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Old Aug 03, 2006, 02:52 AM
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macr0t0r's Avatar
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Maybe. I figured the stacked magnets helped with OVERALL efficiency. Partial throttle efficiency losses are usually due to the ESC.

- Jim
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Old Aug 03, 2006, 03:23 AM
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Dylwad's Avatar
Phoenix,Arizona, United States
Joined Jun 2002
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This is info i recieved from Hacker. I like reminding the hacker guys that they arent the only game out there, I just wish Lehner had better distribution in the states. As for the price, worth EVERY PENNY.

I would like to see an outrunner come close to their 60 gram motors geared on a 13x6.5 prop. 19 OZ 40" flat foamie extra with 3+ lbs of thrust, more than enough pitch speed, and 15 minute flights on a 3s(10C!) 2200. The best flying foamie ive ever had. Used a GWS 400 gearbox, mounted with a stick and G-10 plate using 2 lower screw holes. 100+ flights, no adjustments or stripped gears.

comparative tests using different controllers on the same motor/prop/battery would be nice to see. I would love to see how the Quark ESC's compare to castles. They certainly feel better in flight.

Do you think the resistance of the controller would be the determining factor on part throttle eff. or would it be more of a software/switching rate/timing thing?

Dylan
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Old Aug 03, 2006, 03:52 AM
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Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
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Quote:
Originally Posted by matttay
What is a bit surprising is that on my FutabaT6XH that half throttle is about 1/4 WOT current (not sure, though if this is a function of the ESC, which is a TBird 18 or the radio
There are two effects at work here:-

1. As Lnagel stated, propellor loading is not linear.

2. At lower throttle settings, motor current is higher than battery current. The reason for this is that during the 'OFF' part of the PWM cycle, motor current is maintained by the collapsing magnetic field in the windings. This current is recirculated via the ESC, without going through the battery. At 50% PWM, the motor current could be twice the battery current!

Some ESC's (eg. Castle Creations) have a non-linear throttle curve in an attempt to compensate for these effects.

An ESC may be less efficient at part throttle for several reasons. At part throttle it has to recirculate the motor current. In most ESC's this is done via the FET body diodes, which have a voltage drop of about 0.7V. The higher switching frequency also introduces some loss. Timing may also change, due to PWM voltages upsetting the rotor position detection circuitry.

To see what effect part throttle has on efficiency, I took two sets of measurements. First I measured input power and rpm at WOT with various input voltages. Then I tried to match those rpm by lowering the throttle. Part throttle operation always required more power to get the same rpm. The efficiency loss varied from 2% to 10%, getting generally worse as the throttle was lowered (controller was a GWS ESC-25A).
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Old Aug 03, 2006, 07:00 AM
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meteor's Avatar
Ontario, Canada
Joined Nov 2002
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The motor has a minimal impact on efficiency compared to the part-throttle PWM losses in a brushless ESC.

I have a 50A CC/CV HP lab power supply which I use with a Jeti 70A controller for my testing purposes.


Although not 100% accurate (due to motor efficiency point changing), a simple test is to pick a target RPM and simply dial up the voltage 'till you reach it, and then do the same at 75% throttle.

Your WhattMeter/eMeter will show you the number of watts in.

Simple!

If you do it with enough motors and ESC's, you will probably get a pretty good picture of the effects. (more than likely, a clearly distinct trend will appear...)


Maybe I'll do that when I have a sec.
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Old Aug 03, 2006, 07:31 AM
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Quote:
The power output is the same if the RPM is the same no matter if one motor is at full throttle while another does the same RPM on part throttle.
It sounds as if you had done this test. Here's my results.

http://www.rcgroups.com/forums/showp...&postcount=166

This is the type of result Meteor is expecting.

Greg
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Old Aug 03, 2006, 07:52 AM
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Quote:
Want more efficiency from your motors?

Pull back on the throttle!!! Just like letting up on the gas
pedal!!
Reduced fuel consumption is not to be confused with increased efficiency. Virtually any IC engine has a peak efficiency at peak HP output. Efficiency is measured by comparing the heat value of the fuel input to the engine and crank HP output. The result is termed specific fuel consumption (SFC) with a typical unit of lbs of fuel per hp per hour (Lb/hp/H). A little research will quickly show that SFC is lowest and energy conversion of the fuel highest at wide-open throttle at peak HP. The pressure drop across the closed throttle is the reason for this.

Reduced fuel consumption is the result of keeping your foot out of the throttle, but doesn't improve efficiency. This is analogous to our reducing the throttle and getting less current draw, but efficiency may or may not improve.

Greg
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Old Aug 03, 2006, 08:05 AM
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Quote:
Originally Posted by matttay
Note that the voltage presented to the motor is either full battery or ground. The ESC is dropping the voltage via PWM, so you can indeed just measure the voltage of the battery. Presumably the meter is true RMS and thus the current you see times the input voltage is you see is the true Pin at any throttle setting.
Measuring the voltage at the battery doesn't tell you the average voltage applied to the motor at part throttle. As you reduce the throttle battery voltage goes up but the motor slows down. Less average voltage is being applied to the motor.

Good Luck!
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Old Aug 03, 2006, 08:55 AM
hot air rises...
jfinch's Avatar
Pleasant Grove, UT
Joined Jul 2005
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Quote:
Originally Posted by matttay
Note that the voltage presented to the motor is either full battery or ground. The ESC is dropping the voltage via PWM, so you can indeed just measure the voltage of the battery. Presumably the meter is true RMS and thus the current you see times the input voltage is you see is the true Pin at any throttle setting.
Yes it's pulsed, but you need to average it in some way in order to use all those motor equations your using to calculate efficiency.

edit: oops, didn't see Beaverdam's post before posting mine...
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Old Aug 03, 2006, 10:02 AM
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meteor's Avatar
Ontario, Canada
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Quote:
Originally Posted by gkamysz
It sounds as if you had done this test. Here's my results.

http://www.rcgroups.com/forums/showp...&postcount=166

This is the type of result Meteor is expecting.

Greg

Yup!


But I guess the critical point is where is that extra heat being dissipated; in the ESC, the motor, or both?

If it's mostly the ESC, that's a bad thing, as 10% at 300W would be 30W being dissipated by the ESC, which would probably mean meltdown if not cooled properly!

If it's mostly the motor, then it's not that big a deal...


Hmmmm....
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Old Aug 03, 2006, 10:15 AM
jrb
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Edina, MN, USA
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A while back some one posted that “WOT is pulsed too”.

Not the case for a brushed controller.

The following chart (BL) is probably what meteor sees via his PS; I used different packs – just happened to all of have them.


I’ve recently made a motor change on a plane; the new motor draws a few more amps (35 vs. 31) than the other – both are brushless.

And, I too mostly fly at PP.

The stick is set for the same flight performance not the numbers notches.

As expected, duration is down.
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Old Aug 03, 2006, 10:20 AM
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From what I've seen, it's shared heat dissapation, but more from the ESC. Running the motor at partial throttle seems to make it warmer than running it full blast with a smaller prop (thereby matching power input). The ESC ALWAYS gets toasty at partial throttle.

- Jim
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Old Aug 03, 2006, 10:36 AM
jrb
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Oops, I forgot to add/ask: will this improve when we start getting true sinusoidal BL ESCs?
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Old Aug 03, 2006, 11:13 AM
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Quote:
Originally Posted by gkamysz
It sounds as if you had done this test. Here's my results.

http://www.rcgroups.com/forums/showp...&postcount=166

This is the type of result Meteor is expecting.

Greg
Can you elaborate on the currents during this test and the motor?

Also, you comment in here (and Bruce also noted) that the currents aren't equal on both sides of the ESC. This is true for peak currents, but the average RMS currents absolutely are the same.

Hopefully someone will just repeat the experiment...it takes 5 minutes.

The outstanding question to me is whether or not the motor efficiency on the graph is tied to the RMS current or peak current. I'm leaning towards RMS based on my quick test, seems like others are indirectly stating peak.

Quick caveat: Assume ESC rating is 2X the Ipeak so as to minimize it's switching losses.
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Old Aug 03, 2006, 11:17 AM
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http://www.acpropulsion.com/ACP_PDFs...tor_drives.pdf

Steve Nue had an article about this in QF a couple years ago. Part throttle efficiency was fabulous, but the cost is significant.

Greg
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Old Aug 03, 2006, 11:24 AM
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macr0t0r's Avatar
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Mind-blowing tech, there. Way over my head, and probably won't be seen in any affordable ESC for awhile. However, it does show where some of the partial-throttle losses come from.

With the limited info, it seems the efficiency varies as if the current is being altered as you lower the throttle. However, the prop size and RPM make it look as if the voltage is being altered. Man, what a twist.

- Jim
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