value and the motor current is reduced gradually. With switches  82  S 1  and  84  S 2  remaining off, the voltage on capacitor  92  C 1  is still at a low value or zero. 
   Then, when the voltage on bus capacitor  12  C bus  is high enough, switch  82  S 1  is turned on, which initiates a free resonance between converter inductor  94  L 1  and capacitor  92  C 1 . By properly choosing the time to turn on switch  82  S 1 , the resonant current drawn from bus capacitor  12  C bus  can be higher than the motor current charging capacitor  12 . In other words, there are instances when the net current to bus capacitor  12  C bus  is zero as is the slope of the voltage on capacitor  12  C bus . Similarly, this can be used as an indicator that the current to converter inductor  94  L 1  at those instances equals the motor current i cs  or −i as . 
   Parameter identification is closely related to sensor accuracy. After cross-checking current sensors and voltage sensors as described above, it is safe and reliable to implement passive device parameter identification. Passive device parameter identification can be used to predict component life, evaluate possible failure, and initiate a limited operation strategy. 
   There are three levels of parameter identification:
     1. offline parameter identification;   2. semi-online parameter identification; and   3. online parameter identification.
 
1. Offline Parameter Identification
   

   Offline parameter identification can be performed as a standard check procedure performed at a particular location, such as an automobile dealership, or as a programmed auto self-check routine performed periodically. 
   Step 1. Passive Discharge 
   For a hybrid electric vehicle system without a boost converter, as shown in  FIG. 11 , the quickest check is to charge the bus capacitor  12  C bus  to a certain voltage, then cut the power, and allow bus capacitor  12  to discharge through a bleeding resistor  98 . If the voltage on bus capacitor  12  C bus  drops from V bus1  to V bus2 , the discharge time can be calculated from the following equation:
 
 t=−R   2   ·C   bus  ln(1− V   bus2   /V   bus1 )  (8)
 
   If the measured discharge time is close to the calculated value, then it can be safely concluded that both bus capacitor  12  and bleeding resistor  98  have the correct values. 
   If the discharge time is shorter than expected, either the bleeding resistor  98  is partially shorted or decreased in resistance, or the bus capacitor  12  is deteriorated. In either case, the step two, capacitance and inductance check, shall be performed. 
   For a hybrid electric vehicle system with a boost converter  80  and inverter  14 , as shown in  FIG. 12 , the same principle applies. Two capacitors  92  C 1  and bus capacitor  12  C bus  are charged to the same voltage and discharged through resistors  100  R 1  and  98  R 2 . 
   To identify each capacitor or resistor, boost is needed to ensure that the bus voltage V bus  is greater than V 1  in order to block conduction through diode  86  D 1 . 
   Step 2. Capacitance and Inductance Check 
   Capacitance is checked by an active discharge approach, as described with reference to  FIG. 13 . The bus capacitor bank  12  is first charged to a predetermined voltage  102 . Then two phase legs, e.g., A and B, of the inverter  14  are controlled to achieve current oscillation between the bus capacitor bank  12  and the two motor windings that correspond to the selected phase winding legs. Since the current through the phase windings of the motor is much larger than the leakage current through the bleeding resistor  98 , the effect of bleeding resistor  98  can be ignored. 
   The relationship between the voltage across bus capacitor  12  and the current through the A and B windings of motor  28  is 
                     C   bus     ⁢       ⅆ       V   bus     ⁡     (   t   )           ⅆ   t         =       i   ab     ⁡     (   t   )               (   9   )               
Thus, the capacitance of bus capacitor  13  can be calculated from
 
   
     
       
         
           
             
               
                 
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                   bus 
                 
                 = 
                 
                   
                     
                       ∫ 
                       
                         t 
                         n 
                       
                       
                         t 
                         
                           n 
                           + 
                           1 
                         
                       
                     
                     ⁢ 
                     
                       
                         
                           i 
                           ab 
                         
                         ⁡ 
                         
                           ( 
                           t 
                           ) 
                         
                       
                       ⁢ 
                       
                           
                       
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                         ⅆ 
                         t 
                       
                     
                   
                   
                     
                       
                         V 
                         bus 
                       
                       ⁡ 
                       
                         ( 
                         
                           t 
                           
                             n 
                             + 
                             1 
                           
                         
                         ) 
                       
                     
                     - 
                     
                       
                         V 
                         bus 
                       
                       ⁡ 
                       
                         ( 
                         
                           t 
                           n 
                         
                         ) 
                       
                     
                   
                 
               
             
             
               
                 ( 
                 10 
                 ) 
               
             
           
         
       
     
   
   Since step two is an offline operation, the switching frequency and duty ratio can be adjusted to an optimized value for sample accuracy. One easy way is to close switch  15  S a1  and switch  19  S b2  until V bus  drops close to zero, and then open switches  15 ,  17 . The current will then be conducted through diode  24 , motor windings A and B, and diode  22  to charge bus capacitor  12  C bus . In this way, switching noise can be eliminated from the test. 
   For the system shown in  FIG. 12  having a boost converter  80  and inverter  14 , the following three sub-steps are used to determine the inductance of converter inductor  94  and the capacitance of bus capacitor  12  and converter capacitor  92 . 
   First, charge capacitors  92 ,  12  (C 1  and C bus ) to the same voltage using an external pre-charge circuit, as shown in  FIG. 15 . During this step, there is a chance that the total capacitance of capacitors  92 ,  12  (C 1  and C bus ) can be checked if the pre-charge circuit is very well regulated. 
   Second, boost the voltage in bus capacitor  12  C bus  to a predetermined voltage in continuous current mode by operating switches  84  S 2 ,  15  S a1 , and  19  S b2 , as shown in  FIG. 16 . During this step, the inductance of inductor  94  in the boost converter  80  can be calculated using equation (11) upon measuring the voltage and current 
                     L   1     =         V     C   ⁢           ⁢   1       ·     T   on         Δ   ⁢           ⁢     I     L   ⁢           ⁢   1             ,           (   11   )               
wherein T on  is length of the period during which switch  84  S 2  is on, and ΔI L1  is the inductor current change during that period.
 
   Third, discharge capacitor  92  C 1  and bus capacitor  12  C bus  by changing the states of switches S 1 , S a1  and S b2  on and off, as shown in  FIG. 17 . In this case, switch  82  S 1  is turned off at the beginning. Since bus capacitor  12  C bus  has a higher voltage than capacitor  92  C 1 , only bus capacitor  12  C bus  will be discharged; therefore, the capacitance of bus capacitor  12  C bus  can be estimated using equation (10). When V bus  decreases to the same level as V C1 , switch  82  S 1  is turned on, both capacitors  92 ,  12  begin to oscillate with the motor windings, thus the total capacitance can be estimated. Using the capacitance of bus capacitor  12  C bus  estimated from equation (10) and the estimated total capacitance, the capacitance of capacitor  92  C 1  can be calculated. 
   2. Semi-online Parameter Identification 
   While the vehicle is shutdown, sub-steps 2 and 3, discussed above, can be performed to check the inductance and capacitance. 
   3. Online Parameter Identification 
   When the vehicle is running, inductance of L 1  can always be checked using equation (11). 
   In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.