Solenoid Work Notes

15 years in custom solenoid and solenoid valve design, selection, simulation and failure analysis.

Browse by topic

• Troubleshooting 17 A unit is failing now: no movement, weak force, sticking, heat, noise, leakage.
• Selection and specification 15 Choosing a topology, a winding and a set of tolerances before anything is built.
• Materials, corrosion and sealing 7 Humidity, washdown, dust, water quality and the coatings that decide service life.
• Applications 12 Vending, locks, coffee, medical, automotive, agricultural and HVAC duty.
• Fundamentals 6 Air gap, ampere-turns, saturation and temperature: the four relationships behind every number.

Latest notes

• Ampere-turns in a solenoid: why equal magnetomotive force is not equal force

  Two coils with the same 877 ampere-turns at the same 3 W can differ by 40 % in force, and the difference is entirely in the magnetic circuit rather than the winding. Wire gauge does not change ampere-turns at fixed dissipation, so choosing a gauge is a voltage decision, not a force decision. Here is the arithmetic, the saturation limit, and what actually sets the efficiency.

• Air gap vs. solenoid force: why it is closer to inverse square than linear

  Force falls roughly with the square of the gap, so the last fraction of a millimetre costs more than the whole rest of the stroke. A gap growing from 0.30 mm to 0.35 mm removes 27 % of the force, and to 0.40 mm removes 44 %. Here is how the magnetic circuit divides, how much of it the iron actually consumes, and which tolerances to attack first.

• Copper resistance and temperature: predicting hot-state solenoid force from a cold measurement

  Copper gains 0.393 % of its resistance per kelvin, which means a coil at 90 °C has 27 % more resistance than the same coil at 20 °C and delivers 61 % of its cold force at fixed voltage. That ratio also turns a resistance measurement into a winding thermometer, accurate to about 2 K without a thermocouple. Here is the arithmetic and where it stops working.

• Rotary vs. push-pull solenoids: reading a torque-angle curve

  A rotary solenoid does not have a torque, it has a torque curve, and the curve is not monotonic. It rises, peaks somewhere in the middle of the rotation and falls away again towards both ends, so a selection made on peak torque is wrong at both ends of the stroke. Overlay the return spring and the two worst points are usually not where the datasheet highlights.

• Reading a solenoid force vs. stroke curve: what the datasheet leaves out

  A force-stroke curve is one snapshot taken at one temperature, one voltage and on one sample. Add the omissions back and the picture changes: a winding 70 K above ambient costs about 39 % of the force, a 10 % voltage shortfall costs 19 %, and the low end of a batch costs more again. Here is what to ask for, and in what order.

• Why solenoid force drops after thermal cycling

  When a solenoid is measurably weaker after a thousand hours or a thousand cycles, the magnetic material is almost never the reason. Soft magnetic steel does not lose saturation flux density in service, and the stress left by machining anneals out. The loss comes from geometry: a thermoplastic bobbin creeping under load, a spring relaxing, or joints settling by tens of microns.

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