A dry-type enclosure changes the connected air and radiation paths around the core. Assess the routes serving the yoke and limbs in the represented installation, alongside the appearance requirement. [1] [2]
Trace the core’s path from incoming air to its exit. Include the cover, screens, support beams and installation surroundings in the geometry being tested.
Draw the complete local path
- Supply: identify the openings that actually admit air to the lower and inner core regions.
- Rise: follow the passages beside the limbs and through spaces sharing flow with the coils.
- Top region: show the clearance above the yoke, pockets beneath the cover and obstructions to lateral escape.
- Exit: identify the connected outlet and whether discharged warm air can return to the supply.
Mark which openings belong to a winding bypass and which serve the core route. A nearby vent is ineffective for the latter if an internal panel separates them.
Evaluate more than free area
Screens, louvers and narrow approach spaces create pressure resistance. Their nominal opening percentage does not uniquely determine the flow. Natural circulation is driven by temperature-dependent density around the connected loop, and changing an outlet can redistribute air between paths.
The cover also changes radiation. A previously exposed yoke can exchange radiation with the cover instead of the wider surroundings; its receiving temperature and view matter. Do not add an unchanged “radiation cooling” allowance after modifying the enclosure.
Retain core and winding heat inputs separately. At different duties, the windings can supply a warm plume to the core’s top region or dominate circulation elsewhere. A test with only one artificial heat source may not represent those interactions.
Compare the installed enclosure with its reference
Use matched excitation and loading conditions, initial thermal state, ambient measurements and sensor locations. Record local core temperatures, air temperatures along the mapped path and enclosure surface temperature where relevant.
Compare the hottest location and the local gradients, not only an average air outlet reading. If adding the cover raises the top-region air temperature while nearby lower surfaces change little, investigate the exit and recirculation path before blaming the steel loss model.
Where a fan is part of the design, check its actual operating point and the flow path it serves. A fan nominal rating alone does not validate circulation beneath the cover.
At 1.7 T and 50 Hz, 23ZH90 has a catalogue specific-loss limit of 0.90 W/kg. [3] Assume 800 kg of represented steel, giving a 720 W strip-input budget for an enclosure comparison. At an assumed measured air flow of 0.080 kg/s and constant heat capacity of 1,005 J/(kg·K), assigning that source entirely to air gives a 720/(0.080 × 1,005) = 8.96 K bulk air rise. If the cover reduces flow to 0.060 kg/s, the same balance gives 11.94 K. A roughly 3 K outlet-rise increase warrants reviewing the connected air path. This balance excludes winding heat and radiation and does not predict the core hot spot.
Keep packet geometry, joint quality and clamping state unchanged between enclosure trials. Changing them at the same time changes the core heat source and prevents attribution of the temperature difference to the cover.
Retain enclosure openings, clearances and surrounding installation features in the qualification. Confirm the connected airflow and radiative boundaries for the proposed wall and ceiling conditions.
References
- Z. Chen, Z. He and S. Wang, Transformer Iron Core Temperature Field Calculation Based on Finite Element Analysis, Energies 18, 6537, 2025. DOI: 10.3390/en18246537.
- U.S. Department of Energy, DOE-HDBK-1012/2-92, Thermodynamics, Heat Transfer, and Fluid Flow, Volume 2, 1992.
- Nippon Steel Corporation, Grain-Oriented Electrical Steel Sheets, catalogue D004je, April 2020, specification and typical-property tables.