Robot Motors: Which Type Do You Actually Need?
Choose a robot motor by the kind of motion the joint needs, then size it by torque. Continuous rotation means a DC motor with a gearbox, a held angle means a servo, and precise repeatable steps mean a stepper.
Most builders start with the wrong question. They compare RPM figures across products when the number that decides success is torque at the output shaft after gearing.
The families are not interchangeable. A servo cannot drive a wheel continuously without modification, and a stepper wastes current holding a position a servo holds for free.
On this page: the brushed and brushless split, gearboxes, the torque calculation, servos and steppers, encoders and controllers, then which motor suits which robot.
- Brushed vs brushless DC motors
- Geared motors and why the gearbox matters most
- Sizing robot motors: the torque calculation
- Servos and steppers: when position beats speed
- Encoders and motor controllers
- Which robot motors for which robot
| Motor type | Motion it gives | Needs feedback? | Typical robot use | Watch out for |
|---|---|---|---|---|
| Brushed DC | Continuous rotation, speed varies with voltage | Only if you want closed loop | Drive wheels, simple arms, pumps | Brushes wear out and generate electrical noise |
| Brushless DC (BLDC) | Continuous rotation, high efficiency | Yes — the controller must commutate | Drones, high-performance drivetrains, gimbals | Needs an ESC or BLDC driver, not a plain H-bridge |
| Standard servo | Angle held between roughly 0-180 degrees | Built in | Grippers, pan-tilt heads, legged joints | Torque is rated at stall and drops as it moves |
| Continuous-rotation servo | Continuous rotation, speed commanded | No position feedback | Small differential-drive rovers | You command speed, not distance — add an encoder |
| Stepper | Fixed increments, holds position under power | Usually open loop | 3D printers, CNC, linear axes, camera sliders | Loses steps silently when overloaded |
| Geared DC (gearmotor) | Continuous rotation, torque multiplied | Encoder optional and recommended | Almost every mobile robot drivetrain | Gearbox backlash limits positioning accuracy |
Brushed vs Brushless DC Motors
A brushed DC motor uses physical carbon brushes to switch current inside the motor, and a brushless DC motor moves that switching into an external controller. That single difference explains every other trade-off between them.
Brushed motors are cheaper, simpler to drive, and wear out. The brushes erode, throw carbon dust, and generate electrical noise that upsets nearby logic. For a robot that runs a few hours a week, none of that matters.
Brushless motors are more efficient, run cooler, and last far longer because nothing rubs. They cost more and cannot run from a plain H-bridge — they need an electronic speed controller or a dedicated BLDC driver that knows the rotor position.
Pick brushed for a first mobile robot, a low-duty arm, or anything on a tight budget. Pick brushless when the robot runs for hours, when efficiency decides runtime, or when you need smooth torque at low speed.
- Choose brushed DC when cost, simplicity, and a cheap H-bridge matter more than lifespan
- Choose brushless DC when duty cycle is high, runtime is battery-limited, or heat is a problem
- Coreless motors are a brushed sub-type with no iron core — very light, very responsive, and easy to burn out under stall
- A brushed DC motor controller is an H-bridge; a brushless one is an ESC or field-oriented-control driver. They are not substitutes
Geared Motors: Why the Gearbox Matters More Than the Motor
A gearbox trades speed for torque at a fixed ratio, and it is what makes a small motor useful in a robot. A 100:1 gearmotor delivers roughly 100 times the torque at one hundredth the speed, minus real losses in the gear train.
Gearbox type changes behaviour more than most builders expect. Spur and planetary gearboxes back-drive freely, which is good for compliant arms and bad for anything that must hold a load without power.
Worm gear motors are the opposite. A worm drive is usually self-locking, so the output holds position with the power off, at the cost of lower efficiency and no back-driving.
Backlash is the hidden spec. Every gearbox has a small dead zone when reversing direction, and on a positioning arm that dead zone becomes repeatability error no encoder on the motor shaft can see.
The same backlash tradeoff scales straight up to industrial servo gear units, just with tighter numbers: our SEW-EURODRIVE PxG economy gear unit breakdown covers a factory-grade example where the difference between a 6-12 arcminute tier and a sub-3-arcminute tier decides which axis on a build it belongs on.
| Gearbox type | Efficiency | Back-drivable? | Best for | Trade-off |
|---|---|---|---|---|
| Spur | High | Yes | Budget drivetrains, low-load joints | Noisier, more backlash per stage |
| Planetary | High | Yes | Compact high-torque drivetrains | Costs more for the same ratio |
| Worm | Low to moderate | No — usually self-locking | Joints that must hold load unpowered | Wastes energy as heat, cannot back-drive |
| Harmonic / strain wave | Moderate | Partly | Precision robot arm joints | Expensive, rarely used in hobby builds |
Sizing Robot Motors: The Torque Calculation
Size a drive motor from the torque needed at the wheel, then apply a safety factor of about two. Working the other way — buying a motor and hoping — is how robots end up unable to climb a doorway threshold.
The wheel torque you need is the driving force multiplied by the wheel radius. Driving force has to overcome rolling resistance, any slope you care about, and the acceleration you want.
Work an example. Take a 3 kg robot on 40 mm radius wheels that must climb a 10-degree ramp. The force needed is 3 x 9.81 x sin(10 degrees), or about 5.1 N. At a 40 mm radius that is about 0.20 Nm at the wheel, split across two driven wheels.
Then double it. Real carpet, real bearings, and real acceleration all take a share, and a motor run near its stall torque overheats and drains the pack. Buy for roughly twice the calculated figure and check the result against the current your driver can supply.
Stall current is the number that sizes everything downstream. Read it from the motor datasheet, size the driver above it, and size the battery to hold voltage when two motors stall at once.
Servos and Steppers: When Position Beats Speed
Use a servo when a joint must reach and hold an angle, and a stepper when an axis must move an exact distance repeatedly. Both solve positioning, and they fail in opposite ways.
Standard hobby servos are rated in kg-cm at stall, and that rating is optimistic for continuous holding. A servo held at a heavy load buzzes, heats, and eventually strips its gear train — metal-gear versions survive this longer than nylon-gear versions.
Steppers hold position by energising coils, so they draw current even standing still. They also lose steps silently if overloaded, which means an open-loop stepper axis can drift out of position with no error reported anywhere.
Closed-loop options exist on both sides. A servo with a real feedback pot or magnetic encoder reports its true angle, and a closed-loop stepper adds an encoder that detects and corrects lost steps.
Encoders and Motor Controllers
An encoder turns an open-loop motor into a measurable one, and a motor controller turns a logic signal into the current a motor actually needs. You need both before a drivetrain becomes repeatable.
A quadrature encoder on the motor shaft reports rotation count and direction. Multiply counts per revolution by the gear ratio to get output resolution, which is how a DC motor with an encoder becomes usable for odometry.
Motor controllers are chosen by family and by current. Brushed motors take an H-bridge, brushless motors take an ESC or field-oriented-control driver, and steppers take a driver that sets current limit and microstepping.
Size the controller above stall current, not above running current. A driver rated at the motor's nominal draw will survive a demo and fail the first time the robot pushes something.
Common failure to avoid: powering motor drivers and logic from one unregulated rail. When motors stall, pack voltage sags and the controller resets, which reads as a software bug for days. Give logic its own regulated supply.
- Quadrature encoder — two channels, gives count and direction, standard for closed-loop DC drivetrains
- Hall-effect sensors — built into most brushless motors so the driver knows rotor position
- H-bridge driver — for brushed DC; pick one rated above the motor's stall current
- ESC or FOC driver — for brushless DC; FOC gives smooth low-speed torque, plain ESCs do not
- Stepper driver — sets current limit and microstepping; get the current limit right or the motor cooks
Which Robot Motors for Which Robot
Motor choice follows the robot type more reliably than it follows any spec table. Matching the family to the machine gets you most of the way before any arithmetic.
For a first wheeled robot, use two brushed gearmotors with encoders in the 100:1 to 200:1 range. They are cheap, forgiving, and the encoders let you add closed-loop control later without rebuilding the drivetrain.
For a small arm or gripper, use metal-gear servos and derate the published torque. For a 3D printer, CNC, or linear axis, use steppers with a driver whose current limit you set deliberately.
For a drone, a fast rover, or anything with a real duty cycle, use brushless motors with an ESC. The efficiency gain shows up directly as runtime.
The inversion case worth knowing: on a low-duty demonstration robot, a brushless drivetrain is usually the wrong spend. The motor outlives the project, and the money would have done more in the gearbox or the battery.
Reading motor and kit listings for The Bot Scout, the specification that goes missing most often is stall current. Vendors lead with voltage, RPM, and a torque figure, and leave out the number that sizes the driver and the battery. Treat a listing without it as incomplete and ask before ordering.
For where to buy each of these categories, see the robot parts sourcing directory. For how motors fit alongside the other blocks, see parts of a robot.
What Robot Motors Actually Cost
Robot motor prices split by family more than by brand, so budget by the type you actually need rather than assuming a single number covers "a motor." These are hobbyist retail ranges, not bulk-manufacturing pricing, and a driver or ESC is a separate line item on top.
Stepper motors are the cheapest precision option. A standard NEMA 17 stepper — the size used on most desktop 3D printers and small CNC or linear-axis builds — typically retails for roughly $8 to $15 per unit from US hobby suppliers, with bulk or overseas sourcing running lower and higher-torque or geared stepper variants running higher.
Brushed DC gearmotors from suppliers like Pololu and ServoCity generally run from a few dollars for a small plastic-gearbox motor up into the $20-$40 range for a metal-gearbox unit with an encoder, depending on torque and gear ratio. Standard hobby servos typically run $5 to $20 for plastic-gear units and more for metal-gear, high-torque, or digital servos built to hold load without stripping.
Brushless DC motors cost more up front once the controller is included — a small BLDC motor plus a basic ESC commonly lands in the $15 to $80 range for hobby-scale hardware, climbing well past that for high-performance or field-oriented-control drivers. That added cost is the tradeoff for the efficiency and lifespan brushless motors offer over brushed DC, covered above.
Whatever the family, price the driver alongside the motor. An H-bridge, ESC, or stepper driver rated correctly for the motor's stall current is not optional hardware — skipping it to save a few dollars is the fastest way to burn out a cheap motor on its first stall.
Bottom Line
Robot motors are selected on torque at the output, duty cycle, and the controller they demand — not on headline RPM. Use brushed DC gearmotors for a first drivetrain, brushless with an ESC when runtime and duty cycle matter, servos for held angles, and steppers for repeatable linear axes. Calculate wheel torque, double it, read stall current from the datasheet, then size the driver and battery above that figure. Add an encoder from the start, because retrofitting one usually means rebuilding the drivetrain around it.
Sizing a drivetrain for a specific build? Price the motors, driver, and pack together in the DIY robot cost calculator, then use the sourcing directory to order from a supplier that publishes real datasheets — the stall-current figure is the one you cannot build without.
FAQs
What is the difference between a brushed and brushless DC motor?
A brushed DC motor switches current with physical carbon brushes inside the motor, while a brushless motor moves that switching to an external controller. Brushed motors are cheaper and simpler to drive but wear out and create electrical noise. Brushless motors are more efficient and last far longer, but they need an ESC or dedicated driver rather than a plain H-bridge.
Which motor is used in a robotic arm?
Small robot arms typically use metal-gear servos because each joint must reach and hold a commanded angle. Larger and industrial arms use brushless motors with harmonic or planetary gearboxes and encoders on every joint. Steppers appear on hobby arms where repeatable positioning matters more than holding torque under load.
How do you choose a robot motor?
Start with the motion type: continuous rotation points to a geared DC motor, a held angle points to a servo, and repeatable increments point to a stepper. Then calculate the torque needed at the output, double it for a safety factor, and read stall current from the datasheet so you can size the driver and battery above it.
What is a geared motor and why does the ratio matter?
A geared motor is a motor with a gearbox attached that trades speed for torque at a fixed ratio. A 100:1 gearbox gives roughly 100 times the torque at one hundredth the speed, minus losses. The ratio matters because it, not the bare motor, determines whether the robot can actually move its own weight.
Do I need an encoder on my robot motor?
You need an encoder as soon as you want the robot to travel a known distance, hold a speed under changing load, or use odometry. Without one the motor runs open loop, so commanded speed and actual speed drift apart on carpet, slopes, and low battery. Fitting one later usually means changing the motor, so buy encoder versions from the start.
What is a worm gear motor used for in robotics?
Worm gear motors are used where a joint must hold a load with the power off, because a worm drive is usually self-locking and cannot be back-driven. That makes them useful for lift axes and pan mechanisms. The trade-off is lower efficiency and no compliance, so they are a poor fit for arms that need to yield on contact.
What size motor controller do I need?
Size the controller above the motor's stall current, not its running current. Stall current is the peak the motor draws when the shaft is held, which happens every time the robot hits an obstacle. A driver rated only for nominal draw will survive a demo and fail the first time the robot pushes against something.
How much does a stepper motor cost?
A standard NEMA 17 stepper motor, the size used on most desktop 3D printers and small linear-axis builds, typically retails for roughly $8 to $15 from US hobby suppliers. Higher-torque or geared stepper variants cost more, and a stepper driver to control it is a separate purchase on top of the motor price.
Primary Sources
- Pololu — motors and gearboxes, with stall current and torque data
- Pololu — motor driver and controller selection
- SparkFun — motor and motor driver tutorials
- Adafruit Learning System — DC motors, servos, and steppers
- ServoCity — gearmotor and actuator specifications
- Texas Instruments — stepper motor driver documentation
- ODrive — brushless motor control documentation