wrote on 9/14/2020 7:55 PM:
As someone who has experience working with EVs, this is alarming to read. Hearing this makes me skeptical of the FES's electrical design as a whole. Your electric car doesn't lose all of its performance if the battery gets to 30% (it does lose some, but not THIS much).

I believe the effect you're describing is the speed of the motor being limited by battery voltage. A well designed system should not have this problem, and this is an indication of a poor battery/motor/propping combo. As Emir said, these motors have a KV parameter, which describes how fast the motor will spin at a given input voltage. As the battery voltage drops, the maximum speed of the motor decreases as well. However, for a well-designed system, this voltage-limited speed, even at min battery voltage, is above the prop's 20kW speed. When the battery is fully charged and the system is capable of producing much more power, the software in the inverter limits it to 20kW for thermal protection.

As Emir also stated, the inverter is most efficient running at 100% duty cycle, but the efficiency hit from running at partial power (switching losses) is on the order of 1-2%, which isn't terribly significant in context of the whole electrical system's ~90% efficiency.

Electric cars have solved this problem, and they have to operate over much wider speed ranges and power ranges. This should not be a problem for props, since they operate over a much narrower speed range.

There is a good benefit for the pilot if the designer takes advantage of the power
available when the battery is fully charged and at a high voltage: the glider can
take off sooner and climb faster during the critical few minutes near the ground.
Yes, he could limit the initial power to be the same as the power near the end,
but then to get that desirable strong takeoff, he must provide a larger, heavier,
more expensive battery, ditto for the controller. For an FES glider, that may not
be a desirable trade-off.

The trade-off is likely different for gliders with mast-mounted motors and the
batteries carried in the wings: the propeller can be larger and more efficient,
and the batteries can be larger, as they are not constrained by the non-lifting
weight limit on the fuselage, nor the weight the pilot can carry.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1