Uninterruptible power supply

• US 4,916,329 A
• Filed: 10/05/1987
• Issued: 04/10/1990
• Est. Priority Date: 10/05/1987
• Status: Expired due to Fees

• Current Assignee: Topaz Incorporated Palatine Illinois A Corporation of
• Original Assignee: Square D Company (Schneider Electric SE)
• Inventors: Kuzara, Casey, Dang, Gurcharn S.
• Primary Examiner(s): Shoop, Jr., William M.
• Assistant Examiner(s): IP, SHIK LUEN PAUL
• Application Number:
  US07/104,725
• Time in Patent Office: 918 Days
• Field of Search: 307/64, 307/66, 307/82, 307/83, 307/84, 307/85, 307/86, 307/87, 323/258, 323/343, 323/340, 363/126
• US Class Current: 307/66
• CPC Class Codes: G05F 1/24 using bucking or boosting transformers as final control devices
• CPC Class Codes: H02J 9/062 for AC powered loads
• CPC Class Codes: Y02B 70/30 Systems integrating technologies related to power network operation and communication or information technologies for improving the carbon footprint of the management of residential or tertiary loads, i.e. smart grids as climate change mitigation technology in the buildings sector, including also the last stages of power distribution and the control, monitoring or operating management systems at local level
• CPC Class Codes: Y04S 20/20 End-user application control systems

Abstract

An uninterruptible power supply device uses a transformer with a first primary winding and secondary winding tightly coupled to one another to transmit AC input electrical power to a voltage sensitive load. A series regulator switches taps in the first primary to keep the load output voltage within certain limits during minor variations in the AC input voltage. A standby inverter supplies standby power to the load after a loss or large variation in the AC input voltage by driving PWM pulses into a secondary primary winding of the transformer. The transformer loosely couples the second primary winding to the secondary winding to aid in synthesizing the output sinusoidal wave from the PWM pulses. The regulator opens all of the taps to prevent the inverter from supplying power to the AC input. A voltage monitor continuously senses the AC waveform of an AC sample signal produced from the output load voltage in the normal mode and from the AC input voltage in the standby mode to determine forward and reverse transfers between modes. A clipped AC sample signal detector inhibits transfer in high peak current conditions, and a digital phase-locked loop circuit operating at a fraction of the AC input frequency synchronizes the phase of a precision sine wave signal with the phase of the AC input voltage.

Claims

1. In an uninterruptible power supply for transmitting voltage varying and interruptible AC electrical power received from a AC input source to a load through a transformer during availability of said power from said AC input source and supplying said load with AC electrical power through said transformer from an auxiliary source during interruption of power from said AC input source, a circuit for transferring between sensing the voltage of said electrical power transmitted to said load and sensing the voltage of said received electrical power while supplying said load with AC electrical power, said circuit comprising;

A. AC input terminals to be connected to receive said AC input source of AC electrical power;

B. load terminals to be connected to said load;

C. switch means connected to said AC input terminals and load terminals for selecting an AC sample signal from one of said AC electrical power received at said AC input terminals and said AC electrical power transmitted to said load terminals, said selection respectively occurring in response to receipt of a switch signal of a standby state or a normal state;

D. monitor means receiving said AC sample signal and producing a transfer signal of a normal state while the voltage of said AC sample signal continues between certain voltage limits indicating availability of said AC input source AC electrical power and producing a transfer signal of a standby state when the voltage of said A sample signal exceeds said certain voltage limits indicating interruption of said AC input source of AC electrical power; and

E. sequence control means for producing said switch signal of said normal state to select passing said AC sample signal from said load terminals while said transfer signal continues in said normal state and for producing said switch signal of said standby state to select passing said AC sample signal from said AC input terminals while said transfer signal continues in said standby state.

2. The transfer circuit of claim 1 in which said switch means include two switches, one connected to said AC input terminals and a common lead and the other connected to said load terminals and said common lead, each switch receiving said switch signal, said other switch becoming conductive during said switch signal normal state and said one switch becoming conductive during said switch signal standby state.

3. The transfer circuit of claim 1 in which said uninterruptible power supply includes a regulator circuit selectively for connecting said transformer between said AC input and load terminals to transmit electrical power to said load terminals and for disconnecting said AC input terminals from said transformer while said standby source supplies electrical power to said load terminals respectively in response to receipt of a regulator enable signal of a normal state or a standby state and said auxiliary source supplying said load with electrical power and being disabled respectively in response to an auxiliary source enable signal of a standby state or a normal state, and in which said sequence control means produce said regulator enable signal and said auxiliary source enable signal.

4. The transfer circuit of claim 3 in which said sequence control means change said regulator enable signal from the normal to standby states before changing said auxiliary source enable signal from the normal to standby states.

5. The transfer circuit of claim 3 in which said sequence control means change said regulator enable signal from the standby to normal states before changing said auxiliary source enable signal from the standby to normal states.

6. In an uninterruptible power supply for transmitting voltage varying interruptible AC electrical power received from a AC input source to a load through a transformer during availability of said power from said AC input source and supplying said load with AC electrical power through said transformer from an auxiliary source during interruption of said power from said AC input source, a method of transferring between sensing the voltage of said electrical power transmitted to said load and sensing the voltage of said received electrical power while supplying said load with AC electrical power, said method comprising:

A. connecting AC input terminals to said AC input source and connecting load terminals to said load;

B. selecting an AC sample signal from one of said AC electrical power received at said AC input terminals and said AC electrical power transmitted to said load terminals, said selecting occurring respectively in response to receipt of a switch signal of a standby state or a normal state;

C. producing a transfer signal of a normal state while the voltage of said AC sample signal continues between certain voltage limits indicating availability of said AC input source AC electrical power and producing a transfer signal of a standby state when the voltage of said AC sample signal exceeds said certain voltage limits indicating interruption of said AC input source of AC electrical power; and

D. producing said switch signal of said normal state to select passing said AC sample signal from said load terminals while said transfer signal continues in said one normal and producing said switch signal of said standby state to select passing said AC sample signal from said AC input terminals while said transfer signal continues in said standby state.

7. The method of claim 6 in which said selecting an AC sample signal includes connecting one switch between said AC input terminals and a common lead that carries said AC sample signal and connecting another switch between said load terminals and said common lead, applying said switch signal to each switch, rendering said other switch conductive during said switch signal normal state and rendering said one switch conductive during said switch signal standby state.

8. The method of claim 6 in which said uninterruptible power supply includes a regulator circuit selectively for connecting said transformer between said AC input and load terminals to transmit electrical power to said load terminals and for disconnecting said AC input terminals from said transformer while said standby source supplies electrical power to said load terminals respectively in response to receipt of regulator signal of a normal state or a standby state and said auxiliary source supplying said load with electrical power and being disabled respectively in response to an auxiliary source enable signal of a standby state or a normal state, and in which said producing said switch signal includes producing said regulator enable signal and said auxiliary source enable signal.

9. The method of claim 8 including changing said regulator enable signal from the normal to standby states before changing said auxiliary source enable signal from the normal to standby states.

10. The method of claim 8 including changing said regulator enable signal from the standby to normal states before changing said auxiliary source enable signal from the standby to normal states.

11. In an uninterruptible power supply for transmitting interruptible AC electrical power from AC input terminals to load terminals through a transformer during availability of said power from said AC input terminals and supplying said load terminals with AC electrical power through said transformer from an auxiliary source during interruption of said power from said AC input terminals, and said supply selecting an AC sample signal from said load terminals and from said AC input terminals, the voltage of which represents the availability of said interruptible AC electrical power, an AC voltage monitor for indicating the value of the voltage of said analog AC sample signal applied thereto relative to certain voltage limits, which define availability of said electrical power from said AC input terminals, said monitor comprising:

A. high threshold means for producing an analog AC high voltage limit signal in phase with said AC sample signal, said AC high voltage limit signal attaining a maximum absolute voltage of said ceratin limits;

B. low threshold means for producing an AC analog low voltage limit signal in phase with said AC sample signal, said AC low voltage limit signal attaining a minimum absolute voltage of said certain limits;

C. voltage comparator means for producing at least one voltage comparator signal that continues in phase with said AC sample signal while the voltage of said AC sample signal continues between said AC high and low voltage limit signals, and that goes out of phase with said AC sample signal when the voltage of said AC sample signal exceeds one of said AC high and low voltage limit signals; and

D. phase comparator means for producing a transfer signal of a normal state while said voltage comparator signal continues in phase with said AC sample signal and a transfer signal of a standby state when said voltage comparator signal goes out of phase with said AC sample signal, said transfer signal of said normal state indicating that said AC sample signal continues within said certain voltage limits and said transfer signal of said standby state indicating that said AC sample signal exceeds said certain voltage limits.

12. The monitor of claim 11 in which said high and low voltage threshold means include resistor voltage divider arrays with variable resistors, and said threshold means receive a precision sine wave signal produced by said supply and pass same through said arrays to produce said limit signals.

13. The monitor of claim 11 including zero dead-band means receiving a precision sine wave signal, having substantially a sinusoidal wave form, produced by said supply, said zero dead-band means for modulating said precision sine wave signal to flatten the zero crossing portions of said sinusoidal wave form.

14. The monitor of claim 13 in which said high and low voltage threshold means include resistor voltage divider arrays with variable resistors and said threshold means receive said zero dead-band precision sine wave signal modulated by said zero dead-band means and pass same through said arrays to produce said limit signals.

15. The monitor of claim 14 in which there are two sets of resistor arrays for each threshold means, the first set for producing a normal voltage limit signal while said supply transmits electrical power to said load and the second set for producing a standby voltage limit signal while said supply supplies electrical power to said load.

16. The monitor of claim 11 in which said voltage comparator means include a pair of analog comparators, the first comparator receiving said high voltage limit signal and said AC sample signal to produce a high voltage comparator signal and the second comparator receiving said low voltage limit signal and said AC sample signal to produce a low voltage comparator signal.

17. The monitor of claim 11 in which said phase comparator means include a pair of digital gates each receiving from said supply a squared precision sine wave signal that stays in phase with said AC sample signal, the first gate also receiving a high voltage comparator signal to produce a high phase comparison signal indicating the phase relationship of its two received signals and the second gate also receiving a low voltage comparator signal to produce a low phase comparison signal indicating the phase relationship of its two received signals.

18. The monitor of claim 11 in which said phase comparator means include timing means for delaying producing a transfer signal after a change in the phase relationship between said voltage comparator signal and said AC sample signal.

19. The monitor of claim 18 in which said timing means include two timing circuits providing two different delay periods for delaying producing a transfer signal, one for a change in said AC sample signal exceeding said high voltage limit signal to change the phase of said voltage comparator signal and the other for a change in said AC sample signal exceeding said low voltage limit signal to change the phase of said voltage comparator signal.

20. The monitor of claim 11 including clipping detector means for inhibiting producing said transfer signal upon occurrence of a clipped AC sample signal otherwise exceeding said low voltage limit signal.

21. In an uninterruptible power supply for transmitting interruptible AC electrical power from AC input terminals to load terminals through a transformer during availability of said power from said AC input terminals and supplying said load terminals with AC electrical power through said transformer from an auxiliary source during interruption of said power from said AC input terminals, and said supply selecting an AC sample signal from said load terminals and from said AC input terminals, the voltage of which represents availability of said AC electrical power, a method for indicating the value of the voltage of said AC sample signal applied thereto relative to certain voltage limits, which define availability of said electrical power from said AC input terminals, said method comprising:

A. producing an analog AC high voltage limit voltage signal in phase with said AC sample signal, said AC high voltage signal attaining a maximum absolute voltage of said certain voltage limits;

B. producing an analog AC low voltage limit signal in phase with said AC sample signal, said AC low voltage limit signal attaining a minimum absolute voltage of said certain voltage limits;

C. producing at least one voltage comparator signal that continues in phase with said AC sample signal while the voltage of said AC sample signal continues between said AC high and low voltage limit signals, and that goes out of phase with said AC sample signal when the voltage of said AC sample signal exceeds one of said AC high and low voltage limit signals; and

D. producing a transfer signal of a normal state while said voltage comparator signal continues in phase with said AC sample signal and a transfer signal of a standby state when said voltage comparator signal goes out of phase with said AC sample signal, said transfer signal of said normal state indicating that said AC sample signal continues within said certain voltage limits and said transfer signal of said standby state indicating that said AC sample signal exceeds said certain voltage limits.

22. The method of claim 21 in which said producing said high and low voltage limit signals includes passing a precision sine wave signal from said supply through voltage divider arrays having variable resistors.

23. The method of claim 21 including modulating a precision sine wave signal from said supply to flatten the zero crossing portions of its sinusoidal wave form.

24. The method of claim 23 in which said producing said high and low voltage limit signals includes passing said zero dead-band precision sine wave signal through resistor voltage divider arrays having variable resistors.

25. The method of claim 24 in which said passing includes passing said zero dead-band precision sine wave signal through two sets of resistor arrays for each threshold means, the first set yielding a normal voltage limit signal while said supply transmits electrical power to said load and the second set yielding a standby voltage limit signal while said supply supplies electrical power to said load.

26. The method of claim 21 in which said producing at least one voltage comparator signal includes applying said high voltage limit signal and said AC sample signal to a first analog comparator to yield a high voltage comparator signal and applying said low voltage limit signal and said AC sample signal to a second comparator to yield a low voltage comparator signal.

27. The method of claim 21 in which said producing a transfer signal includes applying a squared precision sine wave signal that stays in phase with said AC sample signal from said supply to a pair of digital gates and also applying to the first gate a high voltage comparator signal to yield a high phase comparison signal indicating the phase relationship of its two received signals and also applying to the second gate a low voltage comparator signal to yield a low phase comparison signal indicating the phase relationship of its two received signals.

28. The method of claim 21 in which said producing a transfer signal includes delaying said producing a transfer signal after a change in the phase relationship between said voltage comparator signal and said AC sample signal.

29. The method of claim 28 in which said delaying includes providing two different delay periods, one for a change in said AC sample signal exceeding said high voltage limit signal and the changing of the phase of said voltage comparator signal and the other for a change in said AC sample signal exceeding said low voltage limit signal and the changing of the phase of said voltage comparator signal.

30. The method of claim 21 including inhibiting said producing said transfer signal upon occurrence of a clipped AC sample signal otherwise exceeding said low voltage limit signal.

31. In an uninterruptible power supply for transmitting interruptible AC electrical power from AC input terminals to load terminals through a transformer during availability of said power from said AC input terminals, and supplying said load terminals with AC electrical power through said transformer from a standby source during interruption of said power from said AC input terminals, the supply selecting an AC sample signal from said load and AC input terminals, the voltage of which represents availability of said electrical power, and said supply including a voltage monitor producing, in response to the voltage of said AC sample signal, a forward transfer signal for effecting forward transfer of said power supply from transmitting electrical power to said load terminal to supplying power to said load terminals, a detector circuit for inhibiting said forward transfer of said power supply upon detection of a clipping of the crests of said AC sample signal voltage wave form that otherwise appears to be a low voltage condition normally causing said forward transfer through said monitor producing said forward transfer signal, said detector circuit comprising:

A. voltage comparator means receiving said AC sample signal voltage wave form and producing a clip detect signal having a width directly varying with the amount of clipping of the crests of said AC sample signal voltage wave form; and

B. switch means in series with said monitor for disabling production of said forward transfer signal during said clip detect signal.

32. The detector circuit of claim 31 in which said voltage comparator means include in series connection, differentiator means for processing said AC sample signal to shift the phase thereof 90 degrees and dV/dT comparator means for making said clip detect signal in the periods during which said phase shifted AC sample signal passes through its fast dV/dT portions.

33. The detector circuit of claim 32 in which said dV/dT comparator means include a pair of voltage comparators connected in parallel to produce output signals when said phase shifted AC sample signal passes through the voltages associated with its zero crossing point.

34. The detector circuit of claim 31 in which said voltage comparator means include precision sine wave means enabling production of said clip detect signal for only a certain length of time centered at each crest of said AC sample signal voltage wave form.

35. The detector circuit of claim 31 in which said monitor includes a timer for delaying production of said transfer signal after recognition of an apparent low voltage condition, and said switch means include a switch connected to said timer for resetting said timer upon receipt of said clip detect signal.

36. The detector circuit of claim 31 including disable means for blocking said switch means from disabling said production of said forward transfer signal upon the voltage of said AC sample signal voltage wave form falling below a specified point regardless of any clipping.

37. In an uninterruptible power supply for transmitting interruptible AC electrical power from AC input terminals to load terminals through a transformer during availability of said power from said AC input terminals, and supplying said load terminals with AC electrical power through said transformer from a standby source during interruption of said power from said AC input terminals, the supply selecting an AC sample signal from said load and AC input terminals, the voltage of which represents availability of said electrical power, and said supply including a voltage monitor producing in response to the voltage of said AC sample signal, a forward transfer signal for effecting forward transfer of said power supply from transmitting power to said load terminals to supplying power to said load terminals, a method for inhibiting said forward transfer of said power supply upon detection of a clipping of the crests of said AC sample signal voltage wave form that otherwise appears to said monitor to be a low voltage condition normally causing said forward transfer through said monitor producing said forward transfer signal, said method comprising:

A. producing a clip detect signal having width directly varying with the amount of clipping of the crests of said AC sample signal voltage wave form; and

B. disabling production of said forward transfer signal during said clip detect signal.

38. The method of claim 37 in which said producing a clip detect signal includes processing said AC sample signal to shift the phase thereof 90 degrees and making said clip detect signal in the periods during which said phase shifted AC sample signal passes through its fast dV/dT portions.

39. The method of claim 37 in which said making said clip detect signal includes voltage sensing when said phase shifted AC sample signal passes through the voltages associated with its zero crossing point.

40. The method of claim 37 in which said producing a clip detect signal includes enabling production of said clip detect signal for only a certain length of time centered at each crest of said AC sample signal voltage wave form.

41. The method of claim 37 in which said monitor includes a timer for delaying production of said transfer signal after recognition of an apparent low voltage condition, and said disabling includes resetting said timer upon receipt of said clip detect signal.

42. The method of claim 37 including blocking said disabling of said production of said forward transfer signal upon the voltage of said AC sample signal voltage wave form falling below a specified point regardless of any clipping.

43. In an uninterruptible power supply for transmitting interruptible AC electrical power from AC input terminals to load terminals through a transformer during availability of said power from said AC input terminals, and supplying said load terminals with AC electrical power through said transformer from a standby source during interruption of said power from said AC input terminals, a phaselocked loop circuit for producing from an oscillator internal said power supply a precision sine wave signal in phase with the phase of the sinusoidal wave form of the AC electrical power, said oscillator being capable of varying the frequency of the precision sine wave signal from a desired frequency equal to that of the AC electrical power in response to the current level of a control signal fed to said oscillator varying from a certain current level, said loop circuit comprising:

A. phase shifter means receiving said sinusoidal wave form of said AC electrical power and producing a shifted signal having a phase 180 degrees from said sinusoidal wave form;

B. first divider means receiving said shifted signal and producing a first divided signal therefrom oscillating at a frequency of half that of said shifted signal;

C. second divider means receiving said precision sine wave signal and producing a second divided signal therefrom oscillating at a frequency of half that of said precision sine wave signal;

D. gating means receiving said first and second divided signals and producing a synchronizing signal having pulses with varying widths directly related to the phase difference between said first divided signal and said second divided signal; and

E. current generator means receiving said synchronizing signal and producing said control signal having current levels directly related to the varying widths of said pulses of said synchronizing signal,

F. so that a phase difference between said electrical power sinusoidal wave form and said precision sine wave signal causes said oscillator to vary the frequency of said precision sine wave signal until the phase of said precision sine wave signal matches that of said electrical power sinusoidal wave form.

44. The loop circuit of claim 43 in which said phase shifter means include a pair of analog comparators, the first receiving said electrical power sinusoidal wave form at an inverting input and the second receiving said precision sine wave signal at a non-inverting input.

45. The loop circuit of claim 43 in which said first and second divider means include, respectively, first and second flip flops receiving their respective signals at clock signal inputs.

46. The loop circuit of claim 43 in which said gating means include an exclusive-or gate receiving said first and second divided signals and producing said synchronizing signal.

47. The loop circuit of claim 43 in which said current generator means include an error amplifier receiving said synchronizing signal and producing said control signal.

48. In an uninterruptible power supply for transmitting interruptible AC electrical power from AC input terminals to load terminals through a transformer during availability of said power from said AC input terminals, and supplying said load terminals with AC electrical power through said transformer from a standby source during interruption of said power from said AC input terminals, a method of producing from an oscillator internal said power supply a precision sine wave in phase with the phase of the sinusoidal wave form of the AC electrical power, said oscillator being capable of varying the frequency of the precision sine wave signal from a desired frequency equal to that of the AC electrical power in response to the current level of a control signal fed to said oscillator varying from a certain current level, said method comprising:

A. producing a shifted signal having a phase 180 degrees from said sinusoidal wave form of said AC electrical power;

B. producing a first divided signal oscillating at a frequency of half that of said shifted signal;

C. producing a second divided signal oscillating at a frequency of half that of said precision sine wave signal;

D. producing a synchronizing signal having pulses with varying widths directly related to the phase difference between said first divided signal and said second divided signal; and

E. producing said control signal having current levels directly related to the varying widths of said pulses of said synchronizing signal,

F. so that a phase difference between said electrical power sinusoidal wave form and said precision sine wave signal causes said oscillator to vary the frequency of said precision sine wave signal until the phase of said precision sine wave signal matches that of said electrical power sinusoidal wave form.

49. The method of claim 48 in which producing said shifted signal includes shifting the phase of said electrical power sinusoidal wave form while maintaining constant the phase of said precision sine wave signal.

50. The method of claim 48 in which said producing first and second divided signals includes respectively clocking said shifted signal and precision sine wave signal through respective flip flops.

51. The method of claim 48 in which said producing a synchronizing signal includes applying the rules of an exclusive-or gate to said first and second divided signals to produce said synchronizing signal.

52. An uninterruptible power supply for transmitting voltage varying and interruptible AC electrical power received from a AC input source to a load during availability of electrical power from said AC input source and supplying said load with AC electrical power from an auxiliary source during interruption of power from said AC input source, said supply comprising:

A. AC input terminals adapted to be connected to said AC input source;

B. load terminals adapted to be connected to said load;

C. monitor means coupled to said AC input terminals and said load terminals for producing a normal mode signal when the AC electrical power from said AC input source continues within about certain voltage limits and for producing a standby mode signal when AC input source electrical power varies substantially beyond said certain voltage limits to be considered interrupted;

D. transformer means connected in series between said AC input and load terminals for transmitting AC electrical power from said AC input terminals to said load terminals;

E. inverter means for supplying AC electrical power to said load terminals through said transformer means upon receipt of an inverter enable signal;

F. regulator means connected in series with said transformer means between said AC input and load terminals for regulating, in response to a regulator enable signal, to within said certain voltage limits the AC electrical power transmitted to said load terminals from said AC input terminals in response to the AC voltage from said AC input terminals varying substantially at said certain voltage limits, and for disconnecting, in response to a regulator disable signal, said transformer means from said AC input terminals;

G. sequence means for producing said regulator enable signal and a switch normal signal in response to said normal mode signal and for producing said regulator disable signal, said inverter enable signal and a switch standby signal in response to said standby mode signal; and

H. switch means for connecting said load terminals to said monitor means in response to said switch normal signal and for connecting said AC input terminals to said monitor means in response to said switch standby signal.

53. The supply of claim 52 in which said monitor means include phase shift detection means for determining whether the AC voltage at said terminals continues within said certain voltage limits to produce said normal and standby mode signals.

54. The supply of claim 52 in which said transformer means include two magnetic paths to said load terminals, a first closely coupled path from said AC input terminals to said load terminals and a second loosely coupled path from said inverter means to said load terminals.

55. The supply of claim 52 in which said transformer means include plural taps between said AC input and load terminals and said regulator means include plural tap switches for connecting said taps in series between said AC input and load terminals.

56. The supply of claim 52 in which said sequence means produce said regulator disable signal before producing said inverter enable signal and produce said regulator enable signal before removing said inverter enable signal.

57. The supply of claim 52 including synchronization means coupled to said AC input and load terminals for synchronizing the phase of said inverter means with said AC input source electrical power.

58. The supply of claim 57 in which said switch means also connect said AC input and load terminals to said synchronization means respectively in response to said switch normal and standby signals.

59. The supply of claim 57 in which said synchronization means include phase-locked loop means having a phase comparator portion operating at a fraction of the frequency of said AC input source electrical power.

60. A method of transmitting voltage varying and interruptible AC electrical power received form a AC input source to a load during availability of electrical power from said AC input source and supplying said load with AC electrical power from an auxiliary source during interruption of power from said AC input source, said method comprising:

A. connecting AC input terminals to said AC input source;

B. connecting load terminals adapted to said load;

C. producing a normal mode signal from a monitor when the AC electrical power from said AC input source continues within about certain voltage limits and producing a standby mode signal from said monitor when the AC electrical power form said AC input source varies substantially beyond said certain voltage limits to be considered interrupted;

D. connecting a transformer in series between said AC input and load terminals for transmitting AC electrical power from said AC input terminals to said load terminals;

E. supplying AC electrical power from an auxiliary source to said load terminals through said transformer upon receipt of an inverter enable signal;

F. connecting a regulator in series with said transformer between said AC input and load terminals for regulating, in response to a regulator enable signal, to within said certain voltage limits the AC electrical power transmitted to said load terminals from said AC input terminals in response to the AC voltage from said AC input terminals varying substantially at said certain voltage limit, and for disconnecting, in response to a regulator disable signal, said transformer from said AC input terminals;

G. producing said regulator enable signal and a switch normal signal in response to said normal mode signal and for producing said regulator disable signal, said inverter enable signal and a switch standby signal in response to said standby mode signal; and

H. connecting said load terminals to said monitor in response to said switch normal signal and for connecting said AC input terminals to said monitor in response to said switch standby signal.

61. The method of claim 60 in which said producing said normal and standby mode signals includes phase shift detecting to determine whether the AC voltage at said terminal continues within said certain voltage limits to produce said normal and standby mode signals.

62. The method of claim 60 in which said connecting a transformer includes furnishing two magnetic paths to said load terminals, a first tightly coupled path from said AC input terminals to said load terminals and a second loosely coupled path from said inverter means to said load terminals.

63. The method of claim 60 in which connecting said transformer includes furnishing plural taps between said AC input and load terminals, said connecting a regulator includes furnishing plural tap switches and further including connecting said taps in series between said AC input and load terminals with said tap switches.

64. The method of claim 60 including producing said regulator disable signal before producing said inverter enable signal and producing said regulator enable signal before removing said inverter enable signal.

65. The method of claim 60 including synchronizing the phase of supplying AC electrical power with said AC input source electrical power.

66. The monitor of claim 65 in which said synchronizing occurs in response to the voltage at one of said AC input and load terminals respectively in response to said switch normal and standby signals.

67. The method of claim 65 in which said synchronizing includes operating a phase comparator at a fraction of the frequency of said AC input source electrical power. PG,124

68. An uninterruptible power supply for transmitting voltage varying and interruptible AC electrical power received from a AC input source to a load during availability of electrical power from said AC input source and supplying said load with AC electrical power from an auxiliary source during interruption of power from said AC input source, said supply comprising:

A. AC input terminals adapted to be connected to said AC input source;

B. load terminals adapted to be connected to said load;

C. monitor means coupled to said AC input terminals and said load terminals for producing a normal mode signal when the AC electrical power from said AC input source continues within about certain voltage limits and for producing a standby mode signal when said AC input source electrical power varies substantially beyond said certain voltage limits to be considered interrupted;

D. transformer means connected in series between said AC input and load terminals for transmitting AC electrical power from said AC input terminals to said load terminals;

E. inverter means for supplying AC electrical power to said load terminals through said transformer means upon receipt of an inverter enable signal;

F. regulator means connected in series with said transformer means between said AC input and load terminals for regulating, in response to a regulator enable signal, to within said certain voltage limits the AC electrical power transmitted to said load terminals from said AC input terminals in response to the AC voltage from said AC input terminals varying substantially at said certain voltage limit, and for disconnecting, in response to a regulator disable signal, said transformer means from said AC input terminals;

G. sequence means for producing said regulator enable signal and a switch normal signal in response to said normal mode signal and for producing said regulator disable signal, said inverter enable signal and a switch standby signal in response to said standby mode signal;

,

H. switch means for connecting said load terminals to said monitor means in response to said switch normal signal and for connecting said AC input terminals to said monitor means in response to said switch standby signal;

I. clip detector means for inhibiting said monitor from making said standby mode signal upon detection of a clipping of the crests of the AC voltage wave form applied to said monitor from said switch means that otherwise appears to be a low voltage condition normally causing said monitor to produce said standby mode signal; and

J. phase-locked loop means for keeping said inverter means synchronized with the phase of said AC input source electrical power.