• 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 / What is a rectifier?

What is a rectifier?

August 20, 2016 By Danielle Collins Leave a Comment

AC motors can be operated directly from AC supply voltage, but this is only suitable if the motor is required to run at constant speed. If the speed or load requirements of the motor change, an AC drive — also referred to as a variable frequency drive (VFD) — can vary the voltage and frequency of power to the motor to control its speed.

What is a Rectifier?

An AC drive is made up of three primary parts: a rectifier, which converts the incoming AC voltage to DC voltage; a DC bus (also referred to as a DC voltage intermediate circuit), which stores the DC power; and an inverter, which converts the DC power back to AC at the necessary frequency and voltage, making it suitable for the motor.

what is a rectifier?
An AC drive consists of a rectifier, a DC bus (intermediate circuit), and an inverter.
Image credit: ABB Group

The process of rectification results in DC power that consists of pulses of current, so when a device requires steady DC current, the DC bus acts as a low-pass filter and smooths the current.

Types of AC drive rectifiers

The simplest version of an AC drive rectifier is a diode rectifier, which is also referred to as a 6-pulse diode bridge. (Each phase of power requires two rectifiers—one that allows current to pass through when the voltage is negative, and one for when the voltage is positive. Thus, three-phase power requires six rectifiers.)

Diode rectifiers are simple and low-cost, but they only allow energy to flow in one direction: from the supply to the motor. So when the motor decelerates and acts as a generator, the regenerated power cannot be fed back into the electrical network—it must be sent to a capacitor via a dynamic braking system.

One alternative that allows for regeneration of power produced by the motor is the thyristor rectifier. Each thyristor bridge uses six thyristors, and bridges are typically applied in pairs. One bridge is used when the motor is consuming electricity (motoring) and the other is used when the motor is generating electricity (regenerating). This allows power to flow in both directions and regenerative power to be fed back to the supply network.


Note that the terms SCR and thyristor are sometimes used synonymously, although SCR is a trade name held by General Electric for a specific type of thyristor.


For regeneration, another alternative is to use an IGBT (insulated-gate bipolar transistor) rectifier. IGBT devices consist of six IGBTs and six diodes, with switching of the IGBTs being controlled by electronics. This allows power to flow in two directions, so energy can be fed back into supply network during regeneration. These rectifiers significantly reduce lower order harmonics. However, they introduce higher order harmonics, so a filter is required, which makes the up-front cost higher for IGBT versions than for other rectifier types. AC drives that use transistors in the rectifier are referred to as having an “active front-end.”

One way to reduce cost in a system with multiple AC motors and drives is to use a single rectifier for multiple devices. The rectifier feeds power to a common DC bus, which is connected to each inverter. Not only does this reduce hardware cost, but it also allows sharing of power between the axes, which increases overall efficiency.

what is a rectifier
Using a single rectifier for multiple axes improves overall efficiency of the system.
Image credit: Siemens Industry

You Might Also Like

Filed Under: AC Motors, Drives + Supplies, FAQs + basics

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