Transmission and Motors
Reading Motor Specs Before Buying Components
Identify duty, load, motor ratings and application requirements.
You need to size or verify a real system using “Reading Motor Specs Before Buying Components”, with units, duty and limits that match the application.
Read motor nameplates, datasheets and torque-speed curves in context, then convert the machine's load, motion, duty, environment and integration needs into a traceable component-selection record.
- Select or check first
- 1. Freeze the operating cases. Record normal, maximum, startup, holding, reversal, controlled stop and credible fault or recovery cases with timing and environmental conditions.
- Controlling assumption
- Load, motion profile, geometry, alignment, duty, lubrication and environment match the selected mechanism and ratings.
- Top risk
- A holding brake is not automatically a service brake or safety brake. Many motor brakes are intended to hold a stationary shaft after motion stops, not repeatedly decelerate the load. Their timing, permissible work, wear and emergency behavior are model-specific.
- Proof
- 7. Verify the built machine. Measure motion, following error, current or torque estimate, temperature, vibration, stopping and defined worst-case behavior before accepting the design.
Start with the basics Optional foundation before “Reading Motor Specs Before Buying Components”
Mechanical-design basics
Mechanical parts must carry loads, guide motion and remain aligned; similar-looking components can behave very differently under real force and duty.
- Force and torque
- Force pushes or pulls along a line. Torque is a turning effect equal to force multiplied by its perpendicular lever arm.
- Speed and acceleration
- Speed describes how fast position changes. Acceleration describes how fast speed changes and often creates the highest inertial load.
- Strength and stiffness
- Strength concerns permanent damage or failure. Stiffness concerns elastic deflection under load; a part can be strong enough yet too flexible for accurate motion.
- Load path
- The load path is the complete route by which force travels through shafts, bearings, fasteners, frames and foundations. Every interface on that route must be checked.
- Tolerance, clearance and alignment
- Tolerance permits controlled variation, clearance provides intentional space, and alignment keeps interacting axes or surfaces in the required relationship.
How to use this article: Sketch the load path and motion direction first, then attach units, worst-case loads, duty and acceptance limits to every calculation.
On this page
A motor specification is a promise under stated conditions—not a universal description of what the motor can do. Rated torque may depend on a particular drive, supply voltage, ambient temperature, mounting surface and cooling method. Peak torque may be available for only a short interval. An enclosure code does not answer every question about connectors, shaft seals or chemicals. Read the number, its unit, its definition and the conditions attached to it before comparing components.
Use four documents together
Diagram description
A servo motor illustration is surrounded by example fields: 400 watts rated mechanical power, 3000 revolutions per minute, 1.27 newton-metres rated torque, 3.0 newton-metres peak torque, 48 volts DC, 10 amperes, rotor inertia of 0.00008 kilograms square metres and encoder feedback.
| Source | What it commonly provides | What to verify elsewhere |
|---|---|---|
| Nameplate | Identity, rated point, voltage, current, frequency or speed, duty and selected construction codes | Full curves, permissible overload, derating, tolerances, connection and drive requirements |
| Catalogue or datasheet | Rated, peak and sometimes stall data; dimensions, inertia, mass, feedback and options | The exact ordering code, test conditions, notes and revision |
| Torque-speed or performance curve | Where continuous and intermittent operation is permitted across speed | Supply voltage, drive, winding, temperature, cooling and time limits used for that curve |
| Motor and drive manuals | Wiring, compatible combinations, protection, tuning, cable, braking and commissioning rules | Whether optional brake, feedback and connectors match the ordered hardware |
| Application or configuration guide | Sizing sequence, duty calculation, inertia and environmental guidance | Rules specific to the selected family and manufacturer |
Match every value to the exact motor suffix and compatible drive. Motors with the same frame can have different windings, brakes, encoders, connectors or shaft options. A servo motor may also have different rated operating points with different drives or supply voltages. Keep a copy of the specific document revision used for the decision.
Translate each field into a physical question
| Field | Meaning | Reading check |
|---|---|---|
| Rated mechanical power, W or kW | Continuous mechanical output at a defined rated point | Do not confuse it with electrical input power or drive size |
| Rated speed, rpm | Shaft speed at the rated operating point | Use the curve for behavior above, below and at base or rated speed |
| Rated or continuous torque, N·m | Torque permitted continuously under stated thermal conditions | Confirm ambient, mounting, cooling, winding and drive conditions |
| Peak or maximum torque, N·m | Short-duration torque inside a stated intermittent region | Find duration, repetition, speed and drive-current limits |
| Voltage and current | Electrical operating values with AC or DC context | For AC, record phase, frequency, connection and power factor where applicable; for servo systems, record the matched drive and supply |
| Rotor inertia, kg·m² | Resistance of the rotor to angular acceleration | Convert units carefully; kg·cm², g·cm² and kg·m² differ by large factors |
| Feedback | Encoder, resolver or other sensing type and interface | Resolution alone does not establish sensor accuracy, machine accuracy or controller compatibility |
| Brake | Usually a spring-applied holding brake option | Verify release voltage, holding torque, wear and whether dynamic stopping is permitted |
| IP and thermal or insulation data | Defined enclosure ingress tests and thermal construction limits | Inspect shaft, connector and cable details plus ambient, altitude, contamination and cooling derating |
| Mounting and shaft limits | Flange, pilot, shaft, key and permissible radial or axial loads | Use the dimension drawing and calculate actual bearing loads |
Check whether torque, speed and power describe the same point
P = T × ω, where ω = 2πn ÷ 60
Use shaft torque in newton-metres and angular speed in radians per second to obtain mechanical power in watts. This checks arithmetic consistency; it does not prove that an illustrative or vendor rating applies to the application.
- P
- mechanical shaft power, W
- T
- shaft torque, N·m
- ω
- angular speed, rad/s
- n
- rotational speed, rpm
For the teaching image, 3000 rpm is about 314.16 rad/s. Multiplying by 1.27 N·m gives about 399 W, consistent with the rounded 400 W label. This says nothing by itself about efficiency, electrical input, peak duration or thermal suitability.
Rated, peak and stall values answer different questions
A rated or continuous point describes operation that can continue while temperatures remain within defined limits. A peak or intermittent region supports acceleration, disturbance recovery or another brief demand, subject to the drive current, motor speed, duration, repetition and thermal state. Stall or static torque occurs at zero speed. Mechanical output power is then zero, although current and winding heating can be high. Never substitute stall or peak torque for the cycle's continuous requirement.
How IEC duty labels such as S1, S2 and S3 help
IEC duty designations describe defined load and rest patterns for rotating electrical machines. S1 is continuous duty long enough to reach thermal equilibrium. S2 is short-time duty followed by enough de-energized rest to return near ambient. S3 is intermittent periodic duty made of repeated constant-load and rest periods, described with a cyclic duration factor. Later duty types cover starting, braking, load and speed variations in other combinations.
The code is useful only with its associated duration or cyclic factor and the manufacturer's rating. Do not assume an IEC duty label grants unlimited use of a servo's peak-torque zone. Servo sizing usually also evaluates the actual speed-time and torque-time cycle, peak torque at speed and an equivalent or root-mean-square thermal demand using the manufacturer's method.
A preliminary equivalent-torque calculation
T_RMS = √[(Σ Tᵢ²tᵢ) ÷ (Σ tᵢ)]
For a repeating servo cycle, this is a common preliminary thermal comparison. Include acceleration, running, dwell, deceleration and applicable holding intervals with signed torque squared. The manufacturer's sizing rules, speed dependence, regeneration, drive limits and cooling conditions still govern final approval.
- T_RMS
- root-mean-square cycle torque, N·m
- Tᵢ
- motor torque during interval i, N·m
- tᵢ
- duration of interval i, s
A low average torque does not guarantee a low RMS value because high-torque acceleration intervals are squared. Check peak demand separately; RMS torque cannot reveal a peak-limit violation.
Collect the inputs before selecting a frame size
Diagram description
A carriage travels along a leadscrew between end supports. Callouts identify payload and external force, travel, linear speed, acceleration, friction and orientation, move and dwell timing, accuracy and repeatability, and electrical and environmental constraints.
| Area | Record | Why it changes selection |
|---|---|---|
| Function and movement | What moves, rotation or translation, travel, directions, positions and homing | Defines the mechanism and required operating envelope |
| Load | Moving mass or inertia, external process force or torque, gravity, friction and credible variation | Sets steady and disturbance demand at the load |
| Motion profile | Speed, acceleration, deceleration, jerk, move time, dwell and emergency or recovery behavior | Sets motor speed, acceleration torque, peak demand and control bandwidth |
| Transmission | Screw lead, pulley diameter, gear ratio, efficiency, backlash, stiffness and preload | Reflects load speed, torque and inertia to the motor |
| Duty | Sequence of moves, starts, reversals and holds; cycle time, cycles per hour and operating hours | Determines peak duration and thermal demand |
| Precision | Accuracy, repeatability, resolution, smoothness, settling and allowable following error | Determines feedback placement, mechanics, control and calibration |
| Environment | Ambient range, altitude, dust, water, chemicals, washdown, vibration, noise and ventilation | Changes protection, materials, seals, cooling and derating |
| Integration | Available supply, drive, controls, network, cables, mounting space, shaft loads and maintenance access | Eliminates electrically or mechanically incompatible options |
| Safety and lifecycle | Hazards, stop and holding needs, service life, spares, repair skill, energy and cost target | Defines risk reduction and the true ownership decision |
Diagram description
A motor, coupling, screw and guided carriage form a complete axis. Requirement panels cover function, force and moment loads, stroke, speed and timing, precision, environment, mounting space, and lifecycle cost.
Convert load requirements through the transmission before reading motor curves. The selected screw lead, pulley radius or gear ratio changes motor speed and torque; efficiency and friction change demand; reflected inertia changes acceleration and control behavior. Keep calculations traceable from load values to motor-shaft values, and identify whether each input is measured, specified, estimated or assumed.
Build a selection record that another engineer can audit
From requirement to purchase decision
- 1. Freeze the operating cases.Record normal, maximum, startup, holding, reversal, controlled stop and credible fault or recovery cases with timing and environmental conditions.
- 2. Model the mechanism.Calculate motor speed, torque and reflected inertia from load motion, process force, gravity, friction, transmission ratio and efficiency.
- 3. Create speed-time and torque-time profiles.Find the peak at the corresponding speed and the full repeating thermal cycle; do not reduce the requirement to one average number.
- 4. Screen exact motor-drive combinations.Use the matching continuous and intermittent curves at the intended supply, winding, drive and cooling conditions.
- 5. Check integration details.Confirm dimensions, shaft and bearing loads, brake, feedback, cables, connector direction, supply, regeneration, controls, protection and environmental construction.
- 6. Record margins and assumptions.Explain each margin and avoid stacking unexplained multipliers that can create an oversized, sluggish or unnecessarily expensive axis.
- 7. Verify the built machine.Measure motion, following error, current or torque estimate, temperature, vibration, stopping and defined worst-case behavior before accepting the design.
Use Gear Ratio, Speed and Torque in Real Machines to translate transmission choices to the motor shaft, Selecting a Complete Linear Axis for the wider axis worksheet, and DC, Stepper and Servo Motors: Which One Fits? when choosing the candidate motor family. Purchase only after the exact motor, drive, feedback, brake and mechanism pass the same documented cycle.
One-minute questions
Questions to answer before you apply this lesson
What should I select or check first for Reading Motor Specs Before Buying Components?
1. Freeze the operating cases. Record normal, maximum, startup, holding, reversal, controlled stop and credible fault or recovery cases with timing and environmental conditions.
Which assumption can change the conclusion?
Load, motion profile, geometry, alignment, duty, lubrication and environment match the selected mechanism and ratings.
What can go wrong if I copy the example directly?
A holding brake is not automatically a service brake or safety brake. Many motor brakes are intended to hold a stationary shaft after motion stops, not repeatedly decelerate the load. Their timing, permissible work, wear and emergency behavior are model-specific.
How do I verify the result in the real system?
7. Verify the built machine. Measure motion, following error, current or torque estimate, temperature, vibration, stopping and defined worst-case behavior before accepting the design.
Sources and further reading
- Motor GuideABB · 2026-10-12Primary manufacturer guide reviewed for IEC duty types, operating conditions and motor-selection context.
- Manual for Induction Motors and GeneratorsABB · 2026-10-12Primary manual reviewed for nameplate fields including duty, connection, insulation, IP protection, cooling, mounting, power, voltage, frequency, speed, current and power factor.
- How to Read a NEMA Motor NameplateABB · 2026-10-12Manufacturer explanation reviewed for the contextual meaning of common motor nameplate fields.
- SINAMICS V90, SIMOTICS S-1FL6 Getting StartedSiemens · 2026-10-12Primary manual reviewed for a servo motor nameplate example, matched drive system and commissioning context.
- S200 Servo Motors CatalogSiemens · 2026-10-12Primary catalogue reviewed for rated, static and maximum torque, current, torque constant, rotor inertia, feedback and inertia-ratio terminology.
- Different Motor Data Depending on the Supply ConditionsSiemens · 2026-10-12Primary technical FAQ reviewed as evidence that a motor can have different rated data with different supply and drive conditions.
- Servo Motor Sizing ConceptsYaskawa America, Inc. · 2026-10-12Primary technical document reviewed for inertia ratio, speed, maximum torque at speed and RMS torque at speed as core servo sizing checks.