SYSTEM FOR PHASE SHIFTING INVERTERS TO OBTAIN A VARIABLE MODULATED WAVEFORM

• US 3,781,615 A
• Filed: 10/24/1972
• Issued: 12/25/1973
• Est. Priority Date: 10/24/1972
• Status: Expired due to Term

• Current Assignee: Westinghouse Air Brake Technologies Corporation
• Original Assignee: Boris Mokrytzki, Udo Meier
• Inventors: Mokrytzki, Boris, Meier, Udo
• Primary Examiner(s): Rubinson, Gene Z.
• Application Number:
  US05/299,932
• Time in Patent Office: 427 Days
• Field of Search: 318/227,230,231
• US Class Current: 318/801
• CPC Class Codes: H02P 21/00 Arrangements or methods for the control of electric machines by vector control, e.g. by control of field orientation

Abstract

A a. c. motor drive system for effecting fundamental voltage modulation by phase shifting inverters coupled in parallel, thereby providing a gradual voltage change between a minimum pulse width modulated waveform and an unmodulated waveform to overcome surges experienced in the transition therebetween. A voltage programmer produces a control signal predicated on vehicle speed so as to follow a preselected profile computed to control the degree of inverter phase shift so that the phase shift modulated fundamental precisely matches the pulse width modulated fundamental at one extreme of the phase shift region of the voltage curve and precisely matches the unmodulated fundamental at the other extreme of the phase shift region.

Claims

1. A control system for energizing an a. c. motor in accordance with a modulated load waveform being generated within a first preselected speed range and in accordance with an unmodulated load waveform being generated within a second preselected speed range, said system comprising:

a. multi-phase load windings forming the stator of said motor, b. a source of d. c. voltage, c. first and second inverter means each having a plurality of phase displaced outputs for connection with said load windings, said inverter means including switch means for periodically reversing the polarity of said d. c. voltage potential at said outputs to effect energization of said load windings, and d. control means for operating said switch means within a first region of said first speed range to provide said modulated waveform by pulse width modulation, and within a second region of said first speed range to provide said modulated waveform by phase shifting said first and second inverter means from a predetermined phase angle effective within said first region to a phase angle which varies with the speed of said motor, thereby effecting a smooth voltage transition from said modulated waveform to said unmodulated waveform.

2. The system, as recited in claim 1, wherein said control means further comprises voltage programmer means for generating a reference signal which varies in accordance with the speed of said motor so as to follow a predetermined profile and thereby effect said phase shift of said inverter means to an extent sufficient to produce said phase shift modulated waveform having a voltage component equal to the voltage component of said pulse width modulated waveform at the transition between said first and second regions of said first speed range, and to provide said predetermined phase angle of said inverter means to effect said unmodulated waveform at the transition from said second region of said first speed range into said second speed range.

3. The system, as recited in claim 1, further characterized in that the degree of phase shift of said first and second inverter means is equal and of opposite sense relative one to the other so that the resultant position of the voltage component of said phase shift mdoulated waveform coincides with the position of the voltage component of said pulse width modulated waveform.

4. The system, as recited in claim 1, wherein said switch means includes solid state switch means.

5. The system, as recited in claim 2, wherein said first and second inverter means are each comprised of a three-phase, solid state bridge network consisting of said switch means.

6. The system, as recited in claim 5, wherein said control means further comprises motor speed responsive drive means for controlling the switching frequency of said bridge network.

7. The system, as recited in claim 6, wherein said control means further comprises:

a. said drive means comprising a ring counter having a plurality of outputs providing phase displaced digital control signals, b. comparator means for providing first and second digital control signals in accordance with the level of said reference signal exceeding the level of a first and a second voltage signal of triangular waveshape having a frequency corresponding to the speed of said motor, and c. logic means subject to said phase displaced digital control signals and to said first and second digital control signals for operating said switching means to produce said phase shift modulated waveform.

8. The system, as recited in claim 7, further characterized in that said first and second voltage signals of triangular waveshape are 180* out of phase so that said switching means are operated to effect said phase shift of said first and second inverter means equally in opposite sense, thereby maintaining coincidence between the relative positions of the voltage component of said phase shift modulated waveforms and said pulse width modulated waveform.

9. The system, as recited in claim 7, wherein said control means further comprises means for integrating a pulse signal having a frequency corresponding to the speed of said motor to provide said first and second voltage signals of triangular waveshape.

10. The system, as recited in claim 1, wherein said multiphase load windings comprise first and second sets of three-phase load windings, one mechanically displaced relative to the other an amount corresponding to said predetermined phase angle of said inverter means.

11. The systEm, as recited in claim 10, further comprising reactor means for coupling the phase displaced outputs of said first and second inverter means with the windings of said first and second sets of load windings, respectively, to form said modulated and said unmodulated waveforms.