Method and device for preventing excessive currents in a welding

Details Method and device for preventing excessive currents in a welding Method and device for preventing excessive currents in a welding

1994-04-07

1996-10-29

Gaffin, Jeffrey A.

Electricity: electrical systems and devices

Safety and protection of systems and devices

Voltage regulator protective circuits

361 44, 361 45, 361 87, 363 80, 21913032, H02H 326

Patent

active

055702546

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The invention is based on a method for preventing excessive currents in a welding converter that generates a two-phase, pulse-width-modulated a.c. current from a rectified mains a.c. current using power semiconductors, which generated current is supplied to the primary side of a welding transformer, as is known in a commercially-available welding machine in which monitoring is provided to prevent excessive current. Also, in this arrangement the magnetization of the welding transformer is adjusted by the control system. Current monitoring is effected from the secondary side of the welding transformer. Because of this, errors in the primary circuit may possibly be falsified or identified too late, which can result in damage to the power semiconductors.
It is an object of the invention to disclose a method and an associated arrangement that permits better protection of the power semiconductor against excessive currents. It is further an object of the invention to disclose an arrangement that permits simplification of the location of the cause of the error when overcurrent occurs in the primary circuit.


SUMMARY OF THE INVENTION

The above object is attained in accordance with the invention by a method for preventing excessive currents in a welding converter that generates a two-phase, pulse-width-modulated a.c. current from a rectified mains a.c. current by means of power semiconductors, which generated current is supplied to the primary side of a welding transformer, comprising the following method steps: pulse-width-modulated a.c. current (I.sub.p) supplied to the primary side of the welding transformer using respective sensors; primary current (I.sub.p) that were measured by the sensors with one another; predetermined peak value, emitting a signal that leads to the deactivation of the power semiconductors; measuring signals, by differentiation according to the time; value; and if the time-derived measuring signal exceeds the predetermined maximum value, emitting a signal that leads to the deactivation of the power semiconductors. responds to an overcurrent within such a short time period that protection of the power semiconductor against destruction is assured. This very rapid response time assures protection of the power semiconductor even in the case of a short-circuit in the primary circuit. The technological realization of the arrangement for executing the proposed monitoring method is effected by means of standard components and standard switching circuits, and can easily be executed by a person skilled in the art. Portions of the individual monitoring functions can advantageously be realized with the aid of software solutions.
In an advantageous manner, the two phases of the primary current are monitored individually. Through continuous comparison of the current behavior in the two phases, effective protection of the power semiconductor results in the case of a short to ground, as does a possibility of finding the cause of the error simply.
In another advantageous feature, the arrangement of the invention permits the prevention of saturation magnetization of the transformer. By means of this, uniform welding performance is assured for the entire duration of welding, which again is beneficial for the welding quality. Excessive primary currents are prevented, as is excessive heating of the transformer. The iron volume of the transformer can therefore be kept to a minimum.
An embodiment of the arrangement is illustrated in the drawing figure and described in detail below.


BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a fundamental representation of a welding converter constructed in accordance with the invention, FIG. 2 shows the effect of saturation identification in a current-time diagram, and FIG. 3 shows a structural representation of expanded saturation recognition.


DETAILED DESCRIPTION OF THE EMBODIMENT

FIG. 1 shows the fundamental circuit diagram of a medium-frequency resistance welding system as it is the basis of the invention. The main components of the sys

REFERENCES:
patent: 5120929 (1992-06-01), Kobayashi et al.
patent: 5196668 (1993-03-01), Kobayashi et al.
patent: 5229567 (1993-07-01), Kobayashi et al.

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