2023 – 2026 · IEEE JESTPE 2026
6.78-MHz Integrated Wireless Motor
A cable-free motor that consolidates the receiving coil, power converter and PMSM into one compact unit — end-to-end hardware design from Class E amplifier to shaft.
- 10.6 W/cm³
- receiver power density, ~6× the closest published kHz-class system
- 69%
- end-to-end efficiency, DC source to shaft (vs 58% benchmark)
- 30 W
- wireless power chain at 6.78 MHz
Problem
Wireless motors replace power cables with a wireless power transfer (WPT) link, which removes cable wear and entanglement in robots and medical devices such as artificial hearts. But existing systems use large kHz-class coils placed far from the motor, so power density is low and the motor’s range of motion is restricted.
Approach
I designed the complete wireless power chain end to end:
- Transmitter — a current-mode Class E power amplifier with soft-switching parametric design.
- Resonant link — operating at 6.78 MHz shrinks the receiving coil to 60 mm × 1 mm (2.8 cm³) and allows compact SMD capacitors.
- Receiver — a high-frequency half-bridge rectifier feeding a three-phase GaN inverter that drives a PMSM.
- Integration — circular PCBs mounted at the motor rear (axially housing-mounted scheme); electrolytic DC-bus capacitors replaced with a 250-V MLCC array to stay compact and remove dry-out failure risk.
- Firmware — STM32G4 V/f control with SPWM whose modulation index auto-compensates DC-bus sag.
Two integration challenges drove the design. The motor’s ferromagnetic structure distorts the coil’s field, so I quantified the inductance drift with Ansys HFSS multi-domain simulation and validated it against impedance-analyser measurements before committing to fabrication. The motor drive also behaves as a dynamic load, so the WPT link was optimised for robustness across wide impedance swings.
Results
- 10.6 W/cm³ receiver power density against 1.8 W/cm³ for the closest published system.
- 69% end-to-end efficiency from DC source to mechanical shaft (WPT stage > 89%, converter stage 92%).
- Holds 600 r/min through a 6× load step (0.045 → 0.27 N·m) with 15 ms settling and no voltage collapse.
- Stable across 1–4 cm coil separation and ±20 mm lateral misalignment (efficiency 69% → 60%).
- 15-minute thermal run: GaN devices 51.7 °C, coils 30 °C, MLCC bank 35 °C.
- Stage-by-stage loss breakdown (motor 42.4%, converter 23.1%, coils 18.4%, amplifier 16.1%) to direct further optimisation.