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Finding the Perfect Fit: Challenges and Guidelines for Motor Drop-In Replacements

October 19, 2023

As an engineer, you’re likely familiar with the challenges that arise when considering a switch to a new motor supplier. From issues with the incumbent motor’s performance or reliability, the need for minor modifications to a catalog motor during product development, or a desire to improve product performance, there are a variety of reasons why this change might be necessary. Other reasons include supply chain disruptions, which make it essential to have additional suppliers for critical assemblies like motors, as well as circumstances like the current manufacturer no longer supporting the original motor configuration.

No matter the reason, switching motor suppliers is a challenging situation for designers and engineers, as it requires careful consideration of all available options to ensure the continued success of the product. One such consideration for electric motors is a "drop-in replacement," which refers to a motor that is such a close match to the original motor, that it can simply be dropped in and operate like the original without requiring any special engineering changes to make it fit. This means there is no need to worry about rerouting power leads and connections, ensuring it provides the right amount of torque and required speed, or making any major changes to electronic circuits.

Finding a drop-in replacement motor can be a challenging task and requires extensive knowledge and experience of both the motor and the application. However, with the right collaboration between the application manufacturer and motor manufacturer, it is possible to achieve a drop-in replacement that meets all requirements.

Selecting a Motor Drop-In Replacement

When it comes to selecting a drop-in replacement motor, it is not solely the responsibility of the motor manufacturer or the customer alone. Instead, it is a joint effort that requires collaboration and communication between both parties. The motor manufacturer needs to be provided with basic motor specifications and application requirements from the customer, while the customer can also place special emphasis on critical motor parameters and assembly requirements. This ensures that the replacement motor is suitable for the application and meets all necessary requirements. We cover the specific steps in more detail below.

Step 1: Understanding the Motor Characteristics

The first step is to understand the maximum possible motor characteristics. One can get the motor characteristics from sources like motor label or marking, electrical and mechanical specification sheet*.  Additionally, one can consider looking other technical and quality related document to gather more information which include history of technical issues with motor or unit currently being used by application manufacturer, physical/space constraints for the replacement motor, critical to quality (CTQ) parameters, value additions like electrical connectors, pulley, gear, brake, encoder etc.

*This can be either from the motor being replaced or the closest motor from the same manufacturer

Step 2: Motor Comparison

The second step is to compare the current motor and the proposed motor. To make an informed comparison, it's critical to identify what factors are most important for the particular application; this allows for a more focused and targeted comparison that accounts for the application’s unique requirements and specifications. Below are some of the motor parameters that can be considered: Kindly note this is not an exhaustive list of parameters and may vary based on the motor technology.

Electrical Parameters

Mechanical Parameters

Nominal voltage

Mounting details (hole dia, or tapping, PCD)

No load speed

Boss dia

No load current

Shaft projection and dia


Overall length and dia


Cable details (AWG, insulation material, type(ribbon/individual, UL) and orientation

Back emf constant

Shaft end play and shaft runout

Torque constant

Maximum radial load on shaft with distance/location

Motor regulation, R/k2

Bearing type

Maximum continuous torque

Lubrication type

Maximum continuous current


Maximum coil temperature

Ambient working temperature

Electrical time constant

Ambient storage temperature


Thermal resistance

Driver/controller details

Thermal time constant

Commutation: Sensor/Sensor less

Mechanical time constant

Commutation optimization

Rotor inertia

Number of magnet poles and slots


Detent torque/Cogging torque

Label/marking details, type, size

Step angle/Step per revolution

Maximum output power

Dielectric withstanding voltage

Maximum continuous speed

Maximum input power

Friction torque

Coil type (Bipolar/Unipolar)

Cable connection details

In addition to motor parameters, special or regulatory parameters should also be considered.

Special or Regulatory Parameters include:

  • RoHS, REACH requirements
  • Noise and vibration level requirement
  • Low or high temperature requirement
  • Compatibility with specific chemicals
  • Sealing/Ingress protection requirements
  • Reliability and life requirements
  • Special qualification test requirements
  • Sterilizability (steam/dishwashing, ETO, H2O2 etc.)

Step 3: Type of Drop-In Replacement Motor

The third step is understanding the type of ideal drop-in replacement motor needed. It will typically fall into one of the below categories:

  1. Catalog Motors. If the motor under consideration is a common technology, then a catalog motor may be a drop-in replacement option. This would be an off-the-shelf motor that is readily available and meets the necessary specifications for the application.
  2. Minor Modifications. In many cases, there may be minor differences in parameters between the original motor and the replacement motor, such as mounting dimensions, performance specifications, winding or feedback characteristics, connectors and pinouts, or even overall physical size. These differences may require some level of re-engineering to adapt the replacement motor to the system. In such cases, minor modifications can be made either by the customer or by the motor manufacturer to ensure the motor is suitable for the application.
  3. Custom Projects. A high level of customization might sometimes be required for the replacement motor to fit the application. This can be a considerable investment in terms of time and cost, but it may be necessary in cases where no other option is available or suitable.

Drop-In Motor Replacement Challenges

If a drop-in replacement motor is not chosen properly, significant issues can arise:

  1. Incorrect Fit. Choosing a motor that does not fit properly can lead to issues with installation, alignment, and performance. Mating parts or housing or nearby parts in the application may also interfere with the motor.
  2. Interfacing Issues. Compatibility with the main and auxiliary controllers, drives, and sensors is essential for proper operation. Selecting a motor that is not compatible can lead to issues with wiring, programming, and communication, which can impact performance and reliability.
  3. Load and Speed Issues. A replacement motor must be able to handle the usual load and speed requirements of the system. It can also result in decreased efficiency, decreased output, or even damage to the motor and other components because of different loads and speeds.
  4. Reliability Issues. If a motor is not carefully selected, it can lead to reliability issues that can be costly and time-consuming to fix. This can result in unplanned downtime, decreased productivity, and increased maintenance costs.


When it comes to replacing a motor, relying solely on a comparison of specification sheets can lead to a variety of issues, as the specifications listed on the sheet may not necessarily cover all the end application needs. This means that it’s essential to evaluate the application requirements in addition to comparing specification sheets.

To overcome this problem, it is recommended to test the closest motor and evaluate its performance to determine if it is suitable for the application. This allows for a more comprehensive evaluation of the motor's performance and ensures that it meets all necessary requirements. By testing the motor, one can determine its suitability for the application and make any necessary modifications or adjustments to ensure optimal performance. This approach ensures that the replacement motor is not only a close match on paper but also performs well in the application.

While specification sheets provide a useful starting point, it is critical to evaluate the motor in the context of the application requirements to ensure the best possible replacement option is chosen. Need assistance in selecting an ideal drop-in replacement motor? Reach out to use here – our engineers are ready to help!