RC Motor Efficiency

So, we see RC motors used for high-power, but there's also a good application for low-power, long-range. If it can handle 2000W for half an hour or an hour, it can definitely handle 500-1000W for a day.

Matt's recumbent gets excellent range, and 14Wh/mile IIRC, but I don't think that's the max.

The Astroflight motors get 93% efficiency at about 1.3kW and drop to 90% as it approaches max speed/torque/power.

What are these motors like before these high power outputs? Does anybody have some nice charts? If these motors are 90% +/- 6%, is there any benefit of gears?

There's enough power on these things to accelerate from a stop, but still get a max speed of 60-120km/h (~37-74mph), so is gearing worth it for long-distance?

How do you measure your efficiency?

Gearing the 6-turn AstroFlight 3210 at 48V down to 40km/h (25mph) takes a lot, especially on a 700c wheel (my current situation). Does using 48V make up for the mechanical loss of reducing by 2, over 24V?

I'm asking this for two reasons: personal interest, and R/C e-bikes are approaching car speeds, car horsepower ratings, and automotive quality (in a good way, not a Ford-quality way). Now, e-cars have the problem of range, but I see no reason why we can't be hitting 20-30mph for 10-12 hours. I personally have a 420mile trip I'd like to make by e-bike over two days.

What's needed in motor and battery efficiency for a practical car replacement, even if half speed?

Playing with one of the motor calculator programs is a good way to learn.
http://www.drivecalc.de/
http://www.micronradiocontrol.co.uk/scorpion_calc.html
http://www.peakeff.com/

Improving motor efficiency is a fairly small part of the long-range problem (Without batteries that cost a whole lot), and most reputable motors have efficiencies in the 85-95% range (Even my Turnigy motor has a peak efficiency around 80-83%), so there's not really that much room for drastic improvements. What's needed is reducing the energy consumption during travel and the absolute biggest one by far is air drag, for e-bikes and other light vehicles. One way of doing this is minimizing your frontal area (recumbents excel in this department) and the other way is improving your "aerodynamics" or the thing that mathematically embodies that, the drag coefficient. Full frontal fairings can have a drag coefficient 3x less than that of a normal bicycle, so given the same frontal area, it would take approximately 3 less the amount of power to go the same speed - it would also take about 3 less the amount of battery capacity, and the cost of batteries (in capacity) is usually the limiting factor for most people in range. Otherwise, I think most people would be just fine with carrying 20-40 pounds of lithium for an "all-day range".

Miles,
I don't know how I missed that program after all my googling and forum reading. Very helpful.

Although these graphs are estimates, I think it's safe to say these R/C motors lose efficiency quickly when outputting low power (<750W). For cruising speed, around 40km/h (25mph), I would only need about 500W output which is close to the sharp drop in motor efficiency. Also, for really long ranges, like 200 miles, that would be 4.5kWh, not something that can just be carried with 20-40lbs.

swbluto, I think you've answered a question I didn't ask, but I've been wondering for a while. Running RC motors through the gears is not an improvement. These high-power RC motors are not efficient at low power and low speed. But there are equally efficient RC motors for lower powers.

What do you think of a dual-motor setup, where one is used for starting and low speed, and one is used for high speed and cruising?

I apologize if I'm thinking out loud, more than contributing to the forum. One day I'll have an ingenious idea!

nsasch said:

Although these graphs are estimates, I think it's safe to say these R/C motors lose efficiency quickly when outputting low power (<750W). For cruising speed, around 40km/h (25mph), I would only need about 500W output which is close to the sharp drop in motor efficiency.

Click to expand...

I think I misinterpreted your observation, after looking back at the graphs. Disregard the original post I made (Well, except the partial throttle part). Anyways, just to make sure we're seeing things the same things, 500 watts output is at or right next to peak efficiency, no?

I think the graph that miles quoted was assumed to be 100% throttle. If you gear your motor so that 100% throttle is close to 25 mph, then the motor will actually be fairly efficient, since the wasted heat will be less (Even though the nominal energy efficiency percentage as measured by power-in/power-out may be low as the power output declines towards no-load) and the actual amperage should be somewhat low - in essence, gearing will determine a lot about how efficient your motor will be at a given speed.

But if you're running at partial throttle, the motor will see a lower voltage and so the 500 watt output will meet at a different section of the efficiency curve, so the motor may actually be pretty efficient at 500 watts output. The ESC, though, takes an efficiency hit at partial throttle of some 4 to 6% (And so do regular bike controllers).

But, the lower power that a motor consumes, it is typically less efficient but the peak efficiencies at different power levels are usually above 70-75%. You kind of have to have a pretty good understanding to apply Miles's graph to the context of e-biking. I think the graphs from my simulator are a little more intuitive and understandable since it actually overlays it over speed, instead of amps.

Click to expand...

swbluto said:

Anyways, just to make sure we're seeing things the same things, 500 watts output is at or right next to peak efficiency, no?

Click to expand...

Well, yes. 500-750W happen to be right in the range of peak efficiency, if this graph is correct. But just short of 500W (100-200) and efficiency drops off very quickly.

If you're running low on power, for example, it's almost better to go at 500W than 100W to get where you're going, since efficiency increases faster than aerodynamic drag increases, in that particular range only.

During acceleration from a stop, you're at high power, but low RPM. I'm not sure where gearing should be. I'm starting to think it might be best to just get an R/C motor set up.
*sigh* There goes my budget.

I am assuming you are running a Cat 700? Mine will be here in a week or so. I will be mounting an RC drive in it. I will be able to give you some observational data then. However, winter is fast approaching here. So, this may not happen untill spring. :?

Anyway, the problem with using the bike's gear range is the DRASTIC reduction required (normally 3 stages) to get to the correct chain speed for the stock gearing. Then you have one more stage to get to the rear wheel. That is 4 stages! You lose alot in 4 stages. However, with a left side drive, you can get away with two stages for an RC drive pretty easily. That is what I am pushing for.

Anyway, Lipo batteries are about 15 pounds per kwh (or less). So, 4kwh would weigh about 60 pounds. Not out of the realm of possibility to carry. Now, a decent RC setup can do 15 wh per mile at 25mph or better. So, you are looking at nearly 200 miles if you are very conservative with the throttle on one charge!

Realistically, though, you will be on the throttle occasionally, but you would probably be pedalling a bit. I would say to run a 2.5kwh pack and expect 18 to 20wh per mile for some faster riding. That will give you 100 miles on a charge.

RC drives are not cheap or easy to setup. However, they are in their infancy still. As popularity grows, price and ease of installation will improve.

Matt