Electricity: power supply or regulation systems – Output level responsive – Using a transformer or inductor as the final control device
Reexamination Certificate
2001-05-02
2002-07-09
Nguyen, Matthew (Department: 2838)
Electricity: power supply or regulation systems
Output level responsive
Using a transformer or inductor as the final control device
Reexamination Certificate
active
06417651
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a voltage stabilizer apparatus for use with alternating current (AC) power sources. In particular, the present invention relates to an AC voltage stabilizer with digitally-controlled emulation of servotransformer operation using tap-switching technology.
2. Discussion of Background
AC power lines are subject to a number of different types of voltage disturbances. These include spikes (i.e., pulses of very high voltage and current), which are most commonly caused by lightning and usually last less than a second; surges and sags, which are periods of less severe, too-high or too-low voltage lasting from several seconds to a few minutes, usually caused by faulty power company switching or by other devices on the line; brownouts, which are longer sags lasting from several minutes to several hours; and blackouts, which are periods of near-zero voltage, which can be caused by blown fuses or tripped circuit breakers after a lightning strike or other line problem.
Users of computers and other electronic equipment are familiar with the problems caused by power-line spikes and surges. So-called “surge-suppressing” (more correctly, “spike-suppressing”) power strips or adapters are now widely available. However, these devices are of little value in protecting against sustained power-line surges and of none at all against sags or brownouts. In areas where severe sags, surges or brownouts occur frequently, voltage-sensitive devices such as motors, computers and other electronic equipment, and even ordinary light bulbs have a limited life expectancy. These problems are ubiquitous in the historically less-developed countries, and even in remote areas of the developed countries where generators or long stretches of low-tension cable may be needed to provide electrical power. In many countries, the infrastructure needed to support widespread use of these technologies is inadequate. In particular, the lack of steady, dependable electrical power has proved to be an important factor in limiting the potential market for electrically-powered consumer goods (refrigerators, air conditioners, television sets, personal computers, etc.).
Many methods and devices are available for stabilizing local line voltage, but typically represent difficult compromises, playing off smoothness and accuracy of control against cost. Most commercially-available voltage stabilizers rely on transformers whose step-up or step-down ratios can be changed to help compensate for input-voltage changes. These devices fall into two broad classes: discrete tap switchers or “relay boxes,” and servo-transformers.
Tap switchers are simple and inexpensive, but do not provide smooth voltage regulation. In such a device, one or more relays select various taps of a transformer (typically an autotransformer) so that the output voltage is raised or lowered in steps to help compensate for changes in the input voltage. This principle is illustrated in
FIG. 1A
for a nominal 230-volt line using a single relay, where an autotransformer
10
is provided with three taps
12
,
14
and
16
, arranged so that an AC voltage applied between taps
12
and
14
results in a higher (“stepped-up”) voltage appearing between taps
14
and
16
. (For clarity, transformer
10
, an iron-cored AC power transformer, is shown as a simple coil or series of windings in FIG.
1
A and the following Figures.) The selected step-up ratio depends upon the particular application, but is typically within the range of about 10-30%.
In an unbalanced circuit, the AC (alternating current) hot line is normally connected to tap
12
through a terminal
12
a,
and the neutral line to tap
14
through a terminal
14
a.
A relay
20
connects either the input voltage at tap
12
or the stepped-up voltage at tap
16
to a terminal
22
. Relay
20
is driven by a control circuit (represented schematically as
24
) which selects tap
12
when the input voltage is generally above a threshold
26
, or tap
16
when the input voltage is generally below this threshold. By “generally” it is meant that the control is not exact; hysteresis effects are usually present, and indeed are desirable to prevent relay chatter and prolong the life of relay
20
. The output is then taken between terminal
22
, which is the new AC hot line, and neutral output terminal
14
b.
Threshold
26
is chosen so that the output voltage remains in the vicinity of a selected voltage represented by a line
36
(FIG.
1
B), over a wider range of input voltages than if no compensation were made.
The result is shown graphically by line segments
30
,
32
and
34
(FIG.
1
B). When the input voltage is below threshold
26
, and neglecting the effects of loading, relay
20
selects tap
16
. The output voltage measured between taps
14
and
16
is higher than the input voltage by the step-up factor of transformer
10
, typically between about 10-30%, as shown by line segment
30
. As the input voltage rises above threshold
26
, control circuit
24
causes relay
20
to change position, selecting tap
12
instead of tap
16
, and the output voltage changes abruptly as shown by line
32
. At still higher input voltages, the output equals the input as shown by line segment
34
.
If the input voltage then drops below threshold
26
, relay
20
again changes position to select tap
16
and the output voltage is again stepped up to compensate for the input voltage drop. For comparison, line segment
38
represents the output voltage if no compensation is made.
When transformer
10
is supplying current to a load, the graph shown in
FIG. 1B
is not completely accurate since some voltage drop occurs within the transformer, and the output voltage is correspondingly lower. Hence, selecting the transformer tap according to the output voltage, rather than the input, would in theory provide superior control. In commercial tap switchers, however, this is not often done because it can cause oscillation between positions, so that the “stabilizer” actually makes the output voltage less rather than more stable.
The quality of voltage regulation available from a tap switcher can be improved by adding more steps, so that the intervals between the steps can be made smaller and/or so that a wider span of input voltages can be handled. More steps, however, require correspondingly more relays and transformer taps, resulting in greater circuit complexity and higher cost. These types of devices normally operate in simple “daisy-chain” fashion, so that a separate tap and relay are required for each step.
For example,
FIG. 2A
illustrates a tap switcher using an autotransformer
10
with four taps (not separately labeled) and three relays
20
a,
20
b,
and
20
c
acting successively at three different thresholds
26
a,
26
b,
and
26
c,
respectively, under the control of circuit
24
. Such a stabilizer might, for example, provide either straight-through operation as shown by line segment
34
(FIG.
2
B), boosts of 20% or 40% for line undervoltages as shown by line segments
30
a
and
30
b,
respectively, or 20% bucking (voltage decrease) for line overvoltages as shown by line segment
30
c.
Line segments
38
a
and
38
b
represent the output voltage if no compensation were made.
As in the circuit of
FIG. 1A
, abrupt voltage steps
32
a,
32
b
and
32
c
occur each time a relay switches. However, a larger number of steps permits either stabilization of the output voltage near a desired level
36
over a wider input voltage range, smaller individual steps so that the output voltage remains closer to level
36
over this range, or both.
Some products designed mostly for audiophile or recording-studio use perform a similar function to the circuits of
FIGS. 1A and 2A
using solid-state switching devices rather than electromechanical relays. This approach speeds response and minimizes switching noise, but tends to be quite costly because of the added complexity of the circuitry.
Servo-transformers provide much better and smoother, nearly stepless reg
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