Why this matters: When transformers work with variable-frequency drives (VFDs), they don’t see a clean sine wave. They see PWM edges, high dv/dt, common-mode voltage, and heavy harmonic currents. “Ordinary” distribution transformers aren’t built for that. Inverter-duty (a.k.a. converter-duty) designs are.
What’s different electrically?
- Non-sinusoidal excitation: PWM carriers and sidebands raise core and stray losses; copper I²R at harmonic frequencies goes up.
- High dv/dt & repetitive surges: Fast edges (and cable reflections) stress inter-turn insulation well beyond sinusoidal duty.
- Common-mode voltage: Causes capacitive displacement currents to ground; needs shielding and grounding provisions.
- Wide frequency span: Transformers may see anything from a few Hz to hundreds of Hz fundamental—flux density and cooling must hold across the range.
Relevant standards/ideas: IEC 61378 (converter transformers), IEEE C57.110 (K-factor/harmonics), and IEC 60076 series for type tests and temperature rise.
Design features you should expect in an inverter-duty build
- Core & flux density
- Lower B_max than ordinary designs to curb harmonic/excess loss and audible noise.
- Step-lap CRGO (or amorphous for lower core loss/noise); robust core clamping to tame vibration.
- Winding geometry
- Interleaved or sectional windings to set leakage L and reduce stray hot-spots.
- Controlled leakage inductance (useful as part of the output filter with VFDs).
- Optimized capacitance (winding-to-winding and to ground) so common-mode current stays manageable.
- Insulation system
- Corona-resistant magnet wire, enhanced inter-turn and inter-layer insulation (e.g., PI/Nomex®/DMD stacks).
- Bigger creepage/clearances, edge taping, end-fillers to avoid partial discharge at high dv/dt.
- VPI (vacuum-pressure impregnation) for dry-type or cast-resin coils in harsher environments.
- Thermal design
- Winding and core loss modeled with harmonic spectrum, not just kVA.
- Higher thermal class (e.g., F/H), embedded PT100/NTC sensors, better ducting/oil channels (for oil-filled).
- Nameplate may include K-factor or an explicit harmonic loading curve.
- Shielding & grounding
- Electrostatic screen between windings with low-impedance bond to PE to drain common-mode currents.
- Grounding lugs/straps sized for high-frequency displacement currents.
- Terminations & EMC
- Foil/braid connections with low stray inductance; short, symmetrical leads.
- Provisions for RC snubbers or external dv/dt/sine-filter interfaces when the transformer is on the drive output side.
Manufacturing & QA considerations
- Process control: Consistent VPI/cure cycles; measured resin pick-up; tight core bolt torque to stabilize sound levels.
- Surge & partial-discharge checks: Routine repetitive-impulse test and PD limits for dry-type; induced and applied tests per IEC 60076 but interpreted for non-sinusoidal duty.
- Temperature-rise test under harmonic load: Use an equivalent RMS/harmonic profile (or K-factor method) rather than pure sine.
- Sound-level verification: PWM harmonics can excite the core—verify dB(A) vs catalog limits.
- Documentation: Provide harmonic capability (K-factor or spectrum), dv/dt withstand, recommended filtering/cabling, and grounding instructions.
Quick chooser: ordinary vs inverter-duty
| Situation | Ordinary Transformer | Inverter-Duty Transformer |
| Pure sine supply, linear loads | ✅ OK | Not required |
| Feeds 6-pulse VFD front-end (line isolation) | ⚠️ Risk of heating/noise | ✅ Sized for harmonics (K-factor), low stray loss |
| Sits after VFD (isolation to motor) | ❌ Not suitable (dv/dt/CMV) | ✅ With shield, dv/dt withstand, tuned L/C targets |
| Long motor cables / EMC-critical sites | ⚠️ | ✅ With screen + specified capacitance |