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Power Systems Talk about motors, ESC speed controllers, gear drives, propellers, power system simulators and all power system related topics

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Old 12-15-2014, 11:32 PM   #1
xray328
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Default If your motor is over amped can you reduce throttle to compensate?

Hey guys. I have a Hextronic 24g 1300v motor running off a 11.1V pack. I have a ton of 9x4.7 props laying around but the combo is over amping the motors which has a max rating of 7.5A so it really needs a 8x3.8 prop. I thought I could just reduce the throttle end point to a level that would reduce amp draw below the 7.5A and I'd be ok but the motor is smoking hot after just a minute or so. It started at near 13A and I quickly lowered the end point to 80% to get under 7.5A put the motor was smoking hot. Maybe I was above the 7.5A too long?

So I guess that isn't an option? Can someone tell me why that doesn't work?
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Old 12-15-2014, 11:41 PM   #2
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No. Change the prop.

Exactly why it won't work doesn't really matter and involves a long explanation.

You will burn up the motor by using the wrong prop.
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Old 12-15-2014, 11:46 PM   #3
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Due to the way the switching regulator in the ESC workd the eSC and motor still see wide open throttle current even when you throttle back. The only difference is that at part throttle the current is turned rapidly on and off so that the average current is lower, but the peak is just as high (if not higher) than when you have the throttle open

More detailed explanation in FAQ #6 on Castle Creations web site: http://www.castlecreations.com/suppo...eral.html#gen6
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Old 12-16-2014, 04:46 AM   #4
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The max wattage of the 24g motor is approx. 72 watts (3W/g) -- rough approx. of the ability of the motor-mass to dissipate heat. At 7.5A and 11.1V =>over 83W. That's probably why the motor is hot. Actually you can protect the motor by throttling back. The motor only experiences the "averaged" current from the ESC. The reduced wattage avoids overheating the motor. However, if the wide-open-throttle amperage exceeds the current rating of the ESC, operating at partial throttle will not protect the ESC.
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Old 12-16-2014, 04:52 AM   #5
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Originally Posted by xray328 View Post
Hey guys. I have a Hextronic 24g 1300v motor running off a 11.1V pack. I have a ton of 9x4.7 props laying around but the combo is over amping the motors which has a max rating of 7.5A so it really needs a 8x3.8 prop. I thought I could just reduce the throttle end point to a level that would reduce amp draw below the 7.5A and I'd be ok but the motor is smoking hot after just a minute or so. It started at near 13A and I quickly lowered the end point to 80% to get under 7.5A put the motor was smoking hot. Maybe I was above the 7.5A too long?

So I guess that isn't an option? Can someone tell me why that doesn't work?
Yeah, explanation is not simple. These motors are actually running on three phase AC voltage as supplied from the brushless ESC. In throttling back from full throttle, the three phase signal is "Chopped" at somewhere around 10,000 cycles per second. This results in peak currents that are still present even though you've throttled down.

The heating effect in the motor is the result of what's called RMS value of current, not the simple average value of current. RMS values take into account the extra heating effects of those high peak currents that take place when chopping the three phase power to the motor.

RMS is Root Means Square in AC power theory.

For those interested, here is more info on what RMS is:
http://www.ask.com/wiki/Root_mean_sq...apn&ap=ask.com

High quality Digital voltmeters can measure current in RMS values, but most of them only work at 60 Hertz line frequencies. They won't read properly at the high frequencies present in our ESC's.

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Old 12-16-2014, 07:17 AM   #6
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I have throttle limited a few models in my quest for speed .. increasing throttle on highly overstressed motor combos ... but it has always been a risk of blowing the motor. It works - but is highly questionable.

It is always best to set-up a combination that is capable of WOT without stress.

Even if motor does not burn out - you risk magnets coming loose or even lost by the high temp. You risk ESC itself ...

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Old 12-16-2014, 08:48 AM   #7
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Originally Posted by gyrocptr View Post
Actually you can protect the motor by throttling back. The motor only experiences the "averaged" current from the ESC. The reduced wattage avoids overheating the motor.
I dont think that's entirely accurate. ESC's have no smoothing circuitry on the output side, so the current that thay pass into the motor during part throttle operation is pulsing rather than smoothed down to average value. There might be some smoothing effect due to the motor's impedance but as there are no capacitors on the output side I'm not sure it would be right to conclude that the motor only sees the average current.

You might 'get away with it' but the safest course of action is to spec the system so that full throttle operation is within the current limit of all of the components in the system. The battery is maybe the one component that would be fully protected by limiting throttle, but there again speccing a battery that's marginal for the current required isnt really a great idea.
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Old 12-16-2014, 09:12 AM   #8
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The amount of 'throttle-back' to stop overheating was significant in the ones I tried it with. To such an extent that it really wasn't worth it. I admit that I was doing this with high power 450 heli motors (3700kv) and 5x5 props and 4S packs ... so it was extreme.
There didn't seem to be a steady relation of throttle EPA and amps draw on the wattmeter. Despite what we usually see when normally doing wattmeter testing throttle and current draw.

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Old 12-16-2014, 12:35 PM   #9
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Originally Posted by xray328 View Post
Hey guys. I have a Hextronic 24g 1300v motor running off a 11.1V pack. I have a ton of 9x4.7 props laying around but the combo is over amping the motors which has a max rating of 7.5A so it really needs a 8x3.8 prop. I thought I could just reduce the throttle end point to a level that would reduce amp draw below the 7.5A and I'd be ok but the motor is smoking hot after just a minute or so. It started at near 13A and I quickly lowered the end point to 80% to get under 7.5A put the motor was smoking hot. Maybe I was above the 7.5A too long?

So I guess that isn't an option? Can someone tell me why that doesn't work?
Cut your 9" prop down to a 8" or 7" prop, balance the prop before using them, no need to buy more props.cut down props work great, as long as your not going to be racing I cut down props all the time,

I may be getting Older, But I Refuse to grow Up I am Having to much Fun to Grow Up LOL
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Old 12-16-2014, 12:57 PM   #10
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Just a tip in cutting back props that I use ....

I cut one blade to length and shape of tip. I do not fine down the thickness yet.
I line up prop flat onto a table with centre boss against table edge. I then take sharp pencil and draw accurately around the prop .... This gives me a reference to cut the other prop.
Turn prop 180 deg and mark prop with length to cut.

Cut prop. Keep going back to template drawn on table till it matches.

Now you start the thinning of tip to reduce tip-weight. Keep going back to template as well and if you do lose some - RE-draw template ...

You should thin a bit one blade - then other .. swapping back and forth so to try and keep same.

Balance to see that you have done this well.

This is MY way and there are plenty other ways ...

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Old 12-16-2014, 02:21 PM   #11
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Since this is the "Power" thread, I'll ask the question. Bought a couple of new 50mm XRP 4200KV 11 bladers (3s/4s version) for my JPowers Skyangels: Installed one in a new AMX. Ran it up and though it's quiet (swooshy) and well-balanced , the thrust seems rather anemic. Just a cursory test with my hand at the back of the fuse.

http://donsrc.com/cart/index.php?_a=...product_id=153

Guys over on RCG have been raving about them as they are much more efficient amp draw wise than the predecessor Chang Sun version.

Do I need to look at the timing?

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Old 12-16-2014, 03:39 PM   #12
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As a 50mm flyer ... I read a lot of crap about what to do with them and claims of fantastic performance.

I really don't know where some of them get their data from ... maybe they dream it ?

It cost me earlier when building the Concorde ... I went against my better judgement and bought Mad Thrust Alloy 3S units ............ they were diabolically crap ! Once I stuck 4S up them - they then came alive ... but still less than my cheapo Haoye units ...

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Old 12-16-2014, 07:06 PM   #13
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Loaded in a freshly charged 1300mah 4S 40C CHL and gave the AMX another scientific thrust test, holding my hand behind the tail pipe. Plenty of thrust for the XRP 3S/4S 11 blade and a nice swoosh wet turbine sound.

This motor fan combo is the bees knees for 50mm jobs.

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Old Yesterday, 01:45 AM   #14
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figured i would ask this here. seems similar. i have a power 32 - 770 kv motor rated at 42 amp constant. im using 75 amp escs. these motors are also rated at 800 watts max. at full throttle i get the 800watts but the max amp is 55 amp. so this seems like the watts are OK but i am clearly going over the motors max amp value. im trying to produce enough thrust for this model and these props are pulling hard! based on the previous conversations, is it not advisable to throttle back to a point my watt meter reads as 42 amps for steady safe flight? i am still in the search for a prop set up that will not exceed the continuous amp ratings for this motor at WOT. i never fly over 3/4 for the most part trying to get decent flight times. these are 3 blade props as well. Walt

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Old Yesterday, 01:56 AM   #15
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Sorry.. if you are reading more amps OR watts than the motor is rated for you are in danger of burning it up.

And reducing throttle is not the correct answer. If the motor won't give the power you want/need without exceeding its ratings then you need a different motor.

Throttle management is not a viable solution.
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Old Yesterday, 02:07 AM   #16
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got it. i will continue the search for different prop combos.i have plenty that are not exceeding the rating but not giving me quite the watts/lb. im hoping for. what is the higher short term amp burst rating for and how is that different?

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Old Yesterday, 03:40 AM   #17
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The short term burst rating is a current limit that can overheat the motor if sustained more than 30 seconds, but won't immediately fry the motor.

You might hit that current during startup when slapping from 0 to full throttle. In that situation the current will spike, but will very rapidly come back down as the rpm increases.
Note that repeatedly doing this you can overheat any motor even if the normal load is well below the motor's ratings.
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Old Yesterday, 06:55 AM   #18
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Walt,

That motor is rated for 60A burst, so if you don't fly hard all the time it should be fine. Also based on my experience any 200g motor of decent quality should be perfectly ok at 55A providing it's got reasonable cooling.

So personally i'd be happy to run it as-is, just check the motor after flying to make sure it not too hot.
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Old Yesterday, 07:15 AM   #19
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Originally Posted by JetPlaneFlyer View Post
Walt,

That motor is rated for 60A burst, so if you don't fly hard all the time it should be fine. Also based on my experience any 200g motor of decent quality should be perfectly ok at 55A providing it's got reasonable cooling.

So personally i'd be happy to run it as-is, just check the motor after flying to make sure it not too hot.
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Old Yesterday, 06:45 PM   #20
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thanks fellas. i have some emp props that are well inside all parameters but only get me 116watts/lb at WOT static. my other power prop set up was 160watts/lb. but it gives me the 55watts at WOT STATIC. as long as 116watts/lb. will fly the model i will try that first and experiment later with the other prop with short flights and motor temp tests. does 116watts/lb. sound like it would fly this 10lb. model decently? i want sport flying characteristics nothing more as a starting point. appreciate all the knowledge and guidance. walt

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Old Yesterday, 07:20 PM   #21
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Originally Posted by walter3 View Post
thanks fellas. i have some emp props that are well inside all parameters but only get me 116watts/lb at WOT static. my other power prop set up was 160watts/lb. but it gives me the 55watts at WOT STATIC. as long as 116watts/lb. will fly the model i will try that first and experiment later with the other prop with short flights and motor temp tests. does 116watts/lb. sound like it would fly this 10lb. model decently? i want sport flying characteristics nothing more as a starting point. appreciate all the knowledge and guidance. walt
Yeah
Normally, 116 watts per pound of airplane will fly the model kind of "OK". It won't be "overpowered", but should get off the ground in a reasonable time. Doing a loop, or acrobatics might be a bit of a challenge.

What we'd need would be the models wingspan, wing area, and the prop diameter, pitch, and RPM of your setup.

The prop requirements for a slow flying Piper Cub type of model versus a fast acrobatic type of model are quite different, even though they might have the same watts per pound.

Programs such as www.motocalc.com will help in this area, especially if you've measured the prop diameter/pitch and RPM. What works is to select your battery pack, prop, and motor. If motocalc doesn't match your motors actual performance, just tweak motocalcs motor KV number to match your actual motor performance.

If you've run glow powered models before, 1200 Watts is roughly the power output of a four stroke 70 sized glow engine. Methinks a 4 stroke 70 glow engine would be a bit on the small side for a 10 pound model.

Model airplane Power Systems
http://www.wattflyer.com/forums/showthread.php?t=73275

fhhuber had a very good posting in wattflyer showing how a model will perform with different watts input per pound of airplane.
Could you locate that and copy it here??

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Old Yesterday, 08:41 PM   #22
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Flight is possible at 30 to 50 watts / lb

Pattern aerobatics of the 1960's 1970's used the equivalent of about 75 to 90 watts/lb with heavy aircraft making use of a dive from altitude to perform their maneuvers. this is when the 11 lb max weight limit was set and only .61 glow engines (that were anemic by today's standards....) were the largest allowed.
You can loop from level flight on 75 watts/lb.

100 watts/lb can sustain brief vertical and can complete a vertical inside-outside 8 (inside 1/2 loop, outside full loop, inside half loop) of good size.

150 watts/lb can sustain vertical for an extended time. Transitioning from level at mid throttle into the vertical, then going full power and straight up out of sight.

200 watts/lb can accelerate vertically out of a hover.

All of this assumes you have made appropriate prop choice... If you use a pylon racer prop on a trainer you won't get as good a result.
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Old Yesterday, 11:49 PM   #23
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well this girl is from the 70's for sure! im still waiting on one more prop set to arrive so i have another to run on the watt meter. my wingspan is 66" each wing is 14.5 at fuse out to 8.5 at tip. props are all 3 blade. 7 pitch and 8 pitch. remember this is a twin. walt

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Old Today, 04:17 AM   #24
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Originally Posted by walter3 View Post
well this girl is from the 70's for sure! im still waiting on one more prop set to arrive so i have another to run on the watt meter. my wingspan is 66" each wing is 14.5 at fuse out to 8.5 at tip. props are all 3 blade. 7 pitch and 8 pitch. remember this is a twin. walt
At 45 Amps and 800 watts per motor, I'm guessing you're running a 4S LiPo pack for each motor? If so, the Power 32 motor should turn a 10 inch diameter, 7 inch pitch prop at around 9500 RPM. These numbers were plugged into www.motocalc.com, with the following results. Motocalc predicts a 1600 Feet per minute climb out ability at around 35 degrees. Not spectacular, but reasonable.

Note that at 30 ounces per square foot of wing area, your model is going to be a heavy weight, fast flyer. Be sure to have a very experienced pilot fly the model on its maiden flights.

MotOpinion - WF Walts Twin
Sea Level, 29.92inHg, 59F

Motor: E-Flite Power 32 Outrunner 770KV; 770rpm/V; 2.4A no-load; 0.02 Ohms.
Battery: Great Planes Electrifly 3200 (20C); 4 series x 2 parallel cells; 3200mAh @ 3.7V; 0.0045 Ohms/cell.
Speed Control: Castle Creations Phoenix 80; 2 controls (separate); 0.001 Ohms; High rate.
Drive System: Edge 540 T Prop; 2 motors (parallel); 10x7 3-bladed (Pconst=1.31; Tconst=0.95) direct drive.
Airframe: Walts Model; 760sq.in; 160.4oz RTF; 30.4oz/sq.ft; Cd=0.055; Cl=0.47; Clopt=0.71; Clmax=1.24.
Stats: 118 W/lb in; 102 W/lb out; 25mph stall; 34mph opt @ 57% (32:25, 98F); 41mph level @ 67% (24:58, 105F); 1790ft/min @ 37.2; -302ft/min @ -5.9.

Power System Notes:

The full-throttle motor current at the best lift-to-drag ratio airspeed (41.3A) falls approximately between the motor's maximum efficiency current (41A) and its current at theoretical maximum output (351.6A), thus making effective use of the motor.
The voltage (14V) exceeds 12V. Be sure the speed control is rated for at least the number of cells specified above.

Aerodynamic Notes:

The static pitch speed (66mph) is within the range of approximately 2.5 to 3 times the model's stall speed (25mph), which is considered ideal for good performance.
With a wing loading of 30.4oz/sq.ft, a model of this size will have flying characteristics suited to an experienced pilot. The plane will fly fast, and be readily able to handle fairly strong winds.
The static thrust (150.7oz) to weight (160.4oz) ratio is 0.94:1, which will result in very short take-off runs, no difficulty taking off from grass surfaces (assuming sufficiently large wheels), and steep climb-outs.
At the best lift-to-drag ratio airspeed, the excess-thrust (96.8oz) to weight (160.4oz) ratio is 0.6:1, which will give steep climbs and excellent acceleration. This model should be able to do consecutive loops, and has sufficient in-flight thrust for almost any aerobatic maneuver.

General Notes:

This analysis is based on calculations that take motor heating effects into account.

These calculations are based on mathematical models that may not account for all limitations of the components used. Always consult the power system component manufacturers to ensure that no limits (current, rpm, etc.) are being exceeded.

DennyV
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looks like you used the 800 watts per motor values that my 8 pitch props produce. based on that the aerodynamic notes seem pretty impressive. if i prop down to a 7 its another ball game all together i would think. yes im using 2 - 3600 4s batts. in the first bold pharagraph near the bottom it says stats:118 watts/lb. in , 102watts/lb. out. what does that mean? are those the watts/lb. that were used for these calculations? the 800watts per motor gives me 160watts/lb. that's why i ask. kinda confused? thanks for the work helping sort this out. much, much appreciated! Walt.

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