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Choosing an electromagnetic linear motor is not simply a matter of selecting the highest force rating. The correct choice depends on motion distance, acceleration, duty cycle, load geometry, and the surrounding machine structure. A motor that performs well on a laboratory bench may overheat in continuous production.
Eric Laithwaite, a pioneering authority on linear motor technology, described the principle clearly: “The linear motor is a motor turned inside out.” That idea remains useful today. Instead of creating rotary torque, an electromagnetic linear motor produces direct thrust along a guideway. This can remove belts, screws, gears, and their related backlash. However, it does not remove engineering responsibility.
Begin with the required peak and continuous force. Then examine stroke length, target speed, positioning accuracy, and moving mass. A 300-millimeter stroke with rapid reversals creates different thermal demands than a slow, steady transfer. Check the cooling method carefully. Air cooling may suit intermittent operation, while water cooling may be necessary for dense cycles. Magnetic attraction can also increase bearing loads, especially when the motor is mounted beneath a metal carriage.
Feedback selection deserves equal attention. Linear encoders can provide excellent accuracy, but alignment, contamination, and cable routing still matter. I would not trust a specification sheet alone. Test the motor with the real guide, payload, controller, and cycle profile. Small mechanical friction can change the result.
The best choice is rarely the largest motor. It is the one that delivers stable motion, manageable heat, predictable maintenance, and enough reserve for imperfect conditions. That last requirement is easy to underestimate.
How to Choose an Electromagnetic Linear Motor?
Define the Required Motion and Operating Conditions
Selecting an electromagnetic linear motor begins with the motion, not the catalog rating. Describe the travel distance, target speed, acceleration, and positioning accuracy. A 300-millimeter stroke means little without its cycle time. Record the complete movement profile.
Measure the moving load carefully. Include the carriage, tooling, cables, and any product being handled. Specify the payload’s center of gravity and the direction of travel. Horizontal motion may need less force than vertical lifting. Gravity changes the calculation.
Operating conditions often expose weak assumptions. Check the duty cycle, peak force duration, ambient temperature, humidity, dust, and available cooling. Note whether the motor operates continuously or makes short, repeated moves. Heat can reduce performance before the motor reaches its advertised limit.
In practical design reviews, engineers should compare continuous force with peak force. Peak force may support acceleration, but continuous force controls sustained operation. Leave a realistic safety margin. Too much margin can increase cost and mass.
Control requirements also matter. Define feedback resolution, settling time, repeatability, and allowable overshoot. A fast move with a heavy load may need stronger control than expected. Cable movement can add resistance and vibration. Do not ignore it.
One overlooked detail is maintenance access. A technically suitable motor may be difficult to inspect inside a sealed machine. Recheck the assumptions after testing. Early calculations are useful, but real friction and heat may disagree. That disagreement is valuable.
| Selection Dimension | What to Define | Typical Engineering Range | Why It Matters | Recommended Evaluation Criteria | Priority |
|---|---|---|---|---|---|
| Travel Distance | Required stroke or continuous travel length Unit: mm or m |
Short stroke: 10–100 mm Medium stroke: 100–500 mm Long travel: above 500 mm |
The available motor architecture, guide arrangement, cable management, and machine footprint depend strongly on the travel length. | Confirm the usable stroke, end-of-travel margin, homing space, and whether the application requires a rotary-to-linear conversion or a direct linear drive. | High |
| Peak Speed | Maximum required linear velocity during the motion profile Unit: mm/s or m/s |
Precision motion: 0.01–0.5 m/s General automation: 0.5–2 m/s High-speed systems: above 2 m/s |
Speed affects back electromotive force, amplifier voltage requirements, power dissipation, settling time, and achievable throughput. | Compare the required speed with the motor's continuous and peak speed ratings under the actual load, stroke, and duty cycle. | High |
| Acceleration and Deceleration | Maximum acceleration, deceleration, and jerk limits Unit: m/s² and m/s³ |
Light payload systems: 1–5 m/s² Fast positioning: 5–20 m/s² Higher values require detailed mechanical validation |
Acceleration determines peak force. Jerk affects vibration, mechanical stress, product handling, and positioning smoothness. | Calculate force using F = m × a, then add friction, cable forces, gravity components, and a suitable engineering margin. | High |
| Moving Mass | Mass of the carriage, payload, tooling, cables, and attached fixtures Unit: kg |
Small mechanisms: 0.1–5 kg Medium automation: 5–50 kg Large systems: above 50 kg |
Moving mass directly influences acceleration force, braking energy, guide loading, and the required motor peak force. | Use the worst-case payload, not the nominal payload. Include tooling, workholding devices, and any mass that moves with the carriage. | High |
| Continuous Force | Force required throughout the operating cycle Unit: N |
Small systems: 5–100 N Medium systems: 100–1,000 N Heavy systems: above 1,000 N |
Continuous force is governed by friction, gravity, process resistance, thermal conditions, and the average duty cycle. | Verify the motor's continuous force at the intended ambient temperature and cooling method. Do not size only from peak force. | High |
| Peak Force | Short-duration force needed for acceleration, cutting, pressing, or disturbance rejection Unit: N |
Often 1.5–3 times the continuous force, depending on the motion profile and duty cycle | Peak force determines acceleration capability and the ability to overcome short-term process loads. | Check peak force duration, repetition rate, thermal recovery time, and amplifier current limits. Confirm that the guide system can withstand reaction forces. | High |
| Positioning Accuracy | Difference between the commanded and actual final position Unit: μm or mm |
General automation: ±0.05–0.5 mm Precision systems: ±1–50 μm |
Accuracy depends on feedback resolution, calibration, thermal stability, mechanical straightness, servo tuning, and external disturbances. | Define absolute accuracy, repeatability, bidirectional error, and measurement conditions separately. A high-resolution encoder alone does not guarantee high absolute accuracy. | High |
| Repeatability | Ability to return to the same position under consistent conditions Unit: ±μm or ±mm |
Industrial systems: ±5–100 μm General-purpose systems: ±0.05–0.5 mm |
Repeatability is important for pick-and-place, inspection, dispensing, assembly, and process registration. | Specify unidirectional or bidirectional repeatability, measurement range, payload, temperature, and approach direction. | High |
| Feedback Device | Linear or rotary position feedback and its resolution Unit: μm, nm, counts/revolution |
Typical linear feedback resolution: 0.1–10 μm Higher resolution is used in precision applications |
Feedback supports closed-loop commutation, position control, velocity regulation, and compensation of mechanical errors. | Prefer direct linear feedback when screw pitch error, belt compliance, or transmission backlash would compromise the required accuracy. | High |
| Duty Cycle | Operating time, idle time, cycle frequency, and force profile Unit: %, cycles/min, h/day |
Intermittent: below 25% on-time Moderate: 25–60% Continuous: above 60% |
Thermal loading is determined by the RMS force and the complete cycle, not by peak force alone. | Calculate RMS force from the full motion profile and verify winding temperature, cooling conditions, and allowable temperature rise. | High |
| Guide and Bearing System | Type of linear guide, preload, stiffness, friction, and allowable load | Rolling guides provide low friction and high stiffness; air or hydrostatic guides support very low friction and high precision when utilities permit | The motor produces thrust but does not normally constrain the carriage against side loads or moments. | Size the guide separately for radial load, moment load, stiffness, service life, contamination, and allowable misalignment. | High |
| Operating Environment | Ambient temperature, humidity, dust, liquids, vibration, and cleanroom requirements | Common industrial ambient range: 5–40°C Special designs may support wider ranges |
Temperature changes affect resistance, force constant, encoder accuracy, lubrication, and structural expansion. Contamination can reduce guide and feedback life. | Specify ingress protection, cooling method, corrosion resistance, cable routing, cleanroom compatibility, and any washdown or vacuum requirements. | High |
| Electrical Supply and Amplifier | Bus voltage, continuous current, peak current, feedback interface, and control bandwidth Unit: V, A, Hz |
Industrial servo buses commonly use DC link voltages from approximately 48–800 V, depending on system size | Available voltage limits maximum speed, while current limits force. The amplifier must support the motor's commutation and feedback requirements. | Match motor force constant, winding resistance, inductance, peak current, continuous current, and regenerative energy handling to the amplifier. | High |
| Thermal Management | Natural convection, forced air, liquid cooling, heat sinking, and allowable winding temperature | Natural cooling is suitable for lower continuous power; forced-air or liquid cooling is used for higher force density | Heat reduces available continuous force and can shorten insulation, bearing, magnet, and electronic component life. | Evaluate RMS force, ambient temperature, mounting surface, cooling flow, thermal resistance, and the temperature rating of nearby components. | High |
| Noise and Vibration | Acceptable acoustic noise, cogging force, force ripple, resonance, and structural vibration | Low-noise applications generally require smooth commutation, rigid mounting, balanced guides, and carefully tuned control loops | Force ripple and structural resonance can cause surface errors, imaging artifacts, settling delays, and reduced product quality. | Review cogging force, ripple specifications, servo bandwidth, notch-filter requirements, structural modes, and the effect of cable carrier forces. | Medium |
| Safety and Commutation | Safe torque-off, position limits, homing, overtravel protection, brake requirements, and magnet-related hazards | Requirements depend on the machine risk assessment and applicable regional safety standards | Linear motors can move freely when unpowered, and some magnetic assemblies can create strong attraction forces. | Provide independent limit switches where required, controlled stopping, suitable guarding, emergency-stop behavior, and a documented risk assessment. | High |
| Installation and Alignment | Flatness, parallelism, air gap, mounting tolerance, cable bend radius, and service access | Typical alignment tolerances vary by motor geometry and guide design; precision systems require measured installation and calibration | Misalignment can increase force ripple, reduce air gap, overload guides, and cause premature wear or collision between motor components. | Define datum surfaces, alignment tools, mounting torque, thermal expansion allowance, and inspection procedures before final installation. | Medium |
| Sizing Margin | Allowance for payload variation, friction changes, temperature, aging, and process uncertainty | A preliminary force margin of 20–30% is commonly used, followed by simulation and prototype validation | Insufficient margin can cause overheating, tracking errors, current limiting, and poor disturbance rejection. | Apply margin after calculating the actual force profile. Avoid excessive oversizing, which can increase cost, moving mass, and control difficulty. | High |
Engineering values are general planning ranges. Final motor selection should be verified against the complete motion profile, RMS force, thermal conditions, guide loads, feedback requirements, and applicable machine safety requirements.
Sierramotion engineers help customers design solutions to complex motion problems. Whether a simple coil, or a precision motion assembly working in vacuum, Sierramotion has the experience to create a solution that works the first time.
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