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Foamies Talk about building, power setups and anything having to do with e-powered foamy planes!

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Old 07-19-2016, 11:43 PM   #1
Champstarr
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Default Where Did I go Wrong?

I recently replaced and upsized the ESC and motor in my E-flite 1.2M T28 Trojan. I upgraded the ESC to an E-flite 70 amp esc and the motor to a Power 25 (1250kv) so I can run 4s lipo batteries. The stock motor is a BL 15(850kv).

After three flights on this power setup the I noticed the 4S battery, esc and motor were very warm. I let it cool down completely and flew again. Then I heard a pop and white smoke began to pour out and someone yelled "fire in the ky!". I landed safely and discovered the motor had blown. Afterwards I reinstalled the stock motor and after five flights with the 4s none of the components were really warm. Where did I go wrong on this supposedly "upgraded" power setup. Why would the smaller stock motor be able to accept a 4s lipo and not the larger Power 25?

Any advice is appreciated. I may not be as lucky next time.
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Old 07-20-2016, 12:45 AM   #2
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Champstarr
Its all to do with the Kv of the motor.
Although the Power 25 is physically bigger its higher kV means it will try to drive the prop faster by 1250/850 = 1.5 times. The speed square law means to do this will require 1.5x1.5 = 2.25 times the current driving the same prop!
Not surprising it got hot!
If you are going to upgrade motor/ESC/battery system you really need to get a Watt meter so you can actually measure what is going on and thus prevent expensive overloads.
You will need to reduce the size of the prop on the power 25 motor to keep the current draw within the motor's and the ESC's design limits.
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Old 07-20-2016, 12:47 AM   #3
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In terms of kv/ hr., your stock motor is much more 4s friendly than your high strung 1250. Same prop ? Did you check the rating in the motor? It might not be designed to handle 4S.

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Old 07-20-2016, 12:49 AM   #4
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That Power 25 has worked fine for my 8x6(e) & 8x8(e) props with 4s.....and 10x10(e) with 3s batteries.........using a Castle 60a esc.

Noticed no mention of prop size used when "a pop & white puff of smoke" (blown motor....?) signaled an issue by the OP.........surprised the esc did not have damage and the craft made it down well enough to fly again.....!

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Old 07-20-2016, 12:56 AM   #5
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Originally Posted by quorneng View Post
Champstarr
Its all to do with the Kv of the motor.
Although the Power 25 is physically bigger its higher kV means it will try to drive the prop faster by 1250/850 = 1.5 times. The speed square law means to do this will require 1.5x1.5 = 2.25 times the current driving the same prop!
Not surprising it got hot!
If you are going to upgrade motor/ESC/battery system you really need to get a Watt meter so you can actually measure what is going on and thus prevent expensive overloads.
You will need to reduce the size of the prop on the power 25 motor to keep the current draw within the motor's and the ESC's design limits.
Thank you for that clear explanation. I should of researched and asked these questions before going off the deep end. I do have a watt meter now and the 4S if pulling 76 ~ 77 amps with the stock prop at WOT which is 6 - 7 amps above the rating of the esc. Should I prop down? I typically only use WOT in 5 second bursts.
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Old 07-20-2016, 01:01 AM   #6
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Originally Posted by quorneng View Post
Champstarr
Its all to do with the Kv of the motor.
Although the Power 25 is physically bigger its higher kV means it will try to drive the prop faster by 1250/850 = 1.5 times. The speed square law means to do this will require 1.5x1.5 = 2.25 times the current driving the same prop!
I'm gonna say it was probably this.

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Old 07-20-2016, 01:09 AM   #7
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Originally Posted by pizzano View Post
That Power 25 has worked fine for my 8x6(e) & 8x8(e) props with 4s.....and 10x10(e) with 3s batteries.........using a Castle 60a esc.

Noticed no mention of prop size used when "a pop & white puff of smoke" (blown motor....?) signaled an issue by the OP.........surprised the esc did not have damage and the craft made it down well enough to fly again.....!
When I upgraded the motor the I proped down from the stock 12X6 three bladed prop to an 11X6 two bladed prop. And Yes I feel fortunate the esc didn't flame up and disintegrate my beautiful plane.
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Old 07-20-2016, 01:20 AM   #8
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Originally Posted by Champstarr View Post
I recently replaced and upsized the ESC and motor in my E-flite 1.2M T28 Trojan. I upgraded the ESC to an E-flite 70 amp esc and the motor to a Power 25 (1250kv) so I can run 4s lipo batteries. The stock motor is a BL 15(850kv).

After three flights on this power setup the I noticed the 4S battery, esc and motor were very warm. I let it cool down completely and flew again. Then I heard a pop and white smoke began to pour out and someone yelled "fire in the ky!". I landed safely and discovered the motor had blown. Afterwards I reinstalled the stock motor and after five flights with the 4s none of the components were really warm. Where did I go wrong on this supposedly "upgraded" power setup. Why would the smaller stock motor be able to accept a 4s lipo and not the larger Power 25?

Any advice is appreciated. I may not be as lucky next time.

FYI..I have this same 1.2 T28 Trojan, I have upgraded to a 65 amp ESC, and use 4s 3200 lipos on the stock motor. I did also have a small problem with heat issues on motor and ESC, I changed to a 11x7 APC 2 blade prop and problem was solved...I wasn't really worried about the scale look by loosing the 3 blade prop. I need to get a Watt meter which would make these changes to save electronics waaay easier.
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Old 07-20-2016, 05:54 AM   #9
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Originally Posted by quorneng View Post
Champstarr
Its all to do with the Kv of the motor.
Although the Power 25 is physically bigger its higher kV means it will try to drive the prop faster by 1250/850 = 1.5 times. The speed square law means to do this will require 1.5x1.5 = 2.25 times the current driving the same prop!
Not surprising it got hot!
It's worse than that, power goes up roughly by cube of RPM, so 1.5^3 = 3.375 times more power!

Then you have the battery voltage increase which again works to the cube: 4/3^4 = 2.37 times more power

Power also changes by cube of prop size so the reduction in prop will mean 11/12^4 = 0.7 times, going to two blade proably brings it down to around 0.6

Taking all into account: 3.375 x 2.37 x 0.6 = 4.8 times more power (even after proping down)

The 870Kv version of the Power 25 would be the one to pick for 4S.

The morale of the story is to always use a wattmeter after changing components in the powertrain.

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Old 07-20-2016, 05:58 AM   #10
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Originally Posted by pizzano View Post
That Power 25 has worked fine for my 8x6(e) & 8x8(e) props with 4s.....and 10x10(e) with 3s batteries.........using a Castle 60a esc.
The power 25 comes in three versions 870Kv, 1000Kv and the 1250Kv 'screamer'

I'm guessing that you dont have the 1250Kv version like the OP tried to use? (Kv makes an enormous difference)
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Old 07-20-2016, 07:09 AM   #11
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There is a rough rule when changing cell count .....

Reduce KV when increasing battery cell count.

I have a little rule I use ....

KV x Cell count = factor.

I use the factor to show a reasonable KV for another cell count.

In OP's case the mistake was going wrong way with KV change .... and result is a massive increase in power demand.

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Old 07-20-2016, 04:41 PM   #12
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Originally Posted by JetPlaneFlyer View Post
The power 25 comes in three versions 870Kv, 1000Kv and the 1250Kv 'screamer'

I'm guessing that you dont have the 1250Kv version like the OP tried to use? (Kv makes an enormous difference)
Had this one in my PZ Trojan (44"ws) for awhile......:

http://www.e-fliterc.com/Products/De...odID=EFLM4025B

The only reason I switched back to the stock 480-960kv was due to enjoying the flight characteristics with a 3s 2200 and longer flight times, much more..........also having plenty of 3s-2200 lipo's and 9.5x7.5 props laying around.....

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