Design World

  • Home
  • Articles
    • 3D CAD
    • Electronics • electrical
    • Fastening & Joining
    • Factory automation
    • Linear Motion
    • Motion Control
    • Test & Measurement
    • Sensors
  • 3D CAD Models
    • PARTsolutions
    • TraceParts
  • Leadership
    • 2020 Winners
    • 2019 Winners
    • 2020 LEAP Awards
  • Resources
    • DIGITAL ISSUES
      • EE World Digital Issues
    • Future of Design Engineering
    • 2020 LEAP Awards
    • MC² Motion Control Classroom
    • Motion Design Guide Library
    • Podcasts
    • Suppliers
    • Webinars
  • Women in Engineering
  • Ebooks / Tech Tips
  • Videos
  • Subscribe
  • COVID-19

What is field weakening and how does it affect servo motor performance?

By Danielle Collins | January 28, 2021

Share

Although servo systems can very precisely control motor speed, position, and torque, in some applications, the motor may not be able to reach the commanded velocity due to limits in the available bus (supply) voltage from the drive. But advanced servo drives offer a technique referred to as field weakening, which alters the motor’s torque-speed curve — specifically, to allow the motor to operate at higher than rated speed with rated bus voltage.

field weakening
Field weakening alters a servo motor’s torque-speed curve, allowing the motor to reach higher speeds for a given bus voltage.
Image credit: Alxion

Field weakening enables higher motor speeds by reducing the back EMF generated by the motor. Recall that back EMF (electromotive force) is voltage created when a coil turns inside a magnetic field, and it opposes the supply (driving) voltage. In order to “weaken” the field that creates back EMF, field weakening relies on a control method known as field oriented control (FOC).

Field oriented control allows the two components of current (d and q) to be controlled independently.
Image credit: MathWorks

Field oriented control (also referred to as vector control) is often used for AC induction motors that require dynamic torque and speed response and improved efficiency. With FOC, the quadrature (torque-producing, “q”) and direct (flux-producing, “d”) components of current are controlled independently, imitating the control scheme for a DC motor.


In normal operation, the goal of field oriented control is to maintain the d-axis current (the non-torque producing component) at zero and to control the q-axis current so that the commanded torque is generated.


Here’s how field weakening is achieved: The d-axis of the stator current is referenced to, or in-line with, the axis of the rotor’s magnetic flux. So adjusting the d-axis current (sometimes referred to as phase shifting) directly influences the rotor’s flux. In other words, positive d-axis current strengthens the rotor flux, whereas negative d-axis current weakens the rotor’s flux. More specifically, negative d-axis current weakens the field of the magnets — hence, the term field weakening.

Recall that in an AC motor, speed is determined by the frequency of the supply voltage. Because back EMF opposes (takes away from) the supply voltage, it is an important limiting factor in an AC motor’s achievable speed.

But back EMF is created by rotor flux. So adding negative d-axis current reduces the rotor flux, which means that back EMF is also reduced. The result is that more of the drive’s bus voltage is used to create (increase) motor speed, rather than being used to overcome back EMF.

MOTION DESIGN GUIDES

“motion

“motion

“motion

“motion

“motion

Enews Sign Up

Motion Control Classroom

Design World Digital Edition

cover

Browse the most current issue of Design World and back issues in an easy to use high quality format. Clip, share and download with the leading design engineering magazine today.

EDABoard the Forum for Electronics

Top global problem solving EE forum covering Microcontrollers, DSP, Networking, Analog and Digital Design, RF, Power Electronics, PCB Routing and much more

EDABoard: Forum for electronics

Sponsored Content

  • Drilling Rig OEM Benefits from a PLC with Edge Computing Technology: IIoT Case Study
  • #1 Reason for Retaining Ring Failure & How to Overcome It
  • Motion controllers: design from scratch or buy ready-made?
  • 4 Key Trends in Machine Engineering
  • Configuration Management: Configuration Integrity IS A Core Driver for Business Success
  • How to Choose a Linear Actuator
Engineering Exchange

The Engineering Exchange is a global educational networking community for engineers.

Connect, share, and learn today »

Tweets by @DesignWorld
Design World
  • Advertising
  • About us
  • Contact
  • Manage your Design World Subscription
  • Subscribe
  • Design World Digital Network
  • Engineering White Papers
  • LEAP Awards

Copyright © 2021 WTWH Media, LLC. All Rights Reserved. The material on this site may not be reproduced, distributed, transmitted, cached or otherwise used, except with the prior written permission of WTWH Media. Site Map | Privacy Policy | RSS

Search Design World

  • Home
  • Articles
    • 3D CAD
    • Electronics • electrical
    • Fastening & Joining
    • Factory automation
    • Linear Motion
    • Motion Control
    • Test & Measurement
    • Sensors
  • 3D CAD Models
    • PARTsolutions
    • TraceParts
  • Leadership
    • 2020 Winners
    • 2019 Winners
    • 2020 LEAP Awards
  • Resources
    • DIGITAL ISSUES
      • EE World Digital Issues
    • Future of Design Engineering
    • 2020 LEAP Awards
    • MC² Motion Control Classroom
    • Motion Design Guide Library
    • Podcasts
    • Suppliers
    • Webinars
  • Women in Engineering
  • Ebooks / Tech Tips
  • Videos
  • Subscribe
  • COVID-19