How to Size a BLDC Gearmotor: Torque, Speed, Ratio, and Duty Cycle

How to Size a BLDC Gearmotor: Torque, Speed, Ratio, and Duty Cycle

Author: Verna Lyu
Technical reviewer: Wen Liu
Published: October 14, 2026
Last updated: October 14, 2026

BLDC gearmotor sizing guide with TC5BLD300-PLE060 planetary gearmotor
BLDC gearmotor sizing guide illustrated with the TENCH TC5BLD300-PLE060 product in a conceptual industrial setting.

A BLDC gearmotor should be sized from the machine shaft back toward the motor. Start with the required output speed, continuous torque, peak torque, motion cycle, inertia, shaft loads, supply, and operating environment. Then select a practical gear ratio and check the motor, gearbox, driver, and thermal limits as one system.

A brushless motor with a gearbox can meet the target speed and torque only when every component is suitable for the same operating point. Motor wattage alone is not enough.

The process is easier to control when it is broken into five decisions:

  1. Define the load-side speed, torque profile, inertia, and mechanical loads.
  2. Estimate mechanical power and choose an initial ratio from the expected loaded motor speed.
  3. Compare continuous, peak, and RMS torque after allowing for gearbox efficiency.
  4. Examine the gearhead's torque, input-speed, shaft-load, backlash, lubrication, and mounting limits.
  5. Confirm the BLDC driver, power supply, feedback, braking, and protection requirements.

The result is a shortlist, not an approval. The final choice still depends on the torque-speed curves, thermal data, gearbox ratings, and application tests for the exact motor-and-gearbox combination.

Five-step BLDC gearmotor sizing workflow and core equations
A first-pass sizing workflow creates a shortlist; final approval still requires model-specific data.

Why BLDC Gearmotor Sizing Starts at the Load

Specify the torque and speed at the machine shaft before choosing motor power. Two applications can use the same mechanical power and still need different gearmotors. A continuously running conveyor may be limited by temperature rise, while an indexing mechanism may be limited by acceleration torque, repeated shock, or backlash. A vertical axis may also need a holding brake even when its running torque is modest. A wattage figure alone does not reveal which of these limits will govern the design.

The motor, gearbox, and drive can limit the system in different ways:

  • A motor-limited system reaches the motor's continuous torque, peak torque, speed, current, or thermal limit first.
  • A gearbox-limited system reaches the gearhead's allowable output torque, input speed, bearing load, backlash, or life limit first.
  • A drive-limited system reaches the controller or power supply's current, voltage, regenerative-energy, or temperature limit first.

The usable gearmotor rating is the lowest applicable limit of the complete system.

1. Define the Output-Shaft Requirements

Record the requirements where the gearmotor connects to the machine. Keep motor-shaft and output-shaft values separate throughout the calculation.

Output shaft requirements for BLDC gearmotor sizing including torque speed inertia and shaft loads
Begin with speed, torque, inertia, shaft loads, supply, control, and environment at the machine shaft.
Required input What to record Why it matters
Output speed Minimum, nominal, maximum, and allowed variation Establishes the ratio and motor operating-speed range
Continuous torque Load torque during the thermally dominant operating period Screens the motor's continuous region and gearbox rated torque
Peak torque Starting, acceleration, jam-clearing, shock, or braking torque Screens motor peak torque, driver current, and gearbox overload capacity
Peak duration and frequency Seconds per event and events per cycle or hour A peak value without time is not a complete specification
Motion cycle Acceleration, constant speed, deceleration, dwell, and reversals Supports RMS torque and thermal evaluation
Inertia Load, pulley, wheel, screw, coupling, and mechanism inertia Determines acceleration torque and dynamic behavior
Radial, axial, and overhung load Magnitude, direction, and distance from the output bearing May determine the required shaft and gearhead bearing capacity before torque does
Power supply Nominal, minimum, and maximum voltage; continuous and peak current Constrains motor winding and driver selection
Environment Ambient temperature, dust, water, vibration, altitude, and cooling Affects thermal performance, sealing, lubrication, and life
Mechanical envelope Flange, shaft, orientation, length, mass, and cable direction Eliminates combinations that cannot be installed
Control objective Speed, torque, or position; accuracy; communication; braking Determines feedback and driver requirements

For a conveyor or wheel, convert the required tangential force to shaft torque:

T = F × r

where T is torque in N·m, F is tangential force in N, and r is the pulley or wheel radius in m. If chains, belts, screws, or other components sit downstream of the gearmotor, include their efficiency, friction, and acceleration requirements in the load model.

Acceleration torque can be estimated from:

T_acc = J_eq × α

where J_eq is the equivalent inertia referred to the shaft being evaluated and α is angular acceleration. Combine friction, gravity, process force, and acceleration torque for each operating phase instead of hiding them inside one generic margin.

2. Calculate Power, Ratio, and Gearbox Efficiency

Mechanical power

At one operating point:

P (W) = T (N·m) × n (rpm) ÷ 9.55

An output shaft delivering 12 N·m at 140 rpm therefore provides approximately:

P = 12 × 140 ÷ 9.55 = 176 W

This is the mechanical power at the gearbox output. The motor must provide more mechanical power at the gearbox input because the transmission is not lossless.

Power is a useful cross-check, but it does not replace torque-speed data. A motor may have adequate rated power but insufficient torque at the required speed, or adequate peak torque but insufficient continuous thermal capacity. The load-first procedure in Oriental Motor's selection-calculation guide provides a useful independent reference for relating the drive mechanism, load torque, inertia, speed, and operating time.

Initial gear ratio and real output speed

For a reduction gearbox:

i ≈ n_motor ÷ n_output

and:

n_output,theoretical ≈ n_motor ÷ i

Use the expected motor speed under load, not the no-load speed or a headline maximum. If the motor is expected to run at 2,800 rpm and the machine needs approximately 140 rpm:

i = 2,800 ÷ 140 = 20

Choose a standard ratio that the manufacturer actually supplies, recalculate the operating point, and confirm that the driver can regulate the motor across the required speed range.

The reduction ratio itself remains fixed. However, the actual motor speed may fall below its nominal or rated value under load or because of controller and supply limits. Measured output speed can therefore be lower than the simple calculation based on nominal motor speed. For example, the TENCH TC5BLD300-PLE060 page notes that actual output speed may be approximately 2%–20% lower than the calculated value depending on load. A machine that needs tightly regulated speed should use closed-loop control and be verified at the real operating point.

Gearbox efficiency

Estimate output torque with:

T_output,estimated ≈ T_motor × i × η_gearbox

To work back from the load:

T_motor,required ≈ T_load ÷ (i × η_gearbox)

where η_gearbox is expressed as a decimal. Use 0.90 only if 90% efficiency is supported for the selected gearhead, ratio, speed, load, and operating condition—or label it clearly as an early calculation assumption.

There is no universal gearbox efficiency. It varies with gear type, number of stages, ratio, speed, load, lubricant, temperature, seals, and design. If a supplier publishes tested output torque for an integrated gearmotor, that model-specific rating is generally more useful than a value reconstructed from motor torque and a generic efficiency.

3. Check Continuous, Peak, and RMS Torque

Continuous torque

The continuous operating point belongs inside the motor's continuous torque-speed region and below the gearbox's permitted continuous output torque. The driver also needs enough continuous current capacity for that point without overheating.

Do not compare a zero-speed or peak-torque value with a continuous running load. Torque capability can change with speed, winding, drive current, cooling, and control method.

Peak torque

A peak-torque value is useful only when its operating context is known. Record:

  • peak duration and repetition rate
  • motor speed during the event
  • driver peak current and current-limit behavior
  • power-supply voltage drop
  • gearbox intermittent or emergency torque limit
  • shaft, key, coupling, and driven-component limits
  • braking or regenerative energy.

A motor's peak-torque rating does not establish how long that torque may be used, and it does not prove that the gearbox can transmit the multiplied value. Treat the motor and gearhead limits separately.

RMS torque for a variable cycle

For a multi-segment cycle, an initial torque-equivalent calculation is:

T_RMS = √[(T₁²t₁ + T₂²t₂ + ... + Tₙ²tₙ) ÷ (t₁ + t₂ + ... + tₙ)]

Calculate every segment at the same shaft. RMS torque is useful for screening copper-loss-related heating when torque is proportional to current, but it is not a complete thermal model. Low-speed cooling, dwell current, ambient temperature, mounting, repeated peaks, iron loss, driver switching, and gearbox heating still require model-specific data or testing.

Conceptual continuous peak and RMS torque duty cycle graph
Conceptual torque-magnitude profile showing why peak duration, repetition, and RMS torque must be checked separately.

4. Verify the Gearbox as a Separate Component

Once the ratio and torque calculation works, examine the gearhead on its own. Bodine Electric's gearmotor selection guide illustrates why duty, output torque, life, mounting, and gearbox rating remain separate application checks even when the motor calculation passes.

Gearbox limit What to verify
Rated output torque Should exceed the required continuous load under the stated duty and conditions
Intermittent or emergency torque Needs to cover the peak load for its actual duration and repetition
Maximum input speed Needs to remain above the highest commanded motor speed
Radial and overhung load Include sprocket, pulley, belt, chain force, and distance from the output bearing
Axial load Include thrust from screws, helical components, or machine forces
Backlash and torsional stiffness Should meet positioning, reversal, and settling requirements
Mounting and lubrication Confirm orientation, lubricant, sealing, and temperature range
Shock and starts per hour Repeated impact can determine gear and bearing life
Output shaft and key Check bending, torsion, fit, coupling, and stress concentration
Noise and life targets Compare values measured under defined test conditions

If the calculated value T_motor × i × η exceeds the gearhead's mechanical rating, the gearbox—not the motor—sets the usable output.

5. Match the Driver, Power Supply, and Feedback

A standard three-phase BLDC motor needs compatible electronic commutation, so the motor and driver should be evaluated as one system:

  • rated, minimum, and maximum DC-bus voltage
  • continuous and peak current on a clearly defined phase-current or bus-current basis
  • Hall sequence, encoder type, sensorless operating range, or other feedback
  • speed, torque, or position-control mode
  • acceleration and deceleration ramps
  • current limit and stall protection
  • dynamic braking, regenerative energy, brake resistor, or supply absorption
  • overvoltage, undervoltage, temperature, and fault response
  • PLC I/O, analog command, pulse/direction, or communication protocol
  • cable length, grounding, shielding, and electromagnetic compatibility.

Hall sensors can support commutation and starting, but their resolution may not be sufficient for precise low-speed regulation or positioning. An encoder may be required for tighter speed control, accurate position, or repeatable low-speed motion. See How Does a BLDC Motor Work? for the underlying commutation and feedback principles.

BLDC gearmotor system showing power command status feedback motor gearbox load and regeneration
The motor, gearbox, driver, supply, feedback, and load must be checked as one system.

Worked Example: A 300 W TENCH BLDC Planetary Gearmotor

This example shows how to build a preliminary shortlist. It is not an approved product selection.

TC5BLD300-PLE060 preliminary BLDC gearmotor sizing example
Published motor-side data is combined with a hypothetical load and a clearly labeled 90% efficiency assumption.

Published motor data and calculation assumptions

The TENCH TC5BLD300-PLE060 combines an 86 mm BLDC motor with a PLE060 precision planetary gearhead. The product page identifies the gearhead as PLE060-□; the box represents the selected reduction ratio rather than a separate motor model. Published data includes:

  • rated power: 300 W
  • rated motor speed: 3,000 rpm
  • rated motor torque: 0.96 N·m
  • peak motor torque: 2.87 N·m
  • available reduction ratios including 20:1
  • model-family winding options for 12, 24, 36, 48, and 310 V, each requiring a compatible drive.

Assume that a machine requires:

  • output speed: approximately 140 rpm
  • continuous output torque: 12 N·m
  • short peak output torque: 30 N·m
  • motor operating speed near 2,800 rpm
  • illustrative gearbox efficiency: 90%.

The 90% value is an assumption for this example, not a published or guaranteed efficiency for the PLE060. Final calculations require the efficiency and torque ratings for the exact ratio, stage count, speed, load, lubricant, and temperature.

Ratio, continuous torque, and power

The initial ratio is:

i = 2,800 ÷ 140 = 20

A 20:1 ratio is listed for this product, so it passes the first availability check.

Required continuous motor torque:

T_motor,continuous = 12 ÷ (20 × 0.90) = 0.667 N·m

This is below the published motor rated torque of 0.96 N·m.

Output mechanical power:

P_output = 12 × 140 ÷ 9.55 = 176 W

Estimated gearbox-input mechanical power:

P_input ≈ 176 ÷ 0.90 = 196 W

Under the stated assumptions, the continuous torque and power calculations fall below the published motor-side ratings. The operating point still has to be checked against the model's torque-speed curve and actual cooling conditions.

Peak torque and the remaining limits

Required peak motor torque:

T_motor,peak = 30 ÷ (20 × 0.90) = 1.667 N·m

This is below the published motor peak torque of 2.87 N·m. If the 30 N·m peak occurs at 140 rpm, however, it represents approximately 440 W of mechanical output and cannot be treated as a continuous condition. Its duration, repetition, motor temperature, driver current, supply voltage, and the gearbox's peak rating all need approval.

Check Preliminary result Evidence still required
Ratio availability Pass: 20:1 is listed Exact order code and gear stages
Continuous motor torque Pass under the 90% efficiency assumption Torque-speed curve and temperature rise
Continuous mechanical power Pass under the 90% efficiency assumption Loaded operating point and cooling
Motor peak torque Numerically below the published peak Permitted duration, repetition, and drive current
Gearbox continuous torque Not verified Rated output torque of the PLE060 gearhead at a 20:1 reduction ratio
Gearbox peak torque Not verified Allowed peak or emergency torque and duration
Output speed 140 rpm when the motor runs at 2,800 rpm Loaded motor speed and closed-loop regulation range
Shaft loads, backlash, and life Not evaluated Application geometry and PLE060 mechanical data
Driver and supply Not evaluated Voltage, current, braking, feedback, and protection data

At this stage, the calculation supports a preliminary shortlist. Product approval begins with the missing gearbox, drive, thermal, and application data in the right-hand column.

Choosing the Gearbox Type

TENCH's Brushless DC Gear Motor range includes planetary, worm, right-angle, and other geared configurations. Select the architecture from the machine layout and performance requirements, then verify the exact product data.

Gearbox type Often considered when Main checks
Planetary Coaxial layout, compact torque density, dynamic motion, or controlled backlash is important Stage count, ratio, efficiency, backlash, input speed, shaft loads, and noise
Parallel-shaft spur or helical Straightforward reduction and cost-effective continuous rotation are priorities Package length, gear stages, backlash, bearing load, noise, and lubrication
Worm A compact 90-degree arrangement and high single-stage reduction are useful Load-dependent efficiency, heat, wear, lubrication, backlash, and backdrivability
Bevel or hypoid right-angle The shaft direction must turn 90 degrees but a worm set is not required Efficiency, ratio range, backlash, offset, lubrication, noise, and backdrivability
Planetary spur helical worm and right angle gearbox configuration comparison
Planetary, parallel-shaft, worm, and bevel or hypoid right-angle arrangements solve different packaging and performance problems.

For a compact coaxial drive, a brushless DC planetary gear motor is often a practical candidate, but it is not automatically the best option for every machine. Packaging, accuracy, service life, cost, and shaft-load capacity can change the decision.

A worm gearbox must not be treated as a certified holding brake. Backdrivability depends on ratio, lead angle, friction, wear, lubrication, vibration, and load. Use a rated brake or another safety mechanism when loss of power must not allow motion.

For miniature mechanisms, TENCH also provides Micro Brushless DC Motors & Gearmotors. The same sizing method applies, although small geartrains can be particularly sensitive to shaft load, shock, thermal path, and the difference between intermittent and continuous torque.

Common Sizing Mistakes

  • Selecting by wattage alone. Rated power is meaningful only with torque, speed, duty, and thermal conditions. Begin with the load-side torque-speed points.
  • Using no-load speed for the final ratio. Use the expected controlled speed under load, then verify the real operating range.
  • Multiplying peak motor torque by the ratio and calling it rated output torque. This ignores efficiency, peak duration, driver current, and the gearbox's mechanical limit.
  • Applying one efficiency to every gearbox. An assumed value is acceptable for an early calculation only when it is identified and later replaced with model-specific data.
  • Adding an arbitrary safety factor everywhere. Separate uncertainties such as shock, load estimation, temperature, starts, wear, and required life. Apply the manufacturer's service factor or design margin with its stated basis.
  • Ignoring braking and regeneration. Decelerating a high-inertia load or lowering a vertical load can return energy to the DC bus. The drive and supply must absorb or dissipate it safely.
  • Ignoring pulley or sprocket loads. The torque calculation may pass while the output shaft or bearing fails the overhung-load check.
  • Treating a calculator result as approval. The final review must also cover backlash, bearing life, connector limits, cable routing, contamination, tuning, acoustic requirements, and temperature rise in the machine.

Information to Send a BLDC Gearmotor Supplier

Provide the following information in one request:

  • application and driven mechanism
  • required output speed range and regulation tolerance
  • continuous output torque
  • peak, starting, and braking torque, including duration and repetition
  • full motion cycle, starts per hour, and duty cycle
  • load and mechanism inertia
  • pulley, sprocket, wheel, or screw dimensions
  • radial, axial, and overhung load at the gearmotor shaft
  • available DC voltage and continuous and peak current
  • speed, torque, or position-control objective
  • Hall, encoder, sensorless, PLC, and communication requirements
  • braking, holding, and power-loss behavior
  • flange, shaft, envelope, mounting orientation, and cable requirements
  • ambient temperature, cooling, ingress, vibration, and altitude
  • required life or maintenance interval under stated operating conditions
  • prototype quantity, annual quantity, and project timing.

If the first decision is whether the machine should use a brushed or brushless motor, review Brushed vs Brushless DC Motors for Industrial Equipment before choosing the gearbox and control system.

Frequently Asked Questions

How do I calculate the gear ratio for a BLDC motor?

Divide the expected loaded motor speed by the required output speed: i ≈ n_motor ÷ n_output. Choose an available standard ratio, recalculate the operating point, and check loaded speed, gearbox efficiency, torque limits, and control range. Do not use no-load speed as the final design point.

Does a gearbox increase motor power?

No. A reduction gearbox decreases speed and increases torque while approximately conserving mechanical power before losses. Friction and other losses make the output mechanical power lower than the input mechanical power.

How much safety factor should I use for a BLDC gearmotor?

There is no universal percentage. The margin should reflect the known uncertainties and the manufacturer's service-factor method, including load estimation, shock, starts, temperature, duty, required life, gearbox type, and consequences of failure. Keep continuous, peak, and mechanical-limit checks separate.

What is the difference between continuous and peak torque?

Continuous torque can be sustained under the manufacturer's stated thermal and operating conditions. Peak torque is permitted only for a limited duration and repetition. The motor, driver, gearbox, shaft, coupling, and power supply must all tolerate the event.

Is a planetary gearbox always better than a worm or right-angle gearbox?

No. Planetary gearing is often used for a compact coaxial layout and dynamic torque transmission. A worm, bevel, or hypoid gearbox may suit a 90-degree layout. Efficiency, heat, backlash, backdrivability, shaft loads, lubrication, noise, cost, and mounting must be compared for the exact products.

What can a gear-ratio calculator tell you—and what can it not verify?

A calculator can screen ratio, speed, torque, and power. It cannot approve the final gearmotor because torque-speed curves, thermal limits, gearbox ratings, shaft loads, driver and power-supply compatibility, feedback, braking, environmental requirements, and validation in the machine still matter.

Request a BLDC Gearmotor Selection Review

Send TENCH the required output speed, continuous and peak torque, peak duration, duty cycle, supply, motion profile, shaft loads, dimensions, environment, feedback, and control requirements. TENCH can then review the motor, gearbox, driver, and feedback as one system. Submit your application details to TENCH.

Technical References

How Does a Servo Motor Work? Components and Selection

How Does a Servo Motor Work? Components, Control Loops, and Selection

Author: Verna Lyu
Technical reviewer: Wen Liu, Engineer — approved
Technical review date: September 21, 2026
Publication status: Published September 21, 2026
Primary reader: OEM design engineers selecting a servo system for an industrial machine
Scope: Rotary industrial servo systems; model-specific ratings and machine-safety validation remain outside this article

A servo motor system controls torque, speed, or position by comparing a command with measured motion and correcting the error in real time. The complete axis normally includes a motor, servo drive, feedback device, motion controller, power supply, cables, and mechanical transmission. Selection therefore cannot be based on motor power alone. An OEM engineer must define the motion profile, calculate peak and root-mean-square (RMS) torque, check speed against the motor’s speed–torque curve, reflect load inertia through any gearing, and confirm feedback, voltage, braking, environmental, and safety requirements. This guide explains that workflow for rotary industrial servo systems. It does not replace the motor and drive manuals, machine risk assessment, or application testing.

Industrial servo motor and drive with separate power and encoder cables
Figure 1. Illustrative AI-generated servo motor and drive system, not a TENCH product photo or wiring reference. The thicker motor-power cable and thinner encoder-feedback cable are shown as separate assemblies connected to different drive ports. Actual connectors, pinouts, shielding, grounding, brake wiring, and cable separation must follow the selected motor and drive manuals.

Key Takeaways

  • A servo is a closed-loop motion system, not one universal motor construction.
  • The drive compares a command with feedback and regulates current or torque, velocity, and, when required, position.
  • Motor-mounted feedback does not measure backlash, compliance, or pitch error elsewhere in the machine. A load-side encoder may be required when the actual load position must be controlled directly.
  • Peak torque must cover acceleration and disturbances; RMS torque must remain within the continuous thermal capability for the complete duty cycle.
  • Encoder resolution is not the same as final machine accuracy. Mechanics, feedback location, tuning, stiffness, backlash, thermal growth, and control architecture all matter.
  • The motor, drive, feedback protocol, power supply, brake, cables, gearbox, controller, and safety functions must be confirmed as a compatible system.

Contents

What Is a Servo Motor System?

A servo system is a feedback-controlled actuator that makes a mechanical axis follow a torque, speed, or position command. In industrial equipment, the word servo usually describes the way the axis is controlled rather than one winding or rotor type.

The motor may be a permanent-magnet synchronous motor, a brushless DC motor, a brushed DC motor, a direct-drive motor, or another machine designed for closed-loop control. Modern industrial rotary servos are commonly brushless permanent-magnet motors, but the label alone does not establish the voltage, commutation method, feedback interface, torque capability, or drive compatibility.

A complete servo axis typically contains:

  1. a motion controller or PLC that creates the command;
  2. a servo drive that closes one or more control loops and switches power;
  3. a motor that converts electrical power into torque;
  4. an encoder, resolver, or other feedback device;
  5. the coupling, gearbox, belt, screw, rack, or other transmission;
  6. the driven load; and
  7. protection, braking, cables, grounding, and safety functions.

This system definition is important. Replacing only the motor with a unit of similar wattage does not prove that the feedback, torque–speed curve, connector pinout, drive algorithm, inertia, brake, or thermal behavior will match.

How Does Closed-Loop Servo Control Work?

The controller sends a target to the drive. The feedback device reports actual motion. The drive calculates the difference, applies the control algorithm, and changes motor current through the power stage. This cycle repeats rapidly so the axis can reject disturbances and follow the commanded motion within the system’s limits.

Servo system closed-loop signal path
Figure 2. Simplified self-produced control diagram. Actual loop placement, update rates, feedback paths, communication buses, and safety architecture depend on the selected controller and drive.

Why Are Current, Velocity, and Position Loops Nested?

Industrial servo drives commonly use cascaded loops. The inner current loop responds first and controls torque-producing current, which is closely related to torque for a permanent-magnet servo motor within its operating range. The velocity loop commands the current loop. The position loop, when used, commands the velocity loop.

Control loop Main comparison Typical output to the next inner function Engineering concern
Current or torque loop Commanded current versus measured current Power-stage switching command Current measurement, motor constants, drive current limits, switching and thermal limits
Velocity loop Commanded speed versus estimated or measured speed Torque or current command Load inertia, friction, stiffness, resonances, filters and gain
Position loop Commanded position versus measured position Velocity command Feedback location, following error, backlash, compliance and trajectory

Not every application uses all three loops in the same place. The drive may close all loops, or a motion controller may close an outer position loop while the drive controls velocity and current. Schneider Electric documents current, velocity, and position loops as a cascade in its servo-drive control architecture, while OMRON notes that the drive, motion controller, or both may process the loops depending on the application.

Motor Feedback Versus Load Feedback

An encoder mounted on the motor shaft measures motor motion. This is often called a semi-closed or motor-feedback system. It can provide excellent motor control, but it cannot directly observe gearbox backlash, belt stretch, screw pitch error, coupling compliance, or movement between the motor and the load.

When the actual table, arm, or load position must be measured, a linear scale or external rotary encoder can close an additional load-side position loop. That can improve load-position accuracy, but it also introduces another feedback path and may expose mechanical compliance or resonances that require careful design and tuning. A higher encoder count at the motor does not by itself remove mechanical error.

Which Components Must Be Matched?

The safest selection method is to treat every electrical and mechanical interface as part of one compatibility review.

Component What it does What must be confirmed
Servo motor Produces torque and motion Rated and peak torque, speed–torque curve, inertia, voltage, current, thermal limits, shaft and mounting
Servo drive Regulates the motor and switches power Motor support, feedback protocol, continuous and peak current, DC bus, control modes, communication and protection
Feedback device Reports motor or load motion Incremental or absolute type, resolution, multi-turn behavior, supply, interface, alignment and cable length
Motion controller or PLC Creates trajectories and coordinates axes Command interface, update rate, synchronization, homing, limits and machine sequence
Power and braking system Supplies and absorbs electrical energy Input voltage, peak demand, regeneration, braking resistor or common bus, fusing and disconnects
Mechanical transmission Converts motor motion to load motion Ratio, efficiency, backlash, torsional stiffness, allowable speed, reflected inertia and service factors
Cables and grounding Carry power, feedback and communication Connector pinout, shielding, routing, bend life, electromagnetic compatibility and flex rating
Brake and safety functions Hold or stop the axis under defined conditions Brake purpose, voltage, release timing, vertical-load behavior, safe torque off and machine risk assessment

In a typical industrial servo axis, the motor main-circuit cable and the encoder-feedback cable are separate assemblies connected to different drive ports. Some systems integrate brake conductors into the motor cable or use different connector formats, but the selected manuals determine the pinout, shielding, grounding, routing, and permitted cable length. Do not infer wiring from connector shape or cable color alone.

A holding brake should not automatically be treated as a dynamic service brake. Its permitted stopping duty, emergency role, release timing, and wear limits must be checked in the selected product documentation.

How Do Servo Motors Differ from Stepper and Basic BLDC Systems?

The categories overlap, so the comparison must be based on the complete control architecture rather than the motor name alone. A brushless permanent-magnet motor can be used in a servo system when the drive, feedback, and control functions support the required closed-loop performance. Closed-loop stepper systems also exist.

Decision factor Industrial servo system Typical open-loop stepper system Basic BLDC speed-control system
Primary strength Dynamic torque, speed, or position regulation with feedback Simple commanded incremental motion without continuous position correction Electronic commutation and regulated rotation speed
Feedback Normally motor feedback; optional load feedback Usually none, although closed-loop stepper products exist Hall sensors or sensorless estimation may support commutation; an encoder may be optional
Position control Integrated or coordinated through the drive/controller Position inferred from commanded steps if synchronism is maintained Not guaranteed by the BLDC label; depends on controller and feedback
Tuning Required to match load inertia, stiffness and dynamics Usually limited for open-loop operation Depends on control mode; basic speed control may require less tuning
Best fit Axes with demanding motion profiles, disturbance rejection, coordinated motion, or monitored following error Cost-sensitive point-to-point motion within verified torque and speed margin Fans, pumps, rollers, conveyors, or other speed-controlled loads when precise axis positioning is not required

This table is a starting point, not a universal ranking. The correct choice depends on required speed, acceleration, settling time, load variation, holding behavior, mechanical stiffness, acceptable fault response, integration effort, and cost.

If the earlier decision is whether to use a brushed or brushless motor, see Brushed vs Brushless DC Motors for Industrial Equipment. For electronic commutation and BLDC control in more detail, see How Does a BLDC Motor Work?.

What Determines the Correct Servo Size?

Servo sizing starts with the machine’s motion profile, not a target wattage. Yaskawa identifies speed, maximum torque at speed, RMS torque at speed, and inertia ratio as core sizing factors. Secondary checks include voltage, feedback, environment, space, cost, regeneration, and transmission design.

Servo motor driving a ball-screw linear axis
Figure 3. Illustrative AI-generated ball-screw axis, not a TENCH product photo or installation drawing. The motor, coupling, bearing supports, screw, carriage, load, and base stiffness all affect sizing and tuning.

Define the Complete Motion Profile

For each segment of one repeatable cycle, record:

  • distance or angle;
  • move time;
  • acceleration and deceleration time;
  • dwell time;
  • direction;
  • external force or torque;
  • friction and transmission efficiency;
  • moving mass or inertia; and
  • expected disturbances or process force.

The fastest segment determines required speed. Acceleration, gravity, process force, and friction contribute to torque. A vertical axis also needs a controlled strategy for gravity, brake release, stopping, and power loss.

Check Peak Torque at the Required Speed

Peak torque occurs during acceleration, deceleration, impact, or process-force events. Calculate it from the load and motion profile, add relevant friction and external forces, then check the required torque at the actual motor speed against the manufacturer’s intermittent region and permitted duration.

Peak torque is not a continuous rating. The drive must also supply the required peak current, and the power supply and DC bus must tolerate the event. Repetition rate and thermal state matter.

Calculate RMS Torque for Thermal Loading

For a cycle divided into segments, RMS torque can be estimated as:

TRMS = √[(T₁²t₁ + T₂²t₂ + … + Tₙ²tₙ) ÷ (t₁ + t₂ + … + tₙ)]

where T is motor torque in N·m and t is segment time in seconds. This is a thermal sizing calculation; it does not replace the peak-torque or speed check.

Consider a two-second example cycle:

Segment Torque Time
Accelerate 1.8 N·m 0.3 s
Constant speed 0.8 N·m 1.2 s
Decelerate −1.2 N·m 0.3 s
Dwell or hold 0.2 N·m 0.2 s

The calculated RMS torque is:

TRMS = √[(1.8²×0.3 + 0.8²×1.2 + 1.2²×0.3 + 0.2²×0.2) ÷ 2.0] ≈ 1.04 N·m

The sign of deceleration torque disappears when squared, but regeneration and braking must still be checked separately. The example axis therefore needs at least 1.8 N·m of permitted peak torque at the relevant speed and a continuous thermal capability above 1.04 N·m under the stated installation and ambient conditions.

Servo motion profile and RMS torque example
Figure 4. Self-produced calculation example. The values illustrate the method only and are not ratings for a TENCH motor or drive.

Reflect Load Inertia Through the Transmission

For an ideal ratio N = motor speed ÷ load speed, load inertia reflected to the motor is approximately:

Jreflected = Jload ÷ N²

If the load inertia is 0.004 kg·m² and the motor turns five revolutions for one load revolution, the ideal reflected load inertia is:

0.004 ÷ 5² = 0.00016 kg·m²

If the chosen motor inertia is 0.00004 kg·m², the idealized reflected-load-to-motor inertia ratio is 4:1. The real calculation must also include the coupling, gearbox, screw, pulley, motor rotor, and other moving components, and it must account for transmission efficiency where appropriate.

There is no universal requirement that every servo axis use a 1:1 inertia ratio. A lower ratio generally makes high dynamic performance and tuning easier, but allowable values depend on the drive, motor, mechanism, move profile, stiffness, control bandwidth, and performance target. Use the selected manufacturer’s sizing guidance rather than a single rule of thumb.

Verify Speed, Voltage, Drive Current, and Regeneration

The required operating points must lie within the motor’s continuous or intermittent speed–torque envelope as applicable. Confirm rated speed, maximum speed, field-weakening behavior if supported, continuous current, peak current, DC-bus or supply voltage, and motor back-EMF compatibility.

During deceleration or lowering of an overhauling load, mechanical energy can return to the DC bus. The drive may need a braking resistor, regenerative unit, shared DC bus, or a longer deceleration time. Do not assume that a resistor included in a drive can absorb every machine cycle.

Select Feedback from the Accuracy Requirement

Choose the feedback type only after defining what must be measured:

  • An incremental encoder reports changes in position and normally requires a reference procedure to establish machine position after power-up.
  • A single-turn absolute encoder retains the angular position within one revolution.
  • A multi-turn absolute encoder can also retain revolution count, subject to its operating principle and system configuration.
  • A resolver can be suitable where robustness is more important than very high digital resolution.
  • A load-side encoder or linear scale measures the mechanism closer to the controlled point.

Final accuracy must be budgeted across encoder quantization, interpolation, mounting alignment, backlash, compliance, screw or gear error, thermal growth, controller update, filtering, and tuning. Resolution should never be published as finished-axis accuracy without that analysis.

A Practical Servo Selection Workflow

Use the following sequence so that the selected motor does not become the starting assumption.

Servo motor selection workflow
Figure 5. Simplified self-produced workflow. Final selection still requires the exact motor and drive manuals, sizing calculations, risk assessment, and machine testing.
  1. Define the load and mechanism. Record mass, inertia, external force, friction, transmission type, ratio, efficiency, backlash, stiffness, shaft loads, and installation orientation.
  2. Create the motion profile. State travel, maximum speed, acceleration, deceleration, dwell, cycle time, reversal, stopping, and process force.
  3. Calculate speed and torque. Determine motor speed, peak torque at speed, RMS torque, holding torque, and regeneration for the complete cycle.
  4. Check inertia and mechanics. Reflect load inertia, include transmission components, verify critical speed, torsional resonance, bearing loads, and gearbox limits.
  5. Choose feedback and control mode. Define torque, velocity, position, homing, absolute-position, load-feedback, synchronization, and communication requirements.
  6. Match motor and drive. Confirm voltage, current, feedback protocol, motor data, brake, cables, connectors, firmware support, I/O, network, and protection.
  7. Check installation and environment. Confirm ambient temperature, altitude, cooling, enclosure, ingress protection, vibration, contamination, cable flexing, grounding, and electromagnetic compatibility.
  8. Validate the machine. Test temperature rise, peak events, repeatability, settling, resonance, stopping, brake behavior, fault response, and safety functions under representative conditions.

TENCH lists a DC Servo Motor & Controller product category for project review. The category page should be used as a starting point; the exact motor, drive, feedback, and mechanical requirements still need to be matched to the application.

Where Are Servo Systems a Good Fit?

Servo systems are most valuable when the machine needs controlled acceleration, high-speed torque, disturbance correction, coordinated axes, monitored following error, or repeatable positioning over a demanding motion profile.

Typical mechanisms include ball screws, indexing tables, robot joints, pick-and-place axes, packaging feed axes, cutting or printing registration, tension control, and automated handling. The industry name alone does not prove suitability. A medical or robotic mechanism, for example, may add safety, noise, cleanliness, validation, or regulatory requirements beyond normal motor sizing.

Servo motor and planetary gearbox driving a rotary indexing table
Figure 6. Illustrative AI-generated indexing-axis arrangement, not a TENCH product photo or installation drawing. The gearbox ratio, backlash, torsional stiffness, output bearing, mounting, and table inertia must be checked for the actual mechanism.

TENCH’s Anjian Medical Equipment project provides a public example of a brushless motor used in an intelligent robotic-arm context. It is application context only, not proof that the same motor, feedback, drive, or safety architecture fits another machine.

Common Servo Selection and Commissioning Mistakes

Selecting by Power Alone

Two motors with the same rated power can have different rotor inertia, rated speed, peak torque, continuous torque, winding voltage, encoder, thermal behavior, and physical envelope. Check the required operating points and complete axis, not the wattage label.

Treating Peak Torque as Continuous Torque

Peak torque is limited by drive current, motor magnetic and thermal limits, speed, and permitted duration. Repeated peak events raise RMS torque and may overload the motor or drive even when each event is brief.

Equating Encoder Resolution with Machine Accuracy

High feedback resolution can improve control information, but it cannot remove mechanical backlash, compliance, pitch error, or thermal movement. Feedback location determines which errors the loop can observe.

Using a Universal Inertia-Ratio Rule

An inertia ratio is a design and tuning input, not a pass/fail number that applies to every axis. Verify it against the selected motor–drive system, motion profile, mechanical stiffness, bandwidth, and settling requirement.

Tuning Around a Mechanical Problem

Loose couplings, backlash, flexible frames, resonance, poor alignment, damaged bearings, or unsuitable gearboxes cannot always be corrected with higher gains or filters. Inspect the mechanics before increasing control aggressiveness.

Dial-indicator check of servo coupling alignment
Figure 7. Illustrative AI-generated pre-commissioning alignment check. The measurement method, allowable runout, coupling installation, and safety procedure must follow the component manuals and the machine builder’s validated process.

Ignoring Regeneration and Vertical-Axis Risk

Fast deceleration and gravity-driven loads can return energy to the bus. Confirm braking hardware, overvoltage behavior, stopping sequence, holding brake, and safe response to power loss.

What Information Should You Send a Servo Supplier?

For a useful first review, send one complete axis requirement rather than a preferred motor wattage:

  • mechanism drawing and transmission type;
  • moving mass and calculated load inertia;
  • travel or rotation per cycle;
  • maximum speed and motor-side speed;
  • acceleration, deceleration, dwell, and total cycle time;
  • continuous, peak, friction, gravity, and process torque or force;
  • required positioning accuracy, repeatability, settling time, and feedback location;
  • available supply voltage and controller or PLC interface;
  • communication network, I/O, homing, brake, and absolute-position requirements;
  • mounting, shaft, cable, connector, and envelope constraints;
  • ambient temperature, altitude, cooling, dust, moisture, vibration, and cable-flex conditions; and
  • prototype schedule and estimated quantity.

If calculations are incomplete, provide the mechanism dimensions, masses, ratios, move distances, and timing so they can be reviewed. Clearly identify which values are measured, calculated, estimated, or still unknown.

Frequently Asked Questions

Is a servo motor always an AC motor?

No. Servo describes closed-loop control of torque, speed, or position; it does not define one motor topology. Industrial servo systems may use brushless permanent-magnet motors, brushed DC motors, direct-drive motors, or other constructions. The selected motor must match the drive’s voltage, current, feedback, commutation, motor-data, and protection requirements.

Can a servo motor run without an encoder?

Some drives support sensorless control in limited modes, but a conventional positioning servo normally relies on feedback from an encoder, resolver, or other device. Whether sensorless operation is acceptable depends on startup, low-speed torque, position, safety, and disturbance requirements. Never assume that a motor marketed as a servo will meet positioning requirements without compatible feedback.

Does a higher-resolution encoder guarantee better positioning accuracy?

No. Resolution describes the smallest feedback increment the system can represent; accuracy describes closeness to the true position. Mechanical backlash, compliance, pitch error, thermal growth, alignment, feedback location, controller update, filtering, and tuning can dominate the final result. Use an axis-level error budget instead of quoting encoder counts as machine accuracy.

How should peak torque and RMS torque be used?

Peak torque verifies that the motor and drive can complete the most demanding acceleration, deceleration, or process event at the required speed and for the permitted time. RMS torque estimates continuous thermal loading over the full cycle. Both checks are required, and each operating point must remain inside the applicable speed–torque region.

Is a 1:1 load-to-motor inertia ratio always required?

No. A lower reflected-load-to-motor inertia ratio can make high-bandwidth tuning easier, but 1:1 is not a universal requirement. Acceptable ratios depend on the drive, motor, transmission, mechanical stiffness, motion profile, settling target, and disturbance level. Calculate the complete reflected inertia and confirm the result with the selected manufacturer’s sizing guidance.

Request a Servo-System Review

Send TENCH the mechanism drawing, motion profile, load inertia, peak and RMS torque, motor-side speed, supply voltage, feedback and communication needs, installation limits, environment, and estimated quantity. TENCH can then review the DC servo motor and controller as one system. Submit the axis requirements to TENCH.

Technical References

About the Author and Reviewer

Verna Lyu prepares practical motor-selection and application content with input from the TENCH factory and engineering team.
Wen Liu, Engineer, reviewed and approved the technical content on September 21, 2026.

Brushed vs Brushless DC Motors for Industrial Equipment

Brushed vs Brushless DC Motors for Industrial Equipment

Author: Verna Lyu
Publication date: September 20, 2026
Technical reviewer: Wen Liu, Engineer
Last technical review: September 20, 2026
Scope: Permanent-magnet brushed DC motors and brushless DC motor systems used in industrial equipment

Choose a brushed DC motor when the application favors simple control, low initial cost, limited operating hours, and easy service access. Choose a brushless DC (BLDC) motor when long duty cycles, reduced routine maintenance, high power density in a compact envelope, or closed-loop speed, torque, or position control matter more. Compare the complete drive system at the required mechanical load in the machine. This guide supports preliminary selection; it does not replace model-specific performance, thermal, controller, environmental, or duty-cycle validation.

Cutaway comparison of a brushed DC motor and a brushless DC motor with controller

Figure 1. Illustrative cutaway, not to scale. A brushed motor switches armature current through brushes and a commutator. A BLDC system uses electronic commutation.

Contents

Quick Selection Summary

A brushed DC motor is usually the starting point for a cost-sensitive mechanism with intermittent duty, basic speed control, and acceptable brush service. A BLDC motor is usually the starting point for extended operation, limited maintenance access, compact packaging, or controlled low-speed and dynamic motion. Neither type can be approved without checking its actual operating point.

Choose a brushed DC motor when most of these conditions apply:

Choose a BLDC motor when most of these conditions apply:

How Commutation Changes the Motor System

The main difference is how current is switched to keep producing torque. That difference affects maintenance, control hardware, wiring, noise, and integration risk.

In a typical permanent-magnet brushed DC motor, stationary brushes contact a rotating commutator, which reverses current in the rotor windings as the shaft turns. The motor can run from a DC source; PWM regulates speed, and an H-bridge can add reversing and braking.

In a three-phase BLDC motor, permanent magnets rotate and the windings remain on the stator. A controller switches phase current according to rotor position, obtained from Hall sensors, an encoder, a resolver, or sensorless estimation. A plain DC connection is not a complete BLDC drive.

Diagram comparing mechanical and electronic commutation in DC motors

Figure 2. Mechanical commutation places the switching function inside a brushed motor. Electronic commutation moves that function into the BLDC controller and feedback system.

Brushed and Brushless Motor Comparison

Use this table for screening; final approval requires candidate-specific data.

Decision factor Brushed DC motor Brushless DC motor
Commutation Mechanical brushes and commutator Electronic controller or inverter
Basic drive Can run from suitable DC; PWM or H-bridge adds control Requires compatible electronic commutation
Routine wear Brushes and commutator are wear items No brushes; bearings and other system components still wear
Maintenance Brush inspection or replacement may be required Usually less routine motor maintenance, but not maintenance-free
Low-speed and start control Direct torque response; controller and supply still limit current Depends on controller, feedback method, tuning, and current capacity
Electrical and acoustic noise Brush contact can cause arcing, EMI, and mechanical noise Removes brush arcing, but PWM, commutation, bearings, and the load can still create noise
Efficiency Must be checked at the operating point Often favorable in comparable systems, but controller and gearbox losses must be included
Initial system cost Often lower for a simple drive Often higher because controller and integration are required
Service interval Often constrained by brush and commutator condition Often longer, subject to bearings, temperature, load, speed, and environment
Typical fit Simple, intermittent, serviceable, cost-sensitive equipment Extended-duty, low-maintenance, compact, or precisely controlled equipment

How to Make the Selection

Start With Duty Cycle and Maintenance Access

Duty cycle changes the value of brushless operation. A brushed motor may suit an actuator that moves for a few seconds per cycle. The same brush service may be unacceptable in a two-shift conveyor or inaccessible equipment.

Record daily operating hours, starts per hour, direction changes, dwell time, peak-load duration, and available cooling. Do not describe an application only as “continuous” or “intermittent” when the real cycle can be measured.

Define Torque and Speed at the Same Operating Point

Rated power alone is not enough. Specify continuous torque, starting or peak torque, permitted peak duration, minimum speed, normal speed, maximum speed, and acceleration time. Review the torque-speed curve and thermal limit at the intended supply voltage.

Mechanical output power can be estimated from torque and speed:

Power (W) = Torque (N·m) × Speed (rpm) ÷ 9.55

This calculation checks whether the stated torque and speed are consistent. It does not establish motor size by itself because winding temperature, controller current, cooling, and gearbox limits still apply.

Define the Control Requirement

Simple one-direction rotation may need only PWM speed adjustment. Reversing, active braking, stable low-speed operation, positioning, synchronization, and torque control may require additional controller functions or feedback.

Traditional sensorless back-EMF detection becomes difficult at standstill and very low speed because the measurable signal decreases with speed. When reliable starting under load or repeatable low-speed motion is required, evaluate Hall sensors or an encoder with the selected controller.

Compare Complete Lifecycle Cost

Compare the motor, controller, feedback, gearbox, integration, maintenance, downtime, and energy use at the real duty point. A lower purchase price may not reduce machine cost, while a lightly used device may not recover the added cost of a BLDC system.

Worked Operating-Point Example

Assume a machine needs 0.20 N·m at 1,800 rpm during normal operation. Its mechanical output requirement is:

0.20 N·m × 1,800 rpm ÷ 9.55 = 37.7 W

Now add the conditions that actually influence selection: the load requires 0.60 N·m for 1 second at startup, operates 16 hours per day, and sits behind a protective cover with limited airflow. The 37.7 W result is only the steady mechanical output. The selected system must also handle the startup current, stay within continuous winding and controller temperature limits, and repeat the cycle without exceeding the motor or gearbox ratings.

For this duty, a BLDC system is a reasonable starting point because long operating hours make service interval and thermal management important. It is not an automatic approval. A brushed candidate could still be valid if product-specific brush-life, temperature, torque-speed, and maintenance data meet the machine's requirements.

Controller and Feedback Checks

A BLDC selection is incomplete until the motor and controller are matched. Confirm voltage, continuous and peak phase current, Hall sequence, encoder supply and signal type, control inputs, braking, protection, and communication.

BLDC motor with controller, Hall sensor board and three-phase current illustration

Figure 3. Simplified illustration, not to scale. Motor construction, power electronics, rotor-position feedback, current control, wiring, and software affect the final result.

Check regenerative energy during deceleration. A supply that delivers current may not absorb returned energy; the controller may need a braking resistor, bus-voltage protection, or a regenerative power source.

For a brushed motor, confirm continuous and peak current, PWM frequency, reversing, braking, and interference suppression. Simpler commutation does not remove controller and supply protection.

Application Limits and Common Mistakes

Selecting by the Technology Label

“Brushless” does not guarantee better efficiency, lower noise, longer life, or better low-speed motion in every comparison. Those results depend on the specific motor, controller, load, installation, and test point.

Confusing Peak and Continuous Torque

Peak torque is time-limited by current and temperature. If the load requires the peak value continuously, the motor is undersized even when it can start the machine once.

Ignoring the Gearbox

A gearbox changes speed and torque but adds loss, backlash, shaft-load limits, and its own thermal and service-life constraints. The calculated output torque must remain within the gearbox's continuous and peak ratings.

Treating the Motor Type as an Environmental Rating

Brushed and brushless motors can both be designed for demanding environments, but the motor type alone does not establish IP protection, corrosion resistance, washdown suitability, or hazardous-location approval. Verify the complete assembly and its documented rating.

Six-step checklist for selecting a motor, controller and gearbox system

Figure 4. Use the same requirement set to compare brushed and BLDC candidates. Do not change assumptions between suppliers.

Information to Send a Motor Supplier

Send the same requirement set for every candidate so quotations are comparable:

TENCH can use this information to review a micro brushless DC motor or gearmotor or a high-power brushless DC motor with its controller and gearbox. See the Anjian medical equipment project for a documented brushless application and the TENCH motor selection FAQ for product-specific controller and duty-cycle questions.

Frequently Asked Questions

Can a BLDC motor run without a controller?

No. A BLDC motor requires electronic commutation. The controller may be external or integrated into the motor assembly, but compatible drive electronics are still part of the system. Confirm voltage, current, feedback, phase sequence, control inputs, and protection before connecting the motor.

Do brushless motors require maintenance?

They do not require brush replacement, but they are not maintenance-free. Bearings, seals, cooling surfaces, cables, gearbox lubrication, and the controller may still require inspection. The interval depends on load, speed, temperature, vibration, contamination, and the product's instructions.

Which motor lasts longer?

A BLDC motor often has a longer service interval because it has no brush-commutator wear. Actual life still depends on bearings, shaft load, speed, temperature, vibration, contamination, and controller setup. Compare product-specific data under relevant conditions.

Is a sensorless BLDC motor suitable for low-speed operation?

It depends on the controller algorithm and the load. Conventional back-EMF methods have the least position information at standstill and very low speed. Hall sensors or an encoder are often the safer starting point when the machine must start under load or maintain repeatable low-speed motion.

Can brushed or brushless motors be used outdoors?

Either type can be used when the complete system has suitable documented protection. Check IP rating, sealing, corrosion protection, insulation, drainage, condensation, temperature, and cable entry. Confirm any regulated or hazardous-location certification separately.

Request a Motor-System Review

Send TENCH your voltage, continuous and peak torque, speed range, duty cycle, installation dimensions, control method, environment, and estimated quantity. We can compare the motor, controller, feedback, and gearbox requirements before recommending a configuration. Submit your application details to TENCH.

Technical References

How Does a BLDC Motor Work? Commutation and Control

How Does a BLDC Motor Work? Components, Commutation, Sensors, and Control

Author: Verna Lyu
Publication date: September 20, 2026
Technical reviewer: Wen Liu, Engineer — approved
Technical review date: September 20, 2026
Primary reader: OEM design engineers integrating BLDC motors into industrial equipment
Scope: Three-phase permanent-magnet BLDC motor systems used in industrial equipment

A brushless DC (BLDC) motor turns because an electronic controller energizes the stator phases in a sequence that keeps the stator magnetic field ahead of the permanent-magnet rotor. The controller needs rotor-position information from Hall sensors, an encoder, or a sensorless estimator. It then uses a three-phase inverter to switch current through the windings. The motor, controller, feedback method, power supply, load, and protection settings must therefore be selected as one system.

This guide is limited to three-phase permanent-magnet BLDC systems. It does not cover brushed DC, stepper, or induction motors. Model-specific voltage, current, torque, speed, thermal, feedback, and controller limits must still be confirmed from the relevant specifications and validated in the actual machine.

Illustrative cutaway of a brushless DC motor connected to a three-phase controller
Figure 1. Illustrative AI-generated cutaway, not a TENCH product photo or dimensional reference. A practical BLDC system includes the motor, controller or drive, power source, feedback or estimation method, and protection settings.

Key Takeaways

  • A standard three-phase BLDC motor without integrated electronics does not commutate itself. A compatible controller and power stage are required.
  • In six-step commutation, each electrical revolution is divided into six 60-degree sectors. Two phases are driven while the third is normally left floating.
  • Hall sensors provide discrete rotor-position information at standstill and low speed. Basic back-EMF sensorless control needs sufficient rotor speed before the signal becomes reliable.
  • PWM regulates phase voltage or current, but PWM is not the same as commutation.
  • Six-step control is comparatively simple. Field-oriented control (FOC) can reduce torque ripple and improve low-speed control when correctly implemented, but it usually requires more detailed current and rotor-angle information, computation, and tuning.
  • “BLDC,” “PMSM,” “sensored,” and “sensorless” are not complete selection specifications. The motor winding, back-EMF shape, pole count, controller algorithm, load, speed range, and thermal limits must be compatible.

Contents

What Parts Make a BLDC System Work?

A working BLDC installation contains more than the motor. At minimum, it needs a permanent-magnet rotor, a wound stator, a three-phase power stage, control logic, a rotor-position source, and electrical and thermal protection.

Part Function Integration check
Permanent-magnet rotor Produces the rotor magnetic field Pole count, magnet temperature limit, inertia, maximum speed
Wound stator Produces the rotating magnetic field when its phases are energized Winding connection, resistance, inductance, voltage, current, thermal path
Bearings and shaft Support the rotor and transmit mechanical output Radial and axial load, mounting, speed, lubrication, service life
Position source Tells the controller where the rotor is, directly or by estimation Hall sequence, encoder format, alignment, signal voltage, sensorless operating range
Three-phase inverter Switches DC-bus power into the motor phases Bus voltage, continuous and peak current, switching devices, heat dissipation
Control and protection Determines commutation, PWM, speed or torque loops, and fault response Algorithm, current limit, overvoltage, undervoltage, temperature, stall behavior

The stator is fixed to the housing, so winding heat can be conducted toward the frame. The rotor carries permanent magnets and turns with the shaft. Removing the brush-and-commutator interface eliminates brush wear and brush arcing, but it does not eliminate bearing wear, winding temperature limits, controller losses, or maintenance elsewhere in the machine.

Exploded illustration of a BLDC motor showing the stator, rotor, bearings, shaft, end caps, and sensor board
Figure 2. Illustrative AI-generated exploded view, not a production drawing. The exact rotor, winding, bearing, and sensor construction varies by motor model.

What Happens During Rotation?

The controller repeats five functions while the motor runs:

  1. It obtains or estimates rotor position.
  2. It selects the phase-current state that should produce torque in the commanded direction.
  3. The inverter switches the motor phases according to that state.
  4. Stator current produces a magnetic field that interacts with the rotor magnets.
  5. The controller advances to the next state as the rotor moves.

This sequence is synchronized in electrical angle, not directly in mechanical shaft angle. If a motor has p pole pairs, one mechanical revolution contains p electrical revolutions:

Electrical frequency (Hz) = pole pairs × mechanical speed (rpm) ÷ 60

For example, a motor with four pole pairs running at 3,000 rpm has an electrical frequency of:

4 × 3,000 ÷ 60 = 200 Hz

This 200 Hz value is the electrical cycle frequency. It is not the controller's PWM switching frequency, which is normally much higher and must be selected with the power stage, acoustic targets, motor inductance, losses, and electromagnetic-compatibility requirements in mind.

How Does Six-Step Commutation Work?

Six-step commutation divides each electrical revolution into six sectors of 60 electrical degrees. In a common 120-degree conduction scheme, one phase connects toward the positive DC bus, a second phase connects toward the negative bus, and the third phase is not actively driven. The controller changes the active pair at each sector boundary.

Step High-side phase Low-side phase Floating phase
1 A+ B− C
2 A+ C− B
3 B+ C− A
4 B+ A− C
5 C+ A− B
6 C+ B− A
Diagram of a six-step BLDC commutation sequence with two driven phases and one floating phase
Figure 3. Simplified self-produced diagram. The phase order and polarity shown are one common forward-rotation example; the actual sequence depends on phase wiring, Hall order, controller logic, and commanded direction.

In a Hall-sensored drive, the Hall-state changes can be used to select the next commutation sector. In a basic sensorless six-step drive, the controller monitors the back electromotive force, or back-EMF, of the floating phase. A detected zero crossing provides a timing reference; the next commutation is scheduled according to the control method and the required phase delay.

Back-EMF is proportional to speed. At standstill and very low speed, a basic back-EMF zero-crossing method does not have a strong enough signal for dependable position detection. The controller must use a suitable starting method before transferring to closed-loop sensorless commutation. The acceptable startup method depends on load torque, inertia, allowed reverse movement, required starting time, and controller design.

Hall Sensors, Encoders, or Sensorless Estimation?

The right feedback method depends on what the machine must do near zero speed, during startup, and under changing load.

Position method Information available Standstill and low speed Main advantages Integration risks
Hall sensors Discrete rotor sectors Available Robust commutation reference, practical startup, moderate wiring Hall order and electrical alignment must match the controller; resolution is limited
Incremental or absolute encoder Higher-resolution position and speed Available, depending on encoder type and homing method Precise speed or position control; useful for servo behavior Added cost, wiring, alignment, interface, and environmental requirements
Basic back-EMF sensorless Zero crossings or phase-voltage information Weak or unavailable at standstill Removes rotor sensors and their wiring; useful when startup conditions are controlled Open-loop startup, filtering, switching noise, minimum reliable speed, load sensitivity
Model-based sensorless estimator Estimated rotor angle from measured electrical behavior Capability depends on algorithm and motor data Can support smoother control without a mechanical sensor Requires suitable motor parameters, current/voltage measurement, computation, and tuning

“Sensorless” does not mean “without feedback.” It means there is no dedicated mechanical rotor-position sensor. The controller still uses measured voltage, current, back-EMF, or a motor model to estimate the information it needs.

What Does the Controller Actually Do?

An industrial BLDC controller or drive normally combines four functions:

  • Power conversion: a three-phase bridge switches the DC supply into the motor phases.
  • Commutation or vector control: the algorithm determines which switches operate and when.
  • Closed-loop regulation: speed, current, torque, or position commands are compared with feedback.
  • Protection and supervision: current, voltage, temperature, stall, and other fault conditions are monitored according to the controller design.
Block diagram showing the DC source, controller, three-phase motor, position feedback, sensorless estimation, and protection paths
Figure 4. Simplified self-produced signal-path diagram. Available feedback, protection functions, and control modes depend on the selected motor and controller.

PWM changes the on-time of the inverter switches within each switching period. It can regulate average phase voltage or current, but it does not by itself determine the correct commutation sector. The controller still needs rotor-position information and a commutation or FOC algorithm.

The term electronic speed controller (ESC) is common in drones, model vehicles, and some traction products. Industrial equipment more often uses motor controller or drive because the unit may also control current, torque, direction, braking, acceleration, communication, alarms, and protection. The name alone does not confirm that a controller is compatible with a particular motor.

Six-Step Control or FOC?

Both six-step control and field-oriented control can operate a three-phase permanent-magnet motor, but they control the phase currents differently.

Decision factor Six-step or trapezoidal control Field-oriented control
Basic method Switches among six discrete voltage states Regulates current components in a rotating reference frame
Typical implementation Hall-sensored or back-EMF sensorless Sensored or sensorless rotor-angle feedback plus phase-current measurement
Low-speed smoothness Commutation torque ripple may be more noticeable Can provide smoother torque when correctly tuned
Computation and sensing Comparatively simple Higher computation, sensing, parameter, and tuning requirements
Best starting point Cost-sensitive speed control with acceptable ripple Precision speed, torque, acoustic, or dynamic requirements
Main caution Startup and low-speed behavior must be validated Poor parameters, sensing, or tuning can cancel the expected benefit

FOC is not automatically the most efficient choice at every speed and load. Actual system efficiency depends on the motor winding, magnetic design, inverter devices, switching strategy, controller tuning, mechanical load, speed, cooling, and operating point. Compare motor-and-drive test data at the required duty cycle rather than selecting from the algorithm name alone.

What Is the Difference Between BLDC and PMSM?

Within the three-phase scope of this guide, BLDC and permanent-magnet synchronous motor (PMSM) are both terms used for permanent-magnet synchronous machines. In common engineering usage, BLDC often refers to a motor designed for trapezoidal back-EMF and six-step commutation, while PMSM often refers to a motor designed for more sinusoidal excitation and vector control.

The boundary is not always strict in product naming. Some motors marketed as BLDC can be operated with sinusoidal control or FOC, and some controllers support multiple algorithms. Compatibility must be confirmed from the motor's winding data, back-EMF waveform, pole count, sensor configuration, current and voltage limits, and the controller manufacturer's guidance.

How Should an Industrial BLDC System Be Selected?

Start with the machine requirement, not the motor label:

  1. Define continuous torque, peak torque, speed range, acceleration, inertia, and duty cycle at the motor shaft.
  2. Confirm whether the machine must start under load and whether any reverse movement is acceptable during startup.
  3. Define speed, torque, or position accuracy, including behavior at zero and low speed.
  4. Set the available DC voltage, continuous current, peak current, regeneration or braking requirements, and power-supply limits.
  5. Check installation space, shaft load, gearbox, mounting, ambient temperature, cooling, ingress protection, vibration, and cable length.
  6. Select the motor winding, controller, feedback, communication, and protection settings as a compatible set.
  7. Validate temperature rise, starting behavior, fault response, acoustic noise, and electromagnetic compatibility in the actual machine.

TENCH publishes separate ranges for micro brushless DC motors and gearmotors and high-power brushless DC motors. The required controller, feedback, and gearbox must still be confirmed for the selected model and operating point.

The Anjian Medical Equipment project provides an application context for brushless motors in an intelligent robotic arm. It should not be used as proof that the same motor or controller fits another machine without reviewing that machine's load, motion profile, installation, and safety requirements.

If the first decision is whether to use a brushed or brushless motor, read Brushed vs Brushless DC Motors for Industrial Equipment before selecting the feedback and control method.

Common Technical Mistakes

The most costly errors come from treating the motor, controller, feedback method, and load as independent items. The following claims should be rejected unless the selected motor-and-drive combination has been checked under the required operating conditions.

Treating a BLDC Motor as a Two-Wire Motor

A standard three-phase BLDC motor cannot be operated correctly by connecting it directly to a DC supply. It needs a compatible controller for electronic commutation. Integrated-drive motors are an exception only because the commutation electronics are already built into the product.

Assuming Hall Sensors Always Improve Efficiency

Hall sensors provide rotor-position information; they do not guarantee higher efficiency. Efficiency depends on the complete electromagnetic and power-electronic design and the operating point.

Assuming Sensorless Control Is Suitable for Every Startup

Basic back-EMF sensing is weakest at standstill. A high-inertia or loaded mechanism may need Hall sensors, an encoder, a more advanced estimator, or a specifically validated startup routine.

Calling a Brushless System Maintenance-Free

Removing brushes reduces one maintenance item. Bearings, seals, gearboxes, connectors, cooling surfaces, cables, and the controller can still require inspection or replacement.

Assuming Brushless Operation Eliminates EMI

Removing brush arcing can reduce one source of electrical noise, but PWM edges, cable current, grounding, shielding, and layout can still create conducted or radiated interference.

Publishing a Universal Service-Life Number

There is no responsible single-hour life claim for all BLDC motors. Bearing load and speed, winding temperature, insulation, magnet temperature, vibration, contamination, starts and stops, controller settings, and gearbox conditions can all affect life.

Information to Send a Motor Supplier

For an initial motor-and-controller review, provide:

  • supply voltage and available current;
  • continuous torque, peak torque, and peak duration;
  • target, minimum, and maximum speed;
  • duty cycle and motion profile;
  • load inertia, acceleration, starting load, and stopping requirement;
  • speed, torque, or position accuracy;
  • Hall, encoder, sensorless, communication, and PLC requirements;
  • shaft, flange, envelope, gearbox, brake, and cable requirements;
  • ambient temperature, cooling, ingress, vibration, and installation orientation;
  • estimated quantity and prototype schedule.

Motor power alone is not enough. Torque and speed must be stated at the same operating point, and both continuous and short-duration peak conditions should be identified.

Frequently Asked Questions

Can a BLDC motor run without a controller?

No—not as a standard three-phase motor with no built-in electronics. Its stator phases must be electronically commutated by a compatible controller and power stage. A product sold as an integrated-drive BLDC motor can accept a simpler external command only because its commutation electronics are already inside the assembly. Always check the wiring and controller requirements for the exact model.

Can a sensorless BLDC motor start from zero speed?

Yes, but only when the controller has a startup method suited to the motor and load. Basic back-EMF detection cannot determine rotor position at standstill, so the controller must first align or accelerate the rotor by another method. Startup torque, inertia, allowed reverse movement, current limit, and the handover speed to closed-loop sensorless operation must all be validated.

Are three Hall sensors always required?

No. Three digital Hall sensors are common in six-step BLDC systems because they identify six rotor-position sectors, but they are not universal. Other systems may use an encoder, resolver, model-based estimator, back-EMF detection, or a different sensor arrangement. The signal voltage, sequence, electrical alignment, connector, and controller input requirements must match the selected motor.

Is FOC always better than six-step control?

No. FOC can provide smoother torque, better low-speed behavior, or improved dynamic control when the motor model, current measurement, rotor-angle information, and tuning are suitable. It is not automatically more efficient at every operating point. Six-step control may be the better engineering choice when its startup behavior, torque ripple, speed regulation, acoustic performance, and cost already meet the machine requirement.

How long does a BLDC motor last?

There is no universal service-life value for all BLDC motors. Life depends on bearing load and speed, winding and magnet temperature, duty cycle, starts and stops, vibration, contamination, controller settings, cooling, gearbox conditions, and maintenance. Use model-specific life or reliability data only with its stated test conditions, then validate the motor in the actual load and environment.

Request a Motor-System Review

Send TENCH the voltage, torque-speed points, duty cycle, startup load, control objective, feedback requirement, installation dimensions, environment, and estimated quantity. The motor, controller, gearbox, and feedback method can then be reviewed as one system. Submit your application details to TENCH.

Technical References

About the Author and Reviewer

Verna Lyu prepares practical motor-selection and application content with input from the TENCH factory and engineering team.
Wen Liu, Engineer, reviewed and approved the current technical content on September 20, 2026.

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