Technical queston - experts needed, please

[Bigfoot]

I have just been asked this question, but I don't know the answer!

"I have asked some club members this question. They have said this is impossible but they have failed to convince me by any technical logic.

Very high power/weight ratio electric models have fairly short duration flights. However, they often only use full power in relatively short bursts
and I suspect a lot of the battery power is lost in heat in the speed controller during low throttle spells.

Now I guess there is all sorts of electronic wizardry inside speed controllers so why can they not include voltage conversion circuits?

For example, full throttle sends maybe 98% of battery volts to the motor and perhaps 94% of battery current (6% keeping Rx and servos alive?). Now at reduced throttle (slow inverted passes?) the motor may only need 49% of battery volts. Normally I guess the remaining 51% of volts is reflected in heating in the speed controller. 100% of the reduced motor current is being drained from the battery. Now with conversion circuitry, at the reduced motor voltage, only 50% of the motor current needs to be drained from the battery as the battery is chucking out this current at 98% battery volts and therefore chucking out all the electrical power necessary to supply power to
the motor at 49% battery volts. This should be possible using a power-capable diode between the motor terminals of the speed controller, and perhaps a capacitor and inductance. For the same amount of time spent at full power, the reduced power period would be doubled, flights would last
longer and the speed controller would not have to dissipate much heat."

If there is anyone out there who would like to hazerad an answer, I'm all ears! And thanks.

[ChrisP]

Originally posted by Bigfoot
I suspect a lot of the battery power is lost in heat in the speed controller during low throttle spells.
Now at reduced throttle (slow inverted passes?) the motor may only need 49% of battery volts. Normally I guess the remaining 51% of volts is reflected in heating in the speed controller.I guess that used to be the case when we were using cheap resistance wire devices with a wiper arm back in the early days of model cars and boats. But surely these days with FET's pulsing the power and not just turning it into heat this no longer holds ?

Or am I not understanding something ?

[DaveSawers]

Speed controllers turn the power on and off in short pulses (many times per second). When the pulse is on, the motor gets full battery voltage. When it's off, the motor gets zero volts. Since the switching happens quickly, the motor acts as if it's getting a continuous, reduced voltage.

Heat emitted from the speed controller is from the switching circuitry, not the dissipation of excess volts. When the throttle is wide open, the power is permanently on and the speed controller is doing little or nothing. This is why speed controllers get hotter when run at less than full throttle.

A sensorless brushless controller controls motor speed by the speed at which each pole in the motor is energized.

[BMatthews]

And added to the high frequency pulsing of the speed control you have the inductance of the motor that smooths the current spikes out.

Sorry Bigfoot but you're barking up the wrong tree here. Any of our high rate switching ESC's consume very little power as waste heat at any throttle setting. It basically comes down to the efficiency of the overall conversion of electrons to thrust. Check out some of the test results of motor and prop combinations and look at the watts consumned vs oz of thrust produced. I think you'll find that the secret is in there. And then there's the irrefutable fact that if you want to move a lot of air it's going to take a lot of power. And high speed flight produces lots of drag. Couple that with not wanting to load down the model with too much weight and you have a hot model that doesn't have the battery watts to weight ratio that others may have.

But even then if you were to dial many of these models back to an altitude holding setting and fly them very delicately I suspect you'd see some remarkable duration figures. But the guys that fly these aren't into flying delicately are they...

[Bill Glover]

Originally posted by Bigfoot
Very high power/weight ratio electric models have fairly short duration flights.

That's rather a generalisation, many lightweight indoor/parkfly models have well over 1:1 thrust to weight and can achieve durations of 1 hour plus with Li Ion or Li Poly packs!

I've certainly logged 64 mins of aerobatic flight with my Mini IFO on a single charge. That's using a coreless motor with a high rate (133 khz) ESC, pretty efficient. I have the same setup in a Stubenfliege which flies at very low throttle settings most of the time (it will climb vertically with ease, and full power is only used in short bursts for tight touch & gos etc.). I don't know what the duration is with that, I've never got near flattening the pack during an evening's flying ;)

[Andy W]

As pointed out, your assumptions are incorrect. Our ESC's do not run at significantly reduced efficiency at lower throttle settings due to the high switching rates (Dave, all the brushless controllers I have have a fixed high-frequency switching rates, but vary the duty cycle - on/off ratio - to vary power). Very old low-rate brushed motor ESC's used to have a problem with heat at low throttle settings, but it still wasn't a huge loss. Large LMR models - F5B for example - use considerable power in very short bursts. The heat in the battery is as a result of internal resistance - current through a resistance generates power - heat - and there's nothing we can do to reduce that. A voltage convertor would make no difference - even at reduced throttle, if you need to provide (say) 100W to the prop (vs. the 400W at full throttle, perhaps), you still need to draw that 100W from the same battery. The current from the battery will be the same (neglecting electronics here) whether it's an ESC or any other mechanism for powering the motor. Therefore the same amount of amps will flow from the battery, and it will heat up just the same. I guess your primary assumption - that our ESC's waste energy at lower power settings - is the primary flaw in your argument.
..a

[Bigfoot]

Thanks for all the remarks, that pretty well confirms what I thought, I wanted to check that I was not barking up the wrong tree.

However, please note that I did not ASK the question, I was passing on a question that someone ASKED me.

Thanks again for the help, where would we be without E-zone and all you guys!