Ohm's Law

[BrianG]

Yeah, you kinda have to think of a motor as a variable resistor which is dependant on mechanical load.

When a motor is running, it's impedance is X, so Y amount of current is pulled with Z voltage.

If you decrease or increase the mechanical load, the impedance will change, so less or more current will flow with the same voltage.

This impedance comes from the coil wires, which has two components: DC resistance and inductance. DC resistance is the pure resistive component and is very small since a coil is simply a length of wire. The inductance is resistance that changes with frequency and called inductive reactance (formula is XL = 2 x Pi x frequency x inductance). Increase in frequency means an increase of inductive reactance. The total impedance is a vector sum of the resistance (0* phase angle) and the inductive reactance (-90* phase angle).

Now a coil with AC voltage running through it creates an electromagnetic field. This field expands and contracts, and while doing this, induces a counter voltage in the adjacent coil windings. This counter voltage is impeding current flow, which in effect, looks like added resistance.

Ok, so now you have a motor which can be mechanically loaded. At no load, a certain amount of back-EMF is generated with a given amount of voltage. This produces a current value. When you mechanically load a motor, you are still applying the same voltage, but since the motor is spinning slower (because of the load), less back-EMF is generated, which appears as less resistance, so more current flows.

Load a motor too much and the it stalls. This results in NO back-EMF and therefore the current is limited only by the wires/battery/ESC and the DC resistance of the coil wires (which you recall is VERY low). So, if a motor has 0.01 ohms of coil resistance, at 14.4v (4s), the current theoretically could be 1,440A (14.4v / 0.01 ohms)! Of course, niether your batteries or ESC or motor can handle this for more than a few milliseconfds without extreme heat (otherwise known as fire ).

So, it's not as simple as pure ohms law, but with a little study, it DOES all tie together.

I see that a lot of people use 1515 1.5d or 1y on 4S and use 1y as a moderate racing motor. But I want to use 6S, so I ordered the 1515 3D to get about the same rpms the the 1y 4S combo, so will I have about the same speed but about double as much runtime?

Originally Posted by Jeff Crosson

I see that a lot of people use 1515 1.5d or 1y on 4S and use 1y as a moderate racing motor. But I want to use 6S, so I ordered the 1515 3D to get about the same rpms the the 1y 4S combo, so will I have about the same speed but about double as much runtime?

the 3d is a bit slow on 6s the motor has a KV rating of 1360 which will give you a RPM total of about 30,000...I like to keep the total @ 35,000-40, 000, I have the 2.5d on 6s and I am happy with the results

Most people use the 1515 1y 2200 kv on 4S and that's 30,000 rpm. The 1515 3D 1360 kv on 6S in 30,000 rpm. So I'm right on. I'm racing it on a medium-sized dry track where too much motor isn't good.

Originally Posted by Jeff Crosson

Most people use the 1515 1y 2200 kv on 4S and that's 30,000 rpm. The 1515 3D 1360 kv on 6S in 30,000 rpm. So I'm right on. I'm racing it on a medium-sized dry track where too much motor isn't good.

I didnt know you are racing on a smaller track, I assumme gearing it for 35mph would be right....I am a basher so my mind was there....