Question about Current on motocalc.com?

[alpinesky1]

What does Current mean at http://www.motocalc.com/data/motor.html ?

Say im trying to compare the,
Hacker C40 8S
5250
3.400
0.0092
5.19
too the,
Hacker B50 8S
4613
3.500
0.0056
7.05

I see the B50 is heavier thats a givein.... But the others i dont get or am not sure.... :confused:

Constant = Kv or Max Rpm per Volt. (Right?)
Weight = Weight of the motor.

Resistance = (how effishent the motor is in converting enrgey to energey, the rest is converted into heat)(right?) :confused:

Current = (what??? explain this too me)? :confused:

Thanks
Gary

[ElectricThunder]

I'm takin a shot in the dark, but i think current is usually used to describe how many amps are being pulled by the motor from the battery. At least that's what i think it is, i'm probably wrong though. I'm sure it has something to do with electricity though. That help any? :confused:

[OptimaMan]

Now you guys are talking my language!!! Current is the no load current draw. Resistance is the resistance of the coils.

Want some interesting formulas? Iron loss is input voltage x no-load current. Copper losses is resistance x operating current ^2. When these two are equal, you have max efficiency. So, moving some formulas around,
the square root of (volt x no load current/resistance) is the amp draw at optimum efficiency.

So, the max efficiency at 7.2 volts of the c40 8 turn is at about 51 amps!!!! The b50 at 7.2 volts is at 66 amps!!!!!!!!!!!!

As you can see, the more current you pull, the more efficient the motor becomes. Now, in a 5 minute race using 3000 mah, you only average 36 amps so you want to find a motor that's more efficient at this range to maximize efficiency. Now you see why I'm always trying to tell people to get a slower motor (lower no load current, higher resistance) motor and put more voltage to it and gear it appropriately.

At certain amp draws, you can figure out how fast your motor is spinning: RPM=kV x (input volt - (motor resistance x no load current))

Interesting formulas huh?

[OptimaMan]

Sorry, forgot to add - resistance has no meaning as to overall efficiency of the motor. Just longer thinner wires have more resistance (more turns) than thick short wires (less turns). Lower resistance is more correlational to higher amp draw and not efficiency.

[Craps]

OptimaMan
Then tell me what to expect with my B50 10XL going from 14.8 volts to 22.2 volts?

Gearing changes?

Thermalling or motor/battery/ESC heat differences?

I am still waiting on the new 10C discharge rate 11.1 volt TP Li-Pos.

Thanks for all the information.

[OptimaMan]

Hacker B50 10XL
1578 RPM/volt
1.240 Amps at no load
0.0118 Ohm.

With 14.8 volts, the max efficiency occurs at 39 amps.
However, if you account for voltage drop, we're probably looking more at 12 volts or so so max efficiency is probably closer to 35 amps and spinning around 19000 rpm at that amp draw.

With 22.2 volts, and accounting for a couple of volts for voltage drop (say 19 volts), the max E occurs at 44 amps or so and spinning at around 30,000 rpm.

Now, how that translates to what gear ratio to use? Assuming you are using 8000 mah batteries, at 14.8 volts, you want to run the batteries out in approx 14 minutes to be at the motor's max efficiency. You're obviously running that motor at less than max efficiency since you're getting like 20 minutes of runtime.

Using 8000 mah at 22.2 volts, you should be running at around 10-11 minutes! (You'll be slightly above the max average draw of your lipos).

When you go to 22.2 volt packs, you'll have your battery and motor pretty much well matched. Max E of the motor occurs at max continuous draw of the pack.

Typically, max efficiency is around 90ish percent. At 1/2 that, you're probably looking at 80ish percent which is where you're probably at on average. However, some might argue that you want the motor at max efficiency when you're accelerating because that's where you use most of the energy. I couldn't tell you exactly but to me it seems you're in the right ballpark. Now, 20 volts at 40 amps is roughly 1 horsepower. Your average output is 1 horsepower which is much higher than a gas 1/8 buggy. Yeah, max output of the gas buggy is like 2-3 HP, but the AVERAGE HP is probably less than 1 and has much lower low rpm torque. So, you're definitely at an advantage.

[Craps]

Thanks for the info.

But with the new 10C 11.1 volt 8200 mahs batteries will give me around 82 amps @ 10C discharge rate????

I am planning on using less mahs due to higher voltage equals more power with less pull of the trigger to obtain the same speed. The batteries should not heat up up as bad due to the higher voltage and smaller gear. 12 tooth is as small as I can go with 15 teeth is the smallest to date. I have a 12 and a 14 coming in.

I just hope the power is controllable with the radio when I install the pair of 11.1 volt Li-Pos even though it is a heavy 4wd vehicle. Can you say too much power.

Thanks again and stay tuned!

I am out of town on business till next week and will not be able to test or race till 4/9.