Electric heating – Metal heating – By arc
Reexamination Certificate
2001-03-28
2003-03-11
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C322S063000
Reexamination Certificate
active
06531685
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the art of engine driven welding power supply systems. More specifically, it relates to engine-driven welding power supply systems with a welding power output and an auxiliary power output.
BACKGROUND OF THE INVENTION
Engine driven welding power supply systems may be driven either by a DC generator or an AC generator (also called an alternator-rectifier). An AC generator generally includes, in addition to an alternator, a reactor followed by rectifiers to provide a DC output. One prior art engine-driven welding power supply system is the Miller Bobcat 225® welding power supply. (Engine-driven welding power supply system, as used herein, includes one or more of the engine, the generator, and the power supply. Welding power supply, as used herein, includes power supplies that provide welding, plasma or heating power, and may include a controller, switches, etc.).
The Miller Bobcat 225® welding power supply includes a generator having a single primary mover, with a single exciter, stator and rotor to provide a welding power output and an auxiliary power output. The auxiliary power is 60 Hz power, 120/240 volts used to operate lights, tools, and other 60 Hz. loads. The auxiliary power is ideally a sinusoidal, constant voltage source (like utility power).
The welding output is derived from a single phase welding output winding, and the auxiliary power is derived from a single phase auxiliary output winding that are part of the generator stator. (Output winding, as used herein, includes a winding connected to be able to provide power to a load.) These windings are electrically isolated from each other, but are in magnetic communication (magnetic communication, as used herein, includes windings wherein a single revolving magnetic field is provided to both windings, and/or the windings are wound about a common stator). The magnetic field is created by passing a dc field current through the winding on the rotor. The dc field current is derived from a single excitation winding in the generator stator—the welding power and the auxiliary power share an excitation winding. (Excitation or exciter winding, as used herein, includes a winding connected to provide current to a field winding).
When there is a current output the load current flows in the stator windings and creates a magnetic field called the armature reaction field. The armature reaction field increases with load current. The combination of the magnetic field from the field winding and the armature reaction field is the net magnetic field that produces welding and auxiliary output power.
The armature reaction field opposes the field produced by the field windings on the rotor and reduces the net magnetic field in the generator, which reduces the output voltage of the generator. The reduction in net magnetic field and output voltage increases as the load current increases, because the armature reaction field increases with load current. Such a voltage reduction is particularly undesirable for auxiliary power, which is ideally a constant voltage source (to mimic utility power).
Prior art engine driven welding power supply systems attempt to compensate for the armature reaction field by increasing the field current. However, the resistance of the field winding (rotor coil) requires that, to increase field current, the voltage applied to the field windings must be increased. The field current is increased by increasing the voltage supplied to the field windings. The increased field voltage is provided by increasing the voltage supplied by the excitation winding on the stator.
One prior art technique to increase the field current is described in U.S. Pat. No. 5,734,147, issued Mar. 31, 1998, entitled Method And Apparatus For Electronically Controlling The Output Of A Generator Driven Welding Power Supply, Bunker et al., incorporated herein by reference. Electronic control and feedback is used to adjust the field current to the desired magnitude. This is an effective way to control the field current, but it requires a relatively sophisticated and costly electronic control scheme.
Another prior art engine driven welding power supply systems, the Miller Bobcat 225®, uses a simple system with no electronics or printed circuit boards. The excitation winding in the stator is connected to a diode bridge to rectify the current to dc, a capacitor for smoothing, and a variable resistor, for controlling the magnitude of the field current. A variable resistor may be included to compensate for temperature drift. Generally, the system provides for an increased exciter voltage (which increases field current) by using the influence of the load current flowing in other windings on the stator. Specifically, single phase load currents cause a harmonic interaction in other windings in the stator.
Prior to explaining how these components compensate for the armature reaction field, a brief discussion of the harmonic interaction is useful. A single phase load current flowing in a stator winding causes a pulsating magnetic field. The pulsating magnetic field can be resolved into two components, one that rotates in the forward direction, with the rotor, and one that rotates in the opposite direction. Stated another way, when the load is unbalanced the magnetic field wave created by the stator currents does not move at the speed as the rotor and may be resolved into two components: a forward component that is in the same direction and at the same speed as the rotor, and a backward component. The forward component behaves as a balanced three phase load. The backward component moves at the same speed as the rotor, but in the opposite direction. Thus, it has a motion relative to the rotor of twice the generator speed. This “moving” magnetic field induces voltage in the excitation winding, which causes a higher output voltage. This phenomena is described in Engine Driven Invertor With Feedback Control, Beeson et al, issued Oct. 19, 1999 as U.S. Pat. No. 5,968,385, which is incorporated herein by reference.
The “backward” component of the magnetic field induces ac current at twice the fundamental frequency in the rotor winding (because the relative speed of the backward component is twice the rotor speed). The second harmonic component of field current in the rotor causes harmonic voltages to be induced in the stator windings. The primary harmonic in the stator windings is the third harmonic (the second harmonic of the field current plus the speed of the rotor).
The relative phasing of the third harmonic and the fundamental influence the shape of the resulting voltage waveform, such as being flat-topped, or reduced shoulders with an increased peak voltage. A high peak voltage provides maximum boosting under load. The Miller Bobcat 225® engine driven welding power supply system captures the high peak voltage with the capacitor connected to the excitation windings, smooths the voltage and applies it to the field winding, which in turn drives more field current, and boosts the output.
The desired relative phasing between the fundamental and the harmonics is effected by the placement of the excitation windings and the output windings—i.e. in which slots the windings are placed. Because there are separate welding output and auxiliary output windings, the relative placement of the excitation and load windings, and the relative phasing of the harmonics, will be different for the welding output and the load output. Thus, the placement of the single exciter winding must be based on desirable welding output, a desirable, auxiliary output, or a compromise therebetween.
The Miller Bobcat 225® engine driven welding power supply system has the exciter winding placed to provide a greater output boost for the welding output windings (to provide a desirable welding output). Unfortunately, providing the additional power for welding results in little output boost for an auxiliary load.
This provides a desirable welding output, but at the expense of auxiliary power. Specifically, the generator folds back as the auxili
Corrigan George R.
Illinois Tool Works Inc.
Shaw Clifford C.
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