RC India

Can i use this combination of battery and motor with 20amp esc  :help:

http://www.rcbazaar.com/products/1745-wolfpack-2200mah-35c-74v.aspx (http://www.rcbazaar.com/products/1745-wolfpack-2200mah-35c-74v.aspx)

http://www.rcdhamaka.com/index.php?main_page=product_info&cPath=100_93&products_id=760 (http://www.rcdhamaka.com/index.php?main_page=product_info&cPath=100_93&products_id=760)

@johnnywalker8pm
If you look at the emax site, you will find that this motor will pull 12-14.4 amps.
A 20amp ESC can handle that
A 2200mah 35C can give 77amps (in theory)
So what exactly is your doubt?

A suggestion for getting better response:
Put your data and query clearly and upfront
Do not expect others to have the patience to look at your links

@KK Sir
I am really sorry for the "link" thing.

Its been few months i am going through this kind of calculation for the combination in this forum but not able to solve the mystery of Volts, amps, mah ets which are related to maths.

 My doubt "I thought it has more mah means it will blow my 20 amp esc"   :'(  :'(

Took suggestion from V2 eagle few times, actually those thing bounced back from my ear :banghead:

I hate "MATHEMATICS" and i am a dumb a** in that too  ;D

Please elaborate a little more to get better answers.
See, its quite simple, first you need to decide the AUW weight of your model alongwith which kind of model your are going for like for Ex: lets suppose, it is a high wing trainer model with AUW 600gms. So, now you need to search for a motor that gives atleast 700-800gms thrust. It is always better to choose a slightly overpowered setup. Now, if motor is final then every motor has got a set of recommended ESC, prop, battery.
If you don't find that and are still confused then look for how much current motor will draw. Lets suppose for the motor you mentioned the current is 15A or less. So, for this a 20A esc is sufficient. ESC also of higher rating is required just so that it can handle load. Now comes the battery, again recommended setup must be given or you can see the voltage required in motor specs and accordingly choose 2S(7.4V) or 3S(11.1V) battery with 1000mAh-1300mAh. Its sufficient because other batteries of higher ratings are much heavy.
Lastly, the prop size, you should only go for recommended prop size. An oversized prop could burn your motor. Remember every motor has different thrust for different prop, battery combinations and it is mostly recommended. Also, remember mostly a lower kv rating motor requires a larger prop whereas higher kv motor requires a small prop.


P.S. : If I'm wrong please correct me.

@johnnywalker8pm
It's easy. Nothing to do with maths. Just a little multiplication.
Here's a brief lecture.

Amp is a measure of current. 1amp=1000milliamps (ma)
Voltage is the potential of the battery, ie, how fast it can potentially run a motor.
Kv is the rpm a motor can run on one volt without propeller
A 2840kv motor will run at 28,400 rpm if you use a 10v battery.
Watt is a measure of Power. It is volts x amps
A Kilowatt is 1000 watts.
A kilowatt-hour is the energy consumed when you use 1000 watts for 1 hour.
It is also called a UNIT, that you pay about Rs 6 for.
A 100w bulb will consume 1 unit in 10 hours.
A 2.5 kw AC will consume 2.5 units per hour.

Now consider a 1000mah 3s lipo.
Fully charged, it will be 12.4v.
Half used it will be about 11.1v.
At about 9.9v it will be fully discharged.
Remember that 1amp is 1000ma.
So a 1000mah is 1 amp-hour.
Assuming average voltage of 11.1v, this battery has 11.1 watt-hours worth of energy.
(11.1v x 1ah= 11.1 watt-hour)
Usually we won't use more than half.
So let us say we have about 5 watt hours available.

If the motor uses 5watts, we could run it for an hour.
If it uses 50watts, we could run it for only 6 minutes.
How many watts it needs depends on the load, ie, the size (dia and pitch) of the prop.
Since the available voltage is limited by voltage of the battery, if we need more power (watts) for a bigger prop, we will be drawing more current (amps).

Contd...

Contd...

The EMax site says this motor will draw 12a with a 6x4 prop turning 13,000 rpm, but for not more than 60 seconds.
So if you use 75% power, it will draw about 9-10amps, and turn the 6x4 at around 9-10,000rpm.
Your 20amp ESC can easily cope with this.

The other limiting factor is how many amps your battery is capable of delivering. A 2.2ah 35C can theoretically give 77 amps. In practice expect 10C, ie, 22amps unless you have a high end battery like used for helis.

So
Your motor will use say 10amps. Your ESC can handle that. Your lipo can give that current.
Ok so far.

Assuming battery voltage of say 8v for your 2s lipo, this means 80watts of power.
This will give about 320gms of thrust (unless you have a high efficiency prop like APC)
A typical trainer with a low wing loading (say 6-9oz/sqft of wing area) will fly with a power to weight ratio of about 0.5. Ie, you could fly a trainer of about 600gms all up weight with about 300gms of thrust.

Next problem...
Contd

Assuming that the motor is in the nose, what is the cross section of the fuselage?
If it is say 3" dia, there will be hardly any space left for a 6" dia prop to deliver thrust cleanly.

There are more issues, but i will stop here with this warning:
DON'T use thin blades slow fly props with little strength at the root, with high kv motors.

@Utkarsh  :thumbsup:

@ KK sir
Its really nice and simple elaboration.  :bow: thanks a ton..
I have already taken a print out of this page. Will go through it thoroughly at home. Any doubt means i will back to KK sir again  ;D

Still i am scared because there are lots of numeric values and multiplication symbols..  :banghead: :banghead:

Excellent Iyer sir ...  De- cluttred my brain wiring thanks !

 :thumbsup: :thumbsup:
Very good explanation Iyer Sir

Four years of engineering in just three posts.Amazing Iyer sir.
I went with the suggested values and learned this later.
Anyone has any idea on the Red quote on the post #2.
This
"I thought it has more mah means it will blow my 20 amp esc"
Ashok.P

No,
More mah is not blow up your ESC.
- It's more stored energy but more weight.
+ It will fly more.
Calculate power flow always from back. That is from prop. keep in mind in electrical engg. Load is the King .
A Prop and motor, with supplied volt will define your power draw (Max).
Your ESC and battery should handle that.
Now your ESC should be of more capacity while motor take maximum current. Keeping a safety factor.
And your battery's discharge current should be within the limit keeping in mind 15% extra current for servo operation (Limited by BEC capacity).
Of course your battery 's weight should have to balance CG.

So can we charge a TX from a car battery adapter(40Ah!!) without any protection like current limiting.

Ashok.P

Here comes another factor,
The load impedence. All rechargable batteries have very low internal impedence (or resistance you can say). Now the current will depend upon voltage differance and entire loop resistance.
If your Tx battery is rechargable and charging through a rechargable battery and without limiting resistor, theoritically current will be infinite.
Unlike ESC and Motor+Prop assembly where load current limited by load (Motor+prop+ESC combination).

correctly known as input impedance of the Load. the current limiting factor of a load. this is why its not advisable to charge a TX directly through a high current supply since the internal electronics of TX is designed to withstand only a few hundreds of milliamps since they are going to be run through a low C rating battery only.

Ashok.P

You better see the make hangar series from beginning to end all your doubts will be cleared

@KK iyer Sir
Can u please  explain about the Center of gravity and other parameters, the same way you explained About motors and props  above :)
As i am thinking about scratch building rc planes so i want to know about the parameters for aeromodelling :)