Frameless Torque Motor Guide: Outrunner vs Inrunner for Robotic Joints

Frameless torque motor selection looks simple on paper and becomes complicated on the bench. Most teams can list the target torque, but fewer teams lock the thermal and control assumptions early enough.

For most programs, the first decision is architecture: outrunner or inrunner.

I usually frame it this way: pick the control behavior first, then pick the motor architecture that can support it.

Outrunner vs inrunner: practical differences

1. Outrunner: larger effective torque radius, often favorable for higher torque density in compact envelopes.
2. Inrunner: lower rotor inertia, usually better for fast dynamic response and higher bandwidth control.

The right answer depends on your joint function, not just a headline torque number.

Parameters buyers must compare on one sheet

Request a normalized comparison with these fields:

• Kt (torque constant) and usable current window
• Kv and rated speed range
• Phase resistance/inductance for controller matching
• Rotor inertia for response and stability tuning
• Cogging torque profile at low speed
• Thermal resistance and allowable continuous temperature rise

Without this set, outrunner vs inrunner decisions are mostly guesswork.

If a supplier gives only one “rated torque” value without the test condition, ask for a corrected sheet before continuing.

What advanced buyers compare beyond torque

Experienced teams check:

• Cogging behavior at low speed and micro-motion
• Thermal path and continuous torque stability
• Rotor inertia impact on control tuning
• Mechanical integration tolerance and assembly repeatability
• Manufacturability at pilot and volume stage

Ignoring these factors creates late-stage redesign costs.

Practical architecture mapping

Use this as a first-pass rule:

Then validate with prototype data, not only catalog interpretation.

Catalog numbers are useful for shortlisting, not for final release decisions.

Typical application mapping

Outrunner-frameless concepts are commonly evaluated for:

• Payload-oriented joints with constrained outer envelope
• Direct-drive or low-ratio designs prioritizing peak output

Inrunner-frameless concepts are commonly evaluated for:

• High-dynamic joints with strict response targets
• Applications sensitive to reflected inertia and control smoothness

Test and acceptance package to require from suppliers

Before supplier down-selection, request:

1. Torque-speed and efficiency curves with test fixture definition.
2. Thermal rise report at defined duty cycle and ambient conditions.
3. Cogging waveform data in low-speed region.
4. Rotor inertia measurement method and tolerance band.
5. Batch-level variation statement for winding and magnet assembly.

If the supplier cannot provide repeatable test context, data quality is weak.

OEM sourcing checkpoints

Before final supplier decision, request:

1. Test method definitions for torque, thermal rise, and cogging.
2. Material and winding consistency controls.
3. Matching recommendations with gearbox, encoder, and driver.
4. Engineering support process for design iteration and NPI.

A supplier that can support the full module stack reduces interface risk.

Common failure modes to screen early

During sample validation, look for:

• Thermal drift: NdFeB magnets typically lose 0.11% of residual flux (Br) per °C rise. A 60°C rise can cause a 6-7% torque drop, requiring the controller to push more current, creating a thermal runaway loop.
• Excess low-speed ripple (Cogging): Causes unacceptable end-effector jitter in surgical or precision assembly tasks.
• Bearing preload decay: Assembly tolerance drift that changes bearing preload feel after high-impact stress.
• Harness strain: Lead wire fatigue under repeated cyclic articulation inside the joint cavity.

Quantitative FMEA (Failure Mode and Effects Analysis) Snapshot:

Early detection here can prevent expensive mechanical redesign in pilot phase.

I strongly recommend doing these checks before locking custom tooling. It is the cheapest point in the schedule to fix architecture errors.

Method used in this architecture comparison

This article uses a buyer-side integration method instead of catalog-only ranking:

1. Normalize electrical constants (Kt, Kv, resistance, inductance) on one sheet.
2. Compare inertia and low-speed smoothness behavior under the same load fixture.
3. Validate thermal stability under your real duty cycle, not no-load marketing curves.
4. Score manufacturability and consistency risk before commercial negotiation.

This order prevents “high peak torque but low project reliability” decisions.

Practical test plan template (sample stage)

Boundaries and assumptions

• This guide compares architecture direction, not brand ranking.
• Final choice must be validated with your own fixture, controller, and duty profile.
• If your project includes strict human-interaction safety constraints, add force-control and compliance checks beyond this motor-level comparison.

Source references

• Kollmorgen frameless motor overview: kollmorgen.com
• Celera Motion frameless motor resources: celeramotion.com
• IEC standards portal (for standards discovery and verification): iec.ch

Last reviewed: 2026-05-25

Related pages

• Frameless torque motor product page
• QDD actuator for backdrivable joint design

Inquiry

For frameless torque motor OEM projects and custom joint integration:

• Email: [email protected]
• WhatsApp: +86 18857971991