Electric powered glider ground connection

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sdupre

Member
Joined
Oct 10, 2014
Messages
8
Location
Colorado
Hi HBA,

Those who know electric powered (self-launch or sustainer) gliders could probably answer this question definitively, but please chime in with your aircraft electrics understanding in any case:

If I add a ~72V battery, motor controller, and motor to my homebuilt all-metal glider, should that high voltage / high amperage system be electrically isolated from the glider's metal frame (to which the glider's existing 12V avionics system is grounded)? Or should it be grounded to the airframe as well?

My inclination is to isolate because the electric propulsion system's thick, dedicated wires provide the DC "source" and "return" paths between the components, which negates the need for an airframe common ground to serve as a return pathway.

Any low-voltage signals (e.g., for throttle, voltage and temperature monitoring) would be grounded to the propulsion system via a connection to the motor battery's negative terminal. So there's no electrical interface between the propulsion and the avionics systems.

Also, the thought of touching a metal airplane that's electrically isolated from a high-voltage system is more appealing than the opposite case.

Am I on the right track? Or am I missing out on why the two systems should/must share a common ground? I believe DC circuits of different voltages can share the same common ground, so that's probably not the issue. Can the 72V and 12V system grounds be joined by a ~5A fused bridge that effects a common ground while minimizing high-voltage shock and avionics-interference risks?

What's the standard practice on gliders with electric motors, if there's a standard practice?
 

TiPi

Well-Known Member
HBA Supporter
Log Member
Joined
Aug 25, 2019
Messages
468
Location
Peeramon (AUS)
Hi sdupre,
I’m not an electric glider specialist, my background is automotive, heavy equipment and solar systems.
I would run the electric propulsion system fully isolated from the frame, power and control circuits.
You definitively don’t want the power circuit to have any contact to the frame as that would potentially lead to stray currents and corrosion between frame connections.
I would also use shielded full dual-way wiring for the control wiring. Main reason here is to remove any potential interference and stray signals.
Also keep the 72V cabling as far away as possible from the 12V and the motor circuit. Again to reduce the interference from the high frequency power circuit.

added: if a common ground is required, it should be through a resistor, not a fuse (to limit any current flow)
 

sdupre

Member
Joined
Oct 10, 2014
Messages
8
Location
Colorado
Thanks for the inputs, TiPi. Your resistor idea for bridging the two grounds sounds better than my midnight brainstorming of a fuse. The idea is to enable a whole-airframe equipotential reference while isolating any unwanted effects from the propulsion side.
 
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