What to put in a compact motor specification
What has to appear in a motor specification so that two suppliers quoting it build the same thing, and a field failure can be attributed to a cause?
Most compact-motor specifications are written as a list of catalog values: voltage, no-load speed, stall torque, diameter, length. A document like that can be met by parts that behave nothing alike in the mechanism, and it gives no basis for deciding whether a field failure was a defect or a misapplication. The specification has to say where the motor operates, for how long, in what conditions, and what counts as failure.
Everything a supplier needs to reason about heat, wear and margin is in the position of those three points relative to the shaded region. The running point sets the winding temperature. The acceleration peak sets whether the drive current limit is reached. The jam condition sets whether the motor or the mechanism is the fuse. A specification that lists only the axis endpoints has said nothing about any of them.
The operating point, stated four ways
State continuous, peak, starting and abnormal conditions separately. They are four different loads, they stress different parts of the machine, and a single torque number cannot represent them.
- Continuous. The torque and speed the mechanism holds for the longest uninterrupted period, and the ambient temperature while it holds it. This is what sizes the winding thermally.
- Peak. The torque during acceleration, breakaway, or a mechanical hard stop, and how long it lasts. A 200 ms peak and a 20 s peak are different specifications.
- Starting. Static friction in a cold mechanism, with cold grease. Starting torque is frequently the real sizing case in a low-temperature application, and it is the case most often left out.
- Abnormal. What happens when the mechanism jams, when a limit switch fails, when a user obstructs a moving part. State whether the motor is expected to survive it, for how long, and whether the drive limits current.
The duty cycle is part of the specification
A duty cycle is not a percentage. It is a sequence: on time, off time, torque during each, reversals, and the repetition rate over the life of the product. Two mechanisms both described as 25 percent duty can differ by an order of magnitude in brush wear, because one runs 15 minutes in every hour and the other starts and stops 900 times an hour.
Starts are the expensive event. Inrush current at each start is limited only by winding resistance until the motor develops back-EMF, so a start draws stall current for a few milliseconds. That current passes through the brush contact and through the commutator segment gaps. A duty cycle expressed only as a percentage hides the number of starts entirely.
Life, with a failure criterion
"20,000 hours life" is not a requirement until it says what failure means. In practice a compact DC motor rarely stops. It degrades: speed drops under the same load, current rises, noise changes, brush wear debris accumulates, grease migrates, backlash grows. A life requirement has to name the parameter and the limit that ends life.
- The measured parameter: output speed at a stated load, current at a stated load, backlash, noise level, or a functional pass or fail of the mechanism itself.
- The limit: a percentage change from the initial characterization of the same unit, not a comparison to a catalog value.
- The duty cycle the life is counted under, and the ambient temperature.
- The sample size and the confidence, because a life number without a sample size is an anecdote. See the article on second-source qualification for how sample size and demonstrated reliability are related.
Writing the criterion as a change from each unit's own initial measurement is the practical choice. It removes unit-to-unit spread from the judgment, and it makes the test data usable even when the population is small.
Noise, with a method
A noise limit without a measurement method is unenforceable. Sound pressure depends on distance, direction, background level, mounting, the load applied, and whether the motor is running in the mechanism or in free air. Two labs following different practice will differ by more than the tolerance most specifications carry.
- Microphone distance and position relative to the shaft axis.
- Background noise level of the room, and the requirement that it sit a stated margin below the limit.
- Mounting: fixture material, clamping method, and whether the motor is isolated or coupled to a load.
- Operating condition during measurement: voltage, direction, load torque, and whether the reading is taken in steady rotation or through a reversal.
- Weighting and averaging: A-weighted or unweighted, and fast or slow response.
The rest of the document
| Line | What it must state | Failure mode when omitted |
|---|---|---|
| Supply | Nominal voltage, range, source type: regulated supply, battery, or PWM drive with its frequency | A motor sized at nominal fails at end-of-battery voltage, or heats on a low PWM frequency |
| Envelope and datums | Maximum envelope, mounting face as datum, pilot, bolt pattern, shaft extension from the face | Parts fit the drawing and not the mechanism, because dimensions were referenced from the wrong face |
| Shaft loading | Radial load, its distance from the mounting face, and axial load in each direction | Bearing life is quoted for an unloaded shaft that does not exist in the product |
| Environment | Temperature range in operation and in storage, humidity, cleaning agents, vibration, orientation | Grease migrates in a vertical-shaft orientation nobody stated; a cleaning solvent attacks a polymer nobody named |
| Feedback | Type, counts per revolution, output format, supply voltage, index requirement, phase relationship | A quadrature signal arrives with the wrong phase order and the mechanism counts backwards |
| Leads and termination | Gauge, length, insulation, exit position, connector part number and pin assignment, polarity marking | The correct motor cannot be assembled, or the harness routes through a moving joint |
| Marking and traceability | What is marked on the part, and what lot record must be retrievable | A field failure cannot be tied to a build lot, so containment covers everything ever shipped |
| Change control | That any change to winding, magnet, brush system, bearing, gear material, lubricant or manufacturing location requires written notice | A silent process change appears as an unexplained field trend two years later |
A one-page skeleton
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Function and mechanism
One paragraph on what the motor drives and how the mechanism moves. A supplier who understands the mechanism catches errors in the rest of the document.
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Interface
Envelope, mounting datum, pilot, bolt pattern, shaft, rotation direction for a stated polarity, lead exit, connector.
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Electrical
Supply and its range, source type, drive method, current limit, and the maximum current the product can allow.
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Operating points
Continuous, peak, starting and abnormal, each with torque, speed, duration and ambient temperature.
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Duty cycle
On and off times, starts and reversals per hour, total operating hours or cycles over product life.
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Life and acceptance
Measured parameter, end-of-life limit as a change from initial, sample size, and the duty cycle life is counted under.
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Noise and vibration
Limit plus the full measurement method. Without the method the limit is a preference.
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Environment
Temperature, humidity, chemicals, vibration, orientation, and any cleaning or sterilization exposure.
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Quality and change control
Incoming characterization requirements, lot traceability, notice of change, and what triggers requalification.
A specification written this way is longer than a catalog line and shorter than most quality documents. It is also the document that makes every later conversation quick: a cross-reference, a second source, a cost reduction and a failure investigation all start by asking the same questions this document has already answered.