Abstract:
A system and method for extracting estimated input voltage and input current values of an AC motor based on sensed power inverter output voltage and output current when the AC motor is connected to a front-end EMI filter and the EMI filter is connected to a power inverter. Where the system includes a current estimator portion, and a voltage estimator portion. The estimated input values are used by a feedback controller in the power inverter to control the motor. A second system and method for extracting estimated an input current of an AC motor based on the actual motor input voltage, and the output voltage of the power inverter.

Description:
BACKGROUND OF THE INVENTION 
   This invention deals with an improved method of controlling a high speed electric motor, which has a front-end Electromagnetic Interference (EMI) filter attached. By extracting estimated voltage and current values of the motor windings based on sensed power inverter output values, a controls scheme that is more accurate than the prior art controls schemes is obtained. 
   In industrial applications, regular electric motors are often driven by variable frequency power source. A variable frequency power source is used because in many motors the frequency of the input power controls the speed of the motor. A variable frequency power inverter converts Direct Current (DC) power into Alternating Current (AC) power. Variable frequency power inverters have the capability to adjust the frequency and voltage of the AC power. This allows the power inverter to control the speed and torque of an AC motor attached to it by adjusting both the frequency and voltage of its power output. In most applications the power inverter is connected directly to the motor with no intermediate EMI filter connections. This practice is common for motors of all sizes in most operations and provides the motor with standard power, though not clean power. Clean power is the power that is provided relatively free of electronic and electromagnetic noises and is typically achieved through the use of an EMI (Electro-Magnetic Interference) filter. 
   Some operations, for example aerospace and aircraft applications among others, have very limited electrical tolerances and thus require clean power. Clean power is achieved once a filter has been applied and electrical noise has been filtered off of the power transmission. To achieve clean power in the present setting it is necessary to introduce an EMI filter between the power inverter and the motor winding terminals. Since EMI filters are often set up in complicated inductance-capacitance formats, the electrical values (such as current, voltage, and impedance) of the EMI filter in high power, high frequency, and high speed applications become complex issues as the power level output of the power inverter increases. Simultaneously the electrical values of the motor parameters decrease as the power level output of the power inverter increases. As a result the electrical values of the EMI filter become substantially large relative to the electrical values of the motor parameters. This results in the output voltage and current of the power inverter being significantly affected by the EMI filter. 
   In the prior art methods of controlling motors in this configuration the voltage and current values at the output of the power inverter have been sensed and then erroneously used to control the motor by assuming them to be the voltage and current values at the input of the motor windings. Feedback controls, such as the ones used in the prior art, work by measuring the value of the voltage and current at the power inverter output and assuming them to be the voltage and current values of the motor, ignoring the effect that the EMI filter has on the voltage and current values. The controller then adjusts the input to the power inverter accordingly using the normal pulse-width modulation switching methods (i.e. depending on the load demands, the power inverter switching pattern can be varied to increase or decrease the output voltage and frequency applied into the motor inputs) The influence of the EMI filter on the voltage and current values increases the amount of time it takes to correct any improper voltage and current values in the motor control system, and it may give some disturbance response during transient due to an imbalance between the supply and demand sides of the electric machine. The duration of time necessary to correct the values is referred to as the response time. Erroneously assuming the power inverter output values to be the actual motor winding values results in a controls scheme that is slowed down or sluggish, as well as less accurate than desired. 
   SUMMARY OF THE INVENTION 
   It would be desirable for both the power inverter output values and the actual motor winding voltage and current to be sensed. This would allow for accurate feedback control, eliminate the need for estimating the values, and dramatically hasten the response time of the controllers. 
   When an EMI filter is placed between the power inverter and the motor it is often economically or technologically impractical to sense the actual motor winding voltage and current. This invention discloses the use of an estimator circuit to extract an estimated motor winding input voltage and input current from an actual power inverter output voltage and output current. The feedback time of the estimator circuit is slower than directly sensing the motor winding voltage and current. However, the estimator circuit is a vast improvement over the prior art method of assuming the power inverter values to be the motor winding values. The feedback this embodiment gives is more accurate and results in a quicker response time than the prior art method of using the power inverter output values for its control signal. 
   If the situation permits a voltage reading at the EMI filter output to be taken, in addition to voltage and current readings at the power inverter output, a second embodiment may be used. In the second embodiment of the invention the motor winding current is estimated using the sensed power inverter voltage and output current, and the sensed EMI filter voltage. Under this embodiment a simpler estimator circuit is used to extract the motor winding voltage and current from the EMI filter output voltage and the power inverter output voltage and current. 
   These embodiments provide for a faster response time then the prior art methods because a more accurate estimate of the motor winding voltage and current outputs is determined than the prior art method of using the voltage and current values of the power inverter output. 
   These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a power circuit with an attached EMI filter in a typical configuration. 
       FIG. 2A  is the general circuit layout of a three phase EMI filter with no specified component values. 
       FIG. 2B  is an example of potential component values of one phase path of a typical three phase EMI filter together with the motor impedance in a single phase form; the back-EMF voltage is not included in the figure for simplicity. 
       FIG. 3  is a block diagram of an estimator used to estimate the motor voltage and current when the power inverter input and output voltages and currents are sensed. 
       FIG. 4  is a block diagram of a simplified estimator in the case that the inverter voltage, the inverter output current and the EMI filter voltage can be sensed. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In the disclosed embodiments of the invention a power inverter  10  is connected to an EMI filter  20  which is then connected to a motor  30  as seen in  FIG. 1 . The EMI filter  20  has a general three phase construction as shown in  FIG. 2A  with each phase having an inductor  22 , which has an equivalent serial resistance  21 , a capacitor  23  connected to ground in a Y format, and connections to capacitors  24  arranged in a delta pattern connecting all three phases. A specific example of a sample EMI filter single phase layout is shown in  FIG. 2B  with sample component values in the first inductor (35 μH)  25  and capacitor (35 μF)  27 . The inductor  26  and resistor  30  represent the motor impedance in a single phase format assuming that the internal back EMF is inherently present but not shown. The single phase layout of the EMI Filter is constructed in  FIG. 2B  by just a simple inductance-capacitance format. 
   Because of the high inductance and capacitance values of a typical EMI filter such as the EMI filter illustrated in  FIGS. 2A and 2B  compared to the motor&#39;s internal impedance, the power inverter output current and voltage values are not accurate estimates of the AC motor winding input values. If it is not possible to measure the values of the EMI filter output, then it is desirable to introduce an estimator to achieve a more accurate estimate of the actual AC motor winding input values than is achieved using the prior art method. 
     FIGS. 3 and 4  illustrate estimator circuits  41  and  42 . In both estimator circuit  41  and estimator circuit  42  the value “RL” in gain block  121  refers to the ESR (equivalent serial resistance) of the overall filter inductor  20 , the value “L” in gain blocks  121  and  122 , refers to the overall inductance of the EMI filter  20 , the value “RC” in gain blocks  222 , and  321  refers to the ESR of the filter capacitor and the value “C” in gain blocks  222 ,  221 ,  321 , and  322  refers to the overall capacitance of the EMI filter  20 . 
   Estimator circuit  41  estimates both the motor voltage, in the voltage estimator  100 , and the motor current, in the current estimator  200 . In the voltage estimator  100  the voltage drop across the EMI filter  20  is estimated. In the illustrated circuit this estimated voltage drop is based on the EMI filter  20  input current and the rate of change of the EMI filter  20  input current, however any method to determine an accurate estimate of the voltage drop could be used. Once the voltage drop estimate has been determined it is then subtracted from the actual power inverter output voltage to obtain the estimated motor winding input voltage. In the illustrated circuit this is done in a summation block  132 . 
   In order to obtain the estimated motor winding input voltage in the illustrated embodiment the following steps should be performed. First the voltage estimator  100  accepts the output current value of the power inverter at an input  111 , and the output voltage value of the power inverter at an input  110 . The value at input  111  is then multiplied by RL in a gain block  121 , and sent to a summation block  131 . The value at input  111  is also sent to a derivative block  190  where its derivative is taken. This value is then multiplied by L in a gain block  122 , and the output value is passed to summation block  131 . Summation block  131  sums the output of gain blocks  121  and  122 . The input value  110  is sent to a summation block  132 . The output of summation block  131  is then subtracted from the input value  110  in summation block  132 . Finally the voltage estimator  100  outputs an estimated motor winding voltage  140 . Once an estimated motor winding voltage is determined it is the output at output terminal  140 . This output may be connected to the power inverter controller input terminals and the value may then be used to control the power inverter  10 . 
   The current estimator  200  works by estimating the effect the EMI filter  20  has on the overall current and subtracting the value of the estimated effect from the power inverter  10  output current. In the embodiment illustrated in Figure the output of gain block  222  is an estimate of the affect the EMI filter  20  has on the current. This estimate is based on the estimated voltage from the voltage estimator  100  and on the overall resistance and capacitance of the EMI filter  20 . The estimated change in current from gain block  222  is subtracted from the actual power inverter output current from input  111  to obtain an estimated motor winding input current. 
   In order to obtain the estimated motor winding current using the method summarized above the following steps should be followed. Summation block  231  accepts the estimated motor voltage from the voltage estimator  100  then subtracts the output of integrator block  290 . The output of gain block  231  is multiplied by 1/RC in gain block  222 . The output of gain block  222  is then multiplied by 1/C in gain block  221  and passed to integrator block  290 . The output of gain block  222  is also sent to summation block  232 . Summation block  232  accepts the power inverter output current from input  111 . Then summation block  232  subtracts the output of gain block  222  from the value of input  111 . The resultant value is the estimated motor current  240 . Once an estimated motor winding current is obtained, it is sent to output terminal  240 . Output terminal  240  may be connected to the power inverter controller inputs and the estimated value may then be used to control the power inverter  10 . 
   While an estimated motor winding input voltage and current can be achieved utilizing the above system, a system where only the motor winding input current is estimated would provide a faster feedback and better controls for the motor. Such a system is possible when the EMI filter output voltage, the power inverter output voltage, and the power inverter output current are measured. Estimator circuit  42  illustrates such a system. Estimator circuit  42  works in the same way as the current estimator  200  from estimator circuit  41 , described above, with one difference. In estimator circuit  42  the voltage value used to make the estimate is not an estimated voltage value, as in estimator circuit  41 , but instead the voltage value used is an actual measured input voltage value  311  of the motor windings. 
   The specific operations of estimator circuit  42  as shown in the illustrated embodiment are as follows. The output of integrator block  390  is subtracted from the EMI filter output voltage in summation block  331 . The output of summation block  331  is multiplied by 1/RC in gain block  321 . The output of gain block  321  is then multiplied by 1/C in gain block  322  and then sent into integrator block  390  where it is integrated. The output of gain block  321  is additionally passed to summation block  322  where it is subtracted from input  310  (the power inverter output current). Summation block  332  then outputs an estimated motor current to output terminal  340 . Output terminal  340  may be connected to the power inverter controller inputs and the estimated current value from output terminal  340  may be used to control the power inverter. 
   The estimator circuit can be in the form of a microcomputer, a digital signal processor (DSP), software or an analog control. This invention extends to any such implementation. 
   Although multiple embodiments of this invention have been disclosed, a worker of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.