• Skip to primary navigation
  • Skip to main content
  • Skip to primary sidebar
  • Skip to footer
  • Advertise
  • Subscribe

Motion Control Tips

Automation • Motion Control • Power Transmission

  • News
    • Industry News
    • Editor Blogs
  • Controls
    • HMIs
    • PC-Based Controllers
    • PLCs + PACs
    • Stand-Alone Controllers
    • Software
  • Drives
    • Servo Drives
    • Stepper Drives
  • Encoders
    • Absolute Encoders
    • Incremental Encoders
    • Rotary Encoders
  • Mechanical
    • Bearings
    • Brakes + Clutches
    • Belt + chain
    • Couplings
    • Gears + Gearing
    • Lubrication
    • Shock + Vibration Mitigation
    • Springs + Rings + Seals
  • Linear
    • Actuators
    • Linear Motors
    • Linear Encoders
  • Motors
    • AC Motors
    • DC Motors
    • Brushless Motors
    • Gearmotors
    • Piezo Motors
    • Servo Motors
    • Stepper Motors
  • Systems
    • Conveyors + linear transport systems
    • Gantries + Stages
    • Rotary Tables
    • Grippers + End Effectors
    • Robotics
  • Networks
    • Connections + Sliprings
    • Fieldbuses
    • I/O
    • Sensors + Vision
  • Resources
    • FAQs
      • Motion Casebook
      • Motion Selection Guides
    • Suppliers
    • Video
You are here: Home / FAQs + basics / Why do servo drives use PWM and how does it work?

Why do servo drives use PWM and how does it work?

September 13, 2021 By Danielle Collins Leave a Comment

The basic function of a servo drive is to convert, or amplify, low-power signals from the controller into higher-power signals to be delivered to the motor windings. Servo drives (also referred to as servo amplifiers) can be either linear or switching types, depending on how power is delivered to the motor from the switching devices (typically IGBTs or MOSFETs) in the inverter section of the drive.

A linear servo amplifier allows voltage to flow continuously to the motor by keeping the transistors in the inverter section of the drive always on to some degree. In contrast, a switching, or PWM (pulse-width modulation), amplifier switches the transistors in the inverter section on-and-off to modulate the voltage flowing to the motor.

So why use a switching, or PWM, drive rather than a linear drive? The primary reason is efficiency. Because the transistors in a linear drive are always on, they experience high power dissipation and, in turn, can generate significant heat. This reduces efficiency and increases size, since the drive needs a suitable heat sink or cooling mechanism to protect the transistors.

PWM drive schematic
A PWM drive switches the transistors in the inverter section on-and-off to provide a simulated sine wave voltage to the motor.
Image credit: M. Pelletier, Yaskawa

PWM drives switch the transistors on-and-off at a very high frequency — typically 10 to 20 Hz — creating a train of square-wave pulses that simulates an analog, sine wave signal.

The duty cycle of the pulses — the ratio of on-time to total switching time (on time + off time) — determines the pulse width, which in turn, determines how much voltage and current are delivered to the motor.

PWM duty cycle
The duty cycle of a PWM waveform refers to the ratio of on (or high) time to the total switching time.
Image credit: Performance Motion Devices

Because the transistors in a PWM drive operate in their linear, or active, region — in other words, they’re either fully on or fully off — power dissipation and heat generation are relatively low, especially when compared to carrier-based PWM. And the faster switching times of PWM improve efficiency because the transistors spend less time in their linear region. However, this quick switching causes a high dv/dt (change in voltage), which can result in undesirable effects such as noise that interferes with other equipment and over-voltages known as reflected waves, so proper filtering methods and cable shielding should be implemented.


The pulses for a PWM drive can be created in one of two ways. In the traditional method, often referred to as carrier-based PWM, the PWM pulses are created using the triangular signal that represents the switching frequency of the inverter, together with a sine wave signal produced by the PWM generator.

PWM generator signals
With carrier-based PWM, the modulated PWM pulses (bottom) are created from the triangular signal that is the switching frequency of the inverter and the sine wave signal produced by the PWM generator.
Image credit: Rockwell Automation

A more advanced method for creating the PWM pulses is known as space vector modulation, or SVM (sometimes referred to as space vector pulse width modulation, or SVPWM). This method uses vector representations of the three-phase voltages and calculates the required duty cycle of inverter switches to synthesize a reference output voltage. (This article explains the theory and operation of space vector modulation in more detail.) Space vector modulation makes better use of the bus voltage, delivering higher voltage to the motor with lower total harmonic distortion and less torque ripple.


Variable frequency drives (VFDs) for AC induction motors also use PWM methods to deliver voltage to the motor. In these applications, the drive manipulates both the duty cycle and the frequency of the pulses, to control the voltage and the motor speed.

You Might Also Like

Filed Under: FAQs + basics, Featured, Servo Drives

Reader Interactions

Leave a Reply

You must be logged in to post a comment.

Primary Sidebar

LEARNING CENTER

Design World Learning Center

Motion Control Handbook

“mct
EXPAND YOUR KNOWLEDGE AND STAY CONNECTED
Get the latest info on technologies, tools and strategies for Design Engineering Professionals.

RSS Featured White Papers

  • Robotic Automation is Indispensable for the Logistics Industry’s Continued Growth and Success
  • Reliable Linear Motion For Packaging Machines
  • Polymers Outperform Metals In Precision Gearing

Footer

Motion Control Tips

DESIGN WORLD NETWORK

Design World Online
The Robot Report
Coupling Tips
Linear Motion Tips
Bearing Tips
Fastener Engineering.
Wire and Cable Tips

MOTION CONTROL TIPS

Subscribe to our newsletter
Advertise with us
Contact us
About us

Copyright © 2025 · WTWH Media LLC and its licensors. 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.

Privacy Policy | RSS