Originally Posted by Fourdan

Hi
Note the definition of Km
Km = Motor constant = Kt / sqr(Rm)
Kt (torque/A) is the reciprocal of Kv
Units can introduce some coeff
Km is a sort of "current capability"
Some are considering Kt^2 / Rm
Louis

Hi Louis

Yes, it's a pity that Km is not mentioned and used more often in these threads. I made a point of including Km in my efficiency calculator.

I know your Scorpion Calc also computes Km, although it only displays "Motor Index" which is Km / weight.

Therefore, I assume to get Km, one has to multiply motor index by weight. However, I don't seem to get a valid Km value if I do that.

Christo

Originally Posted by jammied

Interesting the dt700 bell dropped the Kv by 10Kv and the lo by .2 amps

Maybe the magnets are slightly stronger.

Originally Posted by Skylar

Hi Louis

Yes, it's a pity that Km is not mentioned and used more often in these threads. I made a point of including Km in my efficiency calculator.

I know your Scorpion Calc also computes Km, although it only displays "Motor Index" which is Km / weight.

Therefore, I assume to get Km, one has to multiply motor index by weight. However, I don't seem to get a valid Km value if I do that.

Christo

Hi Christo
Yes I have a coefficient for the "power/weight" motor index
Motor index = 91189000 / Rm / weight / (Kv^2)
Rm (ohm) , weight(g), Kv (rpm/V)
Louis

I'm still trying to figure out what all of the %'s on the calculators determine?

I see a Eta %, A %of Eta and system efficiency percent.

I have noticed that in the calculators that the omega is always dissipating more watts or has more motor loss wattage

Originally Posted by Fourdan

Hi Christo
Yes I have a coefficient for the "power/weight" motor index
Motor index = 91189000 / Rm / weight / (Kv^2)
Rm (ohm) , weight(g), Kv (rpm/V)
Louis

Thanks Louis. That explains it. And I'm not even going to ask where the 91189000 comes from.

Christo

Originally Posted by jammied

I'm still trying to figure out what all of the %'s on the calculators determine?

I see a Eta %, A %of Eta and system efficiency percent.

I have noticed that in the calculators that the omega is always dissipating more watts or has more motor loss wattage

Hi Jammied

Ok, I'll be as brief as I can.

In electronics and electrical engineering, efficiency is typically denoted by the Greek lowercase letter Eta (η). And since efficiency is expressed as a percentage, every motor will have an EtaMax, or maximum efficiency, since no motor is 100% efficient.

EtaMax usually occurs when iron loss equals copper loss. Amps at EtaMax should be self-explanatory. If you exceed this current level, you are trading (some) efficiency for hopefully more power at the prop.

System efficiency also takes into account the losses in the ESC and battery.

Christo

Thank's Skylar,

Makes more sense now.

Still uncertain of what % Is good or a good trade off?

wikipedia_Electrical_efficiency
wikipedia_Mechanical_efficiency

Efficiency governs maximum motor power - RCG

Vriendelijke groeten Ron

Originally Posted by jammied

Thank's Skylar,

Makes more sense now.

Still uncertain of what % Is good or a good trade off?

Jammied

Well, that's the million dollar question. When deciding on a power system for my models or for friends (that happens a lot) I like simulating the motors on Louis' virtual dyno, Scorpion Calc.

I usually go a few props larger than what coincides with EtaMax, until I find one that "looks right". But I'm usually guided more by motor temperature, than efficiency. So, I always look at the simulated temperature after say 2 or 3 minutes motor run time. That's usually where the temperature stabilizes with constant WOT.

If the temperature is too high, I ease back and choose an "easier" prop. That can be as low as 92% of EtaMax, but that's just how I like flying. I'm a speed freak.

Christo

Nice post Ron.

Christo

Originally Posted by Skylar

Thanks Louis. That explains it. And I'm not even going to ask where the 91189000 comes from.

Christo

Hi Christo
91189000 is my Visa Premier secret code !!
Louis

Hi Christo
In fact, my coeff is related to
[Kv Kt = 9549.2965 (rpm/V mNm/A)] ^ 2
Louis

Originally Posted by Fourdan

Hi Christo
91189000 is my Visa Premier secret code !!
Louis

Hi Louis

That's why I was afraid to ask.

Thanks for explaining.

Christo

I will havto look into the Km measurements and calculations. And revisit the LRK wind with 19awg. Think I have the pattern down now! And manuel v stated during the earlier days of this thread that he thought the LRK wind with 19awg might be a good wind if the bell clearance doesn't become a issue.

If your not real careful, I mean REAL careful, first thing you know you'll be a big time knitter.