Apparatus and method for measuring current

• US 6,590,794 B1
• Filed: 10/05/2000
• Issued: 07/08/2003
• Est. Priority Date: 10/15/1999
• Status: Expired due to Fees

• Current Assignee: Siemens AG
• Original Assignee: Siemens AG
• Inventors: Carter, Robert
• Primary Examiner(s): Patel, Rajnikant B.
• Application Number:
  US09/684,957
• Time in Patent Office: 1,006 Days
• Field of Search: 363/98, 363/132, 363/56, 363/59, 363/135, 363/136, 363/37, 363/81, 363/87
• US Class Current: 363/98
• CPC Class Codes: H02H 3/05 with means for increasing reliability, e.g. redundancy arrangements for logic circuits H03K19/003
• CPC Class Codes: H02H 7/0838 with H-bridge circuit
• CPC Class Codes: H02H 7/1227 responsive to abnormalities in the output circuit, e.g. short circuit
• CPC Class Codes: H02M 1/0009 Devices or circuits for detecting current in a converter
• CPC Class Codes: H02M 7/53875 with analogue control of three-phase output

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. (FIG. 2)

Claims

1. An apparatus for determining current flowing between an inverter and a load, said apparatus comprising:

a first current measuring device;

a second current measuring device, said first and second current measuring devices being arranged on a DC input side of the inverter, such that said first current measuring device measures a first current flowing at said input side of said inverter and said second current measuring device measures a second current flowing at said input side of said inverter; and

a processor which calculates a magnitude of load current which flows between the inverter and the load, based on said first and second currents measured by said first and second current measuring devices.

2. An apparatus as claimed in claim 1, wherein:

said first current measuring device is coupled to a plurality of electronic switches forming part of said inverter;

said second current measuring device is coupled to a plurality of diodes forming part of said inverter; and

said first and second current measuring devices are arranged such that said first current measuring device measures a first current, which flows through said plurality of electronic switches, and said second current measuring device measures a second current, which flows through said plurality of diodes.

3. An apparatus as claimed in claim 1, wherein:

said first current measuring device is coupled to a negative DC input forming part of a DC link to said inverter;

said second current measuring device is coupled to a plurality of electronic switches forming part of said inverter; and

said first and second current measuring devices are arranged such that said first current measuring device measures a first current, which flows through said negative DC input, and said second current measuring device measures a second current, which flows through said plurality of electronic switches.

4. An apparatus as claimed in claim 1, wherein:

said first current measuring device is coupled to a negative DC input forming part of a DC link to said inverter;

said second current measuring device is coupled to plurality of diodes forming part of said inverter; and

said first and second current measuring devices are arranged such that said first current measuring means measures a first current which flows through said negative DC input and said second current measuring means measures a second current which flows through said plurality of diodes.

5. An apparatus as claimed in claim 1, wherein said processor calculates a real component of said load current based upon said first and second currents.

6. An apparatus as claimed in claim 5, wherein said processor calculates an imaginary component of said load current based upon said load current and said real component of said load current.

7. An apparatus as claimed in claim 1, wherein said first and second current measurement means are shunts.

8. An apparatus as claimed in claim 1, wherein said electronics switches are transistors.

9. A power module comprising said apparatus as claimed in claim 1.

10. An AC drive system comprising a motor and said apparatus as claimed in claim 1.

11. An AC drive system as claimed in claim 10, wherein said system is an open loop system.

12. An AC drive system as claimed in claim 10, wherein said drive system uses pulse width modulation.

13. An AC drive system as claimed in claim 12, wherein said processing means is arranged to calculate said load current, I_load, using the following equation:

$I_{\mathrm{load}}=\left(A\times <br><br>I_{\mathrm{transistor}}+B\times <br><br>I_{\mathrm{diode}}\right)<br><br>\text{\hspace{1em}<br><br>}<br><br>\left(\frac{C-X}{D}\right)$

where;

I_transistor is said first current flowing through said plurality of electronic switches,

I_diode is said second current flowing through said plurality of diodes,

X is a percentage of modulation, and

A, B, C, and D are modulation dependent coefficients.

14. An AC drive system as claimed in claim 12, wherein said pulse width modulation is symmetrical space vector modulation.

15. An AC drive system as claimed in claim 14, wherein said modulation dependent coefficients are as follows:

A=0.55, B=0.45, C=1000, and D=612.

16. An AC drive system as claimed in claim 12, wherein said pulse width modulation is asymmetrical space vector modulation.

17. An AC drive system as claimed in claim 16, wherein said modulation dependent coefficients are as follows:

A=0.45, B=0.55, C=−

80, and D=−

141.

18. An AC drive system as claimed in claim 12, wherein said pulse width is over modulated symmetrical space vector modulation.

19. An AC drive system as claimed in claim 18, wherein said modulation dependent coefficients are as follows:

A=0.46, B=0.54, C=1000, and D=574.

20. An AC drive system as claimed in claim 12, wherein said pulse width is over modulated asymmetrical space vector modulation.

21. An AC drive system as claimed in claim 20, wherein said modulation dependent coefficients are as follows:

A=0.49, B=0.51, C=80, and D=131.

22. A method of determining load current flowing between an inverter and a load, said method comprising:

measuring a first current flowing at a DC input side of said inverter;

measuring a second current flowing at said DC input side of said inverter; and

calculating load current flowing between an output side of said inverter and said load, based upon said first and second currents.

23. A method of determining load current as claimed in claim 22, wherein:

said inverter comprises a plurality of electronic switches and a plurality of diodes;

said measuring a first current comprises measurement of a current flowing through said plurality of electronic switches; and

said measuring a second current comprises measurement of a current flowing through said plurality of diodes.

24. A method of determining load current as claimed in claim 22, wherein:

said inverter comprises a plurality of diodes and is coupled to a negative DC input of a DC link;

said measuring of a first current comprises measurement of a current flowing through said negative DC input; and

said measuring of a second current comprises measurement of a current flowing through said plurality of diodes.

25. A method of determining load current as claimed in claim 22, wherein:

said inverter comprises a plurality of electronic switches and is coupled to a negative DC input of a DC link;

said measuring of a first current comprises measurement of a current flowing through said negative DC input; and

said measuring of a second current comprises measurement of a current flowing through said plurality of electronic switches.

26. A method as claimed in claim 22, further comprising:

calculating a real component of said load current based upon said first and second currents.

27. A method as claimed in claim 26, further comprising:

calculating an imaginary component of said load current based upon said load current and said real component of said load current.