Two Minute Tips  

Drive Input Imbalances

Noah Bethel
Noah Bethel | Vice President-Product Development, PdMA Corporation

Current imbalances are sometimes referred to as a burglar alarm; generally problematic and need further diagnosis. However, imbalance at the inverter input under lightly loaded conditions is a normal phenomenon of the electrical circuit. The source of this current imbalance is often the connection of the electrical supply to the VFD, not to the motor itself. More specifically, the issue is the VFD’s full-bridge rectifier combined with subtle line voltage imbalances to the VFD. The VFD rectifier will draw from the input as necessary to supply the DC bus, which supplies power through transistors to the motor. The diodes in the VFD rectifier only conduct when the input voltage to the drive is higher than the DC bus voltage that it is supplying.  Consider it a voltage replenishment for the VFD. So, when the VFD is lightly loaded and one of the three phase supply lines has a higher line voltage, it will resupply the DC bus first, and conduct longer than the other phases resulting in a current imbalance on the line side of the VFD.

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About the Author

Noah Bethel
Noah Bethel Vice President-Product Development, PdMA Corporation

Noah has over twenty-five years of broad operations and electrical systems maintenance experience in industrial, commercial, and military settings ranging from nuclear submarines to world class amusement parks. His experience includes high and low voltage, AC and DC, power generation, power distribution, motors, and motor controllers. Noah is currently in charge of product development for new and existing PdM technology at PdMA Corporation.

Noah is a graduate of the University of the State of New York and the Naval Nuclear Power School and Training Unit. He is a Certified Maintenance Reliability Professional, with field experience in motor circuit analysis, current signature, power analysis, thermography, vibration analysis, oil analysis and ultrasonic testing.