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« on: October 14, 2013, 01:48:41 PM »
nairitb
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Hello everyone.

I was going through electric motor basics when a question popped up in my mind. I trying to google it but didn't find any success. Can anyone please help me out? The question is:

If two Brushless Motors of same dimensions and same KV (say 2200kv), but different Max Current ratings (say 21A & 34A) are fitted with the same prop (say 6x3E), will they both draw the same current at a particular throttle (say 50%)?

Hope I didn't forget to provide any vital info. Any clarification would be much appreciated...
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« Reply #1 on: October 14, 2013, 02:42:25 PM »
santhosh-r
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How come two motors of same Kv, same dimensions ,same props have different max current ratings ??...is it different manufacturers...
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« Reply #2 on: October 14, 2013, 03:02:38 PM »
nairitb
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Hi Santhosh.

Yes different manufacturers. Max current draw depends on the copper wire thickness used to make the coils inside the motor. From my knowledge so far, they should draw same current (neglecting heat loss etc), but my knowledge is limited  Sad

Check these out:

http://www.hobbyking.com/hobbyking/store/__12919__D2826_6_2200kv_Outrunner_Motor.html
http://www.hobbyking.com/hobbyking/store/__21481__NTM_Prop_Drive_28_36_2200KV_696W.html
http://www.rcbharat.com/brushless-motors/182-mystery-a2212-6-2200kv-brushless-outrunner-motor.html
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« Reply #3 on: October 14, 2013, 03:10:08 PM »
arun.sreelakam
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Fly High
When the w increases it can handle more big props, hence produce more W..
If you are running both the motors with same prop and same lipo...
The W and Thrust produced will be somewhat similar.
Now if you are going for a bigger prop on the less W motor it may go bad after sometime because of overload..
Where the other can handle that and produce more W and Thrust..
This Simply based upon the build quality and materials used to produce the specific motor..
This is From my little knowledge, Correct me if i am wrong..
Smiley
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« Reply #4 on: October 14, 2013, 03:16:36 PM »
obraganza
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Just for academic purposes let me try to put forth my understanding of DC motors (I think it can be extended to Brushless Motors but i am not 100% sure)
I am assuming same voltage across the motors and same load. Also the motor with higher current capacity will have a thicker gauge of motor winding.

Voltage drop across the motors consists of two components
1. Back EMF across the coils
2. Resistive voltage drop (IR Drop) due to ohmic resistance of the coil

under normal circumstances
The motor with higher current capacity will have a lower coil resistance and hence lower IR drop (Point 2). Hence it will run closer to its rated speed (That is the drop in speed due to load will lower). it will do more work and get less hot.
The motor with lower current capacity will lose more voltage to IR drop across the coil and hence it will run slower (That is the drop in speed due to load will be higher). It will do less work and get more hot

Current flow will be slightly lower in the motor with lower current capacity due to higher ohmic resistance of the coil
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« Reply #5 on: October 14, 2013, 08:33:23 PM »
nairitb
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Yes, but will the current draw difference be large?? say 2-3 Amps?  Head Scratching
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« Reply #6 on: October 14, 2013, 08:34:51 PM »
lastRites
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Are the magnets on the motors identical? If they are not then the current draw will not be the same. The only sure way to get the answer to your question will be do a practical experiment with those motors.
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« Reply #7 on: October 14, 2013, 08:36:40 PM »
lastRites
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Ohhh, I just noticed the links to the motors you posted. The motors are NOT similar. 2836, 2826 and 2222 are the motor sizes. The current draw will not be similar(smaller for the bigger motor I think).
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It is the pervading law of all things organic and inorganic,
Of all things physical and metaphysical,
Of all things human and all things super-human,
Of all true manifestations of the head,
Of the heart, of the soul,
That the life is recognizable in its expression,
That form ever follows function. This is the law.
 

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« Reply #8 on: October 14, 2013, 08:39:13 PM »
nairitb
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If the magnets are not be same, how much difference in amps will it make?
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« Reply #9 on: October 14, 2013, 08:39:58 PM »
lastRites
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@obraganza Here you are assuming the motors to be physically identical except the winding. This is not the case here. The motors nairitb has posted vary in size as well as the in magnet strength. The larger motors tend to be a little more efficient but it all depends on the applied load.
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It is the pervading law of all things organic and inorganic,
Of all things physical and metaphysical,
Of all things human and all things super-human,
Of all true manifestations of the head,
Of the heart, of the soul,
That the life is recognizable in its expression,
That form ever follows function. This is the law.
 

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« Reply #10 on: October 14, 2013, 08:41:03 PM »
lastRites
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You may use ecalc to get an estimate. But the best way is to bench test the motors.
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It is the pervading law of all things organic and inorganic,
Of all things physical and metaphysical,
Of all things human and all things super-human,
Of all true manifestations of the head,
Of the heart, of the soul,
That the life is recognizable in its expression,
That form ever follows function. This is the law.
 

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« Reply #11 on: October 14, 2013, 08:41:47 PM »
nairitb
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My bad. I din't realize their sizes  Sad I posted them for reference, but I am considering between 2836 & 2826.
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« Reply #12 on: October 14, 2013, 08:43:35 PM »
nairitb
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Can someone suggest a good book where I can get detailed info on such kinds of motors?
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« Reply #13 on: October 14, 2013, 08:43:54 PM »
lastRites
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These two motors are different as well. Check their specs and choose the best fit for your application. Tongue
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It is the pervading law of all things organic and inorganic,
Of all things physical and metaphysical,
Of all things human and all things super-human,
Of all true manifestations of the head,
Of the heart, of the soul,
That the life is recognizable in its expression,
That form ever follows function. This is the law.
 

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« Reply #14 on: October 14, 2013, 08:45:42 PM »
nairitb
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How much will be the current draw? Any estimate?
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« Reply #15 on: October 14, 2013, 08:46:45 PM »
nairitb
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To bench test the motors, I would have to BUY them. That's what I am unwilling to do Sad
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« Reply #16 on: October 14, 2013, 08:50:41 PM »
nairitb
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One more thing I needed to ask is this:

I flew my Sky Surfer for almost 20 mins yesterday. At the end of the flight, I found that the ESC had become so hot that the the heat shrink around it had melted. Combination is stock motor (1950KV), stock ESC(20A) & 6x3E props.

Any views?
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« Reply #17 on: October 14, 2013, 09:20:06 PM »
aniket210696
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was it a 20a esc? if so, then you may need to ventilate it... cut a hole In the fuse and pass it through that.. that may help...
now I am curious.... can you post a pic of the melted esc covering? :Smiley

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« Reply #18 on: October 14, 2013, 09:54:31 PM »
nairitb
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Ya 20A ESC. Here's the pic of the ESC...
Burnt ESC.jpg
Re: Electric Motor Doubt
* Burnt ESC.jpg (50.79 KB, 800x558 - viewed 409 times.)
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« Reply #19 on: October 14, 2013, 11:12:36 PM »
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#get_set_fly
@nairitb  i can help u in the skysurfer case call me @8093863974 i have faced the same situation and know wat to do ...
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