A solenoid coil is the wound electrical half of a solenoid valve, converting current into the magnetic force that shifts a plunger across an air gap to open or close the valve seat [S1][S2].
Industrial catalog lines separate coils by current type (AC 50 Hz, AC 60 Hz, DC), system width (16 mm to 36 mm), connector form (DIN A, DIN B, DIN C, M12), and power level (1 to 6 in the nass magnet scheme), with overall widths of 16, 22, 30 and 36 mm covering the bulk of pneumatic and process-valve installations [S1][S2].
Classification Axes Used in OEM Catalogs
The first cut is kind of current. AC/50 Hz and AC/60 Hz are split lines because holding force, inrush VA, and hum behavior differ at 50 Hz versus 60 Hz, so most datasheets publish both, and DC versions are listed as a separate SKU family with a different snubber and rectifier story [S1][S2].
The second cut is system affiliation. Suppliers code mechanical interfaces (System 6, System 8, System 13 in the nass magnet catalog) so a coil only mates with a matching armature tube diameter and yoke footprint; mixing across systems is a hard mechanical mismatch, not a soft preference [S1][S2].
The third cut is power level. Levels 1 to 6 map to roughly ascending holding wattage (and therefore continuous-duty thermal load); level 1 is the lightest, level 6 the heaviest in the catalog's frame, and selecting the wrong level for a given duty cycle is the single most common field failure cause I see on incoming inspections [S1][S2].
Insulation Class, IP Rating, and Connector Geometry
Insulation class governs maximum winding hotspot temperature. Danfoss BA/BD/BB "high performance" coils are wound to Class H per IEC 85, which corresponds to a 180 °C hotspot limit on the wire insulation system, while BF-series industrial coils in the same Danfoss line ship with Class F rated equivalents for 50 °C / 80 °C ambient duty [S3].
IP rating tiers follow the same physical coil, with the connector changing the sealing. Danfoss specifies IP00 with a bare DIN 43650 form A spade connector, IP20 with a snap-on protective cap, and IP65 with a mounted cable plug on the same BA/BD/BB body, so a 0.16 kg coil can be re-rated from a panel-internal IP00 to a washdown IP65 just by changing the connector [S3].
Connector geometry itself is a classification: DIN 43650 form A (the dominant 18 mm footprint, 49 SKUs in one filter), DIN B, DIN C, plus industrial variants like M12 (3 SKUs) and the proprietary "ALT B" footprint (11 SKUs), so a maintenance shop should stock by connector form before by voltage [S1][S2].
Voltage Bands, Frequency, and Tolerance
Standard AC bands run 24 V, 48 V, 115 V, 220-230 V, 240 V, and 380-400 V at either 50 Hz or 60 Hz, with a tolerance of -15% / +10% on most 220/380 V AC coils and -10% / +6% on 230/400 V AC coils per VDE 0580 design rules; DC variants are typically 12 V and 24 V at +/-10% [S3].
Holding power is 9 W on BA, 10 W on BF, 15 W on BD high-performance families, with cut-in VA spiking to 39-54 VA on AC coils because of the inrush through the air gap before the plunger seats; this inrush is why upstream transformer and contactor sizing cannot be done from the holding wattage alone [S3].
Continuous duty rating is standard across these families, meaning the coil is sized for indefinite energization at the rated ambient (40 °C on BA/BD/BB, up to 80 °C on BF) without an interposing relay, which is the spec to verify when retrofitting an older intermittent-rated coil [S3].
Sheathing Materials and Housing Construction
Sheathing material determines chemical, thermal, and impact resistance. Thermoset resin is the dominant choice in nass magnet's pneumatic line (the entire filtered set in [S1] sits under "Thermoset resin"), while thermoplastic overmolds dominate the lower-cost general-purpose market, and metal-clad (steel or stainless) shells appear where solvent, fuel, or mechanical abuse is expected [S1][S2].
For low-power and high-frequency circuits, air-core coils (no ferromagnetic plunger path) are specified instead of solid soft-iron or composite cores, trading force for speed and zero residual magnetism; this is the same trade-off that shows up in relay and contactor selection [S5].
Wire gauge is a hidden classification. Too thin a gauge raises resistance, lowers current-carrying capacity, and burns out the coil on continuous or high-frequency duty, while too thick a gauge starves the magnetic circuit of ampere-turns, so wire diameter is sized jointly with turns count and bobbin window area [S6].
Application-Driven Type Selection
For an air solenoid valve on a pneumatic manifold, the practical pick is a 22 mm or 30 mm width, DIN A connector, System 8 or 13, power level 2-4, thermoset-sheathed AC or DC coil, with the voltage band matched to the cabinet PSU (24 V DC most common in factory automation) [S1][S2].
For process and hazardous-area service, the same 30 mm body ships in an ATEX/IECEx-rated variant, with encapsulation, certified terminal box or conduit hub, and a 3-core cable option rather than a spade connector; the datasheet also lists a junction-box variant for Ex d enclosures [S3].
For medical and dosing equipment, designers pick low-power, high-isolation thermoplastic-housed coils with EMC-conscious winding layouts, because infusion pumps, dialysis machines, and anesthesia gear call for low power consumption, high isolation resistance, and fail-safe behavior on continuous duty [S5].
For automotive and mobile hydraulics, coils are paired with rectifier electronics for DC operation from 12 V or 24 V vehicle buses, used in transmission shifters, starter solenoids, fuel injectors, and ABS modulators, where vibration and temperature swings dominate the spec [S5].
Selection Criteria, Failure Modes, and Sourcing Reality
The first decision is voltage and frequency, fixed by the supply; the second is mechanical interface (system affiliation + connector form); the third is power level, fixed by the valve's required holding force and the duty cycle; the fourth is sheathing and IP, fixed by the environment. Mis-ordering any of those four is what shows up on warranty claims, not exotic winding faults. [S3]
Common failure modes are open windings from thermal overload (over-voltage or blocked plunger), shorted turns from insulation breakdown at the hotspot, and connector ingress when the IP65 cable plug is replaced with a lower-rated substitute. Class H insulation buys margin on hotspot, but it does not save a coil run above its ambient rating or with a contaminated mating face [S3].
For engineers comparing options, the working frame is: AC 50 Hz coils are cheaper and draw higher inrush VA, DC coils are quieter and need a polarity-observing connector, dual-frequency 50/60 Hz parts cover global SKUs but at a small cost premium, and encapsulated/metal-clad housings are required where moisture, dust, or solvent ingress is on the risk register [S1][S3].
For a deeper dive on the valve side that these coils sit on, see this electric ball valve types map, and for the maintenance and wiring side, this electric ball valve installation acceptance walkthrough carries the field-test points that apply to any coil-on-valve retrofit.
For the relevant spec sheets and selection criteria, see solenoid coil.