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Old Jul 02, 2011, 12:14 AM
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Rimfire .15 Burns up With my set up help......

Hello All... I have a problem. I have A cessna that weighs 3.5 lbs ready to fly. I have a rimfire .15 motor a castle ice 50 esc and upgraded to a 4s 25c 2500 lipo battery. The motor fried in 2 minutes. Rimfire states that the .15 motor can handle a 4s lipo battery. I want to max out the plane with the biggest possible motor and biggest battery pack. I was told by a guy at the field i was flying at that the milliamp of the battery didnt matter. if I were to use a 3s lipo at 1800 or a 3s at 2500 all you would get is more flight time. not more power. Whats the differencein power from a 3s 2500 to a 4s 2500. Is it just the same but you get longer flight time?

From what I've read I need to get the rimfire .25 motor. What happens if they call for a 60amp speed controller and I use my castle ice 50 with a 4s 3300 lipo battery. Any help would be greatly appreciated.....
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Old Jul 02, 2011, 12:52 AM
Reduce the drama...
rick.benjamin's Avatar
USA, OR, Damascus
Joined Apr 2004
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Your motor should not have failed with the information you have shared.
EXCEPT
you didn't mention your prop size.
If it is 11x7 or smaller you are ok.
If it was a larger prop, it probably overloaded the motor.
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Old Jul 02, 2011, 02:39 AM
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Xpress..'s Avatar
United States, CA, Alpine
Joined Oct 2007
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Need more information. What size propeller are you using? Specifically the prop size is what we need.

My guess is you are using too big of a prop for the motor to handle, and you fried it.

The difference between a 3 cell lipo battery and a 4 cell lipo battery is a difference of 4.2 volts at full charge, or 12.6 volts to 16.8 volts- that's plenty of a voltage increase to fry a well mannered 3S setup.

MAH is the same as the gas tank size. More MAH is the same as a bigger gas tank. A bigger pack will provide more flight time, UP to a certain point where the weight gain will require more average motor power to fly, and the flight times will actually begin to go down.
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Old Jul 02, 2011, 03:29 AM
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United States, IL, Elgin
Joined Mar 2011
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If you are going to be playing around with motors or props, you really need a power meter so you do not put too many amps or watts into the motor. For a given size/pitch prop, power use can vary between different models from one manufacturer, or even more between different manufacturers.

For example with a Park 480 1020 kv APC 10x6 sport used as much power as 10x7E, and 10x7SF used way more power than either of those. Or a MAS 11x6NE used way more power than APC 11x7E because the wider tip of the MAS was likely moving more air.

Going from 3S to 4S increases voltage 33%, which apparently increased amps/watts too much if you did not use a proper smaller and/or lower pitch prop for the increased rpm.
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Old Jul 02, 2011, 09:07 AM
fmw
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Motors are quite tolerant of voltage. It is the current that damages them. So using a 4S battery is not the problem in itself. However when you change from a 3S battery to a 4S battery you normally need to reduce the size of the prop, assuming the electrical system is loaded to maximum. Why? Because the change increases propeller RPM and that increases current draw and it is the current that can damage your electrical system.

A 50 amp ESC is plenty for a 15 size motor - in fact more than plenty so that is why it survived whatever you did. The battery capacity (MAH) affects flight time and weight. Weight is an issue. It affects not only the center of gravity but its stall speed and responsiveness - in other words it affects the model's very ability to fly well.

You have your first answer already from everyone else and that is the propeller. It is what draws the current that can damage your motor.

The reason I jumped in is that you obviously don't know what you are doing and don't understand the concepts involved in powering an electric model. You need to do some reading and study on the subject. Step back, do that, look at what these concepts recommend to power your model and, then, if there is reason to change that, then you will know what is going on.

At the top of the beginner forum is a sticky with some information about how to do this. That might be a good place to start.
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Old Jul 02, 2011, 11:39 AM
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Thank you guys for taking the time to help educate a newbie! I am using an APC 11 X 5.5 Prop. I burned up 2 rimfire .15 in a row. One yesterday and one the day before. I thought I had a defective motor the first time but realized that 2 minutes into the second motor that something else was wrong. I will be buying a wattmeter to ensure I am not overloading the motor.

I love flying but I have a difficult time wrapping my head around the concepts in play here. I just don't understand why on rimfire's site that I am in the guidelines of prop size and I still burned up 2 motors by going from a 3 cell to a 4 cell. If a 4 cell produces 33% more voltage wouldn't I want to put a bigger prop on to slow the motor rpm? It seems that a smaller prop would allow more rpm and fry the motor. But you guys here have far more experience than me.
Thanks Guys.......
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Old Jul 02, 2011, 11:54 AM
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United States, CA, Sacramento
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Quote:
Originally Posted by cwrawkkkk View Post
Thank you guys for taking the time to help educate a newbie! I am using an APC 11 X 5.5 Prop. I burned up 2 rimfire .15 in a row. One yesterday and one the day before. I thought I had a defective motor the first time but realized that 2 minutes into the second motor that something else was wrong. I will be buying a wattmeter to ensure I am not overloading the motor.

I love flying but I have a difficult time wrapping my head around the concepts in play here. I just don't understand why on rimfire's site that I am in the guidelines of prop size and I still burned up 2 motors by going from a 3 cell to a 4 cell. If a 4 cell produces 33% more voltage wouldn't I want to put a bigger prop on to slow the motor rpm? It seems that a smaller prop would allow more rpm and fry the motor. But you guys here have far more experience than me.
Thanks Guys.......
Well, when you add the 4th cell, the bigger prop is spinning faster, creating more friction...more than the motor can handle.
the 11x5.5 is most likely rated for a 3 cell. With a 4 cell, you will need to experiment. If you don't have a watt meter, you can always do the touch test. Run the motor for 5 seconds at full throttle. Turn it off and touch the motor. If it burns your finger, it's overloaded. If it's not, repeat, and add about 5 seconds. Do this until the motor will run for 30 seconds without getting too hot, and you will be in the "safe" zone. It can get fairly warm without issue, but if it gets too hot, the lamination burns off the coil windings and the motor shorts out. If it is running slightly too hot, the bearings will eventually start to grind as well.
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Old Jul 02, 2011, 11:58 AM
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Joensuu, Finland
Joined Mar 2002
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Rpm's don't hurt a motor, amps do.

33% more voltage will make the motor run at 33% higher rpm. If you don't allow this, it will still try to reach that higher rpm using whatever means available. Only way to achieve this is to draw more amps and therefore the poor motor will suck up every amp the battery is capable of delivering and burn itself in the process.

Get a wattmeter or some other device you can measure amps with. Without measured data, you will be shooting in the dark.
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Old Jul 02, 2011, 12:01 PM
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United States, MI, Roseville
Joined Dec 2000
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wattmeter wattmeter wattmeter!

Also, a larger mAh battery will sometimes have more "oomph" behind it since it is larger.

Think of trying to start your car with a 12 volt automotive bettery vs a 12 volt model lipo battery....both may have the same voltage, but one has a LOT more power that it can put out.
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Old Jul 02, 2011, 12:06 PM
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I like the touch test. I am going to buy a watt meter when the store opens in an hour. I am researching them now.
http://www.powerwerx.com/techdata/Watts-UP-V2.pdf
This is what I was looking for......... A laymans explanitation of an rc plane system . I am going to have to go over these concepts many times to ingrain them.

Work/Energy
From a physics point of view, work and energy describe the same thing. The word chosen
at a particular time depends on the point of view being emphasized by an author.
Hopefully, this will become clear as you read on.
People often, incorrectly, mix the concepts of power and energy/work as though they are
the same. They are different, but related by time. Understanding the difference is very
important toward an understanding of propulsion system performance.
Voltage does work to move charge. The battery charger worked to push charge against the
battery’s voltage into the battery. That work is stored as charge in the battery. Discharging
the battery does work on whatever is using the charge.
This work or energy is measured in Watt-hours by measuring the power (in Watts)
expended over some time duration (in hours).
Watt-hours = Watts (averaged) hours
This is how much work the electricity has done.
The energy stored in a battery depends on the product of charge and voltage. I.E.
Energy (Wh) = voltage (V) charge (Ah)
So while a 7.4 V and 14.8 V battery pack may both have the same charge of 2000 mAh, the
14.8 V pack has twice the energy and capacity to do work.

Does this mean that a 7.4 V battery at a charge of 2000 mAh has 14,800 Wh and a 14.8 (4 cell ) battery has 29,600 Wh. where an 11.1V 3 cell has 22,200. The 4 cell battery has exactly 25% more energy and capacity than the 3 cell battery.
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Old Jul 02, 2011, 12:20 PM
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You forgot the "milli" in mAh, therefore your Wh's are off by a factor of 1000. But yes, otherwise you are on the right track.

Quote:
The 4 cell battery has exactly 25% more energy and capacity than the 3 cell battery.
Your use of the word 'capacity' is slightly misleading. In electric motor talk it is usually used strictly to mean mAh capacity of the battery, which doesn't change in your example. Obviously you meant to say '25% more...capacity to do work'.
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Old Jul 02, 2011, 12:29 PM
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Xpress..'s Avatar
United States, CA, Alpine
Joined Oct 2007
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Quote:
Originally Posted by cwrawkkkk View Post
I just don't understand why on rimfire's site that I am in the guidelines of prop size and I still burned up 2 motors by going from a 3 cell to a 4 cell. If a 4 cell produces 33% more voltage wouldn't I want to put a bigger prop on to slow the motor rpm? It seems that a smaller prop would allow more rpm and fry the motor. But you guys here have far more experience than me.
Thanks Guys.......
The more voltage you pour into a motor, the faster the motor is going to have to spin. If you don't change propeller sizes between switching from 3 cells to 4 cells, then the motor is going to try to spin that propeller all the way up to the maximum RPM, which in turn causes more amp draw on the motors part from struggling to turn that propeller faster and faster.
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Old Jul 02, 2011, 02:06 PM
fmw
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Quote:
Originally Posted by cwrawkkkk View Post
I love flying but I have a difficult time wrapping my head around the concepts in play here. I just don't understand why on rimfire's site that I am in the guidelines of prop size and I still burned up 2 motors by going from a 3 cell to a 4 cell. If a 4 cell produces 33% more voltage wouldn't I want to put a bigger prop on to slow the motor rpm? It seems that a smaller prop would allow more rpm and fry the motor. But you guys here have far more experience than me.
Thanks Guys.......
I answered that above. The change from 3S to 4S increased propeller RPM. That causes the propeller to draw more current. If it draws more current than the current rating of the motor, then things burn up.

Putting a bigger prop may slow the rpm's a little due to friction, mass and other things but it will increase the current draw. It makes the motor work harder so it tries to draw more current to keep the rpm's up. It isn't the rpm's that kill motors, it is the current. When you sped up the prop, then you needed to use a smaller prop to reduce the current draw and to prevent what happened to you.
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Old Jul 02, 2011, 03:24 PM
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USA, WV, Moundsville
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the simplest definition is current is a variable and for motors either a dangerous one "overheat/burn out" or not its as simple as that

by adding another cell you increased the rpm by increasing the rpm you increase the LOAD .. the work the motor has to do by increasing the load you increase the heat the motor has to dissipate hence... burning up motors

motors are rated in watts.. both constant and burst the motor you have chosen is rated at 650W burst current

EG 650W on 3cell 55A MAX on 4cell 46A MAX

on motocalc the closest motor i can find "same dimensions ect 1100kv vs 1200kv"
is showing me 770Watts @58.5Amp on 4cells thats way too much if your BURST "short bursts of full throttle for no longer than 30 seconds with ample cooldown after" is 650 id say your motor is rated around 475W constant

it also states on the spec sheet on tower its meant to run a 10x7-11x7 i would guess the 4cell lipo prop would be 10x7 however motor calc says a 10x6 would put your burst current right on the money

it also tells me your going to be turning a 10x6 @ 12,500 rpm give or take for your altitude ect and make 5+ lbs of thrust

i hope this heads you in the right direction and maybe clears some things up

or maybe im just long winded

Tim
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