Power Quality, Electronic Motor Drives, & Other Sensitive Loads

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

www.powerquality.co.th

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

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

 

Non-linear loads AC drives

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

3 Phase Supply Non-linear loads DC Drive Type

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
  

Harmonics

• 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

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)
    

Solutions

• 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
  

Solutions

• 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.