Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to corona
~eneration devices, and more specifically to an i~proved power
supply circuit and associated corona generator which operate
at high frequency and efficiency~
It is generally known that the capacity of a
corona generator is dependent in part on the frequency of the
power supplies across the plates thereof. Traditionally,
high voltage power is supplied to a corona generator at
the frequency of the primary source, i.e., the line
frequency. To increase the frequency of power applied to
corona generators, it has been suggested that mechanically
driven (motor generator) frequency converters be used. It
also has been suggested that electronic circuits which include
vacuum tubes or solid state devices may also be used to
increase frequency.
It has generally been found that motor generator
frequency converters are expensive and not particularly
durable. Furthermore, frequency converters which utilize
vacuum tubes or solid state circuits are often restricted
with respect to efficiency.
Accordingly, it is an object of the present
invention to provide an improved corona generator device
which operates at high frequency and efficiency.
It is a further object to provide an improved
high frequency power supply circuit which is particularly
suited for operation with corona generators.
It is still a further object to provide an
inexpensive high frequency power supply for corona generators
which is constructed from durable, readily available solid
state components.
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Thus, in accordance with the present teachings,
a corona generating system is provided for supplying at
least about 40,000 watts average power to a corona cell.
The system comprises a source of DC power, a transformer
having a low voltage primary winding and a high voltage
secondary winding with a turns ratio which is adapted to
provide a high voltage power pulse across the secondary
winding from a relatively low voltage impressed across the
primary winding of between about 150 to 600 volts DC. At
least two SCR's are connected in a series circuit
relationship with the scource of DC power and the low voltage
primary winding. Each of the SCR's are unequal to each other
with respect to their forward voltage drop characteristic.
Means are provided for pulsing the gates of the SCR's at a
desired frequency. The corona cell has spaced electrodes
with a surface area large enough to generate a corona discharge
of at least the 40,000 watts of average power and for generating
a reverse commutating voltage of sufficient duration for turning
off at least one of the SCR's within each pulse period with
the corona cell representing the only commutating means
for the SCR's in the system.
These objects and others will become readily
apparent to one skilled in the art from the following
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detailed description and drawings wherein;
Figure 1 is a diagram of the circuit used to
supply high voltage, high frequency power to a corona generator
which is also shown schematically therein;
Figures 2, 3 and 4 show various configurations
of corona generators (with parts broken away) which may be
used in conjunction with the power supply circuit of the
present invention;
Figure 5 is a graphic representation of data
obtained from a prior art circuit wherein voltage is plotted
on the vertical axis and time on the longitudinal axis and
Figure 6 shows comparison data obtained from the circuit
of the present invention (dotted line).
Figures 7 and 8 show preferred voltage
unequalization networks which may be used in the circuit of
Figure 1.
Broadly, my invention involves a corona generator
and associated high voltage frequency power supply which
includes a plurality of silicon controlled rectifiers (here-
inafter referred to as SCR's) which are electrically connectedin series with a source of DC potential and the low voltage
winding of a high voltage transformer, the high voltage winding
of which is connected to the plates of a corona generator.
More specifically, I have found that the circuit
turn-off time of SCR's used as high power, high frequency
electrical switches in a power supply may be reliably increased
by driving two or more SCR's in series.
A more clearly detailed understanding of my
invention may be obtained by reference to Figure 1 of the
drawing wherein the circuit generally outlined by broken
line 10 is a conventional rectifier circuit which supplies
pulsed DC voltage. Rectifier circuit 10 includes a rectifier
diode 11 interconnected with capacitor 12 and a source of 60
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cycle 220 volt AC current indicated as 13~ While the
drawing shows the use of a single rectifier circuit suitable
for operation on single phase current, it is contemplated
that three-phase bridge type rectifier circuits are
particularly suited for commercial application of the
invention.
~ lso shown in Figure 1 is a conventional pulse
generator circuit, the components of which are included
within the confines of broken line generally 20. The pulse
generator circuit includes a transformer 21 and a unijunction
transistor 22. One winding of the transformer 21 is
connected in series with a source of 60 cycle 110 volt AC
potential shown as 23. The other winding of the transformer
21 is connected to diode 24, and to a common collector
conductor for several other circuit components including one
side of capacitors 25 and 26.
Also included in pulse generator circuit 20 is a
variable resistor 27 and a fixed resistor 28. The variable
resistor 27 and the fixed resistor 28 are also connected to
the unijunction transistor 22. One lead from the unijunction
transistor 22 interconnects with fixed resistances 29 and 19.
It is to be understood that a wide variety of commercially
available pulse generators are suitable for use in the present
invention.
Figure 1 also includes an inverter circuit
generally outlined by broken line 30. The inverter circuit
30 includes two SCR's 31 and 32 connected in series. The
SCR 31 has a gate lead 35 and the SCR 32 has a gate lead 36.
Also included in the inverter circuit is a transformer 37
which possesses two secondary windings which are inter-
connected with the gates 35 and 36 of the series connected
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SCR's 31 and 32.
The inverter circuit 30 includes a high voltage
transformer 38 and a corona generator generally 40. The
corona generator 40 is schematically shown and includes an
upper electrode 41 and a lower electrode 42. The upper
electrode 41 has a dielectric layer 43 and the lower electrode
42 has a dielectric layer 44. Between the electrodes 41 and
42, a corona gap 45 is defined.
The corona generator generally 40 is connected
to the high voltage side of transformer 38 by means of
conductors 46 and 47.
Figures 2, 3 and 4 of the drawing disclose in
greater detail a variety of typical configurations of corona
generators which may be utilized in the circuit generally
shown in Figure 1. The corona generator 40, as shown in
Figure 1, may comprise the structure shown in Figure 2 wherein
a reaction chamber 50 is equipped with a gas conduit inlet
51 and a gas conduit outlet 52. Within the container 50, an
upper electrode 53 is mounted along with lower electrode 54.
Electrodes 53 and 54 are provided with a dielectric layer 55
and 56 respectively. The electrodes 53 and 54 are connected
with conductors 57 and 58 which in turn are connected with a
source of high voltage, high frequency potential such as is
produced in the circuit shown in Figure 1. Between the
electrodes 53 and 54 a corona gap 59 is defined.
In Figure 3 an alternative corona generator
structure is shown wherein electrodes 60 and 61 are separated
by a single dielectric plate 62. The corona generator of
Figure 3 possesses two corona gaps, 63 and 64.
Figure 4 shows still another suitable corona
generator configuration for use in the present invention
wherein electrode plates 70 and 71 are separated by means of
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a single dielect~ic plate 72 which is affixed to the uppermost
electrode plate 70. The corona ~enerator configuration of
Figure 4 possesses a single corona gap 73. While Figures 2,
3 and 4 show typical plate type corona generators, it is to
be understood that other well known corona generator
configurations such as the well known tube type generator
may be used.
In operation of the circuit shown in Figure 1,
it is seen that the rectifier/capacitor combination 10
produces a steady DC potential at typically 311 volts
( ~ x 220 volts). This potential from the rectifier circuit
10 is supplied to the inverter circuit 30 through one side of
the high voltage transformer 38, which is connected in series
to the power supply circuit 10 through the series connected
SCR's 31 and 32. The pulse generator circuit 20 is adjusted to
produce the desired frequency of triggering pulses which range
from about l/3 to 20,000 cycles per second. The trigger
pulses from the pulse circuit 20 are fed through pulse
_ _
transformer 37, the output of which appears in the two
secondary windings thereof at the gates 35 and 36 of the SCR's
31 and 32. As is generally known in the art, the trigger
pulses from the pulse circuit 20 will cause the SCR's to
conduct (fire). Upon firing a current is passed through one
side (low voltage winding) of the high voltage transformer 38.
At the high voltage winding of the transformer 38 a high
voltage power pulse appears which will have a voltage from
about 2.0 to 20.0 KV. The high voltage pulse from the
transformer 38 then appears between plates 41 and 42 of the
corona generator 40. This high voltage pulse creates a
corona within the corona gap 45. A gas which is contained
within the gap 45 is subjected to the high voltage corona, and
in the case wherein the gas is or includes oxygen, ozone is
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produced.
Once the SCR's 31 and 32 are in a conducting
mode, it is seen that they must be switched to a non~conducting
mode in order to produce the high frequency pulses of current
through the high voltage transformer 38. In the operation of
the present circuit the SCR's connected in series 31 and 32 are
switched to a non-conducting mode as follows:
When the above described power pulse is applied
to the corona cell 40, the following sequence occurs:
The high voltage burst produces an electrical
discharge at the instant the voltage exceeds the gas sparking
potential in gap 45. The electrons produced are attracted
towards the positive electrode. This electron flow
constitutes the current flow giving rise to the corona power
dissipation at that particular voltage. The electrons cannot
pass the dielectric barrier and hence accumulate on the
dielectric as in a capacitor. Hence, the current flow ceases.
Further corona action stops until another power pulse is
applied. The abrupt current stoppage induces a reverse voltage
pulse in the secondary of transformer 38 by Lenz's Law. This
reverse voltage pulse is transformed to the primary and hence
supplies the reverse voltage needed to turn-off (commutate)
the SCR's 31 and 32.
The benefit of the two series SCR's shown in
Figure 1 will now be explained. First, the present application
taxes the capabilities of any SCR when large average powers
are called for. Large here means about 40,000 watts average
corresponding to 300 pounds per day of ozone. The limiting
SCR parameters are turn-off time and frequency at the peak
currents required. Figure 5 below shows the typical SCR
voltage history for a circuit in which a single SCR is used.
The first firing is at Tl, the SCR voltage drops (switches)
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from Vmax to a ~uch lower value Vmin~ The SCR conducts from
Tl to T2 ttypically 100 microseconds). At T2, the reverse
(commutating) pulse appears~ At T3 turn~off occurs, at T4 the
reverse pulse net effect is zero and the SCR voltage climbs
back to Vmax ready for the next firing. The best available
SCR's require at least 10-15 microseconds turn-off at the high
power levels of interest. That is T2 to T3 must be at least
10-15 microseconds. If, for examplel the voltage goes positive
(at T4) less than 10-15 microseconds after T2, then the SCR
will not turn-off, and will immediately either destroy itself
or blow any protective fuses present.
Figure 6 shows the benefit of reducing the SCR
voltage by using the series connected SCR's of the present
invention. In the circuit of Figure 1, the duration of the
actual reverse voltage is increased by the increment shown as
T4-T4' in Figure 6. Hence, if two SCR's are in electrical
series, and if they are perfectly matched, the turn-off
situation is improved by may~e 30-50%. If the SCR's are
unbalanced, that is if the two (or more) SCR's are unequal
2Q with respect to forward voltage drop characteristics, one SCR
may be improved by as much as 100-200% while the other is
improved only slightly. However~ both SCR's must "latch-up"
before damage occurs since they are in series. Hence, the
one SCR may latch-up frequently but the other never. This
un-balance is in opposition to the normal use of series SCR's.
Normally, SCR's, as any rectifier, are limited in reverse
voltage capability, say 1000 volts. If 3000 volt service is
required, then three series units will do the job if they are
perfectly matched. If not, the weakest one (say 900 volt) will
break down, the rest will follow like a line of dominoes.
In essence, the above preferred unbalanced
series SCR arrangement allows faster (shorter turn-off time)
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operation than with a single SCR. It is obvious that further
improvement can be achieved with three or more SCR's in series~
The unbalanced forward voltage charge
characteristic of the multiple SCR's may be an inherent
electrical characteristic of the SCR's which is produced by
manufacturing techniques. The unbalance may also be produced by
placing unbalanced shunt connected resistance elements with the
SCR's. Such an arrangement is shown in Figure 7 which shows
the series connected SCR's 31 and 32 of Figure l to which have
been added unequalizing network shunt resistors 80 and 810 The
resistors 80 and 81 preferably have a difference in value of
at least about +lO~. Therefore if resistor 80 has a value of
5 ohms, resistor 81 will have a value of about 4.5 or 5.5 ohms.
In Figure 8 a more preferred unequalizing
network is shown which includes capacitors 87 and 88 as well
as the unequal resistors 80 and 81 of Figure 7. Capacitors
87 and 88 serve to protect the SCR's 31 and 32 from unwanted
high voltage transits, and accordingly serve the well known
"snubber" function. The value of the capacitors 85 and 86 is
on the order of 0.2 microfarad +10~.
The circuit set forth in Figure 1 is constructed
of conventional components which are readily available from
commercial sources. In one preferred embodiment of the
present invention the circuit component shown in Figure 1
may fall within the definitions set forth in the tables below.
TABLE I Rectifier Circuit 10
.
Reference # Component Rated Value
ll Rectifier diode lO00 volt-lO00 amp
12 Capacitor 10-300 microfarad
13 Power Supply 220 volts AC
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TABLE II Pulse Generator Circuit 20
Reference # Compone - ~-R-~ted Value
. _ _
21 Transformer 1 amp 25 volt
22 Unijunction transistor U2T
23 Power supply 110 volt AC
24 Diode 1 amp - 100 volt
Capacitor 2000 microfarad -
50 volt
26 Capacitor 0.2 microfarad -
50 volt
27 Variable Resistor 10,000 ohms - 1 watt
28 Resistor 1/4 watt
29 Resistor 1/4 watt
Resistor 1/4 watt
TABLE III Inverter Circuit _
Reference # Component Rated Value
31 & 32 SCR General Electric
Corporation type
394-1000 amps peak at
2000 Hart 3, 600 volts
10 to 15m sec. turn-
off. Or GE type 609
3000 amps peak at
2000 Hart 3, 1200
volts 35 m. sec.
turn-off
37 Pulse Transformer Pulse Engineering
Type 5258
38 Transformer Ratio of windings
9 to 1
As shown in Figure 1, the present circuit
includes a corona generator which is generally shown to be of
the opposed plate type. Various construction of corona
generators are described in my previously filed application,
Serial No. 830,248 now U.S. Patent No. 3,798,457, filed June 4,
1969. As shown in this patent the corona generators contain
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electrode plates which are preferably coated with porcelain
enamel dielectrics~ having a thickness on the order of from
about 0.10 to 0.5 millimeters. The corona gap defined by the
opposed plates is preferably on the order of 0.75 to 2.0
millimeters. These corona generators preferably operate at
high voltages on the order of 2.0 to 30.0 KV at a frequency
ranging from about 1/3 to 20,000 Hz.
In one preferred operation of a corona generator,
oxygen or an oxygen containing gas such as air is converted
to ozone. It is found that the capacity of a given corona
generator to produce ozone from oxygen is to some extent
dependent upon the frequency of the power impressed across the
electrode plates thereof. In the practice of the present
invention the frequency which is produced by the power supply
set forth in Figure 1 may range from about 2000 to 3000 Hz.
Compared to prior art power supplies which normally produce a
frequency on the order of 50 to 60 Hz it is seen the power
supply of the present invention is capable of producing
a superior result in terms of enhancing the capacity and
efficiency of a corona generator.
To specifically illustrate the operation of the
device, the following example is given.
EXAMPLE
The power supply circuit generally shown in
Figure 1 is connected to the leads of a corona generator of
the type shown in Figure 2 and operated at a frequency of
2000 Hz. The area of each electrode plate is 322 cm2, the
electrode gap is 1.1 mm and the thickness of a porcelain
dielectric coating on the plates is on the order of 0.2 mm.
The corona generator was then ~onnected to a conventional
source of 60 Hz pulsed potential. It was found that in the
first instance, using the 2000 Hz frequency, the device was
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capable of producing 4500 grams per hour o~ ozone using an
oxygen feed of 400,000 grams per hour. Using the 60 Hz power
supply, it was found that the generator was capable of
producing 7000 grams of ozone per hour using the same feed.
While the above description and specific
example disclose the use of the present high voltage, high
frequency power supply in conjunction with a corona generator,
the present circuit may be used to provide power for devices
which have similar load characteristics. Thus, in Figure 1
the corona generator 40 may be replaced by a plasma generating
or laser powering device.
