volting up and gearing down

In the Snow beast refurb thread DkAngel makes the above reference . . . . and it makes a whole lot of sense to me.

What I'd like to understand is how far it can be taken, the limits of "gearing down" are pretty obvious, but "volting up" is a little more difficult to get ones head around.

If a motor is rated at say 48v, what are the limitations on taking it to 72v and beyond (apart from the obvious mechanical things like the drive shaft and bearings coping with the extra power produced). Cooling is one that is apparent to me but I guess that it will be roughly proportional to power, but there must be others . . .

The only limits to increasing voltage (and RPM) is the insulation of the windings and phase wires (and your budget for controller and battery), and the ability of the structure of the motor being able to remain together.

The windings are probably good for at least a couple hundred volts.

The RPM limit of the structure...depends on the kind. Outrunners are higher than inrunners, generally.


Other than that, remember that the faster the motor spins, the greater the reduction needed to get down to a normal road speed. Beyond a certain point, you end up needing multiple reductions, because the difference in sprocket or gear sizes is too great to fit in the space, etc. And/or using chain becomes too noisy, so you have to go to belt reductions, etc.

Stuff like that.

The good thing about volting up and gearing down is that a much smaller and lighter motor can make the same amount of power than a bigger one spinning much slower, and it's easier to cool taht motor because you can mostly just stick a fan on the end of it to make it suck it's own airflow thru.

The bad thing is that it adds complexity, noise, etc., and the reduction system can weight as much as or be as large as the slower motor, depending on what you need out of it.


There's other threads about this, with similar topic titles, if it helps
https://endless-sphere.com/forums/search.php?keywords=volt*++gear*&terms=all&author=&sc=1&sf=titleonly&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search

Thanks very much Amberwolf, particularly for the search results, my searches are not that effective at the moment as I'm still figuring out the jargon . . . .

It's always a trade off. If you gear down too much and run the motor at a very high RPM, the iron losses will start to get large and overall efficiency drops. The trick is to gear such that your system stays near peak efficiency most of the time.

fechter said:

It's always a trade off. If you gear down too much and run the motor at a very high RPM, the iron losses will start to get large and overall efficiency drops. The trick is to gear such that your system stays near peak efficiency most of the time.

Click to expand...

Newer 0.35mm lamination motors don't suffer from this so much. At least in a 20" wheel. Go smaller and you might start seeing significant effects.

But to OP - when you drop a wheel size, say from 26" to 20", you drop speed, therefore you have to increase voltage to begin with. But at the same time, you can now produce around 20% more power from the same motor, so you can bump up the volts again to take advantage of that power.

The one similarity between gasoline engines and electric motors is that the faster they spin, the more power they make. Volting up is spinning the motor faster, and works well in any size wheel, to a point. But it works better in a smaller wheel than the motor was rated for

I just climbed a 24% grade with a 750w motor.. only possible on a 20" wheel.. otherwise the poor thing melts at the 1.5kw required to climb at a very sad ~15mph :lol:

Also remember that the controller has capacitors inside that limit your voltage, open it up and see what the labels state.
Also going to a lower Turn Count motor (3T, 4T vs high turn 8T), low turn is exactly the same as a High Kv motor (rpm/v) , they go hand in hand.
Thats also how you get your speed up for a smaller diameter motor. Also remember that your motor will run cooler!
Low Turn Motor means larger and shorter wire, can take more amps for longer. Low resistance in the windings.
then
High Turn Motor meaning shorter and longer wire, cant take the same amps for the same time. This category means there is higher resistance in the windings, higher resistance means gets hotter faster.

https://www.ebikes.ca/tools/simulator.html

https://www.ebikes.ca/tools/trip-simulator.html
Input Type: Google Maps, right click for starting location and another right click for end of trip. Then move the blue line to your actual route.

It would help a LOT, to know what motor you are talking about.

But assuming you mean a typical cheap hubmotor with 28 mm stator, then go ahead and run it in 20" wheel. It helps, but its not really all that much less gear. Not enough to make it turn too fast with your adult weight loading it down. Its kind of surprising how little top speed you sacrifice, by going to 20 inch rims. While you gain much better take offs, and climb hills with less motor overheat, and better efficiency.

And typically, 72v is fine, particularly if you limit amps to around 30. Time till motor overheat will be longer at 30 amps, and perhaps 35 mph top speed than if you use 40 amps at 72v, and have 40 mph.

But if your battery is only 72v 10 ah of lipo, then it will generally get used up before the motor melts, unless you stall it on crazy steep hills.


Don't go to slow motors, unless you mean to ride slow. Like 15 mph all day. If you plan to tow trailers, and never go faster than 18 mph, then a slow motor in 20 inch wheel is perfect. That is what I built this bike specifically to do. It was a tugboat, that typically went 15 mph to maximize efficiency, and run all day towing a trailer, on 200w average, plus me pedaling about 75v.

heat and Wh/mi

If you overvolt too much your cruise efficiency starts to go down, your motor instead of being able to handle let's say 2000 watts starts to hit maximum heat (about 110 Celsius for me) at maybe 1800 or 1500 watts.