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Coninsan
27 February 2011, 1625
Hi

I'm just wondering, while tinkering with the Lennon Rodgers excel sheet, what kind of draw you would face at a given speed..

Lets say that your cruising downtown at 35 mph, which eqautes to roughly 2200 rpm on the motor. At this point, following its power curve, the motor should give off around 22 Kw, will the setup in fact draw this much power off the batteries or would it only draw according to the 1.4 Kw needed to push the bike along at 35 mph?

podolefsky
27 February 2011, 1654
The draw will be whatever is necessary to maintain the speed, plus a bit more to make up for efficiency losses. In your case, if you need 1.4 kW to maintain 35 mph, and your motor is 85% efficient, then the battery will be supplying 1.4 / 0.85 = 1.65 kW.

If you want to accelerate, you will be able to produce up to 22 kW at 2200 RPM. When you're just cruising along, there is less current draw, so less power used. If this weren't true, then you could use up an entire 5 kWh battery pack in about 15 min just cruising at 35 mph.

fixitsan
27 February 2011, 1657
Give or take a bit, yes the system will only draw the power required to push it along.

Power in this case is torque multiplied by a rotational velocity. If you imagine the torque required to turn the back wheel being equal to the loading times distance between the axle and the road, the power is that torque multiplied by the speed of rotation (traditionally in radians/second, but rpm nowadays !)

That total mechanical power is going to have to come from the motor, and assuming you aren't trying to make the motor generate massive torque at a low speed, or next to no torque at an incredibly high speed, or in other words you're in the motor's best operating speed range then after allowing a bit for efficiency losses the motor will not need to be made to generate any more than that same amount of power.

Chris

podolefsky
27 February 2011, 1700
This just made me think - if it takes 1.4 kW to go 35 mph, and I have a 4.5 kWh pack, then I should be able to run for about 3 h at a steady 35 mph. That's over 100 mile range! I'm going to start quoting that when people ask me. :D

Coninsan
27 February 2011, 1707
Thanks guys, now I can get this excel sheet done and get some sleep :)

Actually at 35 mph, my system needs 1337 watts too push it along in average danish weather. Which is a bit funny :p
But at 35 mph I've calculated that I should be able to drive 96 miles on a full charge, untill the LVC kicks in. And with a similar bike and battery pack, you could expect something in the same area. Which is pretty respectable.

podolefsky
27 February 2011, 1719
But at 35 mph I've calculated that I should be able to drive 96 miles on a full charge, untill the LVC kicks in. And with a similar bike and battery pack, you could expect something in the same area. Which is pretty respectable.

I think that might be possible on a very flat track with no accelerations or braking. In the real world, I'm getting more like 40-50 mi out of my 4.5 kWh pack. At some point I need to do a test where I drive a steady 35-40 for as long as I can, just to see.

Coninsan
27 February 2011, 1736
I think that might be possible on a very flat track with no accelerations or braking. In the real world, I'm getting more like 40-50 mi out of my 4.5 kWh pack. At some point I need to do a test where I drive a steady 35-40 for as long as I can, just to see.

Yup, you've got that right, that's how it is calculated, how long the battery pack will last at substained cruise at 35 mph.
Keeping a steady hand while driving might bring you close to replicating those results. But that is more likely to happen on the highway or country roads.
While driving home from a family get-together last night (in my moms car that is) I maintained a pretty steady 50 mph 80% which makes me believe that replicating the calculated results in a realworld environment might be within the realms of possibility.

Tony Coiro
27 February 2011, 1756
At some point I need to do a test where I drive a steady 35-40 for as long as I can, just to see.

You got way more discipline than me, even if it is for the sake of science. :D

Coninsan
28 February 2011, 0741
Wuhu! I'm almost done with this excel sheet, sweet.

But it has made me wonder again...

If the motor only pulls what it needs to at 35mph
Then what influence does the raduction ratio have?
- None, because you are the one controlling the speed and the motor outputs the needed Kw reguardless of RPM
- Some, since the power needed at the given RPM would change if the gearing was changed.

lugnut
28 February 2011, 0805
If the motor only pulls what it needs to at 35mph
Then what influence does the raduction ratio have?
- None, because you are the one controlling the speed and the motor outputs the needed Kw reguardless of RPM


For equal conditions (mph, wind, incline, etc) the power remains the same to motivate the bike regardless of the chain ratio. However the change in ratio will alter the RPM-torque ratio output from the motor to achieve the power point thereby requiring a change in the Volts-Amps ratio input to the motor. And this can be done with the controller by inputting a different PWM command from the throttle.

cycleguy
28 February 2011, 0834
I think that might be possible on a very flat track with no accelerations or braking. In the real world, I'm getting more like 40-50 mi out of my 4.5 kWh pack. At some point I need to do a test where I drive a steady 35-40 for as long as I can, just to see.

This seems more realistic, as it equates to roughly 100wh/mile which is typical for most electric motorcycles.

Coninsan
28 February 2011, 0903
For equal conditions (mph, wind, incline, etc) the power remains the same to motivate the bike regardless of the chain ratio.

Thank you, I thought so too. I just wondered because the Lennon Rodgers sheet's range predictions didn't change with ratio.


This seems more realistic, as it equates to roughly 100wh/mile which is typical for most electric motorcycles.

That depends. Faired bikes should have a lower Wh/mile given the less air resistance, and most Sports type bikes, where you sit hunged over, reguardless of fairing should have a lower Wh/mile than cruiser stile motorcycles.
But it's hard to theorize about since there isn't that much evidence yet to back it up.