I want to build me an electric longboard, and online, everybody has been using the turnigy aerodrive sk3 280kv outrunner brushless motor. They have been out of stock for months now, and I was wondering what specifications made it such a good motor for a longboard, so I could choose a substitute. The original website is here: http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idproduct=18175 I was planning on running it off of a few (4 or 5) 11.1 volt laptop batteries. I would also need a speed controller recommendation, because I am horrible at searching for them. The one I was planning on using is here:http://www.hobbypartz.com/07e-ezrun-150a-pro-esc.html but it is also out of stock... (I'm hoping to be able to go around 30MPH on flat/slight uphill...)
This COULD easily be seen as a "shopping question" BUT if you want something that works as well as possible and if you want to understand how and why it works and the limitations of the design, then it MUST be designed. Throwing enough money at it will work, but knowing where to throw the money is even better.
Maximum laptop batteries allowed in series:
Your motor specifies 6 to 10 series LiPo = 22.2 - 37 Volt nominal. (Cells are specified as 3.7V each but are as much as 4.2V when fully charged).
In all cases "=" means "~=" :-)
Each laptop battery = 3S LiPo (3 x 3.7V = 11.1V) so max in series allowed is 3.333 batteries to not exceed 37V. ie 3 max in series. So you can use 3S (3 cells) or 3S2P (6 cells) but NOT 4 or 5 batteries. (You can use as many as you wish if you are willing to exceed the manufacturer's specifications :-).)
The motor is rated at 37V, 60A, 1.5 kW. max.
The controller is rated at 150A continuous, which is very adequate BUT only 6 cells LiPo = 2 laptop batteries = 3.7V x 6 = 22.2V max.
You need a controller rated for 37V mean max (42V at full charge).
Bruce has commented on this well enough so I'll limit comment.
The ultimate measure of motor performance is power produced.
To achieve this you need the correct volts and amps
so battery has to be suitable
and controller must match.
To get range you need battery energy capacity - bigger is better.
To get the battery smaller you usually need lithium rechargeables (LiIon/LiPo, LiFePO4).
Motor rpm at peak and typical power affects required gearing - usually slower is better.
Direct drive motors for eg in wheels need to be very low rpm.
Power and speed:
Power loss due to drag ~~~= 0.6 x A x V^3 Watts
A in square metres, V in metres/second.
30 mph = 48 kph = 15 m^2.
1 m^2 frontal area gives power of 0.6 x 1 x 15^3 = 2025 Watts . 0.5 m^2 gives 1010 Watts.
Crouched to minimise frontal area you can probably get down to near 0.5 m^2 frontal area. Smooth clothes etc help.
So 1000 - 1500 Watts probably enough to get 30 mph if air drag only aspect.
Power to ascend a slope:
Power from height gain up a slope = mghv
m kg, g = 10 :-), V = m/s.
h = rate of rise per metre An angle of 1 degree rises at tan(1) = 0.017 m/m Assume m = 80 kg
So at 15 m/s power from 1 degree rise = mghv = 10 x 10 x 0.017 x 15 = 25 Watts.
Or about 100 Watts at 4 degrees.
Or power from slope energy gain = 1500 x tan(angle)
To express slope as 1 in N
N = 1/(tan(angle) )
At 4 degrees N = 1 in 14
From the above you can (hopefully) see that "windage" is the major loss at 30 mph. Even on a 4 degree slope power from slope rise = 750 Watts and windage losses aare 1000 - 2000 Watts. Be streamlined!.
Add to the above wheel friction losses and rolling friction (both low relatively)
At 1000 Watts in 3S laptop batteries = 9 cells = 33 volts nominal
draw a current = P/V = 1000/33 = 30A.
That's about the 10C rate (or more) for quality 18650 cells. Be sure they are rated for it.
At 2000 W that's 60A and 20C. Be careful.
You say LiPoly - I'll assume standard laptop battery 18650 cells here.
18650 cells are rated at say 3Ah typically. Varies with intended use , model, brand.
Using 3S2P laptop batteries = 9S2P 1850 cells total Watt.hours ~= 3Ah x 3.7V x 18 cells = 200 Watt hours. At 1000 W load that gives about 200/1000 = 0.2 hours = 12 minutes on the flat at 30 mph well tucked in. As cell Ah rating is at usual = lower loads actual capacity will be lower or MUVH lower. So really maybe 5 minutes.
Here is a Wikipedia table of Performance parameters of electric skateboards. Note that battery capacities are not mentioned. See page above table for a discussion of batteries. You'll see that NONE claim a 30 mph top speed (25 mph max). The windage losses increase by 70%+ going from 25 to 30 mph! ( (30/25)^3 ).
From 20 to 30 mph the windage increases by a factor of (30/20)^3 = 3.4 times.
So where I said above that you may get 5 minutes operation you may instead get 20 minutes +. At 20 mph and 20 minutes the range is about 7 miles. Take the speed down to a useful 12 mph (3 x fast walk) and windage drops by another factor of 5. Somewhere around here other losses such as small elevation changes and friction start to matter.
E&OE. YMMV. DTTAH. IANAL. NSFP.
The important parameters are (in order):-
Weight: this is the primary determinant of torque and power output.
Kv (rpm/V): provides the rpm required to work on a particular voltage.
Dimensions: (case diameter & length, shaft size, mounting holes etc.) which may be required to fit the application.
Resistance (Rm) and No-load Current (Io): determines efficiency curve. For highest efficiency these should both be low.
Number of Poles: high pole count (eg. 14) for low rpm direct drive, low pole count (2-8) for high rpm or geared.
Construction Quality: a well built motor may be better than one which is more powerful or efficient, if it can take more punishment and last longer.
Any brushless model aircraft motor that is about the same size, weight, and Kv will probably work about the same.