Motor Kv & Calculated speeds

Question 1:
How well do these calculations hold up for real world e-longboards with a load?
I recently ordered the 149kv SK3 from Turnigy and the calculations @ 10s with a gear reduction of 3, give you still decent speeds (13m/s -> 29mph). But how do these numbers work out in the real world? I only weight about ~140lbs but in general that is still a lot of mass, large inertia.


Question 2:
Is it still safe for the motor to run it at lower voltage than what it's rated at? I'm pretty sure I've seen it is fine, but let us say HobbyKing says max 12s, so 6s would be fine too, just lower RPM?

Thanks!

1) I am new to this, but have a running board and from my experience it seems pretty close. Most of the energy is used in accelerating, it doesn't take a lot proportionally to keep going the same speed.

2) Yes, I think most are rated at max voltage. I have run my 192kV SK3 @ 12v and 50v.

Answer for Q1: Although, I do not have an electric skateboard yet, this depends on a lot of factors, for example the efficiency of motor,batteries, esc, bearings, wheels, aerodynamics etc.

Answer for Q2:
Thats correct, it will be fine BUT the motor power lets say is 1000w. Power=Voltage*I(current)
So, the motor will try to make that power at any voltage so if you run the motor at 6s the current will be double in contrast with 12s. That is a potential problem because the wires of motor and batteries have resistance R and the power loss from resistance is P=R*I^2 so:


The I for 6s is 1000/(6*3.7)=45amperes
I for 12s is 1000/(12*3.7)=22.5amperes

for the same setup lets say we have an R=0.3ohms in the wires of batteries and motors
so the power loss will be P=0.3*45^2=607.5w for the 6s
P=0.3*22.5^2=151.9w for the 12s

In conlcusion, prefer low KV motor. The 190KV is the sweet spot, and high voltage (10s or 12s) with a good esc like for example the VESC.

Good point on the amperage. I ran mine on 12v just for motor detection.

charles2317 said:

Question 1:
How well do these calculations hold up for real world e-longboards with a load?
I recently ordered the 149kv SK3 from Turnigy and the calculations @ 10s with a gear reduction of 3, give you still decent speeds (13m/s -> 29mph). But how do these numbers work out in the real world?

Click to expand...

You left out your wheel size, but something seems to be wrong with your calculations. I'm running that motor at 12S with 76 millimeter wheels and 2.4 gear reduction. My real world, level ground top speed is 25 mph. What size wheels are you using?

A rule of thumb I've found to be fairly consistent with my boards (spanning over 3 motors now, and a scooter) is that the theoretical speed at nominal voltage is about the max speed on full charge. If nominal voltage will leave me at 40km/h then I can expect this as a max, with about 5% leeway. This is using wee little 42mm sk3 up to the 63mm.

As stated, lower is fine. It wont make the same power, obviously, but the voltage limit is given as a maximum speed the motor can/bearings can safely do. The windings and magnets don't really get fussed about voltage (within reason), just the amperage.

SteveS said:

charles2317 said:

Question 1:
How well do these calculations hold up for real world e-longboards with a load?
I recently ordered the 149kv SK3 from Turnigy and the calculations @ 10s with a gear reduction of 3, give you still decent speeds (13m/s -> 29mph). But how do these numbers work out in the real world?

Click to expand...

You left out your wheel size, but something seems to be wrong with your calculations. I'm running that motor at 12S with 76 millimeter wheels and 2.4 gear reduction. My real world, level ground top speed is 25 mph. What size wheels are you using?

Click to expand...

Huh, well check my math here..

Scratch that. I was using the diameter instead of the radius!

So.. 149kv * 37V / 60(s/min) * 2 * pi * (0.08/2) = 7.69m/s to 17mph.

Note, 80mm kegel wheels.

This is a low rpm motor, would it hurt to use a smaller ratio, maybe 1:2 or 1:2.5 rather than 1:3 to get more speed?

I have a 149kv motor on 83mm flywheel clones with 16/40, 2.5 and it can go more than 17mph but not sure by how much since there are no smooth roads that are long enough anywhere ... , oh and it can also accelerate up to that speed going up hill! but thats on 12s.

get a vesc and 12s... 10s packs are hard to find/more expensive. unless you use the spacecell but yea $$


also if you care about range I get about 8 miles on 12s = 2x 6s 5200 mah lipo. 4.15 - 3.8ish...