Power Quality, Electronic Motor Drives, & Other Sensitive Loads

Published by Power Quality (Thailand) Co., Ltd.


Classes of Sensitive Loads

  • Electronic motor drives
  • Computers
  • Computerized equipment
  • Automated machines that have PLC’s
  • Automatic test equipment
  • Electronic scales

Electronic Motor Drives

  • Types of drives
  • Basic electrical operations
  • Understanding electric motors
  • DC drives
  • AC drives
  • Harmonics and power factor
  • Future

What is an Electronic Drive?

  • The mysterious black box that is connected between the motor and electrical system
  • VFD Variable Frequency Drive
  • VSD Variable Speed Drive
  • FV Flux Vector Drives (AC)
  • Converter
  • Inverter

The magic box

Power Quality, Electronic Motor Drives, & Other Sensitive Loads_1

Why use electronic drives?

  • Advanced control features
    • More precision
    • More flexibility
    • Ease of changing control features
  • Lower cost of sophisticated control
  • Significant power savings
    • 20% to 50% Kwatt hour savings depending on the application

AC Drive

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Non-linear loads AC drives

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AC Drive Basics

  • AC is converted to DC for control and then converted back to AC to power AC motor
  • AC fed to motor can be Pulse Amplitude modulated or Pulse Width modulated to control the speed of the motor

DC Drive

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3 Phase Supply Non-linear loads DC Drive Type

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DC voltage controlled by varying firing angle of SCR’s


DC Drive Basics

  • Drive replaces mechanical or electromechanical speed controls for DC motors
  • Simplified version, DC voltage controls the speed of the motor
  • Electronic control converts speed information to control the DC voltage

Why use Electronic Drives

  • Energy savings
    • downsizes the motor power usage to match the load
  • Replaces mechanical controls such as dampers, variable pitch fan blades, flow control valves
  • Drives use less space than mechanical controls
  • Drives have a longer life than mechanical controls

PQ Problems Caused by Electronic Drives

  • Harmonics
  • Notching
  • RFI interference
  • Sensitivity to sags
  • Power Factor correction capacitor switching event sensitivity

RFI Interference

  • Not Harmonics
  • Switching Frequency of drive inverter is in the 5 to 20 kHz range
  • The voltage notches caused by commutation have fast edges that can couple into other electronic control systems


  • The input rectifiers are a non-linear load on the electrical system
  • The magnitude of the harmonics are determined by the type and number of poles of the rectifier system
  • A series inductor can be added to smooth the current and reduce the harmonics

Example of Harmonics from Electronic Drives

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Electronic Drive Symptoms

  • Trip off
    • High DC buss voltage (PF cap)
    • Low input voltage (Sag)
  • Fuse blown
    • Harmonic resonance (PF cap)
    • Low input voltage (short duration)
    • High input voltage (short duration)
  • Control problems
    • Communication interference (RFI)


  • High DC buss voltage
    • check nominal voltage
    • Is it time of day related?
    • Monitor for PQ anomalies (PF cap)
  • If PF cap switching
    • lock on?
  • Install line reactor 3% to 5%

Unexpected Shutdown with Short Sag

  • Monitor to determine the magnitude and duration of the sag
  • Check to see if a contactor or relay is dropping out caused by a single-phase sag
    • Install coil hold in device
    • Change coil to a DC coil
  • Work with utility to identify source and possible correction of sag


  • Fuse blowing
    • Check nominal voltage
    • Monitor for PQ anomalies
      • PF cap, sags, surges
  • If PF cap switching
    • Lock on?
    • Install line reactors
  • If sags or surges
    • work with utility to identify source

References and Reading for Electronic Drives

  • Electronic drives by Robert Carrow
  • Power Electronics Devices, Drives by Williams, B. W
  • Variable-speed drives, Principles and applications by Spitzer, David

Computer Networks

  • Large quantities in 1 building
    • Co Lo’s
    • Data hotels
    • Data centers
    • Internet service providers
  • Very sensitive to down time
  • $100’s to$1000’s per second

Computerize Equipment Sensitivities

  • Voltage sags
    • greater than 1 cycle below 80%
  • any outage greater than 1 cycle
  • surges greater than 10%
  • high frequency electrical noise
  • common mode noise electrical noise

Automatic Test Equipment

  • Same as computerized equipment
  • Specific units may be sensitive to specific noise frequencies
    • IE audio device testing

Audio and Video

  • Sensitive to the harmonic frequencies
  • Sensitive to power frequency currents in the grounding network

Please feel free to leave a question in the comments section. 


Published by PQBlog

Electrical Engineer

One thought on “Power Quality, Electronic Motor Drives, & Other Sensitive Loads

  1. My partner and I stumbled over here by a different website and thought I might check things out. I like what I see so now i’m following you. Look forward to looking at your web page repeatedly.


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