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How did I fry my Electrifly?

Old 10-03-2010, 08:18 PM
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Default How did I fry my Electrifly?

This is my very first electric and this was my second flight on the plane. The maiden flight was uneventful. The motor is installed in my Mistral glider and as such only sees intermittent use (10% duty cycle). When the motor fried, it had been after a 30sec run at about 75% throttle. Once I noticed the engine was not responding, I deployed the airbrake and landed immediately to inspect the damage. Both the battery and ESC were at ambient temperature, but the motor turned as if it had burned out coils in one or more of the phases. Once home, I opened up the motor and confirmed that the coils in two of the phases had burned out.

Motor: Electrifly Rimfire .10
Prop: Master Airscrew 12x8 folding prop
ESC: GP 35A Silver series
Battery: 2100mAh 3 cell LiPo

My first thought is that the spinner is shielding the airflow from the front of the engine, despite the rather large gap left for cooling. Any help would be greatly appreciated.

Old 10-04-2010, 12:13 AM
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Default RE: How did I fry my Electrifly?

Looks like you over-propped it. They recommend 10x4 - 10x7 for that motor, so you're way over that at 12x8. It's not surprising that it burned out. You should also be suspicious of the published parameters until you can verify them with a wattmeter.

If your Mistral is a 2-meter ship, you should have plenty of power with the right prop. I was able to fly a Gentle Lady with a .049 years ago. That was only a fraction of the power and efficiency you have. Perhaps a 9x5 would be a good starting point.

You must be careful not to overload the battery, ESC or motor which causes destructive heat buildup. When operated in the safe zone, a lipo/brushless system is very powerful and reliable.
Old 10-04-2010, 08:14 AM
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Default RE: How did I fry my Electrifly?

I would agree: way over-propped! I tested one of these 35-30-1250's a few years ago and found that at anything over 25A it got very hot very fast. The GP recommendations/specs for amps/watts for their outrunners are ridiculously high.. 30A/330W/35A surge is far too much.... I found that even a 10x7 APC E on 3s (29.70A) was too much.... and an 11x5.5 APC E (32.60A) got the motor to 78C waay too fast! I hate to think what a 12x8 folder was asking of the motor (40A+???).
Old 10-04-2010, 10:23 AM
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Default RE: How did I fry my Electrifly?


ORIGINAL: Dr Kiwi
I hate to think what a 12x8 folder was asking of the motor (40A+???).
Okay, Thanks. But would the ESC and battery still be cool to the touch? The ESC is only rated for 35 amps. I checked them right after landing, approx 2 minutes later and they were both at ambient temperature.
Old 10-04-2010, 11:07 AM
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Default RE: How did I fry my Electrifly?


ORIGINAL: SushiSeeker


ORIGINAL: Dr Kiwi
I hate to think what a 12x8 folder was asking of the motor (40A+???).
Okay, Thanks. But would the ESC and battery still be cool to the touch? The ESC is only rated for 35 amps. I checked them right after landing, approx 2 minutes later and they were both at ambient temperature.
The motor (or ESC?) probably saved the other components by burning out quickly. The battery should have been very hot to the touch, and the ESC would probably have been smoking if they had long sustained the very high current draw that probably existed. I'd check the ESC for melting or burning.

The comfort zone for this motor will probably be around 20-25 amps which is a good level for your battery and ESC as well. That's about 200-250 watts input which should provide a nice climb for a 2-meter Mistral.
Old 10-04-2010, 11:23 AM
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Default RE: How did I fry my Electrifly?

Tim,

Good suggestion. I bench checked both the ESC and the battery and they are fine. So my motor was the fuse in this circuit.

I was a little concerned during the landing after I fried the motor that since I was using the BEC on the ESC, I could lose receiver power if the ESC fried. My guess is that these are independent circuits.

So it looks like I can step up the motor to a .15 and still use the same ESC. The .15 is 6mm longer but the same diameter so it will still fit in the nose of the plane. If I step down the prop to an 11 x 7 I should still have plenty of power.
Old 10-04-2010, 12:08 PM
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Default RE: How did I fry my Electrifly?


ORIGINAL: SushiSeeker


...I was a little concerned during the landing after I fried the motor that since I was using the BEC on the ESC, I could lose receiver power if the ESC fried. My guess is that these are independent circuits.

So it looks like I can step up the motor to a .15 and still use the same ESC. The .15 is 6mm longer but the same diameter so it will still fit in the nose of the plane. If I step down the prop to an 11 x 7 I should still have plenty of power.
You were right to be worried about losing the BEC. Many BECs (non-switching types) are already stressed because they are generating lots of heat dropping the battery voltage to the 5 volts needed by the receiver and servos. It does not cost much (weight or money) to overrate and ESC by a healthy margin.

Going to the .15 sounds like a good plan. You will be able to run it conservatively while increasing your power and safety margin. That should be a very enjoyable combination.

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