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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 09-14-2017, 12:00 AM   #26
Wildflyer
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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.
When you have flying skills like mine,
You become a master at repair.
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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)

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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.

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