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Old 09-14-2017, 12:00 AM   #26
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Please keep in mind that all props are not created equal even though they may be the same size. And different batteries can change power dramatically.

On my first electric plane I had a geared Himax motor, it could handle 175 watts continuous.

One brand of 11x7 prop gave me 180 watts with the batteries I was using, another brand gave me about 90 watts. So of course I ran the one that gave 180.

When I needed new batteries I bought a Hyperion 11.1v 35c battery.
First battery was never marked with a c rating, it didn't even have a balance plug.

The second battery gave me a heart attack! The dang plane shot out like a golf ball off the tee. (I hadn't checked wattage as it was the same voltage) This battery didn't sag voltage like the old one, it delivered 280 watts with the "180" prop.

I went from 80 to 280 just by changing things that were marked the same.
Luckily I didn't lose any magic smoke.

So unless you know EXACTLY what parts are being used and their condition, you really can't go by other peoples results with certainty.

I do agree with you if it was only a 3 second run it should not have smoked unless it was shorted out. When they wind a motor it is possible to scrape the insulation and cause a short.

Dave R, KI7MTA Proud PGR rider.
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Old 09-14-2017, 04:07 PM   #27
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Originally Posted by Douglas Racer View Post
dereckbc I agree with all you said but I still say running a 9x4.7 for 3 seconds shouldn't have burnt it.
Perhaps not, but is a moot point whether or not the motor was defective from the manufacturer, or from your testing. You cannot fix dead.
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Old 09-14-2017, 04:14 PM   #28
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Originally Posted by Douglas Racer View Post
I thought running it without a prop was bad? Nothing to lose, I'll try that.
If it were a DC Series Wound Motor, then you need to worry about running without a load. It does not burn them up, they fly apart. If you apply an operating voltage to a DC Series Wound Motor without a load, the RPM keeps going up until it explodes and flies apart. That is not possible with a 3-phase AC Induction as RPM is determined by the ESC (3-phase Inverter operating frequency).
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Old 09-14-2017, 05:00 PM   #29
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Originally Posted by Douglas Racer View Post
Nigel I'm not unhappy with the replies because I now know smoke didn't come from a good motor because of a 9x4.7 Did you look at the video I linked? That guy got exactly 21amps with at 9x4.7
Douglas perhaps what keeps tripping you up, or false sense of security is that current is NOT the only determining factor when in fact is only 1/3 the picture.

21 amps input @ 3S voltage is 230 to 260 watts over the operating voltage range of 11.1 to 12.6 volts. On a 2S or 7.4 to 8.4 volts is 157 to 176 watts. So that accounts for 2 of the 3 factors but there is still one more factor, EFFICIENCY.

You may input 200 watts on a motor, but you do not get 200 watts of power out. No motor is 100% efficient. If the motor was 100% efficient, the motor would generate no HEAT. Motor temps would be the same as ambient temp. That is Fantasy and the crap they tech in school today. Fake science class.

I have Drive Calc which is free. If you go in that program and select a motor, ESC, and operating voltage, the program will show what props work with that motor. I am not going to get into any specifics, rather what you will notice as you change props. When you select a prop, it tells you everything like RPM, thrust, current, power in, power out, and EFFICIENCY.

One thing that becomes real clear real fast is the motors we use are most efficient when operating at low power aka small prop. As we increase the prop size, or load, efficiency drops meaning the motor is now being heated up with more power loss.

In the end what you pick up and learn is the motors we use operate as low as 68% and up to around 90% efficiency. So with 200 watts input at 20 amps means the motor is generating 20 watts of heat @ 90% efficiency, and 64 watts @ 68% efficiency.

But what happens if we put an over size prop on the motor? Say efficiency drops 50%. Sure you can input 200 watts at 3S voltage and with only 16 amps of current on a 3S battery. Sure you are well within Power and Current limits for the motor, but at 50% efficiency you have exceeded the Thermal Limit because now your motor is being heated with 100 watts of power and lets the Magic Smoke out.

Go extreme with a way over sized prop where efficiency drops to 10%. Now 200 watts in only gets 20 watt so fpower out, and the motor is being heated with 180 watts and bursting into flames despite you are within electrical specs of 200 watts input @ 20 amps of current. The motor can only handle 50 watts of heat.

Hope that helps and allows you to see the bigger picture. Electrical is only part of that picture, not the whole picture.
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Old 09-15-2017, 12:41 AM   #30
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Originally Posted by dereckbc View Post
... I have Drive Calc which is free. ...
www.drivecalc.de , English/German, Windows/Linux/Mac.

DriveCalc motordatabase contributors a.o.
www.theampeer.org
www.elektromodellflug.de
Motor Data - test bench data for several hundred brushless motors. - RCG

More calculators, e.g. e-Calc and ScorpionCalc
e-flight calculators (compilation) (sticky)

Vriendelijke groeten Ron
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Old 09-15-2017, 12:51 AM   #31
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Originally Posted by Wildflyer View Post
... If I have a motor of unknown Kv, I put a piece of narrow (1/4"-1/2") white tape on the motor casing 180 degrees apart. Shine a LED flashlight on the tape and read RPM with an optical tachometer.
I have the motor connected to my inline wattmeter at the same time.
Now I have RPM and applied voltage so it is easy to figure Kv.
I know my method may not satisfy Ron Van S. but it comes close enough for me. ...
It will do
A LED flaslight or fluorescent tube may cause a problem because they have their own frequency. When in doubt, use sunlight or light from incandescent bulb.
OTOH you could use an FL lamp as a stroboscobe, 100/120Hz, 6000/7200rpm.

Vriendelijke groeten Ron
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Old 05-25-2018, 10:18 PM   #32
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My RCTimer 2830-11 1000kv motor runs statically very hot with an APC 10x4.7SF prop. The motor and prop size are recommended by Hacker (for the Hacker Master Stick. I get good performance with an RCTimer 9x6SF(will almost hover), and better performance(hovers) with an APC 10x5E(thin electric). I tried to get drivecalc (Drive Calculator) to work in Linux, but not successful, yet.

I statically tested the motor vs diffferent props, with a thermal couple tucked directly behind the mounting "X"hardware. The atch chart shows static data I gathered. I chose 11.65V as the test voltage since the 2200mah batteries I use drop to this level after about 1min. Seeing the data, I am thinking of prop selection based on <=70C @90sec static runtime. Note that the temperature is measured at the rear of the motor and thus internal temperatures are actually much greater than recorded. Thirty seconds of coast time, ie, time after shutting the motor off, results in significant temperature increases until peak temperatures are reached. [The chart data is a little more complicated than I would like because my Kepco power supply is only rated at 15V/15A so current limitations were experienced.]

Amps and Volts were measured at the ESC input with calibrated equipment; my watt-meter is off on current by as much as 5%, Volts ~ 3%.

Questions:

1) Outright, why might APC10x4.7 be recommended, but run so hot?
2) Is it possible that unloading of prop during flight would make a significant difference in operating temperature vs static measurement?
3) Is there any rule of thumb for prop unloading in flight, or can it be said that there is always at least some unloading, whatever that percent may be?
4) Has anyone created a wind tunnel of sorts to reduce loading during static testing, eg, attempt to simulate flight conditions and observe prop performance.


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Old 05-25-2018, 11:36 PM   #33
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Have you measured rpm's as well?
Motor rotates freely when pushing or pulling shaft? No grinding, mechanical resistance?
(Magnetic bumps (cogging) are do not matter.)

Current is proportional to Kv cubed (and voltage squared), so minor differences can have considerable effects.
Kv correct?
Four simple methods for determining Kv
www.bavaria-direct.co.za
→ motor constants

Prettig weekend Ron

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Old 05-26-2018, 01:33 AM   #34
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ron_

I was hoping you would show as I went through all your posts on this thread - some a bit over my head with formulae. No, I need to purchase a tachometer, and plan to do so.

I am purchasing a 30V/30A switching power supply so that there is no current limiting during testing. This will make testing a little more consistent. I will also establish a maximum temperature allowed. For instance, something like stop the run if the temperature, as I measure it, exceeds 80C.

I also uncovered today on Wattflyer a post that comments that a nearly rigid E prop may be a better choice than a SF which may change configuration at speed. I wonder if the use of SF props mostly results from prior years when motors were geared down? Which is to wonder, a slow fly might do very well at say 3k rpm, but terrible at 6k rpm?

I previously went to your link for "4 simple methods to determine Kv", but all I see at the website is 3 motors?

thanks
mark
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Old 05-26-2018, 02:14 PM   #35
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On the bottom of that page there is a link that says motor constants. That is where the details are for measuring Kv.

http://www.bavaria-direct.co.za/constants/

Good Luck!
Steve

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Old 05-26-2018, 05:58 PM   #36
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Originally Posted by mark_bower View Post
... I previously went to your link for "4 simple methods to determine Kv", but all I see at the website is 3 motors? ...
Don't forget to click on Motor constants in the top blue ribbon.

www.bavaria-direct.co.za
→ motor constants

Originally Posted by mark_bower View Post
... No, I need to purchase a tachometer, and plan to do so. ...
close out sale: Hyperion watt-meter II (optical tach, servo tester, local&remote logging) - RCG

Keep watt-/multi-meter wires short
too long wires batteryside will kill ESC over time: precautions, solutions & workarounds

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Old 05-26-2018, 06:11 PM   #37
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Originally Posted by mark_bower View Post
... For instance, something like stop the run if the temperature, as I measure it, exceeds 80C. ...
You must measure in the motor, on the stator. Measuring the outside temperature would give you too low a temperature reading.
Depending on quality and strength, magnets can start demagnitizing (permantly!) at 80C.
www.supermagnete.nl/eng/data_table.php


Overheating magnets causes de-magnetizing (irreversible! ), which in turn results in higher Kv.
And since motorcurrent is proportional to Kv cubed ...
Higher current → hotter motor → weaker magnets → higher Kv → higher current → hotter motor → weaker magnets → higher Kv → higher current ⟲⟲⟲ etcetera etcetera, temperature runaway


Determining whether magnets still have original strength, by determining whether Kv motor parameter (in rpm/volt) is still the same, several simple straightforward methods.
www.bavaria-direct.co.za → motor constants


Opening an outrunner

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Old 05-27-2018, 01:31 AM   #38
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o.k., I made it to the bottom of the page, a weakness of mine! I am going to target to do the #2 method to determine Kv. I have 2 RPM tachometers on order in addition to my 30V/30A pwr supply. I do not like the drill press idea in that my drill press has some runout, and this would mandate mounting the motor in some kind of "flexible" mount under the chuck.



I understand the implications of high temperature, in fact, that is why I want to get a good handle and understanding on what is going on. And so yes, 80C probably is a bad choice for cutoff if one does not know limits of the motor's magnets.


I did not see a way to bury the thermocouple in the stator, except maybe just "stuff it"? How do you do it or suggest doing it?


I plan to return at the end of next week with some meaningful data characterizing the motor vs prop vs temp issues. And then revisit the many questions I ask.
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Old 05-27-2018, 12:32 PM   #39
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Mark
I wonder if you are over thinking the problem.
Heat in a motor (or anything else) is generated by the product of the current squared and resistance. Note doubling the current generates four times the heat.
You can't do anything about the resistance built into a motor so the only option is to limit the amps.
The motor will have a specified continuous maximum current (with adequate cooling) and frequently the no load (no prop) current is given as well.
An excess no load current would indicate a motor fault.
If the no load is ok then all you have to do is to keep below the maximum current limit by using a prop that suits the characteristics of the plane, the battery voltage and the limitations of the ESC and battery either by your own experimentation or following recommendations.

All you really need to avoid 'smoke' from the electrical side is being aware of all the component's limits and a suitable Watt meter.
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Old 05-27-2018, 04:57 PM   #40
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I agree with the above, keeping an eye on temperature is when you are pushing for the limits, not for everyday use.

It is a good practice to have some headroom built in.
About derating motors, controllers, batteries, electronics in general:

Vriendelijke groeten Ron
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Old 05-27-2018, 11:01 PM   #41
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Well, I believe you are right: I am over thinking/working the high temp problem. Stepping back,
it seems to me a solid, but simple path for prop selection would be to record the parameters of amps, temperature, thrust. Given a power supply to deliver at constant volts(no current limiting), a force guage, and a way to "accurately" measure internal temperature, then use the steps below.


- decide on max diameter prop, then test that diameter size and smaller in combination with thin E and SF types
- try and verify that the motor is not damaged (eg, overheated magnets)
- run the thermocoupled motor at a selected average flight voltage, eg, 11.7 or 11.65V
- abort running for a given prop if temperature exceeds 70C, prior to 1.5min
- record volts,amps,temperature,and thrust at 1.5min


Simple(if you have the equipment). The best prop to use as tested statically will obviously be that which produces the most thrust at 11.7V, but does not exceed the temperature criteria. That is, which prop puts more energy into thrust versus losses in heat at 1.5min runtime. Recording amps and volts provides for runtime Watts.


Unfortunately, the RCTimer motors I use only give max Watts, which I believe is burst Watts. They do not specify continuous running Amps. I still seek answers to the following three questions taken from my previous posts:


1) Is it possible that unloading of prop during flight would make a significant difference in operating temperature vs static measurement?
2) Is there any rule of thumb (ratio of flight vs static) for prop unloading in flight, or can it be said that there is always at least some unloading, whatever that percent may be?
3) I did not see a way to bury the thermocouple in the stator, except maybe just "stuff it"? How do you do it or suggest doing it?


thanks
mark


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Old 05-27-2018, 11:38 PM   #42
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Unloading in flight can be significant.
How much depends on the achieved speed of the plane compared to the theoretical prop pitch speed at the 'in flight' motor rpm.
In general prop unloading and the additional cooling from forward travel will give a motor an easier time than ground static running.
I suspect you can see where this is going. Unless you have some pretty sophisticated telemetry the degree of unloading can't be easily measured.

As far as measuring static thrust is concerned you have to remember the highest achievable for a given maximum current will be with a large diameter fine pitch prop but this combination is very unlikely to give you the best 'flight' performance. The prop will unload quickly (so reducing the available thrust) to a point where the plane will simply not go any faster.
A smaller diameter coarser pitch prop will give less static thrust for the same current but will continue to give useful thrust at higher plane speeds.
Finding the 'best' prop in terms of diameter and pitch has to be by trial and error along with personal preference on how you like the plane to fly.
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Old 05-30-2018, 01:40 AM   #43
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gurorneng - such good information, what I have been looking for.


My interest is focused on maneuvers vs speed. So the path I have been taking is the correct one, especially as I would like to master hovering. However, I had no knowledge that props with smaller diameter with coarser pitch would unload relatively later vs the larger diameter finer pitch props. I have now initiated my "rule of thumb book"(can't trust memory). And thanks for the response that all props will unload to some extent. After establishing the thrust for a given prop, I will repeat the test and direct a portable fan at the motor with the expectation that thrust(and amps will drop). Crude, but it will be fun to witness.



I have built my test stand for looking at thrust and it looks like it will work great. The pull tester for the test stand is limited to 1kg, but that will cover all my present planes which weigh less than 1kg AUW. Tomorrow my new power supply arrives which will facilitate looking at amps, and if I am correct, I will be able to monitor amps on the power supply's digital read out, and eliminate using an in series low ohm resistor. yea! I just need to be able to make three readings near simultaneously: temperature, amps, elapsed time. Volts as previously stated will be constant.
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Old 05-30-2018, 03:00 PM   #44
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Switching power supply? Make/type/link?

Vriendelijke groeten Ron
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Old 06-04-2018, 09:06 PM   #45
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Please go to my post #48
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Old 06-04-2018, 10:23 PM   #46
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I had a brand new SK3 motor that ran only for 2 seconds once.
The windings somehow came off the stator and got chewed by the magnets.
They replaced it.
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Old 06-04-2018, 10:24 PM   #47
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Please goto my post #48


Attached Images
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File Type: jpg Test Setup 2.JPG (1,006.4 KB, 37 views)
File Type: jpg Thermocouple mounted to rear of motor with modified X mount.JPG (805.0 KB, 37 views)
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Old 06-04-2018, 10:43 PM   #48
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Complete post here, please delete my post #45 & 47. I finally have the "logistics" down!



For the Hacker Master Stick(HMS):
- Recommended specs:
M Force 2830CA-12 Kv 980
Max Pwr: 200W(15s)
Max Amps: 17A (15s)
Propeller 9x4.7 - 10x4.7SF
Battery: LiPo 3s2200mAh


RCT 2830-11 1000Kv specs (in use)
Max Pwr: 210W
Prop: 10x7/8x4
Battery: Lipo 2-4
Rm: .127m ??


Per my original concern, the 10x4.7SF tests "hot" with a 2830 1000Kv motor. I have established test parameters for now to be,
Test Volts: 11.7V
Motor Temp at time = 0 is 25C
Static run the motor
Halt test when the thermocouple reaches 65C


Given these criteria, the 10x4.7SF reaches 65C in just 37s as does the APC 9x6SF. These props "run too hot" if used at WOT. Hacker suggests that WOT should be limited to 15s, but does not give a % throttle value that can be used throughout flight for the 10x4.7SF.


I tested 6 different props - please see attached data sheet. I tested the 10x4.7SF twice: at WOT and at a reduced throttle setting (one division lower than WOT). The data allows selecting a suitable prop. For selection, I used the sequential criteria: Amps @30s <=14.5A(170W), and then best thrust at 30s. And then as a sanity check, verify that temp @ 1.5m(90s) is <=65C.


This selects:
APC 10x5E (slightly lowered throttle, time to 65C=120s), thrust @30s = .759kg
APC 9x4.7SF(time to 65C=108s), thrust @30s = .746kg.


For more thrust, the constrained APC 10x5E can be burst powered at WOT for hover(~ .883kg thrust if battery at 7.11V). HackerMS AUW = .862kg, so not much margin.


Some Comments:
- For testing, I mounted the thermocouple at the rear of the motor. The thermocouple is soldered to a brass washer, copper would be better. Seems impractical to bury a T/C in the stator, although this surely would be an improved setup.
- I tested I0 (I zero) and RPM at 10V on the test motor, and Rm on the test motor and a brand new motor. I0 about .8A, RPM 9.96k, Rm .139. The RPM was measured from the bell of the motor with 2 pieces of black and 2 pieces of white tape attached in ratio 1:3. Each black piece spanned 45.The Kv calculates to 1007Kv vs the 1000Kv specified.


Kv = RPM/(Volts -Amps x Rm) = 9.96k/(10V-.8A x .138) = 1,007Kv

- 65C criteria for making a temperature judgment is conservative, hopefully. This temperature lags the stator temperature, as a large temperature rise is observed after the motor is shut off.
- Ambient temperature will have a definite affect, eg, testing at 23 ambient vs flying at 30C means static testing would be improved by raising the ambient temperature to maybe 30C.
- But on the other hand, some not well defined unloading in flight will contribute to a lower operating temperature.
- 30 seconds was selected as a practical runtime "focus" to look at Amps and thrust since temperatures for some props may rise significantly beyond this run time.
- Somewhere I read a rule of thumb that 150W would be a good run value for a 200W motor - My data seems to take me close to that "ball park" value.

The new power supply worked very well for me, invaluable. I bought it in anticipation of maybe getting into 4s motors, and general use with 24V DC. The link for the p/s is below. I already owned the other equipment used.


https://www.bkprecision.com/products...er-supply.html


Attached Images
File Type: jpg 2380-11 1000Kv Thrust-Temp-Watts vs Props.jpg (1.99 MB, 41 views)
File Type: jpg RPM setup.JPG (780.1 KB, 39 views)
File Type: jpg Test Setup 1.JPG (970.2 KB, 36 views)
File Type: jpg Test Setup 2.JPG (1,006.4 KB, 36 views)
File Type: jpg Thermocouple mounted to rear of motor with modified X mount.JPG (805.0 KB, 37 views)
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Old 06-06-2018, 01:28 AM   #49
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Unfortunately 2 edits. The phrase "then best thrust at 30s" should read "then high thrusts at 30s". Now more than one prop can be selected! On my data chart, the APC 9x5E(left column) should read APC 9x6E.


I have trouble figuring out how to edit on Wattflyer.com. What is the detailed path to delete all images after they have been posted, or delete just one image for that matter?

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