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

Motion Control Tips

Automation • Motion Control • Power Transmission

  • News
    • Industry News
    • Editor Blogs
    • Video
  • 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
  • FAQs
    • Motion Casebook
    • Motion Selection Guides
  • Suppliers
You are here: Home / FAQs + basics / External rotor motor basics: Design and applications

External rotor motor basics: Design and applications

June 6, 2018 By Danielle Collins Leave a Comment

Conventional brushless DC motors are constructed with a permanent magnet rotor located inside a wound stator. But one type of DC motor is designed with the rotor on the outside and the stator and housed inside the rotor. Permanent magnets are mounted on the inner diameter of the rotor housing (sometimes referred to as the ‘bell” or “cup”), and the rotor rotates around the internal stator with windings. This design is often referred to as an external rotor motor, but can also be called an outer rotor motor, an outrunner motor, or a cup motor.

external rotor motor
As its name suggests, an external rotor motor is designed with the rotor on the outside and the stator housed inside the rotor.
Image credit: Nidec Corporation

The external rotor design provides several performance advantages. First, to house the stator, the rotor of an external rotor motor is by necessity larger than the rotor of a conventional DC motor. And a larger rotor means higher inertia, which helps to dampen torque ripple (a common problem in conventional DC motors) and provide smooth, stable operation, even at low speeds.

Another advantage of external rotor motors is that they can typically produce higher torque than comparably sized internal rotor designs. Recall that torque is a product of the magnetic force times the radius of the air gap (length of magnetic flux). For a given motor diameter, external rotor motors have a larger air gap area than inner rotor designs, and the larger air gap allows a higher force to build. They also have a larger air gap radius, which increases the “lever arm” for torque production. The larger diameter (and, therefore, circumference) of the rotor in external rotor designs also means the rotor can accommodate more poles, which further increases magnetic flux.

external rotor motor
Compared to an internal rotor motor, an external rotor motor has a larger area for flux to develop, and a larger air gap radius, which acts as the “lever arm” for torque production.
Image credit: T. Reichert, Power Electronic Systems Laboratory

External rotor motors are axially shorter than inner rotor motors with similar performance characteristics. This compact size and high torque production make them ideal for directly driving the propellers of remote-controlled model airplanes and drones. In high-precision applications, such as optical drives, their smooth, consistent speed is a benefit over other motor types. And in applications with varying loads, such as industrial power tools, pumps, fans, and blowers, the high inertia of external rotor motors can help to “push through” load fluctuations and provide steady output torque.

Fan and blower applications are one of the more common uses for external rotor motors thanks to a specific design benefit: The external rotor can serve as the hub of the fan or blower impeller. This provides a compact package and allows the impeller to act as a large, rotating heat sink and assist with motor cooling.

But integrating the rotor into the impeller also increases the motor’s mechanical time constant — the amount of time required for the motor to reach 63.2 percent of its final speed for a given voltage — an important parameter for ensuring the motor doesn’t overheat.

external rotor motors

τm = mechanical time constant of the motor
R = winding resistance
J = rotor inertia
Ke = back EMF constant
Kt = torque constant

As shown in the equation, the motor’s mechanical time constant depends in part on the rotor inertia. When the rotor is integrated into the impeller, the inertia of the rotor and impeller are considered together. This higher inertia results in a higher mechanical time constant, and therefore, a longer time for the motor to reach its required speed.

You may also like:


  • Three things to consider when selecting a customized DC motor…
  • dc motor
    BLDC motor: Marketing jargon or more?
  • linear behavior
    What is linear behavior for DC motors?

  • FAQ: What is phase lag and what effect does it…
  • torque ripple
    FAQ: What are ways to avoid torque ripple in DC…

Filed Under: DC Motors, FAQs + basics, Featured

Reader Interactions

Leave a Reply Cancel reply

You must be logged in to post a comment.

Primary Sidebar

MOTION DESIGN GUIDES

“motion

“motion

“motion

“motion

“motion

POWER TRANSMISSION REFERENCE GUIDE

RSS Linear Motion Tips

  • Haydon Kerk Pittman launches compact Z-Theta motion platform
  • Three easy ways to specify application requirements for linear motion systems
  • Sourcing linear motion solutions from NSK’s global manufacturing sites
  • Ride the wave of electrification: Off-highway designs with linear actuators
  • What are capacitive sensors and where are they used?
Subscribe Today

RSS Featured White Papers

  • Identifying Best-Value Linear Motion Technologies
  • Learn how to reduce noise and distortion in encoders’ signals
  • Helical Planetary Gearboxes: Understanding The Tradeoffs
Tweets from https://twitter.com/Motion_Control/lists/motion-control-tweets

Footer

Motion Control Tips

DESIGN WORLD NETWORK

Design World Online
The Robot Report
Coupling Tips
Linear Motion Tips
Bearing Tips
Fastener Engineering

MOTION CONTROL TIPS

Subscribe to our newsletter
Advertise with us
Contact us
About us
Follow us on TwitterAdd us on FacebookAdd us on LinkedInAdd us on YouTubeAdd us on Instagram

Copyright © 2021 · 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