US2203467A - Arrangement for converting direct current into alternating current - Google Patents

Description

June 4, 1940. LElFER 2,203,467

ARRANGEMENT FOR CONVERTING DIRECT CURRENT INTO ALTERNATING CURRENT Filed April 19, 1938.

INVENTOR ALFRED LEIFER ATTORNEY Patented June 4, 1940 UNITED STATES PATENT OFFICE ARRANGEMENT FOR CONVERTING DIRECT CURRENT INTO ALTERNATING CURRENT Application April 19, In Germany Claims.

To avoid opening sparks when interrupting a direct current circuit it is already known to shunt a condenser or a series connection of a condenser and ohmic resistance to the switch or to the load. 5 When the condenser is placed in parallel to the load the voltage of the load can only decrease in a'slow fashion, so that during the moment of opening no high voltage can appear at the switch and therefore no appreciable spark. In order to obtain a possibly complete eiTect it would be advisable to choose a large capacity and on the other hand a possibly low resistance. But this condition entails the disadvantage that a strong closing spark appears since at the moment in which the switch is closed a charging current flows into the condenser which at a low resistance will be very large at the first moment. Therefore, a mean value must be chosen for the resistor in order to prevent too large a spark at closing as well as at opening of the switch.

The difficulties due to the appearance of sparks are especially considerable in vibrating direct current converters (choppers) in which the voltage of' a direct current source is chopped and the polarity periodically reversed and applied to the load. The weakening of the switch spark in such vibrating direct current converters is, when using the said kn'own arrangement, not appreciable especially if the load of the direct current converter fluctuates.

In accordance with the invention, in order to weaken the switching spark an electric relaxation oscillation generator is provided operating in synchronism with the switching and which applies to the load connected to the direct current source the electrical energy liberated at the moment in which the switch is opened, this energy being applied in such a manner that the current impulses and voltage impulses appearing at the opening of the switch will be compensated.

More especially, a series connection of an ohmic resistor and condenser is placed in parallelto the load on the one hand, and an auto-transformer on the other hand whose tap is connected through a glow tube to the point connecting the resistor with the condenser.

The advantage of the arrangement resides in that the balancing energy compensating for instance too rapid'a decrease of the voltage of the load at the moment of openingof the switch, can be supplied at suflicient speed in view of the relaxation performance while a storing of energy can take place at such slow speed that no appreciable switching spark can appear at closing of the switch.

1938, Serial No. 202,891

April 21, 1937 An example embodying the idea of the invention is shown in Figure l in connection with a vibrating direct current converter.

The voltage of the direct potential source E is applied across the double pole switch U and with alternating polarity to the line leading to the output terminals A, B between which the load, not shown in the drawing, is placed, said double pole switch being actuated for instance by means of a vibrating spring such that always one pair of opposite contacts will be closed and the other pair opened. There is placed between the terminals A and B, hence, in parallel to the load, on the one hand, a series connection comprising the resistor R and the condenser C and on the other hand, a choke coil L acting as auto-transformer and having a center tap. The point connecting the resistor and condenser to each other is connected through a glow tube G to the center tap of the coil. This tube acts as relaxation oscillator in conjunction withtlie' direct potential source E, resistor R and condenser C. The natural frequency of the relaxation circuit is chosen preferably slightly lower than the double frequency of the vibrating direct current converter. In the ,1 present arrangement, the relaxation oscillation is automatically synchronized by the interruption frequency as will be seen from the following:

' During the time of closing, i. e., as long as the voltage of the source E of, for instance, 100 volts is applied between A and B, the condenser C will be charged across the resistor R to a large percentage of this voltage thus, for example, to 95 volts, so that the ignition voltage of the glow tube G, which may be, for instance, equal to 50 volts, will almost be reached since the center tap of the transformer T has 50 volts. Now, if the switch is opened the voltage between A and B begins to collapse immediately due to the action of the self-inductance L and due to the load. But as soon as it has decreased in the present case to volts the ignition voltage of the glow tube is attained since the voltage of the center tap has fallen to 45 volts while the voltage of the condenser has remained at volts. The condenser now passes across the glow tube a. current into the lower partial winding of the coil L, said current rapidly' reaching a maximum value whereafter it decreases slowly. As a result thereof, the voltage at the center tap not only ceases next to drop still further, but is at least maintained at its value or since the operating voltage of the glow tube is somewhat lower than the ignition voltage, the voltage at the center tap even increases slightly again at the first moment. The current impulse in the lower partial winding of the auto-transformer L now produces, however, through induction an additional voltage also at the terminals A and B which compensates the voltage drop thereby quenching the opening spark. The induced voltage will become zero only when the current has reached its maximum value, and will reverse its direction when the current decreases. The voltage between the terminals A and B will in this way be gradually discharged or reversed. Accordingly, also the voltages at the condenser and at the center tap decrease gradually until the glow tube is extinguished. When the voltage is applied again in the opposite direction no appreciable current impulse can be produced even if the charge of the line A, B has not been reversed exactly to the proper voltage, because the condenser is disconnected from the circuit and its charging must take place across the resistor R.

Eventually, it may be advisable to place a suitably dimensioned resistor in series to the glow tube so as suitably to fix the magnitude of the current flowing at the moment of ignition of the glow tube and suit-able to establish its speed of change.

The tap at the coil need not be exactly in the I center of the winding, and may suitably be so set that at opening of the switch, the voltage-jump will be compensated as effectively as possible.

The choke coil may be the primary winding of a transformer serving for transmitting the alter-, nating voltage.

I claim:

1. A converter device of the mechanical type for converting direct current into alternating current, comprising a source of direct current, an output load, switching means adapted to alternately connect the positive and negative terminals of the source to the opposite terminals of the load, a series connection of a resistance and a condenser connected in shunt to the load, a glow tube connected between the common terminal of said condenser and resistance and an intermediate point on the load, the arrangement being such that with the opening of the switch means and the incident decrease of thevoltage applied to the load, the difference between the condenser voltage and a fixed percentage of the load voltage is so impressed upon the glow tube that the same will be ignited.

2. A converter device of the mechanical type for converting direct current into alternating current, comprising a source of direct current, a pair of output terminals for said device, switching means adapted to alternately connect the positive and negative terminals of the source to the opposite terminals of the load, an inductance connected between the output terminals, a series connection of a resistance and a condenser shunted across said inductance, and a glow tube connected between the mid-tap of the inductance and the common terminal of the resistance and condenser.

3. A converter device according to claim 2, wherein the tapped inductance serves as the primary winding of an output transformer.

4. In a converter device of the mechanical type for converting direct current into alternating current, a source of direct current, a load, means including a switching device arranged to be opened and closed for intermittently connecting said source to the load, said switching device being subject to arcing when opened, a condenser interposed between the source and the load for storing up energy from said source while said switching means is closed, a circuit including a gaseous discharge path, the switching device and said source, for discharging said energy subsequent to the opening of said switching means, said discharge being arranged to produce a pulse of current through said source and the switching means in opposite sense to the flow of current therethrough prior to said opening of the switching device to thereby reduce said arcing.

5. In a converter system for converting direct current into alternating current, said system incorporating a mechanical make and break switching means for intermittently connecting a source of dire-ct current to a load, said switching means being subject to arcing on break during operation thereof, the steps in a method of reducing the in-

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