How to Choose a Torque Motor? An In-Depth Guide

Torque motors can be divided into frameless torque motors and framed torque motors (DD motors). This article will explain the fundamental differences between frameless and framed torque motors, and how to select the right motor for your specific application.

What is a Torque Motor?

A torque motor is a motor designed primarily to control output torque. Its control process focuses more on precise torque output rather than speed or position control. Due to its high torque output and precise control capabilities, torque motors are commonly used in applications requiring high dynamic performance, precise positioning, and stable torque output, such as machine tools, automated production lines, and robotic joints. Torque motors can be categorized into frameless and framed types.

Core Design and Development of Torque Motors

Essentially, torque motors are multi-pole permanent magnet synchronous direct-drive motors. Their core design revolves around three objectives: low-speed constant torque, high torque density, and minimal torque ripple, achieved through breakthroughs in electromagnetics, structure, and materials.

Electromagnetic Design: High Pole Count + Fractional Slots for Smooth Low-Speed Operation

Over the past five years, the main stream torque motors have increased their pole pairs from 12 to 32 or even 64. A higher pole count allows the motor to output rated torque at zero or very low speeds (as low as 0.1°/s), eliminating low-speed crawling or jitter common in traditional motors. Combined with optimized fractional slot concentrated windings (e.g., 48 poles / 324 slots, q=2.25), torque ripple can be reduced to less than 1% of rated torque, achieving ultra-smooth, stall-free operation.

Structural Form: Frameless / Framed Options for All Scenarios

Frameless torque motors (mainstream): No housing, no bearings, no output shaft. The stator is embedded directly into the device, and the rotor mounts directly on the load shaft. Axial length is only about 1/3 of a conventional motor, weight is reduced by 30%+, and hollow structures allow cable routing—perfect for compact spaces like robotic joints.

Framed torque motors (DD motors): Include precision bearings, encoder, and housing. They are plug-and-play and can directly replace servo + reducer systems in rotary stages.

Material Revolution: Rare Earth Magnets + High Conductivity Copper

High-grade NdFeB magnets (e.g., N52H, residual flux ≥1.45T) combined with high-conductivity copper alloys ensure reliable high torque output under wide temperature ranges (-40°C to 125°C) and long-term stable performance.

How to Select a Torque Motor?

In practice, taking a torque motor from datasheet to deployment often encounters the “beautiful specs, failed tuning” dilemma. Here are core guidelines and pitfalls from real-world experience:

Golden Rules for Selection

Torque First, Speed Second: Continuous torque should be ≥1.2–1.5× the steady-state load torque; peak torque should be ≥2× the load impact torque, especially for robot joints with frequent starts/stops.

Inertia Matching: For robot joints, the load-to-motor inertia ratio should be ≤5:1 to avoid vibration or oscillation.

Encoder Accuracy: Standard applications: 23-bit absolute encoder (resolution ≈0.0001°); ultra-precision (semiconductor/medical) may require 29-bit encoders.

Three Critical Pitfalls to Avoid

Misalignment (Fatal): For frameless motors, stator/rotor coaxiality must be ≤0.02mm. Larger deviations cause torque ripple spikes and bearing overheating. Use a dial gauge during installation to ensure strict alignment.

Ignoring Cooling: Torque motors generate high currents at low speed, producing significant heat. For maximum power density or continuous stall operation, design forced liquid cooling or efficient air cooling. In one photovoltaic cleaning robot project, the joint housing acted as a heat pipe evaporator with dielectric coolant circulation, increasing continuous torque density by 4×.

Insufficient Load Rigidity: Direct drive has no reducer buffer; low rigidity can cause resonance. Use integrated hollow structures for robot joints and reinforced cast-iron bases for rotary stages.

Key Tuning Tips & EMI Considerations

Tuning: Enable cogging torque compensation, harmonic suppression, and friction feedforward. Current loop bandwidth should exceed 2kHz (ideally ≥5kHz) to suppress torque ripple. In a surgical robot project, tuning PI parameters to Kp=0.35, Ki=1200 achieved a 0.5ms current response.

EMI: Use a spectrum analyzer to detect EMI.

If noise concentrates at specific frequencies (e.g., 1.2MHz), solutions include three-layer shielding (copper foil + nanocrystalline + conductive fabric) on stator windings and magnetic rings on power lines. Interestingly, increasing PWM frequency from 15kHz to 18kHz can reduce peak EMI by 8dB while increasing switching loss by 5%, avoiding mechanical resonance.

For more information and quotes on frameless and framed torque motors, contact us.