Notes / Selection and specification

Selection and specification

15 notes in this topic

• 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.

• How a return spring eats your usable stroke

  A return spring does not simply subtract a constant from the force. It rises linearly with stroke while the solenoid's force rises steeply as the gap closes, so the two curves fight in different places. A spring pre-load of 0.3 N on a 4 mm stroke can consume half the travel before the plunger moves at all, even when the force at the end of stroke is identical to a spring that costs nothing.

• IP65 for solenoids: what the rating actually requires, and what it does not cover

  IP65 tests a 6.3 mm jet at 12.5 L/min from 3 m, and it says nothing about condensation, steam or chemical washdown. On a solenoid the leak paths are the lead-wire entry, the breather and the plunger joint. A sealed 5 cm3 cavity heated from 20 C to 90 C either pushes out 1.2 cm3 or builds 0.24 bar, and that is the mechanism that pulls moisture in.

• Latching solenoid principle: how they work, and the drive cost nobody budgets for

  A latching solenoid holds on a permanent magnet and moves on a pulse, so holding current is zero. The energy saving scales with hold time: a 12 V, 1 A, 50 ms pulse costs 0.6 J, against 16 Wh for the same load held continuously for eight hours. The bill arrives as a bipolar driver and a capacitor sized from C = 2E/V squared.

• Solenoid coil wire gauge and turns: the tradeoff, and the one point that never moves

  Inside a fixed winding window, ampere-turns go with wire area and copper loss goes with its square, so the same window at the same power gives the same force whichever gauge you wind. A 0.20 mm coil at 24 V and a 0.40 mm coil at 6 V both land at 877 ampere-turns and 3.0 W. Gauge sets the supply and the driver; the window sets the force.

• AC vs. DC solenoids: inrush, hum, and instant burnout

  On DC the current is set by resistance alone, so the force is predictable. On AC the plunger position is part of the magnetic circuit, so pull-in current runs three to ten times holding current, a shading ring is mandatory to stop the force collapsing at every zero crossing, and a coil energised with its stroke blocked sits at the inrush value until it fails.

• How to specify a custom solenoid: the 8 parameters you must define

  A custom solenoid is only as good as the specification behind it, and the parameters that get omitted are consistently the expensive ones. Stroke, force at the worst point of that stroke, the real energisation profile, the supply and its tolerance, the environment, the mounting interface, the life target, and what happens at the end stop. Omit the last and the pole face is the brake.

• Open frame vs. tubular solenoid: when each actually wins

  For the same copper volume an open frame usually shows the higher peak force, because its flat pole face can close to almost zero gap. A tubular solenoid shows a lower peak but holds more of it over a longer stroke, and its closed shell returns its own flux so less leaks away. At equal copper the two curves typically cross between 1 mm and 3 mm of stroke.

• How to calculate solenoid holding force (and the correction everyone skips)

  Holding-force equations assume ampere-turns stay constant, and they do not. Copper resistance rises 0.393 %/K, so a winding stabilising 70 K above ambient sits at about 78 % of its cold current and about 61 % of its cold force. Here is the order to run the calculation, with the gap, fringing and saturation corrections that actually move the answer.

• Plunger material and surface treatment: the coupled choice

  A plunger must be magnetically soft and mechanically hard, and those requirements pull against each other in the same steel. Free-cutting grades such as 1215 machine well because of manganese sulphide inclusions, and those inclusions raise coercivity and give corrosion somewhere to start. Choose the body for magnetics, treat the guided surface for wear, and keep coatings out of the gap.

• HVAC damper actuators: the 24V AC sizing trap

  On an AC solenoid the impedance changes as the plunger moves, so pull-in current is several times the holding current, and a coil energised with its stroke blocked stays at the inrush value until it fails. Size the transformer, the wire and the switch from VA and inrush — a volt-amp is not a watt when the power factor is around 0.5.

• Door lock solenoid battery life: the inrush current mistake

  Battery life is almost never limited by the coil's steady-state current, because the coil does not run at steady state. It runs as a millisecond pulse fed from a bulk capacitor, and the battery pays for recharging that capacitor as a high-rate burst, where its usable capacity is well below the mAh figure on the label. Measure the current on the battery side, not the coil side.

• Solenoid duty cycle in vending machines: why 100% is a trap

  A 100% duty rating is only valid at the ambient it was measured in, and the inside of a vending machine is not that ambient. A coil rated continuous at 25 °C is worth roughly 65% of that at 55 °C, because the allowable temperature rise falls one-to-one with ambient. Read the figure as a property of the installation, not of the coil.

• How hot is too hot: solenoid coil temperature limits

  The number that matters is not an absolute temperature, it is the allowable temperature rise for the insulation class. Subtract the real ambient first — a coil in a 25 °C workshop and the same coil inside a machine enclosure at 55 °C have completely different allowable rises. Class B allows 80 K by the resistance method, class F 105 K, class H 125 K.

Other topics

• Troubleshooting A unit is failing now: no movement, weak force, sticking, heat, noise, leakage.
• Materials, corrosion and sealing Humidity, washdown, dust, water quality and the coatings that decide service life.
• Applications Vending, locks, coffee, medical, automotive, agricultural and HVAC duty.
• Fundamentals Air gap, ampere-turns, saturation and temperature: the four relationships behind every number.