INVERTER SYSTEM HAVING SMOOTH SWITCHING BETWEEN OPERATIONAL MODES

• US 3,870,945 A
• Filed: 02/28/1974
• Issued: 03/11/1975
• Est. Priority Date: 02/28/1974
• Status: Expired due to Term

• Current Assignee: John M. Shabel, Niels P. Pedersen
• Original Assignee: John M. Shabel, Niels P. Pedersen
• Inventors: Pedersen, Niels P., Shabel, John M.
• Primary Examiner(s): Envall, Jr., R. N.
• Application Number:
  US05/446,645
• Time in Patent Office: 376 Days
• Field of Search: 321/9A,18,16,45R 318/227
• US Class Current: 363/42
• CPC Class Codes: H02M 7/525 with automatic control of output waveform or frequency H02M7/517 - H02M7/523 take precedence

Abstract

In an inverter system for the conversion of direct current power to alternating current power of the type including an inverter circuit selectively operable in either a time ratio controlled (pulse width modulation) or a square wave mode there is provided suitable control means and method of operation to insure a substantially constant voltage-frequency relationship throughout the operating range of the inverter circuit, including the transition period between the two modes of operation.

Claims

1. means to develop a signal proportional to the output frequency of said inverter circuit,

2. The invention in accordance with claim 1 wherein said means to determine when a change in the operation mode is to be made includes means responsive to the output frequency of the inverter circuit for developing a level signal proportional thereto.

3. means responsive to said signal to determine when a switch in mode of inverter circuit operation is to be made and to effect a substantially instantaneous incremental change in the output voltage of the source when a switch is made.

4. A control system for an inverter circuit operable in two distinct modes, a pulse width modulation mode and a square wave mode, to provide a substantially constant voltage-frequency relationship over a range of output frequencies from the inverter circuit, said inverter circuit receiving input power from a controllable direct current power source, comprising:

a. first reference means for providing a first control signal operable to maintain the output voltage of said direct current power source substantially constant when the inverter circuit is operating in the pulse width modulation mode;

b. second reference means for providing a second control signal operable to vary the output voltage of said direct current power source when said inverter circuit is operating in the square wave mode;

c. means to scale the relative values of said first and second control signals;

d. determining means for determining when a change in operational mode of said inverter Circuit is to be made; and

, e. switching means responsive to said determining means for selectively rendering said first and second control signals effective whereby, because of the relative values of said control signals, at the time of switching between effective control signals there is provided a substantially instantaneous change in the output voltage of said direct current power source.

5. The invention in accordance with claim 4 wherein said second control signal is proportional to the output frequency of the inverter circuit.

6. The invention in accordance with claim 5 wherein there is further included a third reference means and said determining means responds to a comparison of said second and said third control signals.

7. A control system for an inverter circuit operable in two distinct modes, a pulse width modulation mode and a square wave mode, to provide a substantially constant voltage-frequency relationship over a range of output frequencies from the inverter circuit, said inverter circuit receiving input power from a controllable direct current power source, comprising:

a. first reference means for providing a first control signal operable to maintain the output voltage of said direct current power source substantially constant when the inverter circuit is operating in the pulse width modulation mode;

b. second reference means for providing a second control signal which varies proportionally to the output signal from said inverter circuit;

c. third reference means to provide a third control signal indicative of the operational point of the inverter circuit at which the operational mode changes;

d. switching means responsive to said second and third control signals for removing said first control signal from said direct current power source and causing said second control signal to control said direct current power source to first effect a substantially incremental change in the output voltage and to thence vary said output voltage proportionally to said second control signal.

8. The invention in accordance with claim 7 further including means to scale the relative values of said first and second control signals to thereby effect the incremental change in the output voltage of said direct current power supply.

9. An inverter system for supplying alternating current power of varying voltage and frequency to a load from a controllable source of variable voltage direct current power comprising:

a. an inverter circuit selectively operable in either of two modes, a pulse width modulation mode and a square wave mode;

b. means to connect said inverter circuit to the source of variable voltage direct current power; and

, c. means for controlling the voltage supplied from the source to said inverter circuit including;

10. The invention in accordance with claim 9 wherein the means responsive to said signal is a switching means for selectively utilizing, as a control function for said source of direct current power, said signal and a further signal of constant value.

11. The invention in accordance with claim 10 wherein the incremental change is effected by the relative scaling of said signal and said further signal.

12. The invention in accOrdance with claim 10 wherein the incremental change is effected by scaling the values of said signal and said further signal such that at the time of switching the magnitude of said signal is less than the magnitude of said further signal.

13. A method of maintaining a substantially linear relationship between the output voltage and the output frequency of an inverter circuit supplied by a source of direct current power throughout a range of inverter output frequencies comprising:

a. operating said inverter in a pulse width modulation mode throughout a first portion of the range of frequencies using a substantially constant source of direct current voltage;

b. operating said inverter in a square wave mode of operation through a second portion of the range of frequencies using a varying source of direct current voltage;

c. determining the point of switching between the two modes of operation; and

, d. effecting a substantially instantaneous change in the direct current power applied to the inverter at the time of switching to thereby maintain the linear relationship.

14. The method in accordance with claim 13 where the point of switching is determined by:

a. providing a first signal proportional to the output frequency of the inverter circuit;

b. providing a second signal indicative of the desired point of switching; and

, c. comparing said first and second signals to effect the determination.

15. A method of maintaining a substantially linear relationship between the output voltage and the output frequency of an inverter circuit supplied by a source of variable voltage direct current power throughout a range of inverter output frequencies comprising:

a. providing a first control signal of constant value;

b. supplying said first control signal to said source to maintain its output voltage constant while operating said inverter in a pulse width modulation mode;

c. providing a second control signal having a value which varies proportionally to the output frequency of said inverter circuit;

d. supplying said second control signal to said source to vary its output voltage in accordance with the value of said second control signal while operating said inverter in a square wave mode;

e. determining the frequency of inverter output for changing between modes of inverter operation; and

, f. effecting a substantially instantaneous change in the voltage of said source at the time of switching to thereby maintain said substantially linear relationship.