Updated: The Dark Side of AC VFDs

Darkside of vfd Updated: The Dark Side of AC VFDsAs you know, the benefits of using a variable frequency drives (VFD) are great and many. However, like all machines, they can be “moments” where you stand there, staring at it, pulling your hair out as you’re wondering what the heck is happening.

The dark side of AC drives, as listed below, is by no means a comprehensive collection of symptoms and solutions for an issue with your drive. The goal of this list is to present you with several possible  issues that are the direct result of using a VFD. Should your drive “walk” down one of these paths, contact us and let one of our outstanding field service technicians troubleshoot and remedy your electronic malady.

Issues

Harmonics & EMI/RFI

Symptoms

Harmonics (Low Frequency Noise)

  • Product Failure
  • Can be intermittent
  • Overheating Distribution Transformer
  • Circuit Breaker Tripping
  • Motors Burning Out
  • Power Factor Capacitor Bank(?) Failure

EMI/RFI

  • Static on AM radio band
  • Directly effects sensitive equipment, e.g. inadvertent
    security alarm activation
  • Radiated Issue and Conduction

Band-Aid Solutions

Harmonics

  • Increase load impedance
  • Install line reactors
  • Harmonic Traps
  • Active Filters
  • Drives

EMI/RFI

  • EMC filters
  • EMI/RFI filters
  • Drives

Output Reflective Wave (DV/DT)

Symptoms

  • Identifiable Motor failure (e.g. motor failure in first turn)
  • Occurs only with 380 volts and higher
  • Does NOT occur in 208/230/240 volt systems

Band-Aid Solution

  • Change the distance between drive and motor to a distance of approximately 50 to 350 feet

Bearing Currents

Symptoms

  • Common-mode voltage possibly creates current to the bearings
    leading to motor failure

Band-Aid Solution

  • Proper grounding
  • Common-mode filters
  • DV/DT filters
  • Shaft Grounding Brushes
  • Insulated Bearings

 Parasitic Capacitance: In electrical circuits, parasitic capacitance, stray capacitance or, when relevant, self-capacitance (of an inductor), is an unavoidable and usually unwanted capacitance that exists between the parts of an electronic component or circuit simply because of their proximity to each other. All actual circuit elements such as inductors, diodes, and transistors have internal capacitance, which can cause their behavior to depart from that of ‘ideal’ circuit elements. In addition, there is always non-zero capacitance between any two conductors; this can be significant at higher frequencies with closely spaced conductors, such as wires or printed circuit board traces.” 

  Symptoms

  • Drive shuts down on over-current or ground fault
  • Band-Aid Solution
  • Move the drive closer to the motor, within 600 feet
  • Oversize Drive
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