FULL GUIDE
A practical robot joint motor selection process
A robot motor should be selected as part of the complete actuation assembly. Start with the required joint output, work backward through the reducer and drivetrain, then confirm that the motor can provide the required torque and speed without exceeding electrical or thermal limits.
If a specification is not available in the catalog or supplier documentation, treat it as missing information. Do not infer a motor value from another model, product family, marketing description, or visually similar product.
1. Start With the Joint Requirement
Define what the robot joint must do before comparing motor models.
Capture:
- - Continuous joint torque
- - Peak joint torque
- - Target output speed
- - Acceleration profile
- - Motion range
- - Payload
- - Gravity load
- - External forces
- - Duty cycle
- - Available joint envelope
- - Target mass
- - Bus voltage
- - Reducer architecture
- - Expected operating temperature
- - Control requirements
Separate requirements from preferences.
A lighter motor may be preferred, but torque-speed capability, voltage compatibility, thermal limits, and mechanical integration are requirements that must be confirmed before a candidate is shortlisted.
2. Convert Joint Requirements Into Motor Requirements
If a reducer is used, the required motor torque and speed depend on the selected reduction ratio and drivetrain efficiency.
Motor speed ≈ Joint speed × Reduction ratio
Motor torque ≈ Joint torque ÷ (Reduction ratio × Efficiency)
These equations are simplified sizing relationships.
Actual selection should also account for acceleration, drivetrain losses, inertia, duty cycle, thermal limits, and transient loading.
Do not assume one constant efficiency across every operating condition unless supplier data supports it.
| Input | Why it matters |
|---|---|
| Joint output torque | Defines the required output-side load |
| Joint speed | Determines required motor speed through the reducer ratio |
| Reduction ratio | Transforms motor speed and torque requirements |
| Efficiency | Affects required motor torque and heat generation |
| Acceleration | Adds transient motor torque requirements |
| Duty cycle | Determines whether the motor can sustain the load thermally |
3. Distinguish Continuous Torque From Peak Torque
Robot motors are often marketed using impressive peak torque numbers.
Peak torque should not be treated as continuous capability.
Continuous torque describes the load the motor can sustain under specified thermal conditions.
Peak torque describes short-duration capability and is normally limited by current, winding temperature, magnets, driver limits, or other constraints.
| Rating | Selection question |
|---|---|
| Continuous torque | Can the motor sustain the expected repeated operating load? |
| Peak torque | Can it handle acceleration and transient events for the required duration? |
| Peak current | Can the motor and driver safely support the required transient current? |
| Thermal basis | Under what cooling and ambient conditions was continuous torque rated? |
A motor can pass a short bench test and still overheat during repeated robot operation.
4. Use the Torque-Speed Curve
Motor torque cannot be evaluated independently from motor speed.
A candidate should be checked across the expected operating region, not only at zero speed or one nominal point.
The required robot motion may include:
- - Low-speed holding
- - Acceleration
- - Continuous movement
- - High-speed motion
- - Reversal
- - Deceleration
- - Repeated cycles
The motor should remain inside an acceptable operating region throughout the required profile.
If the supplier publishes a torque-speed curve, use the curve for the exact motor and voltage condition whenever possible.
Do not apply a curve from a neighboring motor model without confirmation.
5. Check Voltage and Current Compatibility
The motor must be compatible with the robot's electrical architecture.
Confirm:
- - Nominal bus voltage
- - Maximum allowable voltage
- - Continuous current
- - Peak current
- - Winding resistance
- - Winding inductance
- - Driver current capability
- - Connector and wiring requirements
A motor that is mechanically suitable may still be a poor system choice if it requires a different voltage architecture or exceeds the available driver current.
| Question | Why it matters |
|---|---|
| What is the system bus voltage? | Defines the electrical operating environment |
| What continuous current is required? | Affects wiring, driver sizing, and thermal behavior |
| What peak current is required? | Determines transient driver capability |
| Is the selected winding compatible? | Motor variants may have different electrical characteristics |
6. Understand Motor Constants
Several motor constants can help compare motor behavior, but they should be interpreted together.
Torque constant (Kt)
Describes torque produced per unit current.
Back-EMF constant (Ke)
Relates motor speed to generated voltage.
Speed constant (Kv)
Commonly expresses speed per volt.
Motor constant (Km)
Provides a useful indication of torque capability relative to resistive heating.
Do not rank robot motors using Kv alone.
A motor with a high Kv is not automatically better or worse for robotics.
The appropriate constants depend on the required torque-speed operating point, voltage architecture, reducer ratio, and thermal design.
7. Evaluate Thermal Limits
Thermal behavior is often one of the most important constraints in compact robot joints.
Copper losses increase with current and winding resistance.
Mechanical and electromagnetic losses can also contribute heat at higher speeds.
The motor is usually mounted close to other heat-producing components such as the reducer, driver, brake, and electronics.
Check:
- - Winding temperature limit
- - Case temperature
- - Ambient temperature
- - Continuous current basis
- - Cooling method
- - Conduction path into the housing
- - Airflow if applicable
- - Duty cycle
- - Neighboring component temperatures
A continuous torque rating is meaningful only together with the thermal conditions under which it was measured.
If those conditions are not published, keep them as an open supplier question.
8. Motor and Reducer Must Be Selected Together
The motor and reducer form one drivetrain.
Changing the reduction ratio changes:
- - Required motor speed
- - Required motor torque
- - Reflected inertia
- - Efficiency
- - Backdrivability
- - Control behavior
- - Thermal load
A motor that looks too small at the joint output may be completely adequate with the correct reducer.
Conversely, increasing the reduction ratio simply to reduce motor torque can create unacceptable output speed or control behavior.
Motor Reducer Output Bearing Joint Output
Read How to Select a Reducer for a Robot Joint for the reducer-side requirement process.
9. Consider Rotor Inertia and Dynamic Response
Motor inertia influences the dynamic behavior of the actuator.
In fast robot joints, acceleration and reversal can require substantial torque simply to accelerate the motor and drivetrain.
Compare:
- - Rotor inertia
- - Reflected load inertia
- - Reducer ratio
- - Acceleration requirement
- - Reversal frequency
- - Control response
The appropriate inertia relationship depends on the complete actuator and control architecture.
Avoid applying one universal inertia-ratio rule to every robot joint.
10. Frameless or Housed Motor?
Robot joints commonly use both frameless motors and conventional housed motors.
| Consideration | Frameless Motor | Housed Motor |
|---|---|---|
| Packaging | Can be integrated directly into the joint | Requires its own motor housing |
| Joint size | Can enable compact custom architectures | May increase total package size |
| Thermal path | Depends heavily on customer housing design | Motor housing may provide a defined thermal structure |
| Bearings | Often relies on the joint's bearing architecture | Usually includes motor bearing support |
| Integration effort | Higher | Typically lower |
| Customization | High | Usually more standardized |
| Prototype speed | Can require more mechanical engineering | Often easier to integrate initially |
| Production potential | Can support highly optimized joint designs | Can simplify sourcing and assembly |
Neither architecture is universally better. Frameless motors can be attractive when optimizing actuator packaging, while housed motors can reduce integration complexity during prototype and lower-volume builds.
11. Check Mechanical Integration
Confirm the exact mechanical interface for the selected model.
Check:
- - Outer diameter
- - Motor length
- - Shaft diameter
- - Hollow shaft if applicable
- - Mounting pattern
- - Pilot diameter
- - Connector orientation
- - Cable routing
- - Encoder integration
- - Brake integration
- - Bearing arrangement
- - Total mass
A motor can satisfy torque and speed requirements while failing the joint packaging requirement.
Evaluate the assembled actuator envelope rather than motor dimensions alone.
12. Evaluate Environment and Reliability
Motor suitability may depend on the robot's operating environment.
Relevant factors can include:
- - Ambient temperature
- - Dust
- - Moisture
- - Vibration
- - Shock
- - Corrosion
- - Repeated thermal cycling
- - Continuous outdoor operation
Do not assume an environmental rating unless it is explicitly documented for the exact motor or finished actuator.
For harsh environments, confirm whether the rating applies to the motor itself or only after integration into a sealed housing.
13. Compare Multiple Motor Candidates
Before design freeze, compare more than one technically viable motor.
Joint Requirement Motor Requirement Candidate Motors Torque-Speed Check Electrical Compatibility Thermal Check Mechanical Integration Supplier / Availability Check Prototype Validation Final Selection
| Parameter | Candidate A | Candidate B | Candidate C |
|---|---|---|---|
| Motor type | — | — | — |
| Rated voltage | — | — | — |
| Continuous torque | — | — | — |
| Peak torque | — | — | — |
| Rated speed | — | — | — |
| Maximum speed | — | — | — |
| Continuous current | — | — | — |
| Peak current | — | — | — |
| Torque constant | — | — | — |
| Kv | — | — | — |
| Rotor inertia | — | — | — |
| Diameter | — | — | — |
| Length | — | — | — |
| Weight | — | — | — |
| Thermal information | — | — | — |
| Lead time | — | — | — |
If a value is unavailable, display:
Not publicly disclosed
Do not invent missing values.
14. Evaluate Supplier and Sourcing Constraints
After technical fit, check whether the exact motor can support the build.
Review:
- - Exact model availability
- - Winding availability
- - MOQ
- - Lead time
- - Prototype quantity
- - CAD drawings
- - Electrical drawings
- - Torque-speed curves
- - Thermal data
- - Customization options
- - Future production capacity
- - Second-source availability
- - Supplier technical support
Do not assume price, inventory, MOQ, lead time, or production capacity when those values are not published.
Use Request Quote to capture missing commercial information.
15. Motor Selection Checklist
| Check | Selection question |
|---|---|
| Joint load | Are continuous and peak joint torque requirements defined? |
| Speed | Is the required joint speed and acceleration profile defined? |
| Reducer | Is the motor being evaluated with the intended reduction ratio? |
| Torque | Can the motor supply continuous and peak torque at the required speeds? |
| Voltage | Is the winding compatible with the system bus voltage? |
| Current | Can the motor and driver support continuous and peak current? |
| Thermal | Is the continuous operating condition thermally sustainable? |
| Motor constants | Are Kt, Ke/Kv, and other published constants available for the exact model? |
| Inertia | Is rotor inertia compatible with the required dynamic response? |
| Packaging | Do diameter, length, shaft, mounting, and cable routing fit the actuator? |
| Environment | Are environmental requirements supported by documentation? |
| Sourcing | Are lead time, MOQ, documentation, and exact-model availability confirmed? |