Serial hybrid power converter starts field trials

Introduction:

The energy density of gasoline is about 75 times that of batteries used in electric aviation. Therefore it is compelling to use an IC engine to power a generator that in turn charges the battery. Such a system is called a serial hybrid system.

This post will discuss and contrast conventional vs our approach to serial hybrid power conversion.

Conventional approach:

A diode bridge rectifier is used to convert 3 Phase AC to DC and charge the battery.

Disadvantages:

i) Battery may get overdischarged or overcharged as no BMS is used

ii) Forcing the motors to operate at the same RMS voltage as the alternator may not be efficient

iii) Choice of engine and its operating RPM is limited

iv) We need to rev the engine to increase voltage and makes the engine function outside its point of maximum fuel economy

v) Lack of telemetry and engine start/restart capabilities without additional logic

vi) Limited system life due to peak diode current

Our approach:

We use the same electronics to start the engine and subsequently act as a synchronous rectifier converting 3 phase AC to DC. Furthermore using an interleaved converter we step up/down the voltage output of the rectifier to match the voltage requirements of the electric motors that power the propellers.

Advantages of our approach:

i) Synchronous rectification is more efficient than a diode bridge rectifier (5% saving)

ii) The FADEC used on our system ensures that engine operates at its optimal RPM

iii) BMS ensures the battery is not overcharged and over-discharged thereby increasing battery lifespan by 70%

iv) Interleaved converter acts like an electronic gear that allows the IC engine and the drive motors to operate at different voltages.

v) FADEC allows IC engine start and restart capability

vi) Higher fuel economy (30% more, that directly translates to 30% increase in flight time)

vii) Extremely power dense architecture converting power at 5 kW/kg

viii) Higher power levels can be acheived by stacking more interleaved converters

ix) High conversion efficiency (> 93%) provides superior endurance and reduced cooling requirements

x) Modular system that can be used on manned and unmanned craft. Fixed and rotary wing.

Pictures of our test setup and coverter follow:

Test setup with engine and alternator

Interleaved converter

Synchronous rectifier and engine starter

We start pilot testing to fine tune the system to various operating conditions and requirements