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AssigneeSMC Electrical Products, Inc.
InventorGeraldo Nojima
Estimating the term…
To protect people and equipment, three-phase AC drives need to be properly grounded, and ground faults need to be quickly detected. Conventional methods of detecting ground faults are affected by external factors that reduce their sensitivity. As an improvement on conventional methods, ground fault detection is accomplished by deriving a neutral point from the three-phase output of an AC drive, decoupling normal operating currents of the AC drive from the neutral point, resistively grounding the neutral point, and sensing when a magnitude of signals between the neutral point and ground exceed a maximum threshold level, ground faults are quickly detected. By effectively filtering normal operating currents from ground fault current, detection sensitivity is improved.
An apparatus for grounding an AC drive having a three-phase output, comprising:
A method of grounding an AC drive having a three-phase output, comprising:
Dependencies are inferred from the claim text.
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1. Field of the Invention
The present invention relates to the grounding of AC drives and the detection of ground faults.
2. Description of the Related Art
FIG. 1 illustrates a AC drive using the conventional neutral point inverter topology.
FIG. 2 illustrates a circuit implementing a preferred embodiment of the invention.
FIG. 3 illustrates a circuit implementing a further preferred embodiment of the invention.
FIG. 2 illustrates a preferred embodiment of the invention. The neutral deriving network comprises a transformer T1 (80) and series resistors R1-R3 (51-53), creating an artificial neutral point (70) from the three outputs (21, 22, 23) of the AC drive (20). The derived neutral point (70) is stable, and is not load bearing.
A grounding resistor RG (60) is connected between the derived neutral point (70) and the system ground. Grounding resistor RG limits the current which flows to ground in the event of a short-circuit from any of the three AC drive output phases to ground. The resistance used for grounding resistor RG is application dependent, and has the objective of limiting the ground fault current to levels which are acceptable to a certain application and industry. For example, in the mining industry, there are applications which allow for a maximum current of 10 amperes, others that are limited to 0.5 amperes, and yet other, more stringent applications which allow for a maximum current of 125 milliamperes.
Any ground fault current is detected by a ground fault voltage relay (GFVR 61) and/or a ground fault current relay (GFCR 63) and the drive is turned off without a catastrophic ground fault current surge occurring. The frame (10) and AC motor (40) are also grounded.
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Term end not estimated