Generalized discontinuous pulse width modulator
First Claim
1. An apparatus for use with a PWM controller and inverter, the controller comparing modulating signals to a carrier signal to generate firing signals for turning PWM inverter switches on and off thereby connecting each of first, second and third inverter output lines alternately between positive and negative DC buses and generating high frequency voltage pulses on each output line, the high frequency pulses on each line together generating low frequency alternating voltages, the positive DC bus having a DC bus voltage value, the apparatus for providing modulating signals which cause minimum harmonic distortion in the generated voltages, the controller providing three initial sinusoidal modulating signals, the apparatus comprising:
- an identifier which receives at least one of the initial modulating signals and determines a modulating index;
a comparator which compares the modulating index to a threshold index, anda selector which;
when the modulating index is less than the threshold index, provides a first signal set including first, second and third PWM signals corresponding to the first, second and third output lines, the first set having identical 120 degree phase shifted waveforms; and
when the modulating index is greater than the threshold index, provides a second signal set including first, second and third PWM signals corresponding to the first, second and third output lines, the second set having identical 120 degree phase shifted waveforms;
wherein, below the threshold index, the first set causes relatively less generated voltage harmonic distortion than the second set and, above the index, the second set causes relatively less generated voltage distortion than the first set.
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Abstract
A method and apparatus for selecting one of several different modulating signals for use with a system including a PWM controller and an inverter to generate low frequency alternating voltages on three motor supply lines, modulating signals selected as a function of a modulating index to minimize harmonic distortion in the alternating voltages, minimize switching losses, maximize overall system gain at high modulating index values and maximize the linear range of system operation.
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Citations
34 Claims
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1. An apparatus for use with a PWM controller and inverter, the controller comparing modulating signals to a carrier signal to generate firing signals for turning PWM inverter switches on and off thereby connecting each of first, second and third inverter output lines alternately between positive and negative DC buses and generating high frequency voltage pulses on each output line, the high frequency pulses on each line together generating low frequency alternating voltages, the positive DC bus having a DC bus voltage value, the apparatus for providing modulating signals which cause minimum harmonic distortion in the generated voltages, the controller providing three initial sinusoidal modulating signals, the apparatus comprising:
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an identifier which receives at least one of the initial modulating signals and determines a modulating index; a comparator which compares the modulating index to a threshold index, and a selector which; when the modulating index is less than the threshold index, provides a first signal set including first, second and third PWM signals corresponding to the first, second and third output lines, the first set having identical 120 degree phase shifted waveforms; and when the modulating index is greater than the threshold index, provides a second signal set including first, second and third PWM signals corresponding to the first, second and third output lines, the second set having identical 120 degree phase shifted waveforms; wherein, below the threshold index, the first set causes relatively less generated voltage harmonic distortion than the second set and, above the index, the second set causes relatively less generated voltage distortion than the first set. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 24, 29, 30, 31)
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19. A method for use with a PWM controller and inverter, the controller comparing modulating signals to a carrier signal to generate firing signals for turning PWM inverter switches on and off thereby connecting each of first, second and third inverter output lines alternately between positive and negative DC buses and generating high frequency voltage pulses on each output line, the high frequency pulses on each line together generating low frequency alternating voltages, the controller providing thee initial sinusoidal modulating signals and the DC bus having a DC bus voltage value, the method for providing modulating signals which cause minimum harmonic distortion in the generated voltages, the method comprising the steps of:
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determining a modulating index using at least one of the initial modulating signals; comparing the modulating index to a threshold index; and when the modulating index is less than the threshold index, providing a first signal set including first, second and third PWM signals corresponding to the first, second and third output lines, the first set having identical 120 degree phase shifted waveforms; and when the modulating index is greater than the threshold index, providing a second signal set including first, second and third PWM signals corresponding to the first, second and third output lines, the second set having identical 120 degree phase shifted waveforms; and wherein, the first set is known to cause relatively less harmonic distortion than the second set when the modulating index is less than the threshold index and the second set is known to cause relatively less harmonic distortion than the first set when the modulating index is greater than the threshold index. - View Dependent Claims (20, 21, 22, 23, 25, 26, 27, 28, 32, 33, 34)
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Specification