• 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 / Why is RMS current important in stepper motor applications?

Why is RMS current important in stepper motor applications?

December 7, 2021 By Danielle Collins Leave a Comment

Most precision stepper motor applications — especially those in automation and motion control — use a control method referred to as microstepping, which creates a sinusoidal (or nearly sinusoidal) current waveform, rather than the square wave of current produced with typical full-step control.


The current rating of a stepper motor depends on the amount of power and, therefore, heat that the motor windings can withstand. Power is directly related to resistance and to the square of current (P = I2R), so current is critical to motor heating. (Keep in mind that resistance is a property of the motor windings, so it remains constant for a given motor.)

To quantify the amount of heating inside the motor, we use the average power — and therefore, the average of I2 — delivered to the motor over time. But because the sinusoidal waveform of microstepping control means that current is constantly changing over time and reversing direction (from positive to negative) — much like an AC current waveform — we need to use the time varying average of the current, which is the root mean square, or RMS, current value.

Microstepping waveform
Microstepping produces a sinusoidal waveform, similar to that of AC current.
Image credit: AMCI

The RMS value of an AC current waveform represents the amount of DC current that would be needed to produce the same amount of heating when flowing through the same resistance.


Root mean square is a statistical method of finding the mean, or average, of a set of values. To find the root mean square, first square each value and sum the squares. Then divide the sum by the number of values. This gives you the mean. Finally, take the square root of the mean, and the result is the RMS value.

RMS equation


The relationship between RMS current and peak current is:

Peak to RMS Current Equation

RMS to Peak Current Equation

This is easy to visualize using the geometry of a right triangle, as shown in this post from Lin Engineering.

In short, if you plot a simple stepping sequence (this example uses 8 steps), with Phase A current on the X axis and Phase B current on the Y axis, you can see that the maximum, or peak, current along the step sequence in either phase is 1.414 A, or 1.414 times the RMS current of 1 A. (This exercise works with any value of IRMS. For example, if IRMS is 3 A, Ipeak will be 3 * 1.414 = 4.242 A.)

stepper motor phase diagram
Image credit: Lin Engineering

Looking at the triangle formed at position 7 and using the Pythagorean Theorem, you can derive the relationship between peak and RMS current:

phase triangle

pythagorean theorem equation

pythagorean theorem equation

pythagorean theorem equation

pythagorean theorem equation


There are two fundamental types of stepper drives: L/R drives, which supply the motor with constant voltage, and chopper drives, which supply the motor with constant current. At high speeds, L/R, or constant voltage, drives have difficulty delivering enough current for the motor to reach the rated torque due to the time constant of the motor windings. This limits the use of L/R drives primarily to low-speed applications.

This is why many stepper motor applications use chopper drives and microstepping control. A chopper drive rapidly switches the voltage on-and-off (a method referred to as “chopping”) to control the amount of current to the motor. At the beginning of each motor step, very high voltage (typically eight times the motor’s nominal voltage) is applied to the windings, causing the current to rise very quickly and reach a higher level. The chopping frequency is controlled by pulse-width-modulation, so that a constant RMS value of current is supplied to the motor, regardless of the voltage.

To avoid exceeding a stepper motor’s peak current capability and to avoid possible motor damage, it’s important to pay attention to whether the motor’s current rating is given as peak or RMS and to the type of drive (L/R or chopper) being used.

You may also like:


  • What is a constant voltage drive for a stepper motor…

  • What is current decay (aka recirculating current) in a stepper…
  • stepper drives
    Stepper drives: What’s the difference between an L/R drive and…

  • FAQ: What drive conditions make stepper motors run poorly?

  • FAQ: How to set a stepper motor’s current limit and…

Filed Under: Drives + Supplies, FAQs + basics, Featured, Motors, Stepper Drives, Stepper Motors

Reader Interactions

Leave a Reply

You must be logged in to post a comment.

Primary Sidebar

POWER TRANSMISSION REFERENCE GUIDE

DESIGN GUIDE LIBRARY

“motion
Subscribe Today

RSS Featured White Papers

  • Specifying electric rodless actuators: Ten tips for maximizing actuator life and system performance
  • The truth about actuator life: Screw drive survival
  • Top Ten Tips: How to specify electric rod-style actuators for optimal performance, reliability and efficiency

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 © 2022 · 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