Motor length vs amp draw

[Alfa Spirit]

A brushless motor NEU 1512 have the same efficiency than a NEU 1521, the value is high, more than 92% .

[gearhd48]

Back in the old drag racing days, they used to say " there's no replacement for displacement" . Now days it's hard to say!

[bhorowitz]

Maybe I'm mistaken, but I always thought if you double the voltage, you cut the amperage in half.

[Alfa Spirit]

Maybe I'm mistaken, but I always thought if you double the voltage, you cut the amperage in half.

That' s right,

P (watts) = U (volts) x I (amperes)

[Speed3]

If you double voltage you cut the ampage in half if you was making a 1000 watts at 10v

And you want to make a 1000 watts at 20v then you cut the ampage in half.

That is not the case if you have an electric boat and using the same motor and same prop, then decide to double the voltage.

[785boats]

You beat me to it Speed.
Totally correct.
If you reduce the kv of the motor to turn the prop at the same RPM that it was doing on the lower voltage, THEN you will reduce the amp draw with the higher voltage.
If you leave the kv of the motor & the prop size the same, you will more than double your amp draw when you double the voltage because the prop will be spinning at twice the revs increasing the load on the motor.

[bhorowitz]

Thanks for that explanation.

[NativePaul]

What were the voltages? Amp draw vs RPM means nothing without a voltage alongside it to indicate the power? It obvously wasnt consistant voltage as you have more RPM with more load on the Leopard, and RPM will drop with increased load if the voltage is constant.

You are also comparing a RTR motor with an aftermarket motor, to me your data says that a Leopard is more efficient than an Aquacraft, which is no surprise. Compare the Leopard 4092 against a Leopard 3360 if you want to draw a conclusion about the efficiency of motor sizes.

[Alfa Spirit]

A brushless motor NEU 1512 have the same efficiency than a NEU 1521, the value is high, more than 92% .

This is correct with same brand and same range motors.

If you compare a poor Leopard and a NEU Motor, efficiency is not the same .

[Speed3]

The purpose of the data log was to show that size of motor does not necessarily increase amp draw.

And yes the leopard is holding the rpm better with the same load.

The voltage with the k42 is 7.4 leopard and 7.4 aquacraft.

[NativePaul]

It is the Leopard I was talking about, it is holding RPM better than itself!

"13389 rpm @ 24.6 amps on 2s with the k42.
13467 rpm @ 30.4 amps on 2s with the x642."

7.4 is the nominal voltage of the battery, I am talking about the actual voltage at the time of the amp and RPM readings, which is needed to calculate input power and compare efficiencies. It's not possible for the motor to provide the results above at the same voltage, the X642 reading was taken at a higher voltage.

[Alfa Spirit]

Use silicone gun oïl for the ball bearings ... LoL

[Speed3]

The 7.4v is the voltage for the k42.

The 642 was 7.7v with the leopard but I think that voltage was a little high because how can the voltage drop to 7.4v at 24.6 Amps.

And the same pack give 7.7v at 30.4 amps without even charging the battery.

[Speed3]

My cells have very low internal resistance.

I did a datalogged at 100amps with a 3s. And the voltage drop is .4volts.

That is if you start at 12v it drops to 11.6v at 100 amps.

[NativePaul]

The 7.4v is the voltage for the k42.

The 642 was 7.7v with the leopard but I think that voltage was a little high because how can the voltage drop to 7.4v at 24.6 Amps.

And the same pack give 7.7v at 30.4 amps without even charging the battery.

Either you measured the k42 later in the discharge cycle when the battery had less juice in it, or your figures are off. What about the voltages for the Aquacraft tests?