Part throttle efficiency

[Brian Parker]

Calmed things down a little by fitting a 2S lipo.
 'Scope set at 2Volts/Div and 0.2ms/Div at X5magnification.

First set of traces.

 No prop.

Trace pattern is from idle up to a fast idle/quarter throttle.

 Note trace freq/volts at no load.

 

 

 

 

 

 

 

 

 

Second set.
5x5 prop.

The three throttle settings to match the above.

Can't guarantee RPM readings (poor light).

3300RPM 4Watt,

5850RPM 10Watt,

7800RPM 25Watt.

Note trace freq/volts/watts at load.

 

 

 

 

[Simon Chaddock]

Thanks Brian

Now I'm a bit more puzzled!

 

In the last 3 traces (5x3 prop) am I correct that it shows for each commutation a rising voltage followed by a period at full power?

 

The lowest power trace seems to show a long and low rising voltage period and a short full voltage bit, the highest power one has the voltage rising to a higher value over a shorter period and with the remainder at full voltage.

 

If this is correct it would suggest that for absolute minimum resistance losses the ESC should run at virtually full throttle but only delivering the minimum current required to keep in the air rather than a higher power set up throttled back.

So for my endurance flying it looks like I am going to need a small but very low Kv (300?) motor turning a big prop. I might have to wind my own.

 

[Brian Parker]

Simon,

Puzzled, you can say that again!
I have (see below) attempted to show the relationship between the three traces.

 

Hope its clear enough to read.

 I did also monitor voltage supply ripple during the experiments. Very little ripple observed. I did think of removing the ESC electrolytic capacitor and observing the difference. Might try this at some stage.
Full throttle, large prop and very low Kv seem to be the indication for endurance together with short leads and the minimum of connections.

Good luck.

 

 

[Martin Harris - Moderator]

I've always been given to understand that the conclusion you've arrived at is the case.  A clubmate who flies F5b (see here if you're unfamiliar with it) which is at the forefront of development of electric flight technology (how does around 4KW strike you?) has always maintained that their ESCs would burn out if run at less than full throttle.

 

I believe it's due to the FETs being less efficient and producing more heat during the switching stage but I'm no expert.

 

If you ever get the chance to watch (and hear) an F5b model in operation I guarantee you'll be "impressed".  Even though I see him practice most weeks, each time he gives the motor a quick test blip, every head in the pits snaps round at the sound of the very stalled 18" x 18" prop (plus another couple of inches pitch from the offset pivots) beating the air around it to a pulp!

Edited By Martin Harris on 22/01/2010 17:48:21

[Brian Parker]

Thanks Martin,
 I've looked at the site, very interesting.

We were trying to find out exactly what was going on in the Battery/ESC/motor setup so as to provide an indication for the best combination for endurance flight.

 As to the FETs its not a case of their inefficiency but the fact that energy is expended (and dissipated) by the fast turn on and turn off rate and by parasitic inductance and  voltage spikes.