Amp Draw question

[kendt]

Will a bigger motor drw more amps in the exact same setup as a smaller motor, because its bigger or will it draw less because its bigger and not working as hard??

[Jesse J]

Bigger? key factor is KV rating. The thing that will ensure lower amp draw is higher voltage in bigger lower KV motor.

[kendt]

I guess i was a bit vague by saying bigger..oops. I,m wondering if a cc1717 which is a 1580kv motor rated @120 amp', draw more or less amps in the same setup as a 4074 1400kv rated @80amps.

[lohring]

If the motor efficiency is the same the current draw should be the same at the same voltage and propeller rpm. Power required is determined by the propeller, hull, and setup.

Lohring Miller

[kendt]

Thanks. Thats what I thought but wanted to ask. I will check the efficiency's of the two motors and see how close they are.

[driftah]

so if I go lower on the kv in my df29 running 6s instead of 4s how much stress will it relieve amp wise?

[NativePaul]

Given that both motors are up to the task asked of them, the same KV and the same efficiency, the bigger motor will pull more amps as its rotor has more inertia to overcome when spooling up, also the boat will be heavier and have more wetted area increasing the amp draw again and slowing it down, so the negatives of using a motor unnecessarily large are more amps and less speed, on the positive side it will be more stable.

In your specific case the 1717 will pull a lot more current as it is significantly higher in KV as well as size.

Good luck checking the efficiencies of the motors, I don't believe either company have published dyno charts, as far as I know only Pletenberg have for there whole range although Neu have for a few of their motors.

Driftah, It takes X Watts to spin your prop at Y rpm (the rpm your present setup spins it at), in order to maintain Y rpm when increasing the voltage by 50% you will have to lower the KV by 50%, your prop spinning Y rpm Takes X Watts, and Watts are Amps x Volts and you have increased the Volts by 50% so the Amps will drop by 50%.
(note this only works when maintaining prop speed, if you went from 4s-6s on the same prop without reducing KV the amp draw would would not just increase by 50% it would be a FAR bigger increase than that.)

[lohring]

Notice that the prop rpm is the same in the above example. If you change the rpm, the power required varies as the cube of the rpm.

Lohring Miller