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Five tips for a successful Servo Crossover

Achieving a successful servo crossover involves identifying particular situations that are most common and their correct order of concern.

There are a number of situations that call for crossing over and replacing an existing motor with a new servo motor. These can include:

  • Product Obsolescence
  • Cost Savings
  • Lead Time Issues
  • Upgrading to Newer Technology

The specifics of each application could lead to an endless number of important factors to consider when achieving a successful servo crossover.

Healthy Motivations

The most important aspect of achieving successful servo crossover is mitigating risk. Mitigating risk is when an organisation introduces specific measures to minimise or eliminate unacceptable risks associated with a specific process.

A successful servo crossover is one that minimises the potential number of issues that may be experienced during the upgrade. If great care is not taken to manage the potential risk of a replacement, a higher potential for system failure will be introduced. 

This means cost should not be the controlling factor for replacements! A good replacement is one that minimises risk AND reduces the cost of a system, versus reducing cost and accepting a higher potential for risk.

Axis Stability

Inertia matching is very important and often overlooked when looking to achieve a successful servo crossover. A servo replacement should have the same rotor inertia as the existing motor, or be as similar as possible. The goal is to keep the stability of a system consistent when the new servo is introduced. This of course assumes the existing system already has the desired stability. 

If replacing a lower resolution system (i.e. tachometer, commcoder, or older resolver based system) it is often worthwhile to consider a high resolution sine-encoder feedback device, with resolution ≥ 220 counts per revolution (CPR). Doing so will give more flexibility when matching rotor inertias.

As a general rule, when improving feedback resolution with a high resolution device and aiming for a successful servo crossover, the servo replacement should have at least one third of the inertia of the existing motor, though it's preferred to have one half. This method has been successfully applied in many applications. 

Speed and Torque

Speed and torque matching is equally as important for a successful servo crossover. The performance of the replacement motor should meet or exceed the performance of the existing motor. It is important to review the catalogue values of each (i.e. continuous torque, rated speed) to ensure there are no shortcomings.

It is also critical to compare torque values over the entire speed range of each motor. Comparing graphically may be a helpful exercise. This can be done by comparing motor speed/torque curves and manually plotting like-values in a spreadsheet. For example, at 1000 RPM the continuous torque for motor A = X Nm and motor B = Y Nm, and so on for the entire speed range. 

Motor Dimensions

Though not critical to the performance of the motor, a retrofit situation becomes streamlined if the mounting dimensions of the replacement servo are identical to the existing motor. The outline drawings of both motors should be reviewed to ensure consistency and to achieve a successful servo crossover.

This is good practice even when replacing motors with industry standard mounts, like NEMA or IEC. Standards typically have consistent pilot and bolt circle dimensions, but often do not maintain the same shaft dimensions. Though you are replacing a NEMA 34 motor, one manufacturer's definition may be drastically different than another's.

Additional Considerations

There are additional considerations when working towards a successful servo crossover. For example, is the motor the only part of the machine being replaced?

Typically a servo replacement will mean replacing the drives, cables, and in rare instances even the controller. In this case the difference in motor windings can become a secondary consideration assuming manufacturer recommendations are followed.

However, careful review will be required if the replacement motor is intended to be used with an existing drive. Winding data (including motor constants: Kt and Ke), feedback device type and resolution, and cable pin-outs are just a few of the pieces that must be closely examined and matched. 

Furthermore, different servo motor manufacturers often utilise different units and commutation methods for these critical parameters.  This can leave a lack of clear distinction between definitions and units of the motors being compared.


Hurley Gill of Machine Design discusses and further lists some of the common specification inconsistencies that should be considered when looking to achieve a successful servo crossover.

This article was originally authored by Josh Bellefeuille and reposted by Heason Technology with the kind permission of Kollmorgen. 

We are the Gold Distribution Partner and Systems integrator for Kollmorgen in the UK.  

Take a look at our Servo Motor page for further product information.

Contact our highly experienced servo engineering team on +44 (0) 1403 792 300 to allow us to help you benefit from deploying Kollmorgen AKM servo motors on your machinery. 

Article published on: 08/12/2015

Article last updated on: 08/12/2015