Apparatus and method for measuring current

Abstract: The present invention discloses apparatus and a method for measuring the motor current in an AC drive system in which the DC link current in a voltage source inverter (20) is separated into a transistor current, and a diode current, and then measured separately with shunts (22, 24). This results in two independent non-isolated current measurements and allows for a more accurate calculation of the magnitude of the motor current.

Description: The present invention relates to the field of current measurement. More specifically, the present invention relates to apparatus and a method for measuring load current in an alternating current (AC) inverter drive system. It is well known in the art that a measurement of the load current can provide means for protecting the drive system during an overload situation. It is also known that knowledge of the load current can be used as a means of measuring the output of the drive system. In one technique, load currents in an AC drive system are measured by fitting isolating current transducers to a motor output. However, drive systems incorporating such devices are expensive. It is also know to measure load currents by sampling the motor currents in the direct current (DC) link of the inverter. However, drive systems employing this sampling technique are sensitive to noise and measurement black spots during the pulse-width modulation (PWM) cycle when data is not available or unreliable. This problem is particularly severe at high switching frequencies. Furthermore, the mean DC link current in the open loop AC inverter drive system can be measured by integrating over an integral number of PWM cycles. This method reduces the effects of noise on the measurement reliability. However, while this method can be used to measure the real component of the load current, the imaginary component of the current is still unknown and must be estimated. Thus any subsequent calculation of the total load current is only as accurate as the estimation of the imaginary component. EP 0 075 357 81 discloses a power supply circuit for a multiphase a synchronous machine where the currents flowing through the transistors and diodes of the power supply circuit are measured Figure 1 is a circuit diagram of a known voltage source DC link inverter 100 which measures the load current by sampling the current flowing through the DC link 2. This type of inverter is often used in open loop AC drive systems. The DC link comprises a positive DC input 3 and a negative DC input 4, which are both in electrical connection with capacitor 6. The current is measured through the negative DC input 4 with current measurement means 12. In this example the current measurement means is a shunt. The use and operation of shunts is well known in the art of electronics. The inverter 10 is a three phase inverter comprising a first switch 14, a second switch 16, and a third switch 18. Each of the first, second, and third switches includes a respective pair of transistors, or other suitable electronic switching element: 14a and 14b, 16a and 16b, 18a and 18b; a respective pair of diodes: 14c and 14d, 16c and 16d, 18c and 18d. Each of the first, second, and third switches also comprises a respective output: 7, 8, and 9. The outputs are electrically connected to a motor (not shown). The shunt 12 functions to measure the DC link current (I DC ) into inverter 10 via the negative DC input 4. The instantaneous value of I DC can be used to provide protection to the device in situations such as drive system output short circuit. The real component of the current (I real ) may be calculated from the mean value of I DC and used to facilitate control of the motor. However, to calculate the actual magnitude of the load current the imaginary component of the current must be estimated. Estimation of the imaginary component of the load current can lead to errors in the calculation of the load current magnitude. These errors are worse at certain frequencies and drive system loads. Furthermore, the calculation of the real component of the current involves a division by the modulation depth of the PWM. This quantity is unacceptably inaccurate at low modulation depths and high switching frequencies. It is an object of the present invention to provide apparatus and a method for measuring the load current in which an accurate calculation of the load current is obtained without the need for estimating the imaginary component. According to the present invention there is provided apparatus for providing and measuring current flowing to a load. The apparatus comprises an inverter which uses pulse width modulation; a first current measuring means arranged to measure a first current flowing through an inverter, a second current measuring means, arranged to measure a second current flowing through said inverter; and processing means coupled to said first and said second current measuring means, wherein said inverter comprises a plurality of electronic switches, and a plurality of diodes. Said first current measuring means, said second current measuring means and said processing means are arranged to measure a current I transistor flowing through said plurality of electronic switches, and a current I diode flowing through said plurality of diodes. The processing means is further arranged to calculate a load current I load based upon said currents I transistor and I diode , using the following equation : I load = ( A . I transistor + B . I diode )( C - X D ) where: X is a percentage of modulation, and A , B, C , and D are modulation dependent coefficients. The first current measuring means may be coupled to the plurality of electronic switches, and said second current measuring means may be coupled to the plurality of diodes, such that said first current measuring means measures the current I transistor and said second current measuring means measures the current I diode . The first current measuring means may be coupled to a negative DC input forming part of a DC link to said inverter. The second current measuring means may be coupled to the plurality of electronic switches, such that said first current measuring means measures a first current flowing through said negative DC input equal to (I transistor - I diode ) and said second current measuring means measures the current I transistor . The first current measuring means may be coupled to a negative DC input forming part of a DC link to said inverter. Said second current measuring means may be coupled to the plurality of diodes, such that said first current measuring means measures a first current flowing through said negative DC input equal to (I transistor - I diode ) and said second current measuring means measures the current I diode . The processing means may be further arranged to calculate a real component of said load current I load based upon said first and second currents, according to the formula I real = (I transistor -I diode ) x 82/X. The processing means may be further arranged to calculate an imaginary component of said load current based upon said load current and said real component of said load current. The first and second current measurement means may be shunts. The electronic switches may be transistors. The present invention also provides a power module comprising the described apparatus. The present invention also provides an AC drive system comprising a motor and apparatus as described. Such AC drive system may be an open loop system. Said pulse width modulation may be symmetrical space vector modulation, in which case said modulation dependent coefficients may be as follows: A = 0.55, B = 0.45, C = 1000, and D = 612. Said pulse width modulation may be asymmetrical space vector modulation, in which case said modulation dependent coefficients may be as follows: A = 0.45, B = 0.55, C = -80, and D = -141. Said pulse width modulation may be over modulated symmetrical space vector modulation, in which case said modulation dependent coefficients may be as follows: A = 0.46, B = 0.54, C = 1000, and D = 574. Said pulse width modulation may be over modulated asymmetrical space vector modulation, in which case said modulation dependent coefficients may be as follows: A = 0.49, B = 0.51, C = 80, and D = 131. The present invention also provides a method of measuring a load current I load flowing from an inverter to a load, wherein said inverter uses pulse width modulation and comprises a plurality of electronic switches and a plurality of diodes. The method comprises the steps of: measuring a first current flowing through said inverter, measuring a second current flowing through said inverter, and obtaining a value of current I transistor flowing through said plurality of electronic switches, and a value of current I diode flowing through said plurality of diodes, from said first and second measured currents. The method further comprises the step of calculating the load current I load based upon said currents I transistor and I diode , using the following equation : I load = ( A . I transistor + B . I diode )( C - X D ) where: X is a percentage of modulation, and A, B, C, and D are modulation dependent coefficients. Advantageously, the present invention provides for all measurements to be DC quantities which are integrated over an integral number of PWM cycles to cancel any adverse effects due to switching frequency harmonic noise. Advantageously, the circulating currents are measured directly thus eliminating inaccuracies in the load current measurement due to divisions by small numbers when operating at low modulations depths. Furthermore, the transistor current measurement may also be used for drive system protection. While the principle advantages and features of the invention have been described above, a greater understanding and appreciation of the invention may be obtained by referring to the drawings and detailed description of the preferred embodiments, presented by way of example only, in which; FIGURE 2 is a circuit diagram of a current measurement arrangement according to an embodiment of the present invention, FIGURE 3 is a circuit diagram of a current measurement arrangement according to a further embodiment of the present invention, FIGURE 4 is a circuit diagram of a current measurement arrangement according to yet a further embodiment of the present invention, and FIGURE 5 is a diagram of a drive system comprising the current measurement arrangement shown in figure 2. Throughout the description, like parts shall bear identical reference numerals. Referring to figure 2, a three phase voltage source DC link inverter 200 is shown comprising DC link 2 and inverter 20. The DC link 2 comprises a positive DC input 3 and a negative DC input 4, which are both in electrical connection with capacitor 6. The inverter 20 comprises a first switch 14, a second switch 16, and a third switch 18. Each of the first, second, and third switch includes a respective pair of electronic switches: 14a and 14b, 16a and 16b, 18a and 18b; a respective pair of diodes: 14c and 14d, 16c and 16d, 18c and 18d. The electronic switches 14a, 14b, 16a, 16b, 18a, 18b are preferably transistors. Each of the first, second, and third switches also comprises a respective output: 7, 8, and 9. The outputs are electrically connected to a motor (not shown). Two current measuring means 22, 24 are disposed in the DC link inverter 200. Each current measuring means 22, 24 is coupled to processing means 26, 28 respectively. Processing means 26, 28 are coupled to further processing means 35. The current measurement means 22, 24 are shunts. Shunt 22 is electrically coupled to transistors 14a, 14b, 16a, 16b, 18a and 18b such that shunt 22 can measure the current, I transistor , through the transistors. Shunt 24 is electrically coupled to diodes 14c, 14d, 16c, 16d, 18c and 18d such that shunt 24 can measure the current, I diode , through the diodes. By arranging the shunts in such a manner, the current flowing through DC link inverter 200 can always be measured. For example, when the current in inverter 20 is "freewheeling" or circulating, the current must pass through shunt 24 and thus be measured. Currents I transistor and I diode are converted from analogue to digital signals by processing means 26 and 28 respectively. Each processing means includes an analogue-to-digital converter for this purpose. The values of I transistor and I diode are communicated to further processing means 35. Further processing means 35 is arranged to calculate the values of I DC , I real , and I load based on the measured values of I transistor and I diode . Both the real component of the load current (I real ) and the load current (I load ) are dependent on I transistor , I diode , and on the depth of modulation. The embodiment shown in figure 2 uses a space vector modulation. The variable X equals the percentage of modulation, with 100% modulation being the maximum modulation possible with a sine wave output. However, slightly higher fundamental output voltages are possible by over modulating. Over modulation results in unwanted voltages at harmonic frequencies. Space vector modulation is a pulse width modulation technique well known in the art of AC drive systems. The DC link current (I DC ) is given by: I DC = I trans - I diode The real component of the load current (I real ) is given by: I real = (I DC x 82) / X The load current (I load ) is then given by: I load = (A x I trans + B X I diode ) ( C - X D ), where A, B, C, and D are coefficients which dependent on the type of modulation being used. For normal symmetrical space vector modulation A = 0.55, B = 0.45, C = 1000, and D = 612. For asymmetrical space vector modulation, where at all times one phase or another is locked to the DC negative potential, A = 0.45, B = 0.55, C = -80 and D = -141. When over modulation is used the modulation coefficients must be altered accordingly. For symmetric over modulation A = 0.46, B = 0.54, C = 1000, and D = 574. For asymmetric over modulation A = 0.49, B = 0.51, C = 80, and D = 131. As will be appreciated by the skilled man, all of the above modulation coefficients may be modified slightly to account for component tolerances or other factors. The modulation coefficients may need to be modified by up to 10 percent of the above values. These coefficients give I real and I load as root mean squared (rms) values. As will be appreciated, other types of modulation schemes could be used and which would require modifications to the values of the modulation dependent coefficients A, B, C, and D given above, although all symmetrical waveforms generally require very similar coefficients. In a further embodiment of the present invention, figure 3 shows a three phase voltage source DC link inverter 300 comprising DC link 2 and inverter 20. Two current measuring means 22, 31 are disposed in the DC link inverter 300. Each current measuring means 22, 31 is coupled to processing means 26, 33 respectively. Processing means 26, 33 are coupled to further processing means 35. The current measurement means 22, 31 are shunts. Shunt 22 is electrically coupled to transistors 14a, 14b, 16a, 16b, 18a and 18b such that shunt 22 can measure the current, I transistor , through the transistors. Shunt 31 is electrically coupled to negative DC input 4 such that shunt 31 can measure the DC link current, I DC , directly. The diode current (I diode ) is given by: I diode = I transistor - I DC The real component of the load current and the load current are calculated in the same manner and with the same equations as detailed above for the embodiment shown in figure 2. In yet a further embodiment of the present invention, figure 4 shows a three phase voltage source DC link inverter 400 comprising DC link 2 and inverter 20. Two current measuring means 24, 31 are disposed in the DC link inverter 400. Each current measuring means 24, 31 is coupled to processing means 28, 33 respectively. Processing means 28, 33 are coupled to further processing means 35. The current measurement means 24, 31 are shunts. Shunt 24 is electrically coupled to diodes 14c, 14d, 16c, 16d, 18c and 18d such that shunt 24 can measure the current, I diode , through the diodes. Shunt 31 is electrically coupled to negative DC input 4 such that shunt 31 can measure the DC link current, I DC , directly. The transistor current (I transistor ) is given by: I transistor = I diode + I DC The real component of the load current and the load current are calculated in the same manner and with the same equations as detailed above for the embodiment shown in figure 2. Referring to figure 5, AC drive system 500 comprises the current measurement arrangement 200 shown in figure 2 and a motor 45. Outputs 7, 8, and 9 from inverter 20 are electrically connected to motor 45. The motor is connected to a drive mechanism (not shown). Calculation of the load current provides protection for the motor during an overload situation in the drive mechanism. Calculation of the real component of the load current provides for control of the motor output. While figure 5 includes the current measuring arrangement 200 shown in figure 2, the further embodiments 300, 400 of current measuring arrangements shown in figures 3, 4 may also be used in the drive system, without departing from the scope of the present invention. As will be appreciated, the drive system shown in figure 5 may be an open loop system. Alternatively, the drive system may be a closed loop system. The processing means 26, 28, 33, and 35 may be microprocessors. As will be appreciated, the imaginary component of the motor current may also be easily calculated from the values of I load and I real . It will be apparent to those skilled in the art that various modifications may be made to the embodiment hereinbefore described without departing from the scope of the present invention

Claims: An apparatus for providing and measuring current flowing to a load (45), comprising: an inverter (20) which uses pulse width modulation; a first current measuring means (22; 31) arranged to measure a first current flowing through an inverter; a second current measuring means (24; 22), arranged to measure a second current flowing through said inverter; and processing means (35) coupled to said first and said second current measuring means, wherein said inverter comprises a plurality of electronic switches (14a, 14b, 16a, 16b, 18a, 18b), and a plurality of diodes (14c, 14d, 16c, 16d, 18c, 18d), said first current measuring means, said second current measuring means and said processing means being arranged to measure a current I transistor flowing through said plurality of electronic switches, and a current I diode flowing through said plurality of diodes, characterised in that said processing means is further arranged to calculate a load current I load based upon said currents I transistor and I diode , using the following equation : I load = ( A . I transistor + B . I diode ) ( C - X D ) where: X is a percentage of modulation, and A, B, C, and D are modulation dependent coefficients. An apparatus as claimed in Claim 1, wherein said first current measuring means (22) is coupled to the plurality of electronic switches, and said second current measuring means (24) is coupled to the plurality of diodes, such that said first current measuring means measures the current I transistor and said second current measuring means measures the current I diode . An apparatus as claimed in Claim 1, wherein said first current measuring means (31) is coupled to a negative DC input (4) forming part of a DC link (3, 4) to said inverter, and said second current measuring means (22) is coupled to the plurality of electronic switches, such that said first current measuring means measures a first current flowing through said negative DC input equal to (I transistor - I diode ) and said second current measuring means measures the current I transistor . An apparatus as claimed in Claim 1, wherein said first current measuring means (31) is coupled to a negative DC input (4) forming part of a DC link (3, 4) to said inverter, and said second current measuring means (24) is coupled to the plurality of diodes, such that said first current measuring means measures a first current flowing through said negative DC input equal to (I transistor - I diode ) said second current measuring means measures the current I diode . An apparatus as claimed in any preceding Claim, wherein said processing means is further arranged to calculate a real component of said load current I load based upon said first and second currents, according to the formula I real = (I transistor -I diode ) x 82/X. An apparatus as claimed in Claim 5, wherein said processing means is further arranged to calculate an imaginary component of said load current based upon said load current and said real component of said load current. An apparatus as claimed in any preceding Claim, wherein said first and second current measurement means are shunts. An apparatus as claimed in any preceding Claim, wherein said electronic switches are transistors. A power module comprising said apparatus as claimed in any preceding Claim. An AC drive system comprising a motor (45) and said apparatus as claimed in any preceding Claim. An AC drive system as claimed in Claim 10, wherein said system is an open loop system. An AC drive system as claimed in Claim 10 or Claim 11, wherein said pulse width modulation is symmetrical space vector modulation, and said modulation dependent coefficients are as follows: A = 0.55, B = 0.45, C = 1000, and D = 612. An AC drive system as claimed in Claim 10 or Claim 11, wherein said pulse width modulation is asymmetrical space vector modulation, and said modulation dependent coefficients are as follows: A = 0.45, B = 0.55, C = -80, and D = -141. An AC drive system as claimed in Claim 10 or Claim 11, wherein said pulse width modulation is over modulated symmetrical space vector modulation, and said modulation dependent coefficients are as follows: A = 0.46, B = 0.54, C = 1000, and D = 574. An AC drive system as claimed in Claim 10 or Claim 11, wherein said pulse width modulation is over modulated asymmetrical space vector modulation, and said modulation dependent coefficients are as follows: A = 0.49, B = 0.51, C = 80, and D = 131. A method of measuring a load current I load flowing from an inverter (20) to a load (45), wherein said inverter uses pulse width modulation and comprises a plurality of electronic switches (14a, 14b, 16a, 16b, 18a, 18b) and a plurality of diodes (14c, 14d, 16c, 16d, 18c, 18d), said method comprising the steps of: measuring (26; 33) a first current flowing through said inverter, measuring (28; 26) a second current flowing through said inverter, and obtaining a value of current I transistor flowing through said plurality of electronic switches, and a value of current I diode flowing through said plurality of diodes, from said first and second measured currents characterised in that the method further comprises the step of calculating (35) the load current I load based upon said currents I transistor and I diode , using the following equation I load = ( A . I transistor + B . I diode ) ( C - X D ) where: X is a percentage of modulation, and A, B, C, and D are modulation dependent coefficients. A method as claimed in Claim 16, wherein said pulse width modulation is symmetrical space vector modulation, and said modulation dependent coefficients are as follows: A = 0.55, B = 0.45, C = 1000, and D = 612. A method as claimed in Claim 16, wherein said pulse width modulation is asymmetrical space vector modulation, and said modulation dependent coefficients are as follows: A = 0.45, B = 0.55, C = -80, and D = -141. A method as claimed in Claim 16, wherein said pulse width modulation is over modulated symmetrical space vector modulation, and said modulation dependent coefficients are as follows: A = 0.46, B = 0.54, C = 1000, and D = 574. A method as claimed in Claim 16, wherein said pulse width modulation is over modulated asymmetrical space vector modulation, and said modulation dependent coefficients are as follows: A = 0.49, B = 0.51, C = 80, and D = 131. A method of measuring current as claimed in any of claims 17-20 , wherein said inverter is coupled to a negative DC input (4) of a DC link (3, 4), and said step of measuring a first current is a measurement (33) of a current equal to (I transistor - I diode ) flowing through said negative DC input, and said step of measuring a second current is a measurement (28) of the current I diode flowing through said plurality of diodes. A method of measuring current as claimed in any of claims 17-21, wherein said inverter is coupled to a negative DC input (4) of a DC link (3, 4), and said step of measuring a first current is a measurement (33) of a current equal to (I transistor - I diode ) flowing through said negative DC input, and said step of measuring a second current is a measurement (26) of the current I transistor flowing through said plurality of electronic switches. A method as claimed in claimed in any of claims 17-22, wherein said method further comprises the step of: calculating a real component of said load current based upon said first and second currents, according to the formula I real = (I transistor -I diode ) x 82/X. A method as claimed in Claim 23, wherein said method further comprises the step of calculating an imaginary component of said load current based upon said load current and said real component of said load current.

Inventors: CARTER ROBERT, CARTER, ROBERT
Applicants: SIEMENS AG, SIEMENS AKTIENGESELLSCHAFT
Application year: 2000
Main classification: G01R19
IPC codes: H02H7, H02H3, H02M7