Traction motor retarding flux reference

• US 8,928,260 B2
• Filed: 10/18/2012
• Issued: 01/06/2015
• Est. Priority Date: 10/18/2012
• Status: Active Grant

• Current Assignee: Caterpillar Incorporated
• Original Assignee: Caterpillar Incorporated
• Inventors: Crosman, Alexander Cameron III, Williams, Joshua M.
• Primary Examiner(s): Leykin, Rita
• Application Number:
  US13/655,019
• Publication Number:
  US 20140111125A1
• Time in Patent Office: 810 Days
• Field of Search: 318/380, 318/382, 318/375, 318/808, 318/86, 318/87, 318/140, 318/143, 318/145, 318/151, 318/152, 318/157, 318/158, 363/125, 363/61, 180/165, 180/65.31
• US Class Current: 318/380
• CPC Class Codes: B60L 3/10 Indicating wheel slip ; Correction of wheel slip
• CPC Class Codes: H02P 21/141 Flux estimation
• CPC Class Codes: H02P 21/16 Estimation of constants, e.g. the rotor time constant
• CPC Class Codes: H02P 2205/03 Power loop, i.e. comparison of the motor power with a power reference
• CPC Class Codes: H02P 2205/05 Torque loop, i.e. comparison of the motor torque with a torque reference
• CPC Class Codes: H02P 23/0077 Characterised by the use of a particular software algorithm
• CPC Class Codes: H02P 23/14 Estimation or adaptation of motor parameters, e.g. rotor time constant, flux, speed, current or voltage
• CPC Class Codes: Y02T 10/72 Electric energy management in electromobility

Abstract

A traction motor system calculates motor flux by generating a real time effective resistance of a resistance grid calculated from motor torque and measured voltage on a DC link. Calculating effective resistance avoids solely relying on DC link voltage, which can be influenced by conditions such as wheel slip and drop out of one or more resistance grids. The effective resistance calculation is based on nominal motor values using known power levels and conditions. From these nominal values and the effective resistance, various scaling factors based on actual motor power can be generated and used to adjust a nominal flux reference to more accurately reflect actual motor flux. The scaling factors include power and torque scaling factors and a resistance scaling factor that is active during conditions such as wheel slip.

Claims

1. A method of adjusting electrical power application in a motor control system using an AC motor driven by an inverter bank that is coupled to a resistive grid during retard operation, the method comprising:

calculating a braking factor as a ratio of a nominal power DC voltage vs. a nominal brake DC voltage;

calculating a resistance scale factor as a ratio of measured grid resistance vs. a base grid resistance;

determining that motor operation is in one of a constant power region or a constant torque region;

multiplying a DC link voltage, the braking factor, the resistance scale factor and one of a power scale factor when in a constant power region or a torque scale factor when in a constant torque region to generate an adjusted flux estimate; and

setting inverter operation to adjust motor torque based on the adjusted flux estimate, wherein the power scale factor is a function of a measured torque times mechanical frequency vs. a characteristic torque times a corner point mechanical frequency and the torque scale factor is a function of the measured torque vs. the characteristic torque.

2. The method of claim 1, wherein the braking factor is calculated as

3. The method of claim 1, wherein the resistance scale factor is calculated as

4. The method of claim 1, wherein the power scale factor is calculated as

5. The method of claim 1, wherein the torque scale factor is calculated as

6. The method of claim 2, wherein the resistance scale factor is calculated as a square root of effective resistance divided by a base resistance defined by the equation:

7. The method of claim 6, wherein the power scale factor is calculated as

8. The method of claim 7, wherein the torque scale factor is calculated as

9. A method of operating an AC motor driven by an inverter, the method comprising:

determining a base resistance constant using characteristics of the motor at a first motor rotation frequency, the first motor rotation frequency defined at a transition point between constant torque operation of the motor and constant power operation of the motor (“

the knee frequency”

);

calculating motor power as a function of the actual torque and the actual motor rotation frequency;

calculating an effective resistance at the inverter as a function of motor power and a voltage on the DC link;

calculating an estimated flux reference as a function of the DC link voltage, the effective resistance, and the base resistance constant; and

adjusting torque output of the motor based on the estimated flux reference.

10. The method of claim 9, further comprising:

calculating a voltage constant as a ratio between a DC link voltage in a propel mode at throttle level 8 at the knee frequency and a DC link voltage in a brake mode at throttle level 8 at the knee frequency,

wherein calculating the estimated flux reference includes calculating the estimated flux reference as a further function of the voltage constant.

11. The method of claim 10, further comprising:

determining that the AC motor is operating in a constant power region; and

calculating a first scaling factor when the motor operation is in the constant torque region, wherein calculating the estimated flux reference includes calculating the estimated flux reference as a further function of the first scaling factor.

12. The method of claim 11, wherein calculating the first scaling factor comprises calculating

13. The method of claim 10, further comprising:

determining that the AC motor is operating in a constant torque region; and

calculating a second scaling factor when the motor operation is in the constant torque region, wherein calculating the estimated flux reference includes calculating the estimated flux reference as a further function of the second scaling factor.

14. The method of claim 13, wherein calculating the second scaling factor comprises calculating

15. The method of claim 14, wherein determining the base resistance constant comprises:

determining a value for V<sub>DC—

DB8</sub>=DC link voltage in brake mode at throttle level 8;

determining a value for T_DB8=torque of the motor during dynamic braking at throttle level 8;

determining a value for ω

_mc=mechanical frequency of the motor at the knee frequency, and

calculating the base resistance constant as R_base=V<sub>DC—

DB8</sub>²/η

·

T_DB8·

ω

_m.

16. The method of claim 15, wherein calculating the effective resistance comprises calculating

17. An alternating current (AC) motor system adapted to adjust motor flux based on motor power, a direct current (DC) link voltage and an effective resistance at an inverter used to drive the AC motor, the system comprising:

an AC generator;

a rectifier that converts an output of the generator to DC power;

a DC link coupled to the rectifier;

a resistive grid selectively coupled to the DC link;

a plurality of AC motors;

a plurality of inverters, each of the plurality of inverters electrically coupling the DC link to a respective one AC motor of the plurality of AC motors;

a controller coupled to the DC link, the resistive grid, and each of the plurality of inverters, the controller including;

a processor; and

a memory storing instructions that when executed on the processor cause the controller to;

calculate an estimated flux based at least in part on an effective resistance of the grid at each inverter of the plurality of inverters, the effective resistance calculated from actual torque, actual motor rotation frequency, the DC link voltage and a base resistance constant; and

adjust a setting for each inverter of the plurality of inverters that controls a torque of the AC motor associated with each inverter.

18. The AC motor system of claim 17, wherein the memory stores further instructions that when executed on the processor cause the controller to:

calculate a resistance scale factor based on the effective resistance the resistance scale factor defined as

19. The AC motor system of claim 17, wherein the memory stores further instructions that when executed on the processor cause the controller to:

use an additional factor to calculate the estimated flux defined as a power scale factor calculated as

20. The AC motor system of claim 17, wherein the memory stores further instructions that when executed on the processor cause the controller to:

use an additional factor to calculate the estimated flux defined as a torque scale factor calculated as