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Battery to motor question - 12/19/2012 6:01 PM   
Fewski


 

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Status: offline 		I'm thinking of buying this motor:

http://www.hobbyking.com/hobbyking/store/__18128__Turnigy_Aerodrive_SK3_6364_245kv_Brushless_Outrunner_Motor.html

Under specs it lists "Voltage: 10S Lipoly". My question: Did they use a 10S Lipoly to get the stats they did and that's just the max amount of battery you can use? Or would I be required to run a 10S Lipoly on that motor?

If I can run a different battery, say a 6S or 7S.. how would I recalculate the stats using a smaller battery pack?

THANKS!

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RE: Battery to motor question - 12/19/2012 9:23 PM   
Goldenduff


 

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Status: offline 		That shouldn't be a problem however it will be very inefficient carrying a large motor and not using its power. Use a lighter motor.

Do not exceed the amp draw. If you wanna use 6cell go for a higher kv motor that is lighter. Just my thoughts...


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RE: Battery to motor question - 12/19/2012 10:51 PM   
Fewski


 

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ORIGINAL: Goldenduff

That shouldn't be a problem however it will be very inefficient carrying a large motor and not using its power. Use a lighter motor.

Do not exceed the amp draw. If you wanna use 6cell go for a higher kv motor that is lighter. Just my thoughts...


The problem with using a high KV motor would mean I would be losing torque, which I need.

To be honest, I'm not using this setup to operate a RC plane, but figured this is the best place to get help since it's all RC components.

I guess ultimately I'm trying to match someone who is doing something similar. He's got a 280kV 2725W motor on a 6S battery. Unfortunately that motor is discontinued and honestly, I would like a little more torque for mine. If I did the math correctly, I get:

For his: 280kV x 6S = 6216 RPM
For mine: 245kV x 7S = 6345.5 RPM

If I run a 7S battery pack on a motor that has 245kV, will I get the same amount of power with more torque than he got running a 6S battery pack on a motor that has 280kV?

I feel like I'm beginning an electrical engineering class.

THANKS!!

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RE: Battery to motor question - 12/19/2012 11:16 PM   
Goldenduff


 

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Status: offline 		I think torque is more related to amp draw which you cannot exceed the specified limit. The amp draw would depend on the gearing (comparable to prop size)- what's the top secret application? Is weight an issue? What are your criteria? Efficiency/run times, torque? Speed? Low cost? 

I should add I'm no guru before anyone had a go at my advice.....all the best


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RE: Battery to motor question - 12/19/2012 11:54 PM   
Fewski


 

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Status: offline 		According to http://rcfoamfighters.com/blog/?p=84 The Maximum Constant Amp Draw for a 4500mAh 35C battery is 157.5A. The motor I'm looking at has a max current of 70A. Apparently the rule of thumb is 1.5-2.0 more than what I'm using and in this case it looks like I should be fine on the max draw possible?

It's for an electric scooter. Weight isn't a big issue here, these motors are so much lighter than the original plan of brushed motors and acid batteries. haha. Criteria would be run time along with general speed (20mph seems achievable easily according to some other people). That's why I need a lot of torque, otherwise the motor wouldn't be able to turn unless I give the scooter a push start.

Thanks again.

< Message edited by Fewski -- 12/20/2012 12:58 AM >

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RE: Battery to motor question - 12/20/2012 1:01 AM   
nonstoprc



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Status: offline 		This URL has a formula to compute the torque as a function of the rated horse power and RPM.

My concern is whether a 6S 5000mah lipo can last long time enough for a scooter. One typical gets 7-8 minutes of fly time with a 2500-2800W motor on 5000mah 10S, with a 11lb pattern plane.

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RE: Battery to motor question - 12/20/2012 8:40 AM   
Goldenduff


 

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Status: offline 		The maximum amp output of a battery is C rating x Capacity in amps.  You are right that manufacturers tend to exaggerate the C rating so best compenstate as you have done. Also the closer you get to using the max amp output the less the actual capacity (mah) due to greater ineffeciencies.

For longer run times you would be better getting your power (amps x volts) by having high voltage. I.e. 100amps x 10v = 1000w and 10amps x 100volts = 1000w however the high voltage set up would run for longer.

Generally a low Kv and high voltage set up will give you long run times.





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