Method and system for controlling an electric motor with variable switching frequency at variable operating speeds

• US 8,456,115 B2
• Filed: 06/28/2011
• Issued: 06/04/2013
• Est. Priority Date: 02/23/2011
• Status: Active Grant

• Current Assignee: Deere & Company
• Original Assignee: Deere & Company
• Inventors: Wu, Long, Shaw, Robert
• Primary Examiner(s): Ro, Bentsu
• Assistant Examiner(s): Bouziane, Said
• Application Number:
  US13/170,216
• Publication Number:
  US 20120212167A1
• Time in Patent Office: 707 Days
• Field of Search: 318/400.13, 318/599, 318/685, 318/55, 318/90, 318/91, 318/92
• US Class Current: 318/400.13
• CPC Class Codes: H02P 6/14 Electronic commutators

Abstract

A data processor establishes a first range of rotational speeds of a rotor of the motor from a first lower limit to a first higher limit and a second range of rotational speeds from a second lower limit to a second higher limit. A hysteresis band or a rotational range of speeds is established such that during operation in the first range the first higher limit is adjusted (e.g., raised by a first amount to be greater than the second lower limit). A sensor detects or measures a rotational speed of a rotor of the motor. The data processor determines whether the measured rotational speed falls within the first range or the second range, as adjusted by the hysteresis band, to identify a selected speed range. A switching frequency of a pulse-width modulation signal is varied in accordance with the selected speed range.

Claims

1. A method for controlling a motor, the method comprising:

establishing a first range of rotational speeds of a rotor of the motor from a first lower limit to a first higher limit;

establishing a second range of rotational speeds of the rotor from a second lower limit to a second higher limit;

establishing a hysteresis band or a rotational range of speeds such that during operation in the first range the first higher limit is adjusted or raised by a first amount to be greater than the second lower limit;

detecting or measuring a rotational speed of a rotor of the motor;

determining whether the measured rotational speed falls within the first range or the second range, as adjusted by the hysteresis band, to identify a selected speed range; and

varying a switching frequency of a pulse-width modulation signal in accordance with the selected speed range.

2. The method according to claim 1 further comprising:

establishing a hysteresis band or a rotational range of speeds such that during operation in the second range the second lower limit is lowered by a second amount to be less than the first higher limit.

3. The method according to claim 2 wherein the first amount is different than or less than the second amount.

4. The method according to claim 1 wherein the first amount is within a range of approximately fifty revolutions per minute to approximately one-hundred and twenty revolutions per minute.

5. The method according to claim 1 further comprising:

setting the first higher limit equal to the second lower limit, subject to adjustment by the hysteresis band.

6. The method according to claim 1 further comprising:

constructing a third speed range through Nth speed range for the motor based on full speed range of the motor operation, where N is an positive integer greater than or equal to four; and

applying one or more hysteresis bands to each transition between adjacent speed ranges of the motor within the N speed ranges.

7. The method according to claim 6 wherein the full speed range of the motor operation ranges from approximately zero revolutions per minute to greater than or equal to 4,000 revolutions per minute.

8. The method according to claim 1 wherein the varying of the switching frequency comprises varying the switching frequency consistent with a preferential switching frequency selected from a look-up table in accordance with the selected speed range and a previous speed range of the rotor, where the selected speed range is associated with a current speed interval and where the previous speed range is associated with a previous speed interval prior to the current speed interval.

9. The method according to claim 1 wherein the switching frequency for a corresponding selected speed range is selected to optimize a phase current imbalance factor or ratio in accordance with the following expression:

10. The system according to claim 1 wherein the switching frequency for a corresponding selected speed range is selected to further optimize current ripple component in accordance with a shaft torque ripple and an electric machine efficiency requirement.

11. The method according to claim 1 wherein the switching frequency for a corresponding selected speed range is selected to maintain a desired frequency ratio for the motor in accordance with the following equation:

12. The method according to claim 1 wherein the switching frequency is restricted from slot harmonic frequency defined as the product of a fundamental electrical frequency of the motor and the number of slots per pole pair of the motor, where the fundamental electrical frequency is proportional to the rotational speed of the rotor.

13. The method according to claim 1 wherein a hysteresis speed band is set at the boundary of two adjacent speed intervals such that the first upper limit of the first range is higher than a second lower limit of the second range to avoid jitter or abrupt discontinuities in a change between selected switching frequencies during a series of successive intervals.

14. The method according to claim 1 further comprising:

accessing an upper limit and a lower limit for a previous step selected speed range;

determining a current speed range based on the current measured speed, the accessed upper limit added by the hysteresis band and the accessed lower limit subtracted by the hysteresis band.

15. The method according to claim 1 further comprising:

varying a current regulation proportional and integral gains in accordance with the selected speed range.

16. A system for controlling a motor, the system comprising:

a data storage device for storing a first range of rotational speeds of a rotor of the motor from a first lower limit to a first higher limit, the data storage device arranged to store a second range of rotational speeds of the rotor from a second lower limit to a second higher limit;

a data processor for establishing a hysteresis band or a rotational range of speeds such that during operation in the first range the first higher limit is adjusted or raised by a first amount to be greater than the second lower limit;

a sensor for detecting or measuring a rotational speed of a rotor of the motor;

the data processor adapted to determine whether the measured rotational speed falls within the first range or the second range, as adjusted by the hysteresis band, to identify a selected speed range; and

a pulse width modulation generation module for varying a switching frequency of a pulse-width modulation signal in accordance with the selected speed range.

17. The system according to claim 16 further comprising:

the data processor configuring or controlling the hysteresis band or a rotational range of speeds such that during operation in the second range the second lower limit is lowered by a second amount to be less than the first higher limit.

18. The system according to claim 16 wherein the first amount is less than the second amount.

19. The system according to claim 16 wherein the switching frequency for a corresponding selected speed range is selected to optimize a phase current imbalance factor in accordance with the following equation:

20. The system according to claim 16 wherein the switching frequency for a corresponding selected speed range is selected to further optimize current ripple component in accordance with a shaft torque ripple and an electric machine efficiency requirement.

21. The system according to claim 16 wherein the switching frequency for a corresponding selected speed range is selected to maintain a desired frequency ratio for the motor in accordance with the following equation:

22. The system according to claim 16 wherein the switching frequency is restricted from slot harmonic frequency defined as the product of a fundamental electrical frequency of the motor and the number of slots per pole pair of the motor, where the fundamental electrical frequency is proportional to the rotational speed of the rotor.