Motor (System) Voltage

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.

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One response to “Motor (System) Voltage

  1. Some good points Ted. Turns out, the controller and motor for a 48V system doesn’t have to be that much beefier. You’ll notice that different manufacturers make controllers in the same size package at different voltages (e.g., Alltrax 4865 vs 7245). The lower voltage controllers are usually higher current. By the ratings, it’s not an even trade off…the higher voltage controller has an easier time making more power. But in your example above, you could substitute a 4865 for a 7245 and probably be OK.

    Motors should be similar. Lower voltage motors have fewer turns of thicker wire to make the same RPM. That means less resistance, so less heat produced. Whether it ends up a wash depends on the particular motor (I can’t find any good side-by-side comparisons in the specs out there, unfortunately).

    And the same goes for batteries. A 48V battery will have to put out 417A vs a 72V battery at 277A. BUT! You can use a larger Ah 48V battery and it will be stressed about the same as the 72V battery for the same power. In fact, larger Ah batteries tend to be more efficient on space, so a 48V 90Ah pack might end up being smaller and lighter than a 72V, 60Ah pack. (For both packs, the current required for 20 kW is about 4.6C).

    Anyway, in theory, you should be able to build either a 48V or 72V system with the same power output that is well balanced. In practice, with components in the real world, 72V systems seem to be capable of more power without melting.

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