Drives and Control Solutions

Motors, Control Solutions, Power Transmission and Advanced Motion Technology                                                                 

April 30, 2020

By Jeff Kardell

The ability to size servo motors correctly is imperative for motion-controlled applications, and it can be much more involved than sizing AC induction motors.  Acceleration, deceleration, and running torque must still be taken into account, but the servo’s ability to dynamically control the load’s speed and position is also important.  During acceleration and deceleration, peak and nominal running torque measurements must be calculated to ensure that the servo motor does not overheat during use.  In addition, inertia matching between the motor and load is necessary to ensure optimum response and system performance.

In this post, we will be examining these keys factors, as well as how KEB can provide a solution to your motion control needs.  Four key servo selection factors I will be detailing are:

While there are other factors to keep in mind for your application, these four are most critical when accurately sizing servomotors.

Inertia Matching

Inertia matching refers to the system inertia.  Specifically, the ratio between the inertia of the load and the motor. This formula definition is below:


The moment of inertia is a measurement of how difficult it is to change the rotating velocity of that object or system.  The JM (inertia of motor/rotor) should be supplied by the motor manufacturer.  KEB’s servo motor data are found on the KEB Drive software, which is a program that aids in choosing motors and gearboxes.  JL (load inertia) consists of all components in the system moved by the motor.  This ratio between the motor and load is important when selecting servomotors, but the following must be considered:

  •  -   Performance of the motor improves as the inertia ratio decreases
  •  -   Control loop tuning and machine performance improves as the inertia ratio decreases
  •  -   A motor with too low of an inertia ratio will be more expensive and have little to no performance improvement
  •  -   System inertia ratios should be designed for a max of 10:1 but are typically 5:1 for ideal performance

When choosing and calculating an inertia ratio, find the smallest motor that has the ability to provide the speed and torque necessary for your application.  If you are finding it difficult to obtain a ratio that works, keep in mind that KEB has the ability to add additional motor inertia.  Also consider that many motor manufacturers offer different servo series with different inertias.  For example, there might be a product line with a “low inertia” and another with a “medium inertia”.


Speed and Torque Profile

Speed and torque profiles are additional critical elements in choosing a servo motor.  While the motors capability is  described by the speed-torque curve (see below), the application requirements are best illustrated using the speed and torque profile. Depending on the application, there are different speed and torque requirements that the motor will need to meet.

Below is a graphical representation of a linear application with a servomotor.  The speed profile is a graphical representation of acceleration, constant speed, and deceleration as the payload reaches its destination.  As you can see by the torque profile, the maximum amounts of torque occur during acceleration.  When the machine starts, the motor must overcome the mechanical friction while accelerating the load from rest.  Once acceleration is complete, a nominal torque is output by the motor to maintain speed and overcome the friction.  The decelerating point in the profile is still associated with high torque, but the friction also aids in stopping the load.

It is important to ensure that the motor can produce the required maximum torque at the application speed.  This ideally falls within the intermittent region of the motors speed-torque curve, so it is not oversized.

RMS Torque

RMS torque is the time-weighted average of the torque during a complete machine cycle (steady state).  In order to size your motor correctly and prevent overheating, this RMS torque will need to fall within the continuous region of the speed-torque curve.  For example, a servomotor with 4 N·m of RMS torque will experience the same heat rise if it produced 4 N·m of constant torque.  Therefore, as long as 4 N·m is in the continuous region of your speed-torque profile, the motor will not overheat.


Speed-Torque Curves

Motor speed-torque curves are essential when selecting and sizing servos.  In order to prevent the motor from overheating during use, ensure that the motor has the specific capabilities required for your application.  These curves ensure that the required torques and speeds, whether continuous or intermittent, can be produced with the servo motor you choose.  By examining the speed and torque profiles and computing the RMS torque, you can now look towards speed-torque curves of specific motors and see if they will fit your application.

Let us look at the image below to provide additional insight on speed-torque curves. This is an image from our KEB Drive software, which will be sure to help you when you are deciding if your servomotor is a valid choice for your application. This specific image is for our TA3S servomotor.

In the image, the blue lines represent the maximum speed/torque based on various input voltages.  For this example, we will look at only one input voltage curve, 460 VAC (6).  The region below the S1 line up until line 6 indicates the continuous running region.  In this region, the servomotor is capable of running at the corresponding speed and torque values without overheating.  Above the S1 line (1) lies the intermittent operation region.  In this region, the servomotor can operate for a small amount of time based on the overall RMS torque of the system.

With the input voltage of 460 VAC, the TA3S can reach full peak torque (8.7 N·m) at speeds up to 2750 rpm.  This is assuming no losses in the drive and that the full 460V is available.  If the inverter were to have an input reactor on the input, there would be a slight voltage drop at the input to the drive, which would shift the blue curve to the left.

As the speed increases, the available torque starts to drop.  If you determine application needs of 4 N·m of torque at 3500 rpm, using 460 VAC would allow you to reach this torque level on an intermittent basis.  However, with a 400 VAC input, you would not have sufficient voltage to reach this speed and torque.

For continuous operation (rated torque), the required voltage difference is less, so we can reach higher speeds before we enter the field weakening range.  With a 460 VAC input, we can achieve continuous rated torque (S1) out to about 4200 rpm.

An important point to remember when using speed-torque curves is to check the input voltage the motor will be operating at and ensure that the motor will run successfully in either the continuous or the intermittent regions.


In conclusion, while sizing servo motors can be challenging, these four key factors are imperative to making a choice that will fit your application.  If interested, you can download KEB-Drive from one of the gearmotor product pages.  This software includes servo motor information for the TA servo motors.  It also includes information to add inline and right angle gearing.


Editor's Pick: Featured Article

Motor Disconnect SwitchesMotor disconnects are more than simple on-off devices to satisfy lockout-tagout requirements. They have become an integral part of the motor control circuit as disconnects increasingly offer control functions, such as auxiliary contact interface, added selector switches and push buttons.

Disconnects can be fusible or non-fusible and can have different pole arrangements and environmental ratings. Multiple disconnects can be packed into one enclosure, saving space and cost while allowing you to address varied requirements, including harsh food processing or washdown applications.

Read More


Advanced Sensing Solutions for Cost-Effective Machine Building

Omron IOLinkSensing technology has a major presence in manufacturing machinery. It provides the foundation for maintaining consistent quality and detecting any lapses in machine performance. If subtle changes in the physical properties of a machine can cause it to fail, then the sensors responsible for detecting those changes can save manufacturers lengthy downtime and repair costs.

In addition to helping cut maintenance costs, sensors also present an opportunity to make the machines more cost-efficient. Whether through their resistance to harsh chemicals or their ability to reduce the overall machine footprint, the right sensors can make a huge difference. 

Read More


Product News

  • Prev
The New eSMART Series HMI products from Exor combine state-of-the-art features and top performance ...
The VLX-60 is a new member of the VLX series of Electric Encoders a product line based on Netzer ...
Siemens presents the Simatic Robot Library for the Simatic Robot Integrator - a new universal robot ...
With Sinamics PCS, Siemens presents a new power conversion system for battery storage applications. ...
With “Control Panel Design” Siemens Smart Infrastructure has integrated new electrical planning ...
Having the ability to effectively inspect, diagnose, and document trouble spots is crucial for ...
Advancing gripping technology is essential for promoting new uses for automated systems. Festo’s ...
Yaskawa Solectria Solar has joined the Tigo Enhanced initiative, aimed at providing customers with ...
Toshiba Industrial Products Canada (TIPCA), a wholly owned subsidiary of Toshiba International ...
Hatch has recently launched an all-in-one software called PneuCalc 7.0.0 that will enable engineers ...

New Product

Flir Thermal and Visible VideoscopeFLIR Systems, Inc. recently announced the FLIR VS290-32, an industry-first, videoscope that combines thermal imaging and a visible camera specifically designed for safer and more efficient.

inspections of hard-to-reach underground utility vaults. The VS290-32 is the company’s first industrial-grade, electrical safety-rated, flexible dual-sensor videoscope on a replaceable, two-meter-long camera probe. For use in the most demanding environments, the VS290-32 is CAT IV 600 V safety rated for electrical inspections, along with an IP67-rated camera tip and IP54 base unit to protect against dust and water. 

Read More


Latest News

  • Prev
The solution will mitigate the environmental impact of building a major dam and hydroelectric ...
In May 2020, ABB’s Motion business area announced that it was reviewing how it served customers in ...
The 18th annual Motor & Drive Systems 2021 virtual program has been unveiled.
Yaskawa, a manufacturer of motion control, robotics, and variable speed drives and Phoenix Contact, ...
PC-based control and TwinCAT provide a foundation for advanced Industrie 4.0 and Internet of Things ...
Join ABB for an in-depth look at how variable frequency drives assist in aeration applications for ...
Leading Canadian forest product company, Chantiers Chibougamau, has selected ABB to help overhaul ...
ABB Motion Canada alongside our HVAC Partners nationwide has introduced a special program for our ...
Brock Solutions is pleased to announce the recent award by the Region of Peel of the Clarkson and ...
Kerrwil Publications Great Place to Work. Certified December 2019 - December 2020

538 Elizabeth Street, Midland,Ontario, Canada L4R2A3 +1 705 527 7666
©2020 All rights reserved

Use of this Site constitutes acceptance of our Privacy Policy (effective 1.1.2016)
The material on this site may not be reproduced, distributed, transmitted, cached or otherwise used, except with the prior written permission of Kerrwil