TOPdawg said:
However, the true speed it will reach depends on winding and what not, and that is one idea I would like to understand a little more about.
Side-note, is it possible to make a DIY motor controller?
Welcome to the forum.
The true speed will be hard to calculate without a wind tunnel and some computer modeling. A good
guess at true speed isn't so complicated though.
To find the motor winding you need, first you need to know the RPM needed for your wheel at the speed you want.
A motor's speed is rated in "Kv", RPM per volt. A motor with a Kv of 10 does 10 RPM at 1 volt, 20 RPM at 2 volts, 480 RPM at 48 volts, etc.
Lots of math later, and your 22" wheel will do 15.28 RPM for every 1 MPH of speed, so 917 RPM for 60mph. So at 72 volts you would need 12.7361... call it 13 RPM per volt. That means a motor with a KV of 13 will hit 60mph unloaded speed at 72 volts actual.
But that's a problem, because the motor will be under load when you ride it, and batteries don't produce constant voltage, they work over a range. your 20S Lithium pack will run from 84v peak to 60v at max discharge, though roughly 2/3 of your batteries capacity will be between 80v and 72v. 72V is a good number to calculate with, but depending on your needs, you could design around a max speed of 60mph, using 84V, or a minimum acceptable max speed, using 60v.
Figuring out how fast you'll go under load depends on your motor's efficiency and how well the bike over come rolling resistance of the drive train (Tires, bearings, etc.), and how well it can push through the wind.
If you're on flat, smooth roads, and there isn't anything wrong with your bike, then the rolling resistance is almost negligible. you can mostly ignore this.
Your wind resistance is a bigger factor. On level ground, this is where maybe 85-90% of a bike's power is used. Without a wind tunnel or accurate computer modeling of your bike, this is impossible to calculate meaningfully. However it's a problem you can overcome by throwing Amps at it. with an 80 amp controller on that motor, this becomes almost a non issue.
So the last factor is the motor's efficiency under load. The Qs 205 V3 is rated at 90.8% efficiency at no load. Under load, that falls off. For your motor and assuming a fairly conventional bike design, you're probably fall into the mid to high 80s. Call it 85%. The more wind resistance you have, or if you have to go up an incline, the more this number will drop, but a 5-6% drop is a good estimate.
This drop is also how much slower than the rated Kv the motor will turn under load. So going back to the original Kv calculations, you now need to add 6% more to the Kv rating. Take that 12.7361 we started with, and adding another 6% gives us 13.5 RPM per volt. So a motor with a Kv of 13.5 or should give you 60mph at 72 volts.
As for building your own controller, Yes you can - if you are both an electrical engineer and a software engineer. Also if you happen to have huge amounts of free time with nothing better to do and a serious shortage of girls and beer.
(unless you already happened to have all the bits to do it and already had a working design and circuit board).
