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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 07-20-2011, 04:19 AM   #1
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Default Gas vs Electric Large propellers

Curious to know who is running an electric prop versus a gas prop on large 50cc + electrics.

It states in Great Planes manual for the 50cc electric motor that they DON"T recommend the use of electric style propellers and standard gas type is recommended .

I ran a carbon gas prop with great results then switched to and carbon electric only to jump dramatically in current and thrust. Like and extra 1000 watts going to a 1" larger prop same pitch.

Interesting ?

Any comments ?

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Old 07-20-2011, 05:10 AM   #2
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I cant think why GP would say that unless the motor cant handle the extra load. Even that doesnt make much sense to me as you can always prop down to hit your watt target.

On second thought - its probably a kV issue. The GP motor must be a relatively hi kV and cant handle hi pitch e-props at reasonable diameters.

On the other hand, you cant really say "all gas props are better" or "all electric only props are better".

Props vary a LOT from one to the next. A Mejzlik 22x12 E prop may work better or worse than a similar gas prop but try a slightly different size and the results could be just the opposite.

The biggest difference between gas and electric props in that power range is pitch. Gas props dont come in hi pitch values. For example, I was running a Mejzlik 22x21 in my 50CC Yak conversion. No gas props come with that hi a pitch in that size.

Electrics have the advantage of being able to run at lower rpms, so they can take advantage of larger props for more thrust. To do that well though they also need the larger pitch to keep top speeds reasonable.

I think I need a signature.
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Old 07-20-2011, 05:15 AM   #3
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I tried my PT Carbon 22 x 10 from a 21 x 10 gas Mejzlik and the current hit 90 amps from 75 at full throttle. Now that being said I can always adjust my throttle to max out at about 80 amps for safety. It has tons of power....too much.
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Old 07-20-2011, 05:30 AM   #4
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Dont reduce your throttle end points to lower the current. The esc will actually run hotter if you do that.

Prop down.

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Old 07-20-2011, 05:35 AM   #5
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Do I just keep it the same and stay off the throttle ??
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Old 07-20-2011, 06:01 AM   #6
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Originally Posted by flyer88 View Post
Curious to know who is running an electric prop versus a gas prop on large 50cc + electrics.

It states in Great Planes manual for the 50cc electric motor that they DON"T recommend the use of electric style propellers and standard gas type is recommended .

I ran a carbon gas prop with great results then switched to and carbon electric only to jump dramatically in current and thrust. Like and extra 1000 watts going to a 1" larger prop same pitch.

Interesting ?

Any comments ?

Flyer88
I'm running an APC-E 19x12 Wide Blade prop on a Hacker A60-16M motor with 12S2P 2300 Mah A123 cells. Maximum current is about 78 Amps, turning the prop at just over 7000 RPM on the ground.

APC-E has an RPM limit on their props, and my setup is just under their maximum RPM rating.

Funny thing, that same motor/battery setup pulled noticeably less current on a wood 20x10 prop. And quite a lot less static thrust.

Going one inch larger in diameter can have a substantial increase in watts input to your motor. Horsepower input is proportional to the ratio of the prop diameters raised to the third power. And your electric motor is going to try to turn the same RPM.

So, going from a 19 to a 20 inch prop could increase your watts by 15-20%. Assuming the same blade width, and accurate indicated pitch angles.

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Old 07-20-2011, 06:05 AM   #7
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Yes I agree . I have noticed a lower amp draw with gas props vs electric props. But never measured the thrust.
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Old 07-20-2011, 06:16 AM   #8
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Originally Posted by flyer88 View Post
Do I just keep it the same and stay off the throttle ??
You need to prop the model so that all components can safely handle FULL throttle.

Trying to take it easy on the esc by throttling back a little bit will actually make it run hotter and kill it sooner.


Here are a few quotes from Patrick del Castillo - owner and founder of Castle Creations and the guy who designs all the esc's.

PWM is used to change the power level. However, you have to realize that power loss in an ESC is not linear -- so you can't setup a 25A controller on a motor that draws 40A, and then limit the throttle to stay under 25A. The controller always sees 40A when in the on PWM state. Because the power loss is i^2R + SW, the power losses are higher. (SW= switching losses)

I know it seems backwards, but partial throttles can actually be harder on a controller than full throttle.
You should always check the full throttle amperage, and size the controller based on that. About 87% throttle is hardest on the controller, and will cause the highest temperature rise in the controller.
First of all, a controller always sees full throttle current, even at partial throttle. So you have to be sure that the full throttle current is not above the rating of the controller, or you will see lots of heating. (for example, you can't prop a motor for 50A, and then just throttle up to 10A and expect the controller to be happy with it. The controller still sees 50A of current.)

Secondly, if you are pushing a motor too hard, the controller will suffer from heating. The reason is that the motor iron can saturate, which causes really high currents for short periods. The iron saturation causes a huge drop in the motor inductance, which creates shoft duration REALLY high currents. These won't show up on a meter (which averages over time) but can be seen on an oscilloscope with a current probe or shunt. These short duration high currents cause TONS of extra heat to be dissipated in the controller.
No, voltage has nothing to do with the life of the Phx-10. Current and heat do. The higher the current, the more likely you are to hurt any controller. If you are constantly "on the edge" (for example, if you run a Phx-10 at 15 amps all the time) you will shorten the life of your controller.

Also, (and especially with helis) you cannot simply reduce the throttle to reduce the current and then run a system which draws too much current for the controller. For example, if you have a setup that draws 20 amps at full throttle, you can't just run 50% throttle at 10 amps and expect the controller to survive.
Typical full throttle controller efficiencies are very very high (99% or better.) But at partial throttles the controller efficiencies drop (and therefore, temperatures rise.)

At full throttle, the mosfets are only switching during commutation. At partial throttles the mosfets switch during both commutation and PWM period. Switching losses are therefore higher. Also, there are additional losses during recirculation, and when the FETs are in reverse recovery. These losses all increase in partial throttle.

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Old 07-20-2011, 06:28 AM   #9
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Originally Posted by Larry3215 View Post
Here are a few quotes from Patrick del Castillo - owner and founder of Castle Creations and the guy who designs all the esc's.
Secondly, if you are pushing a motor too hard, the controller will suffer from heating. The reason is that the motor iron can saturate, which causes really high currents for short periods. The iron saturation causes a huge drop in the motor inductance, which creates shoft duration REALLY high currents. These won't show up on a meter (which averages over time) but can be seen on an oscilloscope with a current probe or shunt. These short duration high currents cause TONS of extra heat to be dissipated in the controller.

Back in the mid 1980's when I was rewinding brush type motors for model airplanes, I ran into this. When the iron core saturates due to severe overloading the motor windings, that pretty much turns the copper winding into a dead short for very brief periods of time. The result is very hot copper windings, blown mosfets on my home design brush type speed control and the like.

And as Patrick del Castillo indicates, you'll only see this type of problem with a good oscilloscope.

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Old 07-21-2011, 12:36 AM   #10
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Thanks Guys.

That's good info......I'm all about learning some new stuff.

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