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Old Jun 28, 2003, 07:45 PM
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Blacksburg, VA 24060 USA
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ESC Efficiency at Low RPM?

Steve Neu, in the August Quiet Flyer Magazine, has done some testing of brushless ESCs for efficiency at less than 100% RPM. His bottom line is that efficiency is not nearly as good at reduced throttle settings as it is at 100%.

This reminds me of the view some years ago about frame-rate ESCs for brushed motors versus high-rate ESCs. It was argued that the high-rate ESC was more efficient at reduced throttle than frame rate ESCs. It was further argued that using a frame-rate ESC at reduced throttle could result in enough heating that the motor magnets would be demagnetized!

My question: Huh? What sort of efficiency can you expect with a high-rate ESC for brushed motors at reduced throttle? I recognize that this is a very general question with a lot of "ifs" and "buts," but maybe some of you ESC mavens can shed some light on this matter.

Jim R
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Old Jun 29, 2003, 03:07 AM
BEC
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Re: ESC Efficiency at Low RPM?

Quote:
Originally posted by JRuggiero
Steve Neu, in the August Quiet Flyer Magazine, has done some testing of brushless ESCs for efficiency at less than 100% RPM. His bottom line is that efficiency is not nearly as good at reduced throttle settings as it is at 100%.

This reminds me of the view some years ago about frame-rate ESCs for brushed motors versus high-rate ESCs. It was argued that the high-rate ESC was more efficient at reduced throttle than frame rate ESCs. It was further argued that using a frame-rate ESC at reduced throttle could result in enough heating that the motor magnets would be demagnetized!

My question: Huh? What sort of efficiency can you expect with a high-rate ESC for brushed motors at reduced throttle? I recognize that this is a very general question with a lot of "ifs" and "buts," but maybe some of you ESC mavens can shed some light on this matter.

Jim R
I'll have to go read Steve's article and see what he says, then comment further.

All that about frame rate ESCs is correct - though demagnetizing is an extreme situation.

At part throttle (brushed or brushless) a motor is being turned off and on at the switching frequency of the the ESC. As such it's always speeding up and slowing down. The faster this happens (up to a point which varies from motor to motor) the more efficient it is from the motor's standpoint. But that may or may not be the best from the standpoint of the switching performance of the MOSFETs.

As in most things in aircraft design - compromises have to be made.
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Old Jun 29, 2003, 11:49 PM
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BEC,

TNX for the response. Many years ago, I used a frame-rate Futaba ESC in a motorglider. I often ran it at a low throttle, just keeping the aircraft on the edge of a stall as I "sniffed around" for thermals. Not knowing that running such an ESC at low throttle was inefficient, I didn't know the difference made a difference!

When I switched to a high-rate ESC and did the same sort of thing, I noted no improvement in low-throttle handling or duration.

But as you point out, there are so many variables...

Jim R
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Old Jun 30, 2003, 12:13 AM
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For one of my next tests I am going to run tests on a brushed motor with a high rate controller--I have a feeling that the results may be interesting.

Steve
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Old Jun 30, 2003, 12:45 AM
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Leavenworth Ks
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efficiency

What do you mean by efficiency? Amount of power per rotation or amount of power per min or what? You make it sound as if the power is being drained faster at half than at full, although there may be more "wasted" power at low throttle , you are still using less power.
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Old Jun 30, 2003, 11:44 AM
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Dave makes a good point. Just because it is less efficient does not mean you are using more power. Lets say in an ideal world, running WOT is 100% efficient, but at 50% throttle, you are loosing 10% in efficiency. In this case you would be at 60% power consumption to get 50% thrust. So you are still using considerably less power (40%), but wasting 10% at the same time.

Now we can really muck things up by acknowledging that at 100% throttle, you will be flying faster and realizing that parasitic drag increases exponentially to speed (twice as fast = 4X drag), what we gain in electrical efficiency may be more than lost in aerodynamic efficiency. But wait thatís not the whole story! If you are not scratching your head yet, let me remind you that lift drag decreases with airspeed, so it helps to cancel some of that increase parasitic drag. Of course all of this varies a great deal with airframe design, airfoil, electrical components, etc.

Just further proof that nothing in the hobby is cut and dry. Just remember, heat is a form of energy, the hotter your components, battery, motor, ESC, the less energy goes into turning the thingy up front.
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Old Jun 30, 2003, 04:01 PM
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Best to look at article before making conclusions. Test results showed about a 25-30% drop in efficiency when the RPM was reduced to 70% of the noload RPM. The test was of the motor and controller--no models were harmed in the tests
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Old Jun 30, 2003, 05:58 PM
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>> For one of my next tests I am going to run tests on a brushed motor with a high rate controller--I have a feeling that the results may be interesting.

Steve, that's excellent news. I've wondered about this for some time now. There have been discussions from time to time that an ultra-high-frequency ESC (~100kHz) might considerably extend the brush life (and therefore useful motor life) of some of the small brushed motors, the IPS motors in particular. If so, the ESC could easily pay for itself many times over in reduced replacement motor costs. Looking forward to your results.
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Old Jul 01, 2003, 09:11 AM
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Quote:
Originally posted by sneu
Best to look at article before making conclusions. Test results showed about a 25-30% drop in efficiency when the RPM was reduced to 70% of the noload RPM. The test was of the motor and controller
So is this a 30% drop in overall efficiency at 70% throttle? I doubt that the controller is eating all the lost power because that could equate to a lot of heat! However, it would make more sense if the motor was also less efficient at lower voltage, and/or the non-smooth current flow caused more motor losses (as with Frame-rate control).

Another interesting experiment might be to compare two setups, one running at full throttle, and another at 70% throttle but with increased voltage to achieve the same input power. This should remove motor voltage effects, but would still include FET switching losses, and increased motor losses if switching frequency was too low.
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Old Jul 01, 2003, 03:14 PM
jrb
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Steve, hopefully you can shed some light on a specific motor/esc combo; my Aveox 27/39/2 using their SH-48?

Was told by an ESC dsgnr/mfg that the SH, is and is of DE origin; doesnít have good part power performance.

TIA,

Jim
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Old Jul 01, 2003, 03:31 PM
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Quote:
Originally posted by member73

Just remember, heat is a form of energy, the hotter your components, battery, motor, ESC, the less energy goes into turning the thingy up front.
Some people have a way with words.
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Old Jul 01, 2003, 07:35 PM
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Quote:
Originally posted by jrb
Steve, hopefully you can shed some light on a specific motor/esc combo; my Aveox 27/39/2 using their SH-48?

Was told by an ESC dsgnr/mfg that the SH, is and is of DE origin; doesnít have good part power performance.

TIA,

Jim
Check out the next issue if Quiet Flier for the graphs. One of the tests was done with the SL series(low voltage version of the SH).

I guess I don't understand the second question--The SL/SH controllers are made by Avoex in this country. The Avoex's part throttle efficiency is about the same as the "other" U.S made controller.

The poorest controllers in the tests came from Eastern Europe--I now have some fresher versions of those that I will test soon to see if the latest ones have improved.

Steve
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Old Jul 02, 2003, 08:27 PM
jrb
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Thanks Steve Ė usta respect the otherís comments; but this time it seemed more like a sales pitch; though yours sounds like a pitch for QF/SEM Ė I like many didnít re-new.


'Edited for profanity inuendo" FB
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Old Jul 02, 2003, 10:43 PM
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Steve I read you article and it is interesting. I have a few questions.

You tested the 1412/2Y I think. Don't have it here with me. Kv = 1450. This motor has a 2.7A no load current. So we can very simply estimate low current performance to be 50% efficient when current is only 5.4 A, Even at full throttle.

Correct?

The motor at 10V could very easily be propped for 40A or 37ozin of torque. With a prop power is proportional to the cube of the RPM. So 400W flies our model fine we throttle back to cruise it might be 200W. 200W would be 20A from the source but would be 70% duty cycle. RPM would be 70% of full throttle, torque would be 70%. That makes for 26ozin of torque. 20A source current at 70% duty cycle still is very high on the efficiency curve in your tests. Even at 50% duty cycle this setup would be at 100W and 10A source current, with 18ozin or torque.

My question is when in a model airplane or heli would you ever see a 50% efficiency?

It seems that you would never really get to 5-10 oz in of torque and actually be flying the model to get that low of a system efficiency.

Can you prop the motor for 40A at full throttle then actuallly throtle the ESC and record torque and efficiency?

Greg
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Old Jul 03, 2003, 11:56 AM
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Quote:
Originally posted by sneu
The Avoex's part throttle efficiency is about the same as the "other" U.S made controller.

The poorest controllers in the tests came from Eastern Europe--I now have some fresher versions of those that I will test soon to see if the latest ones have improved.

Steve
Gee Steve, you can't even mention our name? One thing I was wanting to ask is, what timing mode did you use for the controllers? In high efficiency mode, I would expect the CC controller to outperform the Aveox from a pure efficiency standpoint, however, full throttle power would be reduced. What switching frequency did you use on the P-45?? Higher rates usually give better performance at partial throttle, but depends heavily on the type of motor (inductance.)

In working with Aerovironment, we found that the Hacker motors actually worked at highest efficiency above 100khz switching rate.... However, a high power 100khz switching controller is difficult to do at a reasonable $$$. Many of the new generation of ultra-low Qg FETs are pretty pricey, and they are lower power than the large die FETs we are using now.

Also, I believe you were probably running a controller I gave you at the Mid-Winter. Our newer controllers should yield even better results.


Patrick del Castillo
Castle Creations
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