Method and apparatus for improved efficiency in a pulse-width-modulated alternating current motor drive
First Claim
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1. A polyphase alternating current electric motor drive comprising:
- (a) a polyphase motor;
(b) a direct current power source;
(c) a polyphase power conversion bridge for converting direct current power from said source to alternating current power for application to said motor;
(d) means to provide a speed signal representing motor rotational speed;
(e) means to provide current signals having values representing corresponding phase currents of the alternating current power applied to said motor;
(f) means to provide a torque command signal having a value representing a desired torque of said motor; and
,(g) control means responsive to said speed, current and torque command signals for controlling said power conversion bridge in a pulse-width-modulated mode of operation, said control means comprising frequency modulation control ;
means for enabling a variable number of modulating pulses in each half-cycle of the phase currents responsive to said torque command signal to provide a higher number of modulating pulses at values of said torque command signal representing lesser values of desired torque than at values of said torque command signal representing higher desired values of desired torque and to provide a constant number of modulating pulses for values of said torque command signal representing values of desired torque higher than a predetermined value of desired torque;
(h) wherein the higher number of modulating pulses produce a power wave shape more closely approximating a sine wave to thereby lower harmonic heating of the motor when switching currents are lower, and the lower number of modulating pulses reduce switching losses of the power conversion bridge when switching currents are higher.
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Abstract
A scheme for optimizing the efficiency of an AC motor drive operated in a pulse-width-modulated mode provides that the modulation frequency of the power furnished to the motor is a function of commanded motor torque and is higher at lower torque requirements than at higher torque requirements.
44 Citations
22 Claims
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1. A polyphase alternating current electric motor drive comprising:
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(a) a polyphase motor; (b) a direct current power source; (c) a polyphase power conversion bridge for converting direct current power from said source to alternating current power for application to said motor; (d) means to provide a speed signal representing motor rotational speed; (e) means to provide current signals having values representing corresponding phase currents of the alternating current power applied to said motor; (f) means to provide a torque command signal having a value representing a desired torque of said motor; and
,(g) control means responsive to said speed, current and torque command signals for controlling said power conversion bridge in a pulse-width-modulated mode of operation, said control means comprising frequency modulation control ;
means for enabling a variable number of modulating pulses in each half-cycle of the phase currents responsive to said torque command signal to provide a higher number of modulating pulses at values of said torque command signal representing lesser values of desired torque than at values of said torque command signal representing higher desired values of desired torque and to provide a constant number of modulating pulses for values of said torque command signal representing values of desired torque higher than a predetermined value of desired torque;(h) wherein the higher number of modulating pulses produce a power wave shape more closely approximating a sine wave to thereby lower harmonic heating of the motor when switching currents are lower, and the lower number of modulating pulses reduce switching losses of the power conversion bridge when switching currents are higher. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
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9. A method of controlling a pulse-width-modulated alternating electric current motor drive of the type including a polyphase power conversion bridge for providing pulse-width-modulated electrical power to a motor as a function of current switching signals, said method comprising the steps of:
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(a) providing a torque command signal having a value representing the desired torque output of said motor; and (b) generating said current switching signals, said current switching signals serving to effect operation of said bridge at a fundamental frequency proportional to desired motor speed and at a modulation frequency as a function of the value of said torque command signal while enabling a variable number of modulating pulses in each half-cycle of phase currents responsive to said torque command signal to provide a higher number of modulating pulses at values of said torque command signal representing lesser values of desired torque than at values of said torque command signal representing higher desired values of desired torque and to provide a instant number of modulating pulses for values of said torque command signal representing values of desired torque higher than a predetermined value of desired torque; (c) wherein the higher number of modulating pulses produce a power wave shape more closely approximating a sine wave to thereby lower harmonic heating of the motor when switching currents are low, and the lower number of modulating pulses reduce switching losses of the power conversion bridge when switching currents are high. - View Dependent Claims (10, 11, 12, 13, 14, 15)
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16. A polyphase alternating current motor drive of the type in which the motor is provided electrical power from a controllable direct current to alternating current power inverter, said motor drive comprising:
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(a) means to provide a torque command signal having a value representing a desired output torque of the motor; and (b) control means for controlling the power inverter in a pulse-width-modulated mode in response to said torque command signal, said control means comprising frequency modulation control means for effecting relatively higher rates of pulse modulation in response to values of said torque command signal representing low values of motor output torque than to values of said torque command representing high values of motor output torque and to provide a constant number of modulating pulses for values of said torque command signal representing values of desired torque higher than a predetermined value of desired torque; (c) wherein the higher number of modulating pulses produce a power wave shade more closely approximating a sine wave to thereby lower harmonic heating of the motor when switching currents are low, and the lower number of modulating pulses reduce switching losses of the power inverter when switching currents are high. - View Dependent Claims (17, 18, 19, 20, 21, 22)
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Specification