Note: Descriptions are shown in the official language in which they were submitted.
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;~ BATTERY CHARGER
Background of the Invention
Field of the Invention
This invention lies in the field of battery chargers
and, more particularly, chargers utilizing transformers with
ferroresonant circuits.
Description of the Prior Art
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In the present industrial market, there is a very
; strong demand and need for battery charging equipment which
; is relatively simple and inexpensive, but reliable and capable ,
of charging battery loads with a desired battery characteristic.
The basic approach to providing such a battery charging circuit
is to design a transformer and output rectifying circuit which
transforms three phase power and provides it in rectified form
across the battery terminals, the battery itself acting as an
effective filter. Available equipment does, and has done, this
job for a number of years, but there remains a need for improving
the charging characteristics of such chargers, and also improving
their reliability, while simultaneously reducing construction
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Summary of the Invention
It is an object of this invention to provide a battery
charger of simplified construcl:ion compared to the prior art,
which is relatively inexpensive, and reliable, and which
provides a charging characteristic which is stable within
predetermined limits for the entire range of battery loads for
which it is used.
. In accordance with the above objective there is
provided a battery charging circuit for charging a battery
having an effective load capacitance, said circuit having a
charging characteristic within predetermined limits, said
characteristic being a functional dependence of charging
voltage on amperes per predetermined unit of ampere hours,
and said characteristic being substantially independent of
the size of said battery, which charging circuit further
; comprises: (a) a pair of transformers connected in Scott-T
configuration; (b) an input connecting means for connecting
said transformer to a three phase line; (c) an output
connecting means for connection to a first terminal of said
2~ battery comprising a separate secondary output winding and
rectifier circuit, for each of ,said ~ransformers; (d) a
ferroresonant output circuit means associated with each of
said transformers for normally maintalning said transformers
in saturation, and for cooperating with said output connecting
means to provide said charging characteristic, said means
.1 comprising a further transformer secondary winding with only
a capacitance connected thereacross, said capacitance
. predetermined in combination with said effective load
~ capacitance to maintain said charging characteristic within
;~ 30 said predetermined limits.
The invention also consists of a method of providing
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a batter~ charglng circuit for charging a battery loadhaving an effective capacitance, said charging circuit being
characterized by having a charging curve within predetermined
limits, comprising: (a) connecting a pair of transformers
in a Scott-T configuxation, each of said transformers having
a primary input winding, and a pair of secondary output
windings; (b) connecting said input windings to terminals
adapted for connection to a three phase power line; (c)
connecting a first one of said secondary output windings from
each of said transformers to a rectifier circuit; (d)
connecting said rectifier circuit to said battery load
terminals, said battery load having said effective capacitance;
(e) connecting across a further of said secondary output
windings on each of said transformers only a capacitance,
which cooperates with said effective capacitance so as to
provide a charging curve within said predetermined limits; and
(f) removing said load from said battery load termina~s.
Brief Description of the Drawings
Fig. 1 is a circuit diagram showing a charging
circuit according to an embodiment of this invention~
Fig. 2 is a graph showing typical limits within
which the charging characteristic for the battery charger is
found.
Description of the Preferred Embodiment
Referring to Fig. 1, there is shown a circuit diagram
of a battery charger. Lines 21, 22 and 23 are connected
between a three phase power line comprised of Ll, L2, and L3,
and the primary windings of a C,cott-T connected transformer 24.
As seen in Fig. 1, the transformer has two sections, 25 and 26,
comprising two input windings, with the end of one input
winding connected, at point 26~, to the center of the other
input winding. Transformer 24 has four secondary windings, 25S
and 25R, and 26S and 26R. Windings 25S and 26S are
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secondary output windings, which are center tapped at 25C
and 26C respectively, the two center taps being connected
together. Diodes 40, 41, 42 and 43 are connected to respective
ends of windings 25S and 26S, in a four phase star configuration.
The cathodes of the four diodes are coupled together and connected
to output terminal 46. The common connection of the center taps
is connected to output terminal 47. The battery load 50 is
placed between terminals 4G and 47 for charging.
Output windings 25R and 26R are physically positioned
with respect to windings 25S and 26S respectively, so that the
currents through 25R and 26R cause some saturation in trans-
formers 25 and 26 respectively, so as to provide a degree of
regulation to line voltage variation. Across winding 25R is
placed a capacitance 28, the winding 25R and capacitance 28
forming a resonant circuit which carries a capacitive load, i.e.
capacitive volt amperes. Likewise, winding 26R and capacitor 29
form a path for capacitive volt amperes. These arrangements
- constitute a ferroresonant transformer circuit which provides
the desired taper characteristic or charging characteristic for
recharging the battery load 50. The capacitors 28 and 29 may
either be integrated within the winding of the ferroresonant
~ transformer, or mechanically integrated as part of the transformer.
; The desired magnetic conduction path through the transformer may
; be obtained either by conventional interleaving construction, or
by the use of butt stacking.
Referring now to Fig. 2, there are shown curves repre-
senting typical limits of the charging characteristic of the
battery charger of this invention. The curves shown are per
; unit curves, the Y axis representing charging volts per cell,
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and the X axis representing charging amperes per 100 ~mp hour
capacity of the battery being charged. By way of example, if a
500 amp hour (AH) battery is charged by a battery charger in
accordance with Fig. 2, when the volts per cell was at 2.3,
the charging current would be between 70 amps and 75 amps. It
is noted that for this typical charging characteristic the start
rate is limited to about 20.4 amps per 100 AH, whereas when the
volts per cell has reached about 2.52 volts, the charging current
is reduced to between 4 and 5 amps per 100 AH. The saturation
effect contributed by the ferroresonant circuit is a prime
determinant of the current limiling portion of the charging
characteristic.
The efficiency and reliability of the charger of this
invention is to be appreciated in comparison with a conventional
three phase transformer type charging circuit. In this circuit
of this invention, only two transformer sections are required,
instead of three, each transformer handling about 1.5 times the
-~ KVA of the normal three phase transformer section. This design,
in allowing two transformers instead of three, makes possible
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;, 20 a savings in the amount of labor, as well as savings in the
number of diodes, since only four diodes are necessary instead
of six. The arrangement also contributes to increased reliability,
because of the reduced number of components, both transformer
¦ components and diodes.
In addition to the above, a substantial improvement
is appreciated in the size of the external capacitors 28 and 29
required for the ferroresonant circuit. In the Scott T four
phase star configuration of this invention, the effective capaci
tance of the battery load 50 is reflected into the transformer
circuit, thereby reducing the amount of external capacitance
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which needs to be connected to the circuit in order to obtain
the deslred operating conditions. It is well known that batteries,
from a circuit viewpoint, constitute large capacitive loads.
However, in a conventional three phase diode brid~e o~tput
configuration, the capacitive load is not sensed at the secondary
windings as it is with the four phase star configuration of the
invention.
To illustrate the difference, a test comparison was
made, using a conventiQnal three phase configuration and the
Scott T four phase star configuration of this invention. In
both configurations, the charging circuits were designed to give
substantially the same chaxging curve for a six cell 540 A~ lead
acid battery. Also, in both con~igurations, the voltage across
the output winding of each ferroresonant circuit was about 400
volts. It was found that with the conventional three phase
configuration, each capacitor in each ferroresonant circuit was
re~uired to be within the range of 12-16 microfarads. By
contrast, with the four phase star con~iguration, each of the
two external capacitors 2g and 29 was required to be only 7.5
` 20 microfarads. Thus, it is seen that in the circuit of this
invention a sukstantial improvement is achieved and smaller
capacitors can and must be utilized, resulting in a decrease in
cost and an increase in reliability. Further, since the capacitors
are smaller, the capacitor volt amperes carried in the windings
are smaller, such that the required wire size can be smaller.
This is another source of cost reduction derived from the arrange-
ment of this invention.
In practice, the charger of this invention is designed
by connecting the two transformer sections in a Scott T configura-
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tion, and then selecting the capacitors for the ferroresonant
circuit so that the resulting charging characteristic is within
the design limits. If the selected capacitor is too large,
corresponding to a given winding voltage in a ferroresonant
5 circuit, the charging current is too high throughout the charging
range. Conversely, if the capacitance of the selected capacitor
is too low, the charging current is too little, such that regula-
tion becomes a problem and the charging circuit requires too
. great a time to achieve the desired charge on the battery load.
The selection of the fer.roresonant capacitors is made with an
effective load 50 across terminals 46 and 47, so that capacitors ~.
28 and 29 are chosen to cooperate with the effective capacitance
I o:E the load.
It is to be noted that, while the circuit diagrammed
-I 15 in Fig. 1 shows simply a single arrangement for connection to .
a power line, a dual voltage arrangement for connection to
either a high voltage or low voltage line is also within the :
:~ ~ scope of this invention~
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