US3365612A

US3365612A - Dual source spark machining power supply

Info

Publication number: US3365612A
Application number: US479152A
Authority: US
Inventors: Schierholt Hans
Original assignee: Ingersoll Milling Machine Co
Current assignee: Ingersoll Milling Machine Co
Priority date: 1965-08-12
Filing date: 1965-08-12
Publication date: 1968-01-23
Anticipated expiration: 1985-01-23

Description

Jan. 23, 1968 H. SCHIERHOLT v 3,365,612

DUAL SOURCE SPARK MACHINING POWER SUPPLY Filed Aug. l2, 1965 Carre/zi 50e/rde INVENTR.

United States Patent 3,365,612 DUAL SOURCE SPARK MACHINTNG PWER SUPPLY Hans Schierholt, llseriohn, Germany, assigner to lngersol Milling Machine Company, Rockford, Ill., a corporation of Iliinois Filed Aug. 12, 1965, Ser. No. 479,152 4 Claims. (C1. 315-119) ABSTRACT 0F THE DHSCLQSURE A spark machining power supply in which there is provided a high voltage, low current source connected through a ring switch to ionize a gap defined between an electrode and a conductive workpiece, and a low voltage, high current source connected through a power switch to provide high current electrical discharges through the ionized gap. A blocking diode is connected to prevent current flow through the power switch when the voltage across the gap is higher than the voltage of the low voltage, high current source, and a protective relay is connected to open the power switch when the current How through the power switch rises above a predetermined level.

This invention relates to arc discharge or spark erosion machines.

In general, the invention pertains to improvements in the nature and eiiiciency of the power supply systems for arc discharge machines.

The nature of the invention may be understood from the following detailed description, when read with reference to the accompanying drawings in which:

FIG. l is a schematic representation of a certain priorart arrangement;

FIG. 2 is a schematic representation of an improvement on the circuit of FiG. 1; and

lFIG. 3 is a schematic representation of the presently preferred arrangement.

Controlled generators which do not operate by capacitor discharge have become increasingly popular for the operation of spark erosion machines because with the aid of such generators the characteristics of the discharge, such as its duration, current intensity and spark frequency, are adjustably controllable within Very wide limits. However, this advantage is partly offset by their interior eiiciency.

This may be explained by reference to FIG. 1. In principle the generator comprises a source 1 of current delivering a voltage applied through controllable switch means 2 (in practise this may be a transistor) and a resistor R3 to the working gap 4. A discharge across the gap 4 takes place for as long as switch 2 is closed. The voltage drop across the discharge gap is about 20 volts. The voltage supplied lby source 1 must, however, be at least 60 volts before the dielectric in the working gap breaks down and a spark is initiated. In many instances an even higher voltage is desirable. The eiciency of the arrangement cannot therefore exceed 33%. This is undesirable not only from the point of view of power consumption by the generator but also because of the heat loss in resistor R3, the semi-conductor elements in the generator being very sensitive to heat. Efciency can be improved by providing an arrangement which has been known from the early days of spark erosion (British Patent 637,872, FIG. 12). The voltage of source 1 is reduced and the discharge initiated by a separate source 5 providing a higher voltage connected through a tiring switch 6 and a resistor 7 to the working gap 4. The current from the tiring source 5 can be small, so that its ICS power capacity can likewise be small, the major proportion of the discharging current being supplied by source 1 as soon as the spark gap has been ionized after the tiring switch has been closed. In this arrangement the voltage of source y1 need only slightly exceed the discharge voltage across the gap and efficiency is therefore considerably improved.

However, in practise these arrangements give rise to troubles which greatly impair their value.

In the first place, when transistors are used as the switch means, it often is impossible brieily to raise the voltage across the working gap by the tiring source 5 to a considerably higher potential than the voltage of ysource .1 because contrary to au ideal switch most transistors pass some current when a voltage is applied thereto in a direction opposite to that of normal operation. This happens during the tiring process. The potential of the p-ole of switch 2 connected to resistor R3 must then exceed the voltage of source 1, whereas it is normally less than that of source 1. Consequently, when firing, switch 2 functions as if it were closed and the voltage across the spark gap cannot rise to the level required for spark initiation.

The same difiiculty also occurs when more ideal types of switch means are used because for the purposes of simplicity in switch control both switches 2 and 6 are normally operated simultaneously.

The present invention aims at overcoming this difticulty. In the circuit of FIG. 2, a diode 8 is connected in series with switch 2, the diode polarity being such that it will carry a current when the voltage at the working gap is less than that of source 1, whereas it will block when the voltage at the spark gap exceeds the voltage of source 1. This diode functions as if switch 2 were not closed until after ring has taken place. Consequently the voltage build-up at the spark gap to a level substantially exceeding t'ne voltage of source 1, as required for spark initiation, will not be impeded.

Another diiiiculty arises in the operation of the generator. rhe spark gap may not be an optimum, and it may in fact be short circuited. Since `for achieving a satisfactory efficiency the voltage across source 1 should only slightly exceed the discharge voltage, say 30 volts, in the even't olf a short circuit the current carried by switch 2 rises to about three times its normal operational value. This means either that the expensive switch Z (e.g., a transistor) is destroyed in the event of a short circuit or that its size must be such that in normal operation it will carry only about one-third of its rated current.

In order to eliminate this disadvantage it is proposed to connect a protective relay 9 (which may be electronic) in series with switch 2, that relay measuring the current through switch 2. and opening switch 2 and/or cutting ofi source 1 without delay when the measured current exceeds a predetermined level.

The presence of this protective relay makes it unnecessary to design the expensive switch 2 to carry the high short circuit current but only to carry the normal operational current. In the conditions which often apply in normal practise, this means that the same switches are rendered capable of handling three times the hitherto possible operating current.

A special embodiment of the invention will be described by reerence to F'IG. 3. This arrangement comprises a generator with a power switch 2 in the form of a transistor which is controlled by a pulse generator 10. Otherwise this embodiment corresponds to that in FIG. 2. However, a special circuit is provided for the protective relay 9. This comprises a resistor 11. The potential drop across this resistor due to the current flowing through switching transistor 2 is a measure of the magnitude of the current. Resistor h1 is connected to a transistor 1 2. The voltage applied to the emitter of this transistor is adjustable by a resistor 13. The collector orf transistor 12 is connected through a relay 14 to the source of supplementary voltage 15. Relay 14 is a highspeed relay. When the voltage across resistor 11, which measures the current owing through transistor 2, eX- ceeds the voltage applied to the emitter of transistor 12, current can llow through transistor 12 and relay 14 will respond, cutting ott the voltage supply 1S to the pulse generator 10, or otherwise disabling supply 10. Transistor 2 is therefore now uncontrolled and has the effect of an open switch.

Relay 14 might be replaced by a resistor, the voltage drop across the resistor, when the predetermined current is exceeded, being used to block the pulse generator 10.

What I claim is:

1. A power supply for spark machining by providing high current electrical discharges through an ionizable work gap defined between an electrode and a conductive workpiece, the power supply comprising the combination of a high voltage, low current source; a firing switch for intermittently connecting the high voltage, low current source across the electrode and workpiece to ionize the gap; a low voltage, high current source; a power switch for intermittently connecting the low voltage, high current source across the electrode and workpiece to provide the high current electrical discharges through the ionized gap; a blocking means connected in series with the power switch for preventing current flow therethrough when the voltage across the gap is higher than the voltage of the low voltage, high current source; and protective means coupled t0 the power switch and responsive to the current ow therethrough to cut-oil said current tlow [when the magnitude thereof rises above a predetermined level.

2. The power supply of claim 1 wherein the protective means includes a relay connected in series with the power switch and coupled to open the power switch when the current flow through the relay rises above said predetermined level.

3, The power supply of claim 1 wherein the power switch includes a transistor and a pulse generator, said transistor having a collector-emitter circuit connected `across the low voltage, high current supply through the gap and a base-emitter circuit, said pulse generator being connected to apply pulses to the base-emitter circuit to intermittently render the collector-emitter circuit conductive; and wherein the protective means includes an electronic device connected to the collector-emitter circuit to be rendered conductive when the current low through the collector-emitter circuit rises above said predetermined level, and cut-oft means coupled to the electronic device and to the pulse generator for terminating the application ot pulses to the base-emitter circuit in response to conduction of the electronic device.

e. The power supply of claim 3 wherein the electronic device is a second transistor and the cut-off means is a relay coupled to disable the pulse generator in response to conduction of the second transistor.

References Cited UNITED STATES PATENTS 3,052,817 9/1962 Branker 315-171 3,158,728 11/1964 Webb 219-69 3,202,599 8/1965 Schierholt 317-33 X 3,257,583 6/1966` Pfau 315-227 3,259,795 7/1966 Schierholt 315-227 3,280,367 10/1966 Poerschke 315-227 3,289,040' '1l/1966 Pfau 315-227 JOHN W. HUCKERT, Primary Examiner.

R. Fi POLISSACK, Assistant Examiner.

Disclaimer and Dedication 3,365,612.Han8 Scherholt, Iserlohn, Germany. DUAL SOURCE SPARK MACHINING SUPPLY. Patent, dated Jan. 23, 1968. Disdimmer and dedlcatlon filed Apr. 2l, 1975, by the assignee, Amsted I mdustwies Incorporated. Hereby disclaims and dedeates to the Public the remaining term of said patent.

[Ocwl Gazette August 5,1975]