INVERTER SYSTEM WITH AUTOMATIC RIDETHROUGH

• US 3,593,103 A
• Filed: 03/04/1969
• Issued: 07/13/1971
• Est. Priority Date: 03/04/1969
• Status: Expired due to Term

• Current Assignee: Arthur M. Roberts, Edgar F. Chandler
• Original Assignee: Arthur M. Roberts, Edgar F. Chandler
• Inventors: Chandler, Edgar F., Roberts, Arthur M.
• Application Number:
  US04/804,203
• Time in Patent Office: 861 Days
• US Class Current: 363/174
• CPC Class Codes: H02M 7/525 with automatic control of output waveform or frequency H02M7/517 - H02M7/523 take precedence

Abstract

For enabling a variable frequency electric power inverter to ride through a temporary loss or reduction of input power, means is provided for sensing an excursion of DC bus voltage beyond a predetermined normal magnitude range and for deriving first and second signals respectively proportional to the magnitude of the excursion and to its integral, and the frequency of the inverter is varied according to the difference between a given signal and the sum of said first and second signals.

Claims

1. In a control circuit for a variable frequency inverter which comprises a plurality of controlled switching elements interconnecting a DC bus and a set of AC terminals, said DC bus normally being energized by a source of electric power and said AC terminals being adapted to be connected to a variable speed electric motor load, the improvement which comprises:

a. means including a variable frequency oscillator for supplying said switching elements with gate pulses which turn on said elements in a predetermined sequence and at a frequency determined by the frequency of said oscillator;

b. means for providing a reference signal whose magnitude can be varied to change the frequency of said oscillator as desired, c. first means responsive to the voltage of said DC bus for providing a first signal representing the magnitude of deviation of said voltage from a predetermined magnitude range;

d. second means responsive to said DC bus voltage for providing a second signal representing the integral of said deviation magnitude; and

e. means for varying the frequency of said oscillator according to the sum of said first, second and reference signals.

2. The improvement defined in claim 1 wherein said source of electric power includes a power rectifier having input terminals adapted to be energized by a source of AC power, said rectifier including outpUt capacitance means coupled to said DC bus, the voltage across said capacitance means being regulated by said control circuit.

3. The improvement defined in claim 1 wherein said first and second means include in common (i) voltage sensing means for monitoring said DC bus and (ii) a voltage deadband circuit connected to said sensing means for establishing said predetermined magnitude range, said deadband circuit being operative whenever said DC bus voltage is below a first voltage setpoint to produce a control signal having a certain polarity and a magnitude proportional to the magnitude difference between said DC bus voltage and said setpoint, said first and second means being so constructed and arranged that said first and second signals are proportional respectively to the magnitude of said control signal and to the integral of said magnitude and are both applied to said frequency varying means in a sense opposing said reference signal.

4. The improvement defined in claim 3 wherein said deadband circuit is also operative whenever said DC bus voltage is above a second, relatively high voltage setpoint to produce said control signal which then has a polarity opposite said certain polarity and a magnitude proportional to the difference between said DC bus voltage and said second setpoint, said first means being so arranged that said first signal is applied to said frequency varying means in a sense aiding said reference signal whenever said control signal has said opposite polarity.

5. The improvement defined in claim 1 wherein said first and second means include in common (i) voltage sensing means for monitoring said DC bus and (ii) a voltage deadband circuit connected to said sensing means for establishing said predetermined magnitude range, said deadband circuit being operative whenever said DC bus voltage is above a predetermined setpoint to produce a control signal having a certain polarity and a magnitude proportional to the magnitude difference between said DC bus voltage and said setpoint, said first and second means being so constructed and arranged that said first and second signals are proportional respectively to the magnitude of said control signal and to the integral of said magnitude and are both applied to said frequency varying means in a sense aiding said reference signal.

6. In a system for maintaining variable frequency electric power to an AC machine from an electric power converter, said converter having DC input terminals, AC output terminals, and controlled switching elements arranged to provide conductive paths between said input and output terminals, said DC terminals normally being energized by a source of power, the improvement which comprises:

a. gating means coupled to said switching elements for providing gating signals thereto, the frequency of said gating signals determining the actual frequency of said converter;

b. a voltage sensitive frequency regulator comprising i. first means for providing a reference signal having a magnitude related to a desired frequency, ii. voltage sensing means coupled to said source of power for producing a first control signal representing the amount and direction of any input voltage magnitude excursion beyond predetermined limits, and iii. output signal providing means coupled to receive said reference and control signals and including integrating means for producing a second control signal representing the integral of said first control signal, said output signal providing means being operative to provide an output signal having a frequency dependent on the sum of the magnitudes of said reference signal, said first control signal, and said second control signal; and

c. means applying said output signal to said gating means for controlling the frequency of said gating signals.

7. The improvement defined in claim 6 wherein additional means is provided for controlling the magnitude of voltage at said AC output terminals as a function of thE actual frequency of said converter.

8. The improvement defined in claim 6 wherein said voltage sensing means includes an undervoltage detector for sensing an input voltage excursion below a lower voltage setpoint, and an overvoltage detector for sensing an input voltage excursion above an upper voltage setpoint.

9. The improvement defined in claim 8 wherein said output signal providing means is so constructed and arranged that the control signal produced upon operation of said undervoltage detector tends to decrease the output signal frequency while the control signal produced upon operation of said overvoltage detector tends to increase the output signal frequency.

10. In a system for maintaining variable frequency electric power to an AC machine from an electric power converter, said converter having DC input terminals, AC output terminals, controlled switching elements arranged to provide conductive paths between said input and output terminals, and gating means coupled to said switching elements for providing gating signals thereto, the frequency of said gating signals determining the frequency of said converter, an improved voltage sensitive frequency regulator comprising :

a. voltage sensing means responsive to the voltage of said DC terminals for providing a control signal representing the magnitude of deviation of said voltage from a given value;

b. proportional amplifying means coupled to receive said representative control signal and operative to provide a second control signal having a magnitude substantially proportional to the magnitude of said representative control signal;

c. integrating amplifying means coupled to receive said representative control signal and operative to provide a third control signal having a magnitude substantially equal to the integral of said representative control signal;

d. means for providing a reference signal having a magnitude related to a desired frequency;

e. a variable frequency source having input means coupled to receive said reference signal, said second control signal and said third control signal and having an output for providing an output signal having a frequency that depends on the magnitudes of said signals received over said input means, and f. means for applying said output signal to said gating means to control the frequency of said converter.

11. In a control circuit for a variable frequency inverter which comprises a plurality of controlled switching elements interconnecting a DC bus and a set of AC terminals, said DC bus normally being energized by a source of electric power and said AC terminals being adapted to be connected to a variable speed electric motor load having a predetermined natural coast-down rate, and gating means coupled to said switching elements for cyclically triggering the same in a predetermined sequence, the improvement which comprises:

a. control means coupled to said gating means and responsive to a given frequency reference signal for normally determining the operating frequency of the inverter according to the magnitude of said signal, said control means including b. coasting means coupled to said DC bus and responsive to deenergization thereof by said source of power for reducing said frequency initially just enough to permit regeneration by said motor load and thereafter at a rate approaching said coast-down rate, said coasting means including c. means operative upon reenergization of said DC bus by said source for ensuring a gradual increase of said frequency to a level determined by the magnitude of said frequency reference signal.

12. The improvement defined in claim 11 wherein said coasting means comprises means for reducing said frequency by an amount proportional to the sum of the deviation of the DC bus voltage below a predetermined minimum limit and the integral of said deviation.