6500kv 1105's vs 4000kv 1105's on 3s. Which would be best for serious power and speed? And what prop to make it so?


Also is there a difference to a motor as far as heat goes if it's given 2amp @ 6v vs 1amp @ 12v?

Hey Redazu, I have no experience with brushless, but from what I understand, you would be better off using the higher KV for smaller props, the lower KV will have a higher torque so better for larger props.

I would guess that 2A @ 6v would get hotter than 1A @ 12v, despite the watts being the same. Check the thin wire for a 220v lamp then look at the fat wires attached to the 12v battery in a car... High Amps = Heat.

Also in real world application 12v will be spinning motors faster than 6v which means there should be more airflow over the exterior of the motor also helping to keep it cool. 6v might be spinning larger props moving more air overall, but more of the airflow will be further away from the motor itself.

This is just using my minimal knowledge of electrics and some common sense, if I am talking  , someone please say so..?

Yeah seems to make sense. So higher voltage and lower amps is always better right? Like if you can hover with 1amp 12v, then that's better than hovering with 2amp 6v? You're spending twice the current to do the same thing.

So if you over-gear ( high kv ) a setup with a 7500kv motor with a 3s instead of a 2s and you're able to hover with say 1amp 12v on the 3s but it's using 2.4amps to hover with 8v then the 3s is better right? And it would run cooler while hovering?

There are limits as to how far you can push this.

A motor's kv rating is RPM per volt. If you push the motor speed beyond what it is designed to handle, the motor temperature and current consumption will increase exponentially and destroy the motor and possibly the ESC. It's best to stay within design parameters.

(20-Aug-2017, 07:39 AM)Redazu Wrote: So higher voltage and lower amps is always better right?

Its always dangerous to say always, especially with aerodynamics!

Volts vs Amps is the same as Power vs Torque, Volts / Power - good at working fast (ie. move 1/4 bucket every second,) where Amps / Torque - good at working hard (ie. 1 full bucket every 4 seconds.) They each take their toll on the hardware in a different way, higher torque force/amps push the components physically harder, wires get hotter carrying more amps, where if a bearing is designed to for a life span of 1 million rotations, higher Voltage / power run them faster getting to the end of their life quicker.

(20-Aug-2017, 07:39 AM)Redazu Wrote: ...you're able to hover with say 1amp 12v on the 3s but it's using 2.4amps to hover with 8v then the 3s is better right? And it would run cooler while hovering?

Not necessarily... Amperage is also called - draw. The same motor turning a larger prop will 'draw' more amps to turn the extra weight, in addition to this there will be a higher draw due to the physical amount of air the larger prop is able to move. Remember KV is thousand rotations per volt, so because of the higher voltage of the 3s, it will turn the motor faster than the 2s. However, each rotation encounters a certain 'amount' of resistance from the air and due to the 3s turning faster, it encounters this 'amount' of resistance more often, drawing more amps to provide the extra torque to overcome that resistance.

Again please correct me..?

Yes but I mean with the same prop. With any given prop, it requires them to spin a certain rpm in order to gain the required amount of thrust to hold the given weight of the quad in the air. So just because the higher voltage makes the motor spin faster doesn't matter there because either way the motor is spinning at the same speed in order to hover.

So let's say the prop has to spin at 12,000rpm in order to hover. If it takes 1amp @12v to spin that fast to hover with a 1,000kv motor with only 40% throttle... then it's better than 2.4amp @8v with the same motor but at 50% throttle right? Because only the amps is what causes the windings to heat up correct? So less amps to do the same work is always better?

Now I'm sure things get more tricky when actually flying of course. Like if you try to full throttle a high kv motor with a high cell lipo then it's gonna try to spin at a ridiculous speed and won't be able to and it will require more amps to try to reach it and burn things out. But I'm just talking about hovering.

Maybe I don't have a correct understanding of basic electronics. If you have a wire rated at 10v 1amp... and you push 1amp 10v through it, then the wire would probably be a little warm right? Cause it's hitting it's max? So if you push 1.5amp 10v through it, it would probably burn? But what about if you pushed 500mA at 20v through it? Would it act the same way as 1amp 10v? Or would it heat up more?

(20-Aug-2017, 07:14 PM)Redazu Wrote: So let's say the prop has to spin at 12,000rpm in order to hover. If it takes 1amp @12v to spin that fast to hover with a 1,000kv motor with only 40% throttle... then it's better than 2.4amp @8v with the same motor but at 50% throttle right? Because only the amps is what causes the windings to heat up correct? So less amps to do the same work is always better?

What your saying does make sense, but if you look at the spec sheet of a motor, generally you will find that it actually runs more efficient at a lower voltage, just looking at the thrust table for the emax 2205's on BG... Max throttle on 3s = 20.7A draw, 712g thrust, 20A x 12v = 248 Watts which equates to an efficiency (Grams of thrust per Watts of power) of 2.87, with an RPM of 20080. On 4s max throttle = 29.9A draw, giving 1024g thrust, 30A x 16V = 480 Watts = an efficiency of 2.14 with an RPM of 24560. See that the amperage draw on 4s is pretty much exactly 1/3 higher than on 3s but, RPM and thrust produced is not as much as 1/3 higher which leads to the drop in efficiency.

(20-Aug-2017, 07:14 PM)Redazu Wrote: Maybe I don't have a correct understanding of basic electronics. If you have a wire rated at 10v 1amp... and you push 1amp 10v through it, then the wire would probably be a little warm right? Cause it's hitting it's max? So if you push 1.5amp 10v through it, it would probably burn? But what about if you pushed 500mA at 20v through it? Would it act the same way as 1amp 10v? Or would it heat up more?

No, if anything 500MAh at 20V would run cooler than 1A at 10V and 2A at 5V will be hotter again. The further a current has to travel along a wire the more resistance it will encounter, this resistance manifests as heat and lowers the amount of amps you get at the other end. Power lines use high voltage because volts are better at covering distance, once the destination is reached, the power is converted from high voltage to high amperage so it can do the work required by your appliances at home.

That's what I was thinking and yeah I see what you're saying. I wasn't looking at the efficiency really, I was thinking better as in less heat to kill your motors from hovering haha.

Like my problem is I have some 1105 6500kv's that do pretty good on 3s. They are fairly warm from cruising and kinda hot when flying hard but it's really hot here outside. I have some 1106 7500kv's that I hoped would be insane at the cost of the extra amps if I wanted a quick punch. I wanted to simply moderate how hard I fly it so not to push it too hard but still have the option. However these 7500's are like on fire no matter what I freaking do.

I've tried like 5 different props, smallest being some 1935's and even hovering they are so damn hot and I don't understand why. The 6500's are pulling 9.1amps max throttle with 2535's. The 7500's are pulling 14.7amps with same props max throttle but have insane more thrust doing so. ( on emax site there's multiple props that show pull around that kind of current for their own rated 2s batteries so apparently that's not abnormal?? ) But they draw less current than the 6500's to hover and somehow are just burning up?? I even put the 1935's on it and at max throttle they only draw 10.5amps and yet... still hitting around 160-170F just hovering around. They too hot to hold finger on just from running them 40% throttle from motor's tab in a cold room... are these emax motors just garbage? Before I lost my whole quad in my landlord's forsaken swamp of a yard he never has mowed down in years, I had some ipower 1104 7500's from amazon that ripped hard and were only a bit toasty back when it was cool outside. Blah...

Its at this point that my inexperience shows and my theoretical knowledge becomes useless!

The only reasons I can think of for getting so hot under normal loads is if there might be some intermittent contact/shorting between the motor housing and windings? I do also remember someone talking about loop times and if they update too quick then the 'damped light' braking system can kick in for a fraction of a sec when its not supposed, to making the motors work much harder than they should..?

But yeah, now I am going to sit back with my and try and learn from someone who might actually have a clue!

If the motors are being run from the motors tab and the quad is held still (with props) then the actual load at 40% throttle will be more like the real load at 70% throttle in flight.
If you run them without propellers, they will get hot anyway as there's nothing to cool them down and they will also be rotating much faster than they would with propellers fitted.

In real life, the amount of mechanical noise and how it is filtered is a major factor in how hot the motors will get. If the D-term is really noisy and this noise is being output to the motors then the constant changes in throttle signal will rapidly cause the motor to overheat.

Ambient temperature makes a huge difference to how hot the motors get in flight. Just ten degrees warmer can make the motor significantly hotter.

Yeah that's why I ran them in motor's tab to eliminate any software settings causing it ( unless some bl heli setting is doing it? ) And yes normally holding the motor still while spinning the prop will have the highest load but around 40-45% throttle is where it hovers at normally so that's the same as holding it still either way. The quad is sitting still in the air with equally matching force against it the same as me holding it still. I read about that dampening light thing but in bl heli configurator it doesn't have any setting named that. It's got breaking setting but I have it turned off. Timing is set to medium and I tried high just to see but no difference on that. As far as the screws touching, it came with multiple length screws and the smallest ones fit my frame perfectly. The tips just barely make it out of the motor base with plenty of room before touching the windings on these 1106's.

I did notice something strange though. With my IR thermometer, it always shows my #1 motor as like 172F regardless of what temp it actually feels. The moment I unplug the battery it goes back down to actual temp. But it does that on any motor I put on it and that one motor never feels any different than the rest and flies great.

If your flight controller can record a black box log, it would go a long way towards answering questions about vibration and tuning. It will also help explain why motor 1 is warmer than the others. Note that the rear motors always have to work harder than the front motors in forward flight due to the dirty air that they pick up.