The more I learn, the more I revisit some basics and start asking questions… it feels like questions I didn’t know enough to even ask, but I guess that’s how you learn. Anyway, I’m starting a dirt bike project, and I have a nice little 48V motor, and started wondering if maybe I should plan around a 48V system instead of the 72V I’ve worked with in the past bikes. This led me to ask, why not?
The answer, (no surprise) is about balance.
Everything starts with Watts (or horsepower, or however you want to describe “work done”). Let’s say you want to go a certain speed… you need a predictable amount of power to do that. Remember, Watts = Amps x Volts. I promise this is going to be Math that even I understand, so don’t worry. Just pulling some numbers out of my hat, let’s say we need 20kw to go 70mph.
At 72V, you’d need around 278A to get 20kw. 20,000 (W) = 72 (V) x 277.77 (A) Since my system, and most systems, can dish maybe 450A before getting melty, then I’m in my safe zone. My motor, too, is probably happy.
At 48V I need a lot more. Pulling 20kw from 48V needs 417A or so. 36V is even worse, at a little over 550A.
Not only is my controller going to have to be overly beefy, but everything else is too, especially my brushes and other parts of my motor. Can you do it? With the right gearing, I think so. But is it a good idea? Nope. If I want an ultimate goal of 70mph, I’m going to have to live with 20kw. To get there in a good, balanced way, I want to run higher voltage… not just because of what I thought before – higher voltage gives you higher RPM – but because it gives you a better balanced system, and all the parts are going to play nice.