MIL-PRF-27 Transformers
Selecting qualified transformers and inductors for aerospace and defense
When your program calls out MIL-PRF-27, you need transformers and inductors that survive the platform environment and carry their own qualification evidence into your review. MIL-PRF-27 is the U.S. military performance specification for transformers and inductors of the audio, power, and high-power pulse types. It defines the design, environmental, and quality-assurance requirements a transformer or inductor must meet for military and aerospace use, and it supersedes the earlier MIL-T-27.
Abbott Technologies is a listed manufacturer on the DLA Qualified Products List for MIL-PRF-27 (QPL-27, CAGE 58910), holding government designations M27/312 through M27/354 and more than 800 standard designs approved to the specification. We build 400 Hz and 60 Hz transformers in single- and three-phase configurations, plus inductors and custom designs, from a catalog of proven, qualified baselines.
A part called out to MIL-PRF-27 is classified two ways that every specifying engineer should understand up front: by Grade (construction) and by Class (maximum operating temperature). This page explains both, shows the qualification each Grade carries, and lists the Abbott series and their MIL-PRF-27 designations.
Choosing the Right Grade for Your Packaging and Environment
MIL-PRF-27 uses a single-digit Grade to describe how a unit is built, and, through the specification’s Table I, the environmental tests it is qualified against. Select the Grade that matches how the part is packaged and mounted in your system.
| Choose | When your application needs | Qualified against (MIL-PRF-27 Table I) |
|---|---|---|
| Grade 4 sealed, metal-encased |
A sealed metal enclosure for the harshest moisture and ingress environments | Seal, thermal shock, immersion, moisture resistance, vibration, shock, and salt atmosphere (when specified) |
| Grade 5 encapsulated |
A molded or potted unit, the most common choice for airborne and vehicle electronics | Flammability, thermal shock, immersion, moisture resistance, vibration, shock, and salt atmosphere (when specified) |
| Grade 6 open type |
An open unit you will pot or embed inside your own assembly | Thermal shock, vibration, and shock |
Environmental methods are drawn from MIL-STD-202: thermal shock (Method 107), immersion (104), moisture resistance (106), vibration (204), shock (213), and salt atmosphere (101). A sealed Grade 4 unit and an encapsulated Grade 5 unit can meet the same electrical requirement while answering different sealing and flammability constraints.
Matching the Temperature Class to Your Thermal Environment
The Class is a single letter for the unit’s maximum operating temperature, defined as your maximum ambient plus the transformer’s temperature rise. Add your worst-case ambient and expected rise, then choose the class at or above that total.
| Class | Q | R | S | V | T | U |
|---|---|---|---|---|---|---|
| Max operating temperature | 85 °C | 105 °C | 130 °C | 155 °C | 170 °C | 170 °C+ (as specified) |
The class denotes maximum operating temperature only and has no relation to the insulation material. Temperature rise is rated at sea level and increases with altitude, so confirm that your ambient plus rise stays within the chosen class at operating altitude.
Why a QPL-27 Qualified Source
A commercial transformer that meets the electrical specification on the bench can still fail your program at qualification, through thermal-shock cracking of the potting, seal or insulation breakdown, vibration-induced winding or lead failures, or moisture ingress. MIL-PRF-27 exists to force those failure modes out on the test bench, not in the field. Sourcing from a QPL-27 qualified manufacturer gives your qualification and compliance team documented, standardized MIL-STD-202 test evidence up front, rather than a vendor’s assurance. Abbott is a listed QPL-27 source (CAGE 58910) with more than 800 designs approved to the specification, held under full configuration control and not obsoleted, so the part you qualify remains available for the life of the program.
Abbott MIL-PRF-27 Series and Government Designations
Each Abbott transformer and inductor series is qualified to specific MIL-PRF-27 government designations (M27/). Find your frequency, phase, grade, and power, then follow the series to its product page or datasheet.
| Series | Frequency / phase | Grade / Class | Government designation | Power range | Datasheet | |
|---|---|---|---|---|---|---|
 |
P | 60 Hz, single-phase | Grade 4, Class S | M27/312 to M27/322 | 5 to 300 VA | |
 |
6C | 60 Hz, single-phase | Grade 4, Class S | M27/326 to M27/331 | 5 to 300 VA | |
 |
6DM | 60 Hz, three-phase | Grade 5, Class S | Designed to MIL-PRF-27 | Three-phase power | |
 |
4A | 400 Hz, single-phase | Grade 5, Class S | M27/332 to M27/337 | 7 to 200 VA | |
 |
E | 400 Hz, single-phase | Grade 5, Class S | M27/338 to M27/343 | 2 to 175 VA (high-temperature) | |
 |
TM/TF | 400 Hz, single-phase | Grade 5, Class S (Grade 6 available) | M27/350 to M27/354 | 15 to 100 VA (toroidal) | |
 |
R | 400 Hz, single-phase | Grade 5, Class S | Designed to MIL-PRF-27 | 1.5 to 9 VA (PCB-mount) | |
 |
4D | 400 Hz, three-phase | Grade 5, Class S | M27/344 to M27/349 | 60 to 5,000 VA (delta or wye) | |
 |
Inductors and Chokes | DC to 100 kHz | Per requirement | Designed to MIL-PRF-27 | 8 uH to 10,000 uH | |
 |
Modified and Custom | Per requirement | Per requirement | To your requirement | Per requirement | Design form |
For 60 Hz power above 300 VA, the 6HM and 6JM series extend to 10 kVA single-phase and 15 kVA three-phase. Related: transformer rectifier units use Abbott MIL-PRF-27 transformers to convert AC to bulk DC.
Selecting a MIL-PRF-27 Transformer
A short checklist to bound the selection before an RFQ:
- Electrical: input voltage and frequency (60 or 400 Hz), phase (single or three), output voltage(s) and VA, regulation, and no-load requirements.
- Thermal: maximum ambient plus expected temperature rise, which sets the required Class (remember the altitude effect above).
- Construction and environment: sealed metal (Grade 4), encapsulated (Grade 5), or open for board-level potting (Grade 6); mounting and SWaP constraints.
- Qualification: which Table I tests your program invokes (thermal shock, vibration, shock, moisture, seal, salt) and any platform overlays such as MIL-STD-810.
- Program: lifecycle and obsolescence expectations, configuration control, and any NSN requirement.
Modified and Custom Designs From Qualified Baselines
Most requirements do not need a clean-sheet part. When an off-the-shelf unit is close but not exact, Abbott can start from one of its 800+ qualified MIL-PRF-27 designs and modify voltage, VA, footprint, or class, so you keep the qualification heritage of a proven parent design while avoiding the cost and schedule of a ground-up development. When your requirement genuinely needs a custom transformer or inductor, our engineers design to your electrical, thermal, and environmental targets and support the qualification effort with you. In every case the result is held under configuration control and remains available for the life of the program.
Frequently Asked Questions
What is the difference between MIL-PRF-27 Grade 4 and Grade 5?
Grade describes construction, not a quality ranking. Grade 4 units are sealed and metal-encased and are uniquely subject to the Seal test; Grade 5 units are encapsulated, such as molded or potted, and are uniquely subject to the Flammability test. Both are qualified against thermal shock, immersion, moisture resistance, vibration, shock, and, when specified, salt atmosphere. Choose based on how the unit is packaged in your system.
What is MIL-PRF-27 Class S?
Class S designates a maximum operating temperature of 130 °C, defined as maximum ambient plus the transformer’s temperature rise. The class letter refers only to temperature, not to a particular insulation material.
Is MIL-PRF-27 the same as MIL-T-27?
MIL-PRF-27 is the current specification and supersedes MIL-T-27. Parts previously called out to MIL-T-27 are now qualified to MIL-PRF-27.
Send us your electrical, thermal, and environmental targets and we will identify a qualified baseline or a custom path. Request a quote or complete the magnetics design form.