

Mar 01, 2013, 10:12 PM  

That is because they are usually spinning bigger props at higher voltage... Higher voltage and lower kV motors are more efficient as a system getting power from the battery to the prop, IME. However, Ken with motrolfly would be able to wind motors big or small to your kV needs. He wound me a motor to 216kV for my 71" slick to run on 12s, while others are using the same motor on 6,8,10s.

Mar 02, 2013, 01:00 AM  

A larger motor provides the potential for more power, but not necessarily more power in itself. Also, just because a motor is larger doesn't mean it is more efficient. Motor efficiency is dependent upon no load current and winding resistance, and each motor has a current level that will give maximum efficiency. Running the motor at a higher or lower current will decrease efficiency.
So you can easily have the case where the smaller motor and the larger motor both have the same maximum efficiency, just at a different current level. If the smaller motor is running at its max efficiency current and that current level happens to be below the max efficiency current level of the larger motor, then power will actually decrease with the larger motor running the same prop, battery and having the same Kv. Larry 
Mar 02, 2013, 01:17 AM  

Quote:
As for higherKv motors being more efficient, this isn't really specifically true. Motors with higher Kv values can handle more current since the lower phase resistance results in less copper losses for a given current flow (I^2(R)), but then you're working with totally different prop ranges due to the Kv shift. By adjusting voltage to leave the prop unchanged, everything actually ends up fudging out pretty much the same. For example, doubling Kv and halving voltage will result in double the current, the same power, and the same RPMs for a given prop and motor chassis (assuming optimal copper fill). There's no free lunch... UNLESS you want to toss aftermotor gearing into the mix, in which case there actually is free lunch if you use a motor with a high Kv value and gear it down to a useful level. Since a given motor with twice the Kv can take twice current (and so 2x the power for a given voltage), it's effectively got twice the powertoweight ratio as a lowerKv wind of itself. A 2:1 gearbox with that motor will mean you have twice the power available to the prop with only a little extra weight. As for the original question, the practical answer is no. A slightly larger motor won't give you any appreciable increase in actual performance. There's a long story there and I am pressed for time 

Mar 02, 2013, 02:01 AM  

As stated above, the simple answer is 'no' there should be no significant difference if all else is identical. To increase power you either need to spin the prop faster (higher Kv) or fit a larger prop. If the motor was larger and had the same Kv then it should be able to handle more amps than the smaller motor, so you have the option to use a larger prop.

Mar 02, 2013, 03:25 AM  

My 0.02c on this:
The larger and slower the prop the more efficient it is. If I want to increase the output of a 2000 KV motor by 2 times I only have to increase the KV (rpm per volt) to about 1.25 times. But look at what happens on the other side (electrical): Since your cell voltage is the same say 11.1v the input current has to increase by 2 times and a bit more as this results in a higher current and higher I^2RT loss (4 Times) for the original windings. To keep the current low we can increase the cells from 3 cell to 6 cells or double the voltage but the battery pack gets inefficient. On the propeller side with a smaller prop and a higher rpm we have a less efficient propulsion system. To get the most efficiency manufacturers use thicker windings (larger motor), Low KV and Larger Prop. 
Mar 02, 2013, 07:32 AM  

The motors are NOT identical if they are not approximately the same weight and Kv, so the question is really relating only to the weight or mass.
Motors do not produce power. Motors convert the potential, stored, energy of the battery into kinetic energy, the ‘work’ of spinning a prop, in this instance, and also an unwanted byproduct of radiant energy, heat. Three Examples: The only relevant difference in the three example motors is the mass. Derkum DPower AL5005, generic 5052512, 278g/9.8 oz. Turnigy 5055A400, generic 5055512, 310g/10.9 oz. Hacker A5012S, generic 4954507, 370g/13 oz. At 14.8v into the ESC the 278g Derkum turns an APC 14x7E at 6580 RPM. Pout is 269.7W. Vnet is 6580 RPM / 512Kv or 12.85V and Inet is 20.99 amps for a power system efficiency of 269.7W (Pout)/340.1W (Pin) or 79.3%. For the almost 1 oz. heavier Turnigy, with everything else the same, the RPM is 6172. Pout is 217.1W. Vnet is 6172 RPM / 512Kv or 12.05V and Inet is 18.01 amps for a power system efficiency of 217.1W (Pout)/297.3W (Pin) or 73%. The Hacker, the heaviest motor, with all other variables the same, turns the same APC 14x7E at 6871 RPM. Pout is 312W. Vnet is 6871 RPM / 507Kv or 13.55V and Inet is 23.02 amps for a power system efficiency of 312W (Pout)/373.6W (Pin) or 83.5%. All of the math is based on Drive Calculator (http://www.drivecalc.de) data and WILL vary from actual measured data at the user’s elevation AMSL (above mean sea level) and ambient temperature. The general trend is that the heavier motor with the same Kv will usually supply ‘more power’ to the same prop with all other variables being equal, as shown by a comparison of the Derkum and Turnigy to the Hacker. Remember that 'more power' may also mean shorter flight time. The comparison of the Derkum to the Turnigy demonstrates that this is not ALWAYS true, but it is the general trend. 
Mar 02, 2013, 07:42 AM  

One thing to bear in mind when comparing different motors is that two motors with the same claimed kv do not necassarily have the same Kv in reality. there are numeropus methods of measuring Kv and there is a fair variation in the results. many manufacturers also round the Kv up or down to arrive at a round number.
Winding resistance also plays a part in how many watts a motor will pull and generally for the same kv a larger motor will have more copper cross section area so will have less resistance and will probably then pull a few more watts. But in simple practical terms if you want to increase power simply putting in a larger heavier motor with the same Kv, and changing nothing else, is not the way to go. 
Mar 02, 2013, 07:46 AM  

Quote:
Hardly any wonder then that it spins the prop slower and draws less amps. 

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