Most selection errors are vocabulary errors.

A stall torque read as an operating torque, a duty cycle quoted without its period, a PPR compared against a count after decoding. 21 terms, the units and synonyms an engineer actually types, and the misuse each one causes.

Load and operating point

The four numbers that decide whether a motor survives the application, and the points on the curve they are measured at.

Torque

Also typed as moment, load torque, output torque; mN·m, N·m, oz-in, in-lb, kgf·cm

Rotational effort at the shaft. Continuous torque is what the thermal path allows indefinitely at a stated ambient and mounting. Peak torque is a short excursion the winding can absorb without reaching its temperature limit.

Common misuse Quoting stall torque as available output torque. Stall torque is the torque at zero speed, at maximum current, with no rotation to help cooling. Designing to it will destroy the motor.

Output torque calculator Gearmotor configurations

Speed

Also typed as rotational speed, shaft speed, rpm, min-1, rad/s

Shaft revolutions per unit time. On a gearmotor, output speed is motor speed divided by the reduction ratio. On a brushed DC motor at fixed voltage, speed falls close to linearly as torque rises.

Common misuse Comparing a catalog no-load speed with the speed you need under load. The two are different points on the same line, and the gap grows with load.

Gear ratio calculator Operating point calculator

Current

Also typed as draw, amps, A, mA, no-load current, stall current, inrush

Current drawn at an operating point. Above the no-load offset it rises roughly in proportion to torque, so current is the most convenient field measurement of load.

Common misuse Sizing the supply from no-load or rated current. Starting and stall current are several times higher, and a supply that folds back at startup looks exactly like a weak motor.

Operating point calculator

No-load

Also typed as free run, unloaded, NL speed, NL current

The operating point with nothing on the shaft. What remains is friction, windage and, on an iron-core motor, core loss. It is the fastest and coolest point on the curve.

Common misuse Treating no-load speed as an application speed. It is a characterization point, useful for comparing motors and for detecting a dragging bearing, not for sizing.

Motor windings

Stall

Also typed as locked rotor, zero speed, stall torque, stall current

The shaft held stationary with voltage applied. Speed is zero, torque is maximum, and current is limited only by winding resistance, so all input power becomes heat in the winding.

Common misuse Using stall torque as a design point, or assuming a jam is survivable. A coreless winding has very little thermal mass and can be destroyed in seconds at stall. Protect it in the drive.

Duty cycle and thermal calculator

Rated point

Also typed as nominal point, rated torque, rated speed, continuous duty point, S1

The manufacturer's continuous operating point at rated voltage: the torque, speed and current the motor sustains thermally in a stated mounting and ambient.

Common misuse Carrying the rated point into a sealed enclosure or a hot cabinet unchanged. The rating assumes a heat path. Change the mounting and you have changed the rating.

Motor selector

Duty cycle

Also typed as percent ED, on/off ratio, intermittent duty, S1 S2 S3

The fraction of a repeating period during which the motor is energized, together with the length of that period. Both halves are required.

Common misuse Stating a percentage with no period. Twenty-five percent over ten seconds is a thermal average; twenty-five percent over thirty minutes is a series of full-load runs. Which one you have depends on the thermal time constant.

Duty cycle and thermal calculator

Thermal time constant

Also typed as winding time constant, thermal tau, seconds, minutes

The time for a winding to reach about 63 percent of its final temperature rise at constant load. A small coreless winding is measured in seconds; the housing it sits in is measured in minutes.

Common misuse Applying an intermittent rating whose period is long compared with the winding time constant. When the period is long, the peak load is the load, and averaging it hides the failure.

Duty cycle and thermal calculator

Reflected inertia

Also typed as load inertia at the motor, inertia ratio, J load over i squared; kg·m², g·cm²

Load inertia as the motor shaft sees it: load inertia divided by the square of the reduction ratio. A 100:1 gearhead divides the load inertia by 10,000.

Common misuse Comparing an inertia ratio without including rotor and gearhead inertia, or applying a rule-of-thumb ratio instead of checking that the acceleration torque actually fits inside the peak torque.

Reflected inertia calculator

Mechanical fit

What a mechanism engineer measures on the bench and writes on the drawing.

Reduction ratio

Also typed as gear ratio, ratio, i, X:1

Input revolutions per output revolution. Output speed is motor speed divided by the ratio. Output torque is motor torque multiplied by the ratio and by the gearhead efficiency.

Common misuse Multiplying torque by the ratio and stopping there. Efficiency is not optional, and it falls as stages are added. Catalog ratios are also often exact tooth ratios rounded for print, which matters when a mechanism has to index.

Gear ratio calculator

Stage

Also typed as gear stage, planetary stage, reduction stage, train

One meshing reduction inside a gearhead. Each stage multiplies the ratio and adds length, lost motion, loss and noise.

Common misuse Adding efficiencies instead of multiplying them. Three stages at 0.90 give 0.73, not 0.90, and the difference lands in your thermal budget.

Gearhead platforms

Backlash

Also typed as lost motion, play, angular backlash; degrees, arcmin

Free angular motion at the output when the input reverses direction, measured at a stated measuring torque.

Common misuse Treating it as one number for a gearhead family. It depends on ratio, stage count and the torque it was measured at, and it grows with wear. A positioning mechanism that cannot tolerate it should approach from one direction.

Gearhead platforms Noise and backlash

Endplay

Also typed as axial play, end float, axial clearance; mm

Axial movement of the shaft between its mechanical limits, measured under a stated axial load.

Common misuse Confusing it with axial load capacity, or specifying zero. Endplay is removed by a preload, which is a design decision with a cost in friction and life, not a tolerance you can simply tighten.

Testing and qualification

Runout

Also typed as TIR, total indicated runout, eccentricity, concentricity; mm, µm

The total indicated reading of a shaft or pilot surface as the shaft is rotated against a stated datum. It combines the form error and the eccentricity of that feature.

Common misuse Reading it as straightness or as a diameter tolerance. Runout is a rotational measurement and it is meaningless without the datum it was taken against.

Testing and qualification

Construction and feedback

How the motor is built decides what it tolerates, and what the feedback actually reports.

Coreless

Also typed as ironless, ironless rotor, skew wound, basket winding, hollow rotor

A brushed DC motor whose rotor is a self-supporting copper winding with no iron laminations. No cogging, very low rotor inertia, high efficiency and a very small thermal mass.

Common misuse Overloading it because it runs smoothly and quietly. The same small thermal mass that makes it accelerate well gives it a very short overload window.

Motor windings

Iron core

Also typed as slotted, laminated rotor, conventional brushed DC

A brushed DC motor with windings in the slots of a laminated steel rotor. Higher inertia, more thermal mass, more overload tolerance, and cogging torque you can feel by hand.

Common misuse Expecting coreless smoothness at low speed. Cogging is inherent to the slotted construction and shows up as speed ripple and audible tone where the mechanism amplifies it.

Motor windings

Brushless

Also typed as BLDC, EC motor, electronically commutated, sensored, sensorless

A permanent-magnet rotor whose windings are switched by a drive rather than by brushes, using Hall sensors or back-EMF for position. Wear life is set by the bearings, not by a sliding contact.

Common misuse Treating it as a drop-in for a brushed motor. It cannot run on a bare supply; the motor and its drive together define the performance, and the drive is part of the qualification.

Motor windings Adding feedback

Commutation

Also typed as commutation angle, six-step, trapezoidal, sinusoidal, field oriented control, Hall sequence

Switching winding current so that torque keeps acting in one direction: mechanically through brushes and a commutator, or electronically by a drive reading rotor position.

Common misuse Comparing torque constants across motors without stating the commutation scheme and how it was measured. Six-step and sinusoidal drives produce different torque ripple from the same motor.

Feedback options

Brush material

Also typed as precious metal brush, graphite brush, carbon brush, silver palladium

The sliding contact between commutator and terminals. Precious-metal brushes give a low, stable contact drop at low current and low voltage. Graphite carries higher current and tolerates higher voltage and arcing.

Common misuse Running precious-metal brushes at a current, a voltage or a PWM scheme they were not designed for. The contact film that makes them work is destroyed, and the motor fails early for a reason no specification sheet appears to explain.

Motor failure analysis

PPR

Also typed as pulses per revolution, lines, resolution, CPR, counts per revolution

Encoder output pulses per revolution of the shaft the encoder is mounted on. On a gearmotor that is normally the motor shaft, before the reduction.

Common misuse Mixing PPR with counts per revolution after quadrature decoding, which is four times higher, and quoting an output-shaft resolution that silently includes the gear ratio.

Feedback options

Quadrature

Also typed as A and B channels, two channel, x1 x2 x4 decoding, index, Z channel

Two output channels 90 electrical degrees apart, so that both direction of rotation and four edges per cycle are recoverable by the counter.

Common misuse Calling a single-channel tachometer output an encoder. One channel gives speed only. Without the second channel there is no direction, and no amount of firmware recovers it.

Feedback options

A term is only useful if the number beside it carries its basis.

Every specification on this site says whether it was published, calculated, typical, a maximum, a target, or simply not established. The terms of use for each of those are on the terms page.