Adjustable speed A-C motor drive with smooth transition between operational modes and with reduced harmonic distortion
First Claim
1. Improved static electric power apparatus having relatively positive and negative d-c input terminals adapted to be connected to a source of voltage, a plurality of a-c output terminals adapted to be connected to a variable frequency alternating current load, and means for connecting each of said output terminals to both of said input terminals in a manner to convert unipolarity voltage applied to said input terminals into alternating voltage at said output terminals, said connecting means comprising at least one pair of alternately conducting controllable electric valves interconnecting one of said output terminals and the respective input terminals of said apparatus, firing and commutating means for periodically switching the conducting states of said valves, and means responsive to frequency and amplitude command signals for controlling the operation of said firing and commutating means so as to vary, as functions of said command signals, the frequency and the amplitude of the fundamental component of said alternating voltage, wherein the control means includes:
- a. first means responsive to said frequency command signal for generating a periodic reference signal having a frequency which depends on said frequency command signal;
b. second means keyed by said reference signal for causing said firing and commutating means to switch the conducting states of said valves at certain moments during each cycle of the fundamental component of said alternating voltage, the timing of a predetermined first switching moment in each half cycle of said fundamental component of voltage being established by said reference signal and any subsequent switching moments in the same half cycle occurring in accordance with a schedule comprising at least;
i. a moment following said first switching moment by a variable angle alpha (α
) measured in electrical degrees of a full cycle of fundamental voltage,ii. a moment following said first switching moment by another variable angle beta (β
) measured in electrical degrees of a full cycle of fundamental voltage, β
being larger than α
but less than 90°
,iii. A moment occurring 180°
-β
after said first switching moment, andiv. a moment occurring 180°
-α
after said first switching moment;
c. third means for varying α
as a function of β
so as to minimize selected harmonics of said alternating voltage; and
d. fourth means for varying β
as a function of said amplitude command signal to thereby vary the amplitude of the fundamental voltage component.
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Accused Products
Abstract
Alternating voltage for energizing an adjustable speed a-c motor is derived from unipolarity input voltage by means of electric power inverting apparatus comprising at least one pair of alternately conducting controllable electric valves. The conducting states of these valves are periodically switched by control means responsive to frequency and amplitude command signals, whereby the frequency and the amplitude of the fundamental component of the alternating voltage are varied as functions of the respective command signals. The control means includes means for implementing a triangle interception mode of pulse width modulation (PWM) of the alternating voltage so long as the amplitude command signal does not exceed a predetermined reference value (which is less than 1.0 per unit) and the speed of the motor does not exceed a predetermined reference speed, and the control means additionally includes means for implementing a "transition" PWM mode of operation whenever the amplitude command signal exceeds its reference value or the motor speed exceeds said reference speed. As disclosed herein the transition PWM mode implementing means is a dual d-c level set scheme in which the lower level is varied as a function of the higher level so as to minimize selected harmonics of the alternating voltage and the higher level is varied as a function of the amplitude command signal thereby to vary the amplitude of the fundamental voltage component, and means is provided for smoothly transitioning to a square wave mode of operation as the amplitude command signal approaches 1.0 per unit.
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Citations
23 Claims
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1. Improved static electric power apparatus having relatively positive and negative d-c input terminals adapted to be connected to a source of voltage, a plurality of a-c output terminals adapted to be connected to a variable frequency alternating current load, and means for connecting each of said output terminals to both of said input terminals in a manner to convert unipolarity voltage applied to said input terminals into alternating voltage at said output terminals, said connecting means comprising at least one pair of alternately conducting controllable electric valves interconnecting one of said output terminals and the respective input terminals of said apparatus, firing and commutating means for periodically switching the conducting states of said valves, and means responsive to frequency and amplitude command signals for controlling the operation of said firing and commutating means so as to vary, as functions of said command signals, the frequency and the amplitude of the fundamental component of said alternating voltage, wherein the control means includes:
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a. first means responsive to said frequency command signal for generating a periodic reference signal having a frequency which depends on said frequency command signal; b. second means keyed by said reference signal for causing said firing and commutating means to switch the conducting states of said valves at certain moments during each cycle of the fundamental component of said alternating voltage, the timing of a predetermined first switching moment in each half cycle of said fundamental component of voltage being established by said reference signal and any subsequent switching moments in the same half cycle occurring in accordance with a schedule comprising at least; i. a moment following said first switching moment by a variable angle alpha (α
) measured in electrical degrees of a full cycle of fundamental voltage,ii. a moment following said first switching moment by another variable angle beta (β
) measured in electrical degrees of a full cycle of fundamental voltage, β
being larger than α
but less than 90°
,iii. A moment occurring 180°
-β
after said first switching moment, andiv. a moment occurring 180°
-α
after said first switching moment;c. third means for varying α
as a function of β
so as to minimize selected harmonics of said alternating voltage; andd. fourth means for varying β
as a function of said amplitude command signal to thereby vary the amplitude of the fundamental voltage component. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17)
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18. Improved static electric power apparatus having relatively positive and negative d-c input terminals adapted to be connected to a source of voltage, a plurality of a-c output terminals adapted to be connected to a variable frequency alternating current load, and means for connecting each of said output terminals to both of said input terminals in a manner to convert unipolarity voltage applied to said input terminals into alternating voltage at said output terminals, said connecting means comprising at least one pair of alternately conducting controllable electric valves interconnecting one of said output terminals and the respective input terminals of said apparatus, firing and commutating means for periodically switching the conducting states of said valves, and means responsive to frequency and amplitude command signals for controlling the operation of said firing and commutating means so as to vary, as functions of said command signals, the frequency and the amplitude of the fundamental component of said alternating voltage, wherein the control means includes:
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a. first means responsive to said frequency command signal for generating a periodic reference signal having a frequency which depends on said frequency command signal; b. second means keyed by said reference signal for determining the consecutive moments during each cycle of the fundamental component of said alternating voltage at which the control means causes said firing and commutating means to switch the conducting states of said valves, the timing of a first switching moment in each cycle being established by said reference signal, a subsequent switching moment occurring one-half cycle later, and additional switching moments occurring after said first moment at variable angles measured in electrical degrees of a cycle of fundamental voltage, the per-cycle schedule of said additional switching moments comprising; i. a moment following said first switching moment by a variable angle beta (β
),ii. an immediately succeeding moment occuring 180°
-β
after said first moment,iii. a moment following said subsequent switching moment by the same angle β
, andiv. another moment occurring 180°
-β
after said subsequent moment;c. third means for varying β
between predetermined maximum and minimum angles as a function of said amplitude command signal to thereby vary the amplitude of the fundamental voltage component; andd. fourth means operative when β
is equal to or smaller than a predetermined value closely approaching 0°
for eliminating the above-listed additional switching moments from said per-cycle schedule;e. said fourth means including means responsive to said reference signal for preventing an initial operation thereof except during predetermined intervals which are spaced from said first and said subsequent switching moments by at least said predetermined value. - View Dependent Claims (19, 20, 21, 22, 23)
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Specification