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Old Jan 06, 2013, 01:44 AM
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MOTOR watt Question

Question-

if a motor says Max Efficiency current 51a
Input Watts: 375W <--- I guess that means MAX watts???

as long as I stay under the max amps (51a)...will exceeding the watts ( lets say from the above 375W to 456W).... Will that kill the motor?


I have a motor with the above numbers

Tacon 10 2820-1100

13x4 apc prop
3s1p (65c) lipo
456W @ 48a <-- the motor says max 51a & I will use a 60A Esc

So, is running it to 456W going to fry my motor?
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Old Jan 06, 2013, 02:38 AM
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The motort specs are rated at 2S ... 7.4V x 51A = 377W

But specs also say that motor is capable up to 4cell ... which is 4S.

So you may find motor is warm with your 3S ... but I'd do it ... careful watch on amps as powering up when test.

The main thing is to keep the amps down as you increase volts ... usually means propping down ... the specs are most likely prop on that 2S ..

Nigel
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Old Jan 06, 2013, 02:46 AM
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I would say the watts figure is a misprint, if you use the 3 watts per gram rule, that motor should be good for 420 watts plus or minus depending on quality. So i would stick with the 51 amp and under and monitor the heat. According to your figures your voltage drops to 9V when you got these figures, is that correct?
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Old Jan 06, 2013, 02:50 AM
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Thats why this MOTOR is driving me nuts...the MFG. cant help and the Only info ( above) Makes NO sense at all.

No wonder this motor was cheap It does turn tho'...I just would have liked to KNOW some of the figures for this thing.
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Old Jan 06, 2013, 02:58 AM
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You gave us the figures, did you use a watt meter? I am not sure what you are asking. If this motor weighs 140 grams there is a simple rule - a motor will produce 3 watts per gram (motor weight). 140 x 3 = 420 watts. Now 420 divide by your regular voltage reading under load (prop spinning full throttle) lets say 11.5 volts = 36 amps, thats a nice and safe amp draw figure. 456w divide by 46 amps = 9.9 volt, to me that is too much a voltage drop to get any decent efficiency out of this motor.
Sorry if i blabbered but it is fundamental that you know, not so much ohms law but that power(watts) = volts x amps, volts = watts divide amps, amps = watts divide volts. The 3 watts per gram rule is one made (i think) by dr kiwi on the power systems forum, and he knows what he is talking about.

Me personally, i think you have the right setup, just monitor the heat
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Old Jan 06, 2013, 02:58 AM
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A 2820 size motor will be good for 700+ Watts if you run it on 4s. In fact on other motors of the same stator size I'm pulling over 1kW in bursts.
It's the Amps that you have to watch, so your figures look fine.
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Old Jan 06, 2013, 06:00 AM
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Quote:
Originally Posted by JetPlaneFlyer View Post
A 2820 size motor will be good for 700+ Watts if you run it on 4s. In fact on other motors of the same stator size I'm pulling over 1kW in bursts.
It's the Amps that you have to watch, so your figures look fine.
JPF and I agree ....

It's fine all this 3W per gr and all that ... but that assumes that windings and general construction are good or similar across the board. We all know that is not true..... otherwise we'd all be using the cheapest motors.

The specs in OP show that motor is capable of 4S ... what I don't like is that the specs don't give any indication of prop size recc'd.

When I buy a motor - I always check specs for prop size ... if it's not listed then I search reviews etc.

Nigel
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Old Jan 06, 2013, 09:10 AM
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It's all a matter of waste heat and heat dissipation. The waste heat comes from the resistance of the motor windings. I agree with the other posters that a 5 oz. motor should be good for something around 500 watts. Even a fairly generic motor.

Duty cycle and cooling are important. That's why many of the specs give burst ratings, ie. some number of watts (or amps) for some number of seconds. By limiting the duty cycle, you limit the temperature rise in the windings.

The motor's Kv also has some bearing. If you compare motors of the same mechanical dimensions (case size) and different Kv values, you will almost always find that the higher Kv motors yield a higher watts-per-ounce figure.
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Old Jan 06, 2013, 05:55 PM
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And I Agree with all of you...I think My "As long as i dont go over the MAX allowed AMP's" thinking...it should be fine.

Also Agree I look for prop. size range....lipo count range...etc... But this motor havd NONE....so I email the MFG.... guess what....THEY DIDN'T KNOW either....

So, will go with my best guess...and STOMP on this motor ( lucky for this motor I only have 3 cell lipos)
will see if I got lucky...or not.

I do not want to loose my plane OR my ESC....but guess i can afford to melt the motor.
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Old Jan 06, 2013, 09:37 PM
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The 3 watts per gram rule is one made (i think) by dr kiwi on the power systems forum, and he knows what he is talking about.

No, no, no... I don't know enough physics to have formulated a definitive rule.....

I don't know where that "rule of thumb" came from (presumably from someone far smarter than I am), but from lots of static testing I find that in practice 3W/g is an extremely useful guideline!

Really el cheapo motors should not be pushed quite that hard, but good quality ones are fine with it, and with good in-flight cooling can, to a limited extent, exceed it. In reality, due to I^R, the current is the most important factor, so for the same watts-in, higher voltage, lower amp draw is beneficial.

Cheers, Phil
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Old Jan 06, 2013, 11:46 PM
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wait wait wait... so Watts is just Amps * Voltage? That makes sense i guess, since Volts is kinda like the "octane" rating of a battery, Amps is a measure of how much current/flow and Watts is just power, like horse power. So multiplying the known "flow rate" and a current rating will give you the amount of power the combo will produce. I love when things make sense momentarily!
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Old Jan 07, 2013, 12:16 AM
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Quote:
Originally Posted by seeingeyegod View Post
wait wait wait... so Watts is just Amps * Voltage? That makes sense i guess, since Volts is kinda like the "octane" rating of a battery, Amps is a measure of how much current/flow and Watts is just power, like horse power. So multiplying the known "flow rate" and a current rating will give you the amount of power the combo will produce. I love when things make sense momentarily!
Think of volts as a bit like pressure that is pushing the current through the wires. The higher the pressure for the same resistance, the higher the current.
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Old Jan 07, 2013, 12:49 AM
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Quote:
Originally Posted by scirocco View Post
Think of volts as a bit like pressure that is pushing the current through the wires. The higher the pressure for the same resistance, the higher the current.
Yeah that is a much better analogy. An electronics class I took once described it just that way, pressure. For some reason it sounded dumb when I typed it though
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Old Jan 07, 2013, 10:55 AM
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Wow Guy's Lot's of great help here!! Thanks...and if anyone thinks of anything else ...Feel free to add it

I.E. - Is the a simple Math equation to find the best Prop? (other than checking amp. draw)

Again Thank you..
& JetPlaneFlyer...Sorry I PM'd you this question too...I was searching in other post..saw you "Motor Talking" and PM'd you..B4 I game back here to see your post Opps
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Old Jan 07, 2013, 11:03 AM
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Some Motor Info

Hey..I just came acrossed this..and thought I would share Incase anyone else is haveing Elec. Motor questions Like I was.

--------------------------------------------------------------------------------------------------------
KV Rating

KV rating is referred to the RPM of the brushless motor per volt. This rating is determined without load. If you're wondering what "load" is referring to, its if there is weight, or a load, that the motor is pushing/pulling/moving. If it does have weight attached to it, obviously the RPM will be different.

Here is a very general KV Rating for Brushless Motors.

Micro size: 4000 low 160000 high
1/18th: 3000 low 7800 high
1/10th: 2000 low 6000 high
1/8th: 1050 low 2000 high
1/5th: 700 low 900 high

You take the KV rating of the motor, and you multiply it by the voltage of the battery to get the total RPM.

For Example: A Brushless Motor with a KV Rating of 3500 and you're using a LiPo battery that has 11.1 V, the equation will look something like this:

3500 (KV) x 11.1 (V) = 38,850 RPM.

Another thing you should understand about the KV is that the higher the KV rating, the faster the motor will be, but it will have less torque. The lower the KV rating, the higher the torque. So what does that mean? High KV = High Top Speed but Low Acceleration. Low KV = Low Top Speed but High Acceleration.

Watts/Amps/Current

Watts are the power rating of your brushless motor. Every 100 Watts = 1lb of weight movement. In other words, 100 Watts can move 1 pound. You find the Watts by multiplying the AMps with the Battery Voltage. Usually you can find the Watt, Amp, and Current rating of the brushless motor on the Spec Sheet or User Manual that comes with the motors. Most of the times, they will show it on the box as well, since this is all important information when buying your motor.

This bit of information is extremely important for aircrafts. Planes, helicopters, quadcopters are all weight dependent. If a motor does not have produce enough Watts to move the weight, then there is no way that the aircraft can fly. So remember to keep this in mind when you buy your motor.
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