Motor current reconstruction via DC bus current measurement

Abstract

A technique for reconstructing phase currents based on measurements of a DC bus current. Non-observable regions of DC bus current samples to reconstruct phase currents are reduced in size by using two phase space vector modulation. The non-observable regions can be further reduced by omitting deadtime insertions for switch actuations when output current is higher than a given threshold. Voltage command vectors in non-observable areas can be formed by two additive vectors of differing phase and magnitude to obtain an observable DC bus current reflecting phase current. The additive vectors have the same combined time average value as that of the voltage command vector.

1 . A method for reducing a non-observable region in a PWM inverter drive system, comprising: 
 forming a command voltage using two phase space vector modulation;    increasing a time spent on any given basic vector to be above a specified threshold; and    localizing an amount of time spent on any given basic vector, such that an available current measurement time is increased, whereby the non-observable regions are reduced.    
   
   
       2 . The method according to  claim 1 , further comprising using an alternate zero vector when a voltage angle is larger than 30° to maximize an available observation interval.  
   
   
       3 . The method according to  claim 2 , wherein zero vector 111 is used as the alternate zero vector.  
   
   
       4 . The method according to  claim 1 , further comprising eliminating a deadtime insertion when motor current is higher than a given threshold, thereby reducing a time period of the non-observable regions.  
   
   
       5 . The method according to  claim 1 , further comprising sampling a DC bus current at least three times in each PWM cycle.  
   
   
       6 . The method according to  claim 5 , further comprising: 
 defining a time interval over which DC bus current samples are obtained during a PWM cycle; and    spacing the DC bus current sample within the defined interval to permit output phase current reconstruction in at least two phases.    
   
   
       7 . A method for obtaining phase current in a power inverter by measuring DC bus current, comprising: 
 determining a non-observable sector having a width;    forming a first voltage vector with an observable phase angle and a magnitude being greater than the non-observable sector width when a command voltage vector is in the non-observable sector;    forming a second voltage vector having a phase and magnitude such that said command voltage vector results from summing said first and second voltage vectors, such that a time average of the sum of the first and second voltage vectors is approximately equal to a time average of the command voltage vector.    
   
   
       8 . The method according to  claim 7 , wherein the non-observable sector width is related to a DC bus current sample time lag.  
   
   
       9 . The method according to  claim 8 , wherein the non-observable sector width A is given by the equation  
     
       
         
           
             A 
             = 
             
               
                 
                   T 
                   min 
                 
                 
                   T 
                   pwm 
                 
               
               × 
               
                 π 
                 
                   3 
                 
               
             
           
         
       
     
     wherein T min  is the minimum time required to obtain a DC bus current sample and T pwm  is a PWM cycle time interval.  
   
   
       10 . A processor programmable to produce representations of phase output current based on DC bus current samples in accordance with the method of  claim 7 .  
   
   
       11 . A program code for execution by a processor to provide representations of phase current reconstructed from measuring DC bus current in an inverter, comprising instructions for operating the processor in accordance with the method of  claim 7 .  
   
   
       12 . A method for obtaining phase current in a power inverter from DC bus current, comprising: 
 forming a command voltage vector using two phase space vector modulation;    arranging a combination of basic voltage vectors to produce the command voltage vector with a non-observable region having a reduced width; and    when the command voltage vector is in the non-observable region:    forming a first voltage vector in an observable region to have a magnitude greater than the width;    forming a second voltage vector to add to the first voltage vector;    adding the first and second voltage vectors to form a third voltage vector having approximately a same phase and magnitude as the command voltage, such that a time average of the sum of the third voltage vector is approximately the same as a time average of the command voltage vector.