Quick Question

[Marvin]

Now, it is generally accepted that same power, higher voltage (lower current) is more efficient than same power, lower voltage, (more current).

To have higher efficiency, you surely must have less resistance. R=V/I. If current increases (and as resistance and current are inversely proportional), resistance must decrease?

I understand that more current = more heat, which increases resistance, but surely, the above equation shows that resistance does not increase with current.

What have I missed?

[BrianG]

No, you have it right. Same power, higher voltage is better because that means current is lower and resistance is higher. High currents create larger voltage drops along various wires/connections (making less voltage available to the motor) and stresses the ESC and battery more.

Generally, to decrease current with higher voltage, you use a lower kv (higher resistance) motor, and/or gear down. Gearing down places less of a load on the motor and increases the inductive reactance, which is "AC resistance" to simplify it. Don't go by the coil resistance of the motor because that is DC resistance of the coil wire itself and present only a mere fraction of the current-opposing total resistance. Actually, the only thing DC coil resistance is good for is to calculate the current of a stalled motor. As you can imagine, the difference between a coil resistance of 0.001 ohms and 0.006 ohms is all but meaningless. At 14.8v, 0.001 ohms equates to 14,800A, and 0.006 ohms equates to 2,466A. No reasonable battery can handle those levels of current so you see it is meaningless. It's the inductive reactance from a spinning motor that really determines the current.

[Marvin]

So resistance isn't necessarily a bad thing. Heat is what kills efficiency - of course - and higher current means more electrical energy is converted to heat - which isn't factored into the resistance equation.

Ahhhhh. I get it now!


Probably.



Oh, and one more (unrelated) thing. What connectors are used on the MMP, or are the wires directly soldered to the board?

[BrianG]

Motor and power wires are direct soldered, just like the regular MM. The motor wires come with CC's female 4mm bullets, which are a bit shorter than the bullets that come on the regular MM.

[Marvin]

Thanks!