4 Key Factors to Consider When Sizing Servo Motors

Selecting the right servo motor involves closely matching the motor’s capabilities with the specific demands of your application. You should focus on key parameters like speed, required torque, gear ratio, and load dynamics.

Getting this match right ensures optimal performance in sophisticated motion-controlled environments. Factoring this information can be more complex than sizing AC induction motors, particularly during servo vs. induction motor selection decisions.

How to Select a Servo Motor: 4 Key Factors

When it comes to servo motors, acceleration, deceleration, and running torque all affect motor speed. Factoring in this info can sometimes be more complex than sizing AC induction motors.

The servo motor’s dynamic control over load speed and position is critical for optimal performance. This control ensures precise alignment with system requirements.

Conversely, selecting the right servo size requires analyzing your application’s load and speed requirements, as well as its operational dynamics.

Let’s examine these key factors and how KEB can provide a tailored solution to your motion control needs.

Four key servo selection factors we’ll discuss in detail are:

• Inertia matching
• Speed and torque profiles
• RMS torque
• Speed-torque curves

While there are other factors to consider for your application, these four are most critical when accurately sizing servo motors.

1. Inertia Matching

Inertia matching refers to the system inertia. Specifically, it refers to the ratio of the load’s inertia to the motor’s inertia. See the formula definition below:

The moment of inertia measures how difficult it is to change the rotating velocity of that object or system. The motor manufacturer should supply the JM (motor/rotor inertia).

KEB’s servo motor data are available in the KEB drive software, a program that helps select 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 servo motors, but consider the following:

• Performance of the motor improves as the inertia ratio decreases
• Control loop tuning and machine performance improve as the inertia ratio decreases
• A motor with too low 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, select the smallest motor capable of providing the speed and torque required for your application. If you find it challenging to obtain a ratio that works, remember that KEB can add additional motor inertia.

Also, 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.”

2. Speed and Torque Profile

Speed and torque profiles are additional critical elements in choosing a servo motor. While the speed-torque curve describes the motor capability (see below), the application requirements are best illustrated using the speed and torque profile.

Depending on the application, the motor may need to meet different speed and torque requirements. In a typical rotary index table sizing example, acceleration torque spikes can be significantly higher than steady-state running torque.

Below is a graphical representation of a linear application with a servo motor. The speed profile represents acceleration, constant speed, and deceleration as the payload reaches its destination. As shown in the torque profile, the maximum torque occurs during acceleration.

When the machine starts, the motor must overcome mechanical friction as it accelerates the load from rest. Once acceleration is complete, the motor outputs a nominal torque to maintain speed and overcome friction. The decelerating point in the profile is still associated with high torque, but friction also helps stop the load.

Ensuring the motor can produce the required maximum torque at the application speed. This torque ideally falls within the intermittent region of the motor’s speed-torque curve, so it is not oversized.

3. RMS Torque

RMS torque is the time-weighted torque average during a complete machine cycle (steady state). RMS torque must fall within the continuous region of the speed-torque curve to prevent overheating. Achieving this indicates you have sized your motor correctly.

For example, a servo motor with 4 N·m of RMS torque will experience the same heat rise if it produces 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.

4. Speed-Torque Curves

Motor speed-torque curves are essential when selecting and sizing servos. Make sure your motor meets the application’s requirements to prevent overheating.

These curves show whether the servo motor can deliver the required speeds and torques, both continuous and intermittent. After reviewing your speed and torque profiles and calculating the RMS torque, compare them to the motor’s speed-torque curve to confirm they fit your application.

The image below will provide additional insight into speed-torque curves. The image is from our KEB drive software (for our TA3S Servo Motor) and should help you determine whether your servo motor is suitable for your application.

In the image, the blue lines represent the maximum speed/torque based on various input voltages. For this example, we will consider only one input voltage curve: 460 VAC (6).

The region below the S1 line up to line 6 indicates the continuous running region. The servo motor can run at the corresponding speed and torque values without overheating in this region. Above the S1 line (1) lies the intermittent operation region. In this region, the servo motor can operate for a short time based on the system’s overall RMS torque.

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 assumes no losses in the drive and that the full 460V is available. If the inverter had an input reactor, there would be a slight voltage drop at the drive input, shifting the blue curve to the left.

 
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As speed increases, available torque decreases. If you determine the application needs of 4 N·m of torque at 3500 rpm, using 460 VAC would allow you to reach this torque level intermittently. 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 lower, so we can reach higher speeds before entering the field-weakening range. With a 460 VAC input, we can achieve continuous-rated torque (S1) up to about 4200 rpm.

Important: When using speed-torque curves, check the input voltage the motor will operate on. Then ensure the motor runs successfully in both the continuous and intermittent regions.

How Do I Know What Size Servo to Get?

Start by looking at your application’s peak torque (the highest torque required during acceleration or deceleration) and running torque (the torque needed to maintain speed). Your servo motor must handle both without overheating.

To ensure reliable operation, accurately calculate the peak and nominal running torque, which helps prevent the servo motor from overheating.

Next, check the inertia matching between the motor and the load. Even if the torque numbers look correct, poor inertia matching can lead to slow responses or unstable performance. A properly sized motor should deliver the kind of speed and torque you need while remaining efficient and responsive.

Overall, when considering these technical parameters, size your servo motor to align with the specific needs of your application. You should strike a balance between power, efficiency, and control.

Industry Plays a Part in Servo Size

Industry also plays a major role in sizing decisions.

For example, in high-speed packaging applications, fast acceleration and short cycle times can require higher peak torque and tight inertia control.

Meanwhile, in CNC environments and other metalworking machinery, consistent torque and smooth motion make a difference for precision and surface finishing.

And in robotics, lightweight loads and rapid directional changes make inertia matching and dynamic response especially important.

FAQs

What inertia ratio is acceptable for servo motors?

An acceptable inertia ratio is typically up to 10:1 (load to motor), though 5:1 or lower is preferred for optimal performance and easier tuning.

What happens if I oversize a servo motor?

Oversizing a servo motor increases cost and inertia, can reduce responsiveness, and often provides little to no performance benefit.

How does ambient temperature affect servo motor sizing?

Higher ambient temperatures reduce a motor’s ability to dissipate heat, which may require derating the motor or selecting a larger size to prevent overheating.

Get Expert Help with Servo Sizing and Automation Solutions

Choosing the right servo motor doesn’t have to be a trial-and-error process. If you’re working through required torque calculations, inertia matching, voltage considerations, or speed-torque curve analysis, our team can help you size the right solution with confidence.

If interested, you can download the KEB-Drive software from one of the gearmotor product pages. This software includes servo information for the TA servo motors. It also provides information to add inline and right-angle gearing.

If you’d like support with servo sizing or a complete motion control solution, contact our team for a consultation. We’re here to help you get it right the first time.