• 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 / Stepper drives: What’s the difference between an L/R drive and a chopper drive?

Stepper drives: What’s the difference between an L/R drive and a chopper drive?

April 19, 2018 By Danielle Collins Leave a Comment

There are two fundamental types of stepper drives: L/R drives and chopper drives.


L/R stepper drives are also referred to as “constant voltage” drives, because they supply constant voltage to the motor windings. The current produced in the windings depends on the motor’s time constant, which is the relationship between its inductance and resistance (L/R). The time constant causes current to increase slowly with each voltage pulse. So for very high pulse rates (i.e. high motor speeds), full current, and thus full rated torque, may not be reached. This limits the use of L/R drives to primarily low-speed applications.

stepper drives
Torque vs. pulse rate for a typical unipolar L/R drive with a 15 degree step angle.
Image credit: AMETEK Inc.

Current in the windings can be increased by using higher supply voltage, but motor temperature rise then becomes a problem. When the applied voltage is high, the “off” time (when no power is applied) must be long enough to prevent the motor from overheating. Another option is to add resistors in series to improve the time constant (L/2R or L/4R, for example) and increase the voltage. But resistors waste power through heat generation, so efficiency is reduced.


Resistance (R) affects the maximum current in the windings, according to Ohm’s law, I = V/R.

Inductance (L) affects the rate of change of current in the windings, according to the relationship, dI/dt = V/L.


Because of the torque and speed limitations of L/R stepper drives, an increasing number of stepper motor applications now use chopper drives. These drives are also referred to as “constant current” drives because they supply constant current to the motor windings, by “chopping” the output voltage (i.e. turning the output voltage on-and-off very rapidly).

With each motor step, a very high voltage (typically eight times the motor’s nominal voltage) is supplied to the motor windings. This high voltage gives a very short current rise time, according to the relationship for an inductor (dI/dt = V/L), and it causes higher current to be produced, according to Ohm’s law (I = V/R).

stepper drives
Torque vs. pulse rate for a typical bipolar chopper drive with a 15 degree step angle.
Image credit: AMETEK Inc.

Voltage chopping also modulates the width of the output pulses (pulse width modulation) and is typically done at a frequency of 20 kHz or higher — above the audible range. The impedance in the windings varies with the motor speed, so it plays a role in determining the voltage on-time.

At slow motor speeds — and low impedance — the chopper drive provides a short on-time for the voltage, producing a small pulse width. Alternatively, at high speeds —and high winding impedance — the chopper drive provides a long on-time for the voltage, producing a large pulse width and allowing time for the current to rise sufficiently to produce the rated torque.

stepper drives
Relationship between voltage and current in a (chopper) drive.
Image credit: Oriental Motor USA Corp.

A current-sensing resistor placed in series with each winding regulates the current and ensures the shortest time possible for current to build up and decline. As the current increases, voltage develops across the resistor. This voltage is monitored by a comparator, and at a predetermined reference voltage, the output voltage from the drive is turned off (chopped) until the next pulse occurs. This allows current to build and decline as the voltage is switched on and off.

By controlling the duty cycle of the drive through chopping, the average voltage and current are kept equal to the motor’s nominal voltage and current ratings. The result is precise torque control, and more importantly, higher torque production at high motor speeds.


Microstepping drives are common versions of chopper drives.


Feature image credit: AMETEK Inc.

You may also like:

  • stepper motor
    What is a stepper motor’s velocity limit?

  • Electric motors: Trends in stepper, battery-powered, and integrated motion designs 
  • stepper drives
    What’s the difference between ac and dc stepper drives?
  • Time Constant
    Why is the electrical time constant important for stepper motors?

  • FAQ: What features need to be compatible for a stepper…

Filed Under: FAQs + basics, Featured, Stepper Drives

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