Abstract: A system contained on a single isolation panel determines the hazard current of a dual voltage, ungrounded, polyphase system. The system uses a single isolation transformer having dual secondary windings for generating the dual voltages. A first line isolation monitor or LIM, is microcontroller based and continuously measures the fault impedance of one of the output voltages of the polyphase, ungrounded or isolated system. A second LIM continuously measures the fault impedance of the other output voltage of the system. The fault impedance is determined by the LIM by injecting a continuous sine wave measurement current into a ground terminal to generate a measurement voltage across the fault impedance. Using the measurement voltage and current, the LIM calculates the fault impedance and, using this impedance, calculates the hazard current based on the maximum line to ground voltage of the ungrounded system.

Abstract: The present invention converts an alternating current input into a direct current output by switching a current load at high speeds to create a voltage envelope having a maximum allowed voltage and a minimum allowed voltage. Complementary switching elements are employed to take advantage of both the positive and negative portions of an AC cycle. Load switching is arranged so that the current vector which represents the load is nearly coincident with the voltage vector which represents the power source. The system can operate in an open loop mode at a constant frequency, in a closed loop mode, or in a hysteresis mode. During open loop operation, the voltage envelope is not rigidly defined as the switching frequency remains constant. During hysteresis mode control, the voltage envelope remains fixed and the current switching frequency varies.

Abstract: A non-ferrous current sensor is used to continuously measure the charging current magnitude in step-graded foil and paper insulation systems used on high voltage measurement and control apparatus such as instrument transformers and on the condenser bushings of power transformers and circuit breakers. The low voltage measured signal is conditioned by an electronic circuit such that it modulates a DC signal in a system current control loop to provide continuous remote monitoring and alarm functions. Remote power is supplied to the sensor and conversion circuits through the current loop. Alarm level thresholds and alarm time delay are embodied in a control device which may be located in a remote location, away from the high voltage equipment. Step-graded insulation systems form a capacitively divided circuit between the high potential and ground. As the dielectric between the foil layers breaks down, the total capacitance of the insulation system increases, producing an increase in the charging current.

Abstract: A microprocessor based uninterruptible power supply generates an AC electrical output power from an AC input source when the AC input source is within settable voltage limits and from an auxiliary source of DC power when the AC input source is outside said the voltage limits. The microprocessor provides monitoring of the AC input voltage to determine its amplitude and frequency, and uses this information to set the magnitude and frequency of the AC output voltage and to select the source of the AC output power. A power factor improvement circuit boosts the selected source to a high DC voltage which is inverted by a chopper circuit to produce a high frequency AC voltage is rectified and sent to a PWM inverter to produce the AC output power. An average current feedback loop provides the output voltage regulation and utilizes current limiting to provide output current limiting for protection against short circuits and overloads.

Abstract: A dry type transformer has an iron core, high voltage windings embedded in cast resin, and low voltage windings resin encapsulated. The low voltage winding is constructed with flexible sheet conductors coincident with layer insulation sheets that extend beyond the edge of the sheet conductor. Cooling air ducts are fabricated in the low voltage windings as it is wound. To prevent the extension of the layer insulation sheets from collapsing over the formed air ducts during a vacuum and pressure resin impregnation process, reinforcing means are added to the extensions to prevent them from collapsing.

Abstract: An uninterruptible power supply for supplying a load with AC output power generates the AC output power from an AC input source when the AC input source is within settable voltage limits and transfers to generating the AC output power from an auxiliary source of DC power when the AC input source is outside the voltage limits. A microprocessor provides monitoring and sampling of the AC input voltage to determine its amplitude and frequency, and uses this information to set the magnitude and frequency of the AC output voltage by generating a sine reference signal to control the AC output frequency and voltage phase angle. The microprocessor maintains a voltage phase relationship between the AC input power and AC output power to eliminate voltage and current transients at transfer times between the AC and DC input sources that would be present if the AC output suddenly had a drastic, voltage phase angle change.

Abstract: A closed loop pulse width modulator (PWM) inverter corrects for variations and distortion in the output AC voltage waveform caused by non-linearities of the switching devices or changes in the DC link voltage. A signal is generated that is a volt-seconds representation of the voltage error between a voltage command and the actual AC output voltage of the PWM inverter. The volt-seconds error signal becomes a controlling means in the closed loop of the PWM inverter to regulate the output AC voltage of the PWM inverter. Another signal that represents changes in the DC link voltage also modifies the voltage command signal. The system will compensate for the non-linear behavior of the PWM inverter due to deadtime, minimum on-times and off-times, and DC link voltage variations and voltage drops across the switching devices, and will also allow the operation of the inverter in a linear fashion for the region of operation when one or more of its phases are saturated, i.e., either full on or full off.

Abstract: A weld controller utilizing a constant current control employs a stepper program for increasing current boost in predetermined percentage increments or steps following a predetermined stepper current profile to provide compensation for contact tip wear. The weld controller acquires weld power data in the form of current and voltage samples for each step of the stepper program, and computes Volt Amperes or power from this data during an acquisition run through the stepper program. The stepper program specifies a constant current at each step. Associated with each incremental current step, weld power that is required to produce that weld current is determined by the weld controller to generate a characteristic power curve. The power curve will be over the range of currents as specified by the combination of the programmed weld current and the range of current boost as specified by the stepper schedule. The weld controller will build a table of values relating weld current and delivered power or Volt Amperes.

Abstract: A method and apparatus provides for a measurement technique which approximates the true rms value of a current waveform. The technique involves converting the current waveform to a voltage by rectification and determining the peak value and the average value of the resultant voltage. A summing circuit combines the two values at a predetermined ratio to create a DC voltage that is approximately equal to the rms value of the input currents. The DC voltage output is suitable for a wide range of applications. A combination of the approximation circuit with a time integrating circuit and a level detecting circuit produces a low cost solid state overload relay in a preferred embodiment of the invention. Other uses include metering and use as a current feedback control signal for controlling PWM inverters and the like.

Abstract: A line isolation monitor (LIM) indicates the maximum hazard current of an ungrounded polyphase power distribution system. The LIM is microcontroller based and continuously monitors a fault impedance for the distribution system. The fault impedance is determined by the LIM by injecting a continuous sine wave measurement current into a ground terminal to generate a measurement voltage across the fault impedance. Using the measurement voltage and current, the LIM calculates the fault impedance and, using this impedance, calculates the hazard current based on the maximum line to ground voltage of the ungrounded system. The LIM has means for self-calibration and self-testing while on-line and during a power-up sequence to verify that components in the measurement circuitry are within specified tolerances.

Abstract: A weld controller utilizes a neural network to compute power factor of a secondary circuit of a welding transformer supply power to a workpiece through a pair of welding electrodes. Phase controlled switches supply power to the transformer and the neural network uses the phase angle at the time of energization of the switches and the length of time that the switches conduct to compute the power factor. The computed power factor is compared with previous computations of power factor online and any changes are interpreted as changes in resistance of the secondary circuit. This provides a measure of contact wear and the weld controller can compensate for these changes by increasing weld power. The neural network is adaptable for use with other types of control systems.

Abstract: A line isolation monitor (LIM) indicates the maximum hazard current of an ungrounded polyphase power distribution system. The LIM is microcontroller based and continuously monitors a fault impedance of the distribution system. The fault impedance is determined by the LIM by injecting a continuous sine wave measurement current into a ground terminal to generate a measurement voltage across the fault impedance. Using the measurement voltage and current, the LIM calculates the fault impedance and, using this impedance, calculates the hazard current based on the maximum line to ground voltage of the ungrounded system. The hazard current is displayed and if it exceeds a predetermined threshold, the LIM will provide audio and visual alarms. A serial communications channel allows the LIM to communicate with other intelligent devices. The LIM has means for self-calibration and self-testing while on-line and during a power-up sequence. Line frequency is also determined by the LIM.

Abstract: A line isolation monitor (LIM) indicates the maximum hazard current of an ungrounded polyphase power distribution system. The LIM is microcontroller based and continuously monitors a fault impedance for each phase of the distribution system. The fault impedance is determined by the LIM by injecting a continuous sine wave measurement current into a ground terminal to generate a measurement voltage across the fault impedance. Using the measurement voltage and current, the LIM calculates the fault impedance and, using this impedance, calculates the hazard current based on the maximum line to ground voltage of the ungrounded system. The LIM has means for self-calibration and self-testing while on-line and during a power-up sequence. These tests compare measured and theoretical values of circuit parameters of voltage and current sensing circuits.

Abstract: A pulse width modulation DC to DC converter operates in an open loop mode to convert the DC output of the AC to DC converter to a desired, predetermined DC level using pulse width modulation techniques and a switching configuration substantially similar to that of the AC to DC converter.

Abstract: A dry type transformer has an iron core, high voltage windings embedded in cast resin, and low voltage windings resin encapsulated. The low voltage winding is constructed with flexible sheet conductors. Insulating material includes an apparatus to secure the windings in place during a vacuum and pressure resin impregnation process. The result is a coil that exhibits high short circuit protection due to the tightly bond conductors comparable to completely resin encased molded transformers at a substantially reduced cost.

Abstract: A weld controller employs a stepper program for increasing heat boost in one percent increments or steps to provide compensation for contact tip wear. The weld controller acquires weld current data in the form of current samples for each step to create a characteristic curve or nominal current profile for the stepper program. Subsequent operations of the weld control utilize high and low current limit thresholds that track the nominal current profile. The thresholds can be defined as a fixed offset from the profile or as a percentage of the profile at each step of the stepper program. This allows the use of current limits to be established in close proximity to the nominal weld current at all times, enabling the use of tighter tolerances.

Abstract: A dry type transformer has an iron core, high voltage windings embedded in cast resin, and low voltage windings resin encapsulated. The iron core is manufactured with stacked core steel. After the core is stacked, instead of using banding straps, a heat shrinkable material is wrapped around the core legs and is heated to enable the shrunken material to apply uniform compression over the core legs. This will facilitate coil loading, reduces audible core noise and reduce core losses. In place of shrink wrappable material, a film material with elastic properties could be used.

Abstract: A current mode control apparatus generates an AC power output from a DC input source. The apparatus includes a pulse width modulator (PWM) to produce gate drive signals for driving a power output section having an output inductor and connected across the DC input source. An inner current error control loop coupled between the AC power output and the PWM, controls the average current in the output inductor by generating a current error signal for inputting to the PWM for conversion to the gate drive signals. The current error signal is the difference between the average current and a voltage controlled input signal. An outer voltage error control loop coupled between the AC power output and the inner current error control loop generates a voltage error signal based on the difference between the AC power output and a reference AC voltage.

Abstract: A power output driver circuit that converts a DC voltage into an AC voltage includes a gate driver circuit for turning on electronic switching devices. The gate driver circuit allows for a rapid turn-off of the conducting electronic switching device by having a first conduction path for the gate drive signals that turn-on the switching devices and a second conduction path that rapidly discharges stored energy in the gate/drain capacitance of the switching devices, resulting in a faster turn-off time. A transistor in the gate circuit shunts the stored charge directly to the source terminal of the switching device.