Direct-Drive Motor Sizing Guide

The direct-drive motor definition covers what direct drive is and why it demands more from the motor. This guide covers the other half: given an application, how do you actually arrive at the torque, speed and envelope numbers that size the motor?

Step 1: Establish the Duty Cycle, Not Just a Peak Number

A single torque figure is not a specification. Before anything else, write down the load's torque as a function of time: continuous running torque, any peak or transient torque, how long the peak lasts, and how often it repeats. A direct-drive motor sized only to a peak figure will be thermally oversized for continuous duty, or sized only to a continuous figure will be unable to produce a required transient.

• Continuous torque is what the motor must sustain indefinitely without exceeding its thermal limit. This is almost always the binding constraint for direct drive, per the low-speed thermal problem covered in the direct-drive definition.
• Peak or stall torque is only meaningful with a stated duration attached. A motor holding stall torque for two seconds and one holding it for two minutes are different sizing problems even at the same torque figure.
• Duty cycle percentage, the fraction of a repeating cycle spent at each load, lets you compute an RMS-equivalent continuous torque if the load genuinely varies over a short, repeating period rather than sitting at one continuous value.

Step 2: Add Margin Deliberately, Not by Habit

A sizing margin absorbs uncertainty in the load estimate, friction and windage not captured in a first-pass calculation, and future changes to the application. A common starting point is 20–30% on continuous torque, but the right number depends on how well the load is actually characterised: a load measured on an existing prototype needs less margin than one estimated from a CAD model and a guess at friction. Applying a large margin by default, on every axis at once, compounds into a motor two or three times larger than the application needs — size the margin to the actual uncertainty, not as a reflex.

Step 3: Fix the Speed Range and Supply Voltage

Direct drive means the motor turns at the load's actual speed, so the operating speed range comes directly from the application's kinematics, not from a motor datasheet. Note the nominal operating speed and, separately, any maximum speed the application might briefly demand.

The DC bus voltage available (battery pack, supply rail) sets an upper bound on back-EMF at maximum speed, which in turn constrains the winding design. This is the same back-EMF matching question the electrical supply side of any BLDC design has to answer, direct drive or not.

Step 4: Check the Thermal Path, Not Just the Winding's Rated Current

The low-speed thermal problem means a direct-drive motor at or near standstill has little to no rotor-driven airflow to help remove heat. Two motors with identical continuous torque ratings on a datasheet can behave very differently in your application if one assumes forced airflow and yours has none. Establish early what cooling path the motor will actually have: natural convection, contact with a mounting structure that can sink heat, or forced air, and treat the datasheet figure as conditional on matching that assumption, per the general electric motor temperature considerations.

Step 5: Decide How Much Torque Ripple the Application Can Tolerate

Without a gearbox to smooth it, torque ripple and cogging torque reach the load directly. For a servo axis or a precision positioning application this is often a harder constraint than the torque number itself, since ripple shows up as a felt vibration or a tracking error rather than as a simple insufficient-torque failure. A coreless stator removes cogging torque outright rather than reducing it, which is why coreless construction and direct drive pair well for applications where smooth low-speed motion matters.

Step 6: Convert Torque and Envelope Into a Torque Density Target

Once continuous torque, duty cycle and available envelope (the volume or footprint the motor must fit within) are fixed, the sizing question becomes a torque density question: does a motor that fits the envelope exist at the torque density needed, or does the envelope need to grow? This is usually the step that reveals whether direct drive is actually feasible for a given envelope, or whether the application needs to reconsider its packaging constraint.

Worked Example

An application needs to hold a 2 kg load at the end of a 0.3 m arm against gravity, continuously, with occasional moves to a new position. The static holding torque is 2 kg × 9.81 m/s² × 0.3 m ≈ 5.9 Nm. Applying a 25% margin for friction and an imperfectly known payload gives a continuous torque target of about 7.4 Nm. The joint has no meaningful continuous rotation, so speed sizing is dominated by how fast a move needs to complete rather than a sustained RPM. Cooling is natural convection through the mounting bracket only, so the continuous torque figure has to be checked against that specific thermal path rather than a forced-air datasheet number. Ripple matters here, since a jerky hold would be visible at the load — a coreless stator's ripple-free holding torque is a genuine requirement, not a nice-to-have.

Related Terms

• Direct-drive motor — what direct drive is and what it demands, which this guide builds a sizing method on top of.
• Electric motor starting torque — the torque a motor produces from standstill, the operating point direct drive is most often sized against.
• Torque and power density — the reference guide for whether a sized motor actually fits a given envelope.
• Cogging in permanent magnet motors — why a coreless stator removes rather than reduces the ripple that reaches the load in a direct-drive application.
• Sizing your motor before you configure — once you have the numbers this guide walks you to, this is how they map onto the Configurator's fields.

Configure a direct-drive motor →