Using RC motors on E-bikes [Archive]
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John in CR
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My question regarding a dual motor rig is how wide is the operating band of good efficiency for these RC motors and do types of electric motors differ in this respect? I want to ride my bike like a car is driven when it comes hills, ie maintain speed to the extent possible. My thought is to use 2 of the $60 2400W motors instead of the $149 6kw motor. Then use a pair of clutch bearings to combine their output to a simple single drive train, whether it's a chain and sprockets, belts and pulleys, or planetary gears. That way I can use either motor or both with a relatively simple rig, giving me more power on demand for hills.
Also, how difficult is reversing these sensor-less motors...what about other brushless motors? I find the simplicity of the retro direct 2 gear system an elegant solution perfectly suited to electric motors that can be reversed. If it's relatively easy, then a dual motor rig in the high gear for most situations, while allowing a low gear for the steepest grades seems to make sense.
John
Also, how difficult is reversing these sensor-less motors...what about other brushless motors? I find the simplicity of the retro direct 2 gear system an elegant solution perfectly suited to electric motors that can be reversed. If it's relatively easy, then a dual motor rig in the high gear for most situations, while allowing a low gear for the steepest grades seems to make sense.
John
recumpence
1 GW
Reversing the motor is as simple as reversing any two of the three motor wires.
Your double motor setup with oneway bearings sounds very doable from my perspective.
Matt
Your double motor setup with oneway bearings sounds very doable from my perspective.
Matt
If you need a lot of power you could be dissapointed by $60 motors i'm afraid.John in CR said:
Very efficient motors should have very strong magnets and tight mechanical tolerances (high mounting precission/accuracy)
Two motors instead of single one is a bright idea, but you really need to know what you want achieve - clear goals.
John in CR said:
Light low diameter motors have low inertion, so it shouldn't be hard to reverse them. But the key issue would be efficiency - how much would be freewheel loss ? It shouldn't spoil efficiency at heavy loads, but at light loads it could be an issue i suppose.
Best regards
Miles said:
If you are not limited to 1kW power budget than we could calculate efficiency for 18% grade at 20km/h not only at 9km/h.
180N *5.55m/s =1000 W Uw=20V, P_out =~ 1140 W Iw=57A
I=57+2.4A =59.4A
U=20+59.4*.04=22.38V
P_in =1329 W
n=1000/1329=75.2% much better than 70.5% for 9km/h at 20% hill.
So you can see the right balanced choice is possible (for two gears only) if you are ready for some sacrifices.
So we should check that experimentally. What is the idle curent with and without extra one-way clutch.
We also should check how efficient is high geared transmission before make the final motor choice.
So we need much more data than we have so far.
John in CR
100 TW
eP said:
ScottClarke hit 52mph with just one of the $60 1/2kg motors I'm thinking of, so with 2 I doubt power will be an issue even at my weight. Plus at that low price spare motor(s) become a reality. see his thread at http://endless-sphere.com/forums/viewtopic.php?f=3&t=5156. I've yet to decide on what top end speed I want, though 35mph is about my comfort limit with my current upright frame. My primary concern wrt efficiency is heat and how to dissipate it to prevent destructive issues. My feeling is that with so much power from these little motors that they must be pretty darn efficient or else the heat would rapidly destroy them, and that's not the kind of feedback I see about them from the RC guys. Granted their load is nothing like with our bikes, but that gets back to Matt's concern too (what's really going on efficiency-wise during acceleration with these motors?). I'm not going to quibble about a few % efficiency. Instead I want impressive acceleration and a top end of my choice, along with being able to take on hills like a mountain goat instead of slowing to a crawl.
John
John in CR said:
Not power but power loss could be an issue i'm afraid.
If you use moderate efficient motors you would need dense gears to keep efficiency at moderate level, or you could get a huge power loss in other way.
If you use very efficient motors with ( strong magnets and dense winding -low Rm) then you will be satisfied with two gears or two motors at extremly wide conditions (loads) spectrum.
If you want high power than you don't need to care about Io (until it isn't extremly high) but you need low Kv and low Rm as much as possible.
Also transmission efficiency (high gear ratio - higher ratio lower ouput Kv at the same Rm ) is very important for you.
safe
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Perfection? 8-Speed? One Speed?
I made this up last year and was unable to locate the original thread I put it into. The idea here is that more gears improves the width of the powerband.
View attachment 1
It's pretty obvious that there is a point of diminishing returns with gears and the NuVinci (being a CVT) is the theoretical "ideal" of perfection. An 8-speed comes close and even a two speed is better than one. It will depend on the exact circumstances of your bikes needs to determine the number of gears that make the most sense.
Suffice to say a "One Speed" is always the worst...
P.S: The numbers on the left of the chart are watts and the idea is that we are looking at the power that can be delivered to the rear wheel after the gearing changes the motor rpm. This represents the "best case" scenario and does not include errors such as "Short Shifting" where the rider with gears selects the wrong gear as a result of the "Ev Grin". It's this rider error that makes ACL so attractive because it further refines the system and forces the rider into a more disciplined choice of gear selection.
Same Idea With 2-Speed
...I actually ride a six speed and it's gearing range is 200% and in my opinion 300% range is the ideal. Eight gears is enough and six or five would be enough if the range was wide. The NuVinci is unique in that it's range is 350%... which means you cover everything.
Wouldn't that be great?
Combine the RC motor with a CVT like NuVinci.
Maybe Optibike Version 2.0? :wink:
I made this up last year and was unable to locate the original thread I put it into. The idea here is that more gears improves the width of the powerband.
View attachment 1
It's pretty obvious that there is a point of diminishing returns with gears and the NuVinci (being a CVT) is the theoretical "ideal" of perfection. An 8-speed comes close and even a two speed is better than one. It will depend on the exact circumstances of your bikes needs to determine the number of gears that make the most sense.
Suffice to say a "One Speed" is always the worst...
P.S: The numbers on the left of the chart are watts and the idea is that we are looking at the power that can be delivered to the rear wheel after the gearing changes the motor rpm. This represents the "best case" scenario and does not include errors such as "Short Shifting" where the rider with gears selects the wrong gear as a result of the "Ev Grin". It's this rider error that makes ACL so attractive because it further refines the system and forces the rider into a more disciplined choice of gear selection.
Same Idea With 2-Speed
...I actually ride a six speed and it's gearing range is 200% and in my opinion 300% range is the ideal. Eight gears is enough and six or five would be enough if the range was wide. The NuVinci is unique in that it's range is 350%... which means you cover everything.
Wouldn't that be great?Combine the RC motor with a CVT like NuVinci.
Maybe Optibike Version 2.0? :wink:
It's not very hard at all to wire up a switch that turns a cheap 12v30ah LiFePO4 battery into a 6V60Ah LiFePO4 battery, and it's not very hard to flip the switch either. I don't think that this is useful for SLA batteries only. Low "C" rate lifepo batteries are the way to go if you want lots of range for low $, and it's what I have. Amps determine how much torque you get, and if you're running high electrical power but low torque you'll lose efficiency. Using numbers from http://ebikes.ca/simulator/ :
50A limit, 100% throttle, 33v A123 battery, eZee geared hubmotor, at speeds between 8mph and 16mph, efficiency progressively increases from 52% to 75%. Power drops off to nothing at higher speeds.
50A limit, 100% throttle, 66v A123 battery, eZee geared hubmotor, at speeds between 8mph and 16mph, efficiency progressively increases from 43% to 72%. Drop the throttle to a lower level and efficiency stays about the same. (at these low speeds, 100% throttle = 1500 watts, although your losses to air resistance are low, which they won't be at higher speeds).
I suspect these are the same sorts of efficiency differences you'd see if you were comparing a 1-2 ratio of gearing instead of a 1-2 ratio of voltage.
One of the complaints that the singlespeed bike crowd has about multispeed bikes is that you lose efficiency when your chain that must run through derailleur pullies and get pushed left and right (rather than be on a rear cog that is always directly behind the front chainring). Plus, you lose reliability.
Safe - I do think your idea of a 4-to-1 gearup to the motor and then a derailer system that allows anything from a 1 to 1 (high gear) between motor and wheel and a ~ 1 to 3 (11t - 34t) geardown would be practical and efficient.
safe
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"Throttle Fiddling" is a valid technique... hard to do in practice without some kind of gauge of your current and it's something that works against the "urge" to turn the throttle wide open.cerewa said:
Armature Current Limiting is essentially a "Throttle Fiddling" technique that is hard coded into the controller. If your current limit happens to be set at your peak efficiency current limit (usually very low compared to what people like to use) and you are using ACL then that's "perfection" when it comes to efficiency... a motor cannot be run more accurately at it's peak efficiency than that.
But when it comes to power it's not possible for a one speed to have power AND efficiency at all rpms. The more gears you have (all the way up to infinite with a CVT) the closer you get to a system where the motor is always running at it's peak efficiency all the time... or at least it can be like that... one is free to set the power level higher than the efficiency peak and just waste some energy.
We need to remember that what defines the electric motor is it's magnets... the magnets on a permanent magnet motor are "permanent" (thus the name) so we are always dealing with a motor that prefers to behave in a certain way based on the magnets. We really can never be "free" in an electrical sense until we control the magnet strength and then you have yourself an Induction motor. But the Induction motor is apparently impractical at small sizes... so we are stuck with simple magnets, powerbands, and ultimately gears if we want to improve things.
The RC motor is remarkable in that they use small magnets, but then crank the speeds up really high so that the smallness does not matter. If you tried to spin an RC motor at low rpms you would have no power at all... but at 10,000 rpms you've got a lot of power.
tostino
10 kW
I really don't think it would be that hard... I still am working on my e-bike, and don't have it in a ride-able condition yet. However, I do have a dirt bike, and I know that when I ride that (unless racing) I do not just open up the throttle and go. I use how ever much I need to reach a certain speed in a certain amount of time. I honestly think, that in the 4 years i've been riding my dirt bike, i've only fully opened the throttle a couple times (I guess it helps that it's a 40 horse motor though)...safe said:
I really don't see why on an e-bike, it seems necessary to just open it up and go. I know that's not how you ride a motor cycle, so why is that the mentality for a bicycle?
tostino
10 kW
How inefficient is the NuVinci? It may be somewhat heavy, but can you explain how it's less efficient than a regular 7-9 speed freewheel?Miles said:
The truth is, we don't know - but reading between the lines of the way that it is marketed.....
For sure, it will be less efficient than a derailleur. I doubt if its average efficiency is over 90% - probably less.
We need someone to test one at different power levels.
For sure, it will be less efficient than a derailleur. I doubt if its average efficiency is over 90% - probably less.
We need someone to test one at different power levels.
Miles said:
Safe is more funny than you are able to imagine how much he is.
He want to use extremly expensive CVT hub or derailer with cheap and crap very inneficient brushed motor - 75-80% efficient at max with high Rm.
Lets look at his powerband characteristics.
John in CR
100 TW
Miles said:
Wow, I didn't realize that thing was 4kg. My Comet Torq-A-Verter CVT doesn't weigh much more than that. If the belt holds up on my E-moto, then it may be a decent teammate for that 130Kv HXT.
safe
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No you could choose a high efficiency motor and get the same result. The chart was about POWER... it was in watts...eP said:
In all cases a one speed will be worse than a multiple speed... if you compare apples to apples (power and efficiency of the starting motor to be the same) then the one speed always loses.
I mean come on... there's a point where it's beyond debate as we've effectively proven this over and over again.
All permanent magnet motors are limited in what they can do because their magnets are fixed... it's just so basic and obvious that gearing improves performance and since I've got something like 4,200 miles on a multispeed ebike I know from riding experience that it's just the way it is....
safe
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Trust me... when you get your bike going and start to learn how these powerbands are different than gas motors you will see what I'm talking about. The first time I rode my bike I thought:tostino said:
"What the heck is this?"
...because the power has such bizarre behavior.
Once you ride you will say:
"Okay... I get his point... the power does drop off in the weirdest way..."
The "EV Grin" will hit you at low rpms in such a way that you will understand.
dirty_d
10 kW
safe is about power, he wants to have the max power possible to the wheel at any speed to maximize the acceleration, and being able to go the fastest possible speed in any situation, like up or down hill. the charts and stuff are pretty straightforward. but as a result you also get better efficiency too.
safe
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PMG132... A "Well Published" Motor Example
http://www.enigmaindustries.com/PMG_132/PMG_132.htm
The PMG132 is probably the most well published motor around that is commonly used in electric motorcycle racing. If we can't make decisions based on it then it's hard to find any common ground.
First we look at the efficiency at 48 volts using the prescribed (lower) 110 amp as a current limit. This produces an efficiency curve that "peaks" at 89%.
Now we take the same data and produce the same chart for the NuVinci verses 2-Speed verses One Speed and get a look at the actual power that will be delivered to the rear wheel based on the gear ratios in use for each type:
...we still don't see that much difference.
The summation on all this is that when power is limited (like with a legal cap such as the 750 watt legal limit in America) that the best way to increase power is to widen the power by using gears.
If there are no power limits then the easiest thing to do is just produce more power and then you can compensate for the inherent losses that exist with a motors powerband.
The main point is that even a motor running at 89% peak efficiency can't equal it's geared equivalent... there's still room to improve even when you are starting with 89% as your best "peak" efficiency value because all the improvement is in your midrange performance.
Obviously a NuVinci hub would probably explode with that much power, but you get the idea... a CVT does improve things a bit. Actually I did see one guy with a nice electric motorcycle using a CVT and his performance specs were a little better than the others. If I can find it again I'll post it here.
Found it:
http://www.evalbum.com/716
http://www.enigmaindustries.com/PMG_132/PMG_132.htm
The PMG132 is probably the most well published motor around that is commonly used in electric motorcycle racing. If we can't make decisions based on it then it's hard to find any common ground.
First we look at the efficiency at 48 volts using the prescribed (lower) 110 amp as a current limit. This produces an efficiency curve that "peaks" at 89%.
Now we take the same data and produce the same chart for the NuVinci verses 2-Speed verses One Speed and get a look at the actual power that will be delivered to the rear wheel based on the gear ratios in use for each type:
...we still don't see that much difference.
The summation on all this is that when power is limited (like with a legal cap such as the 750 watt legal limit in America) that the best way to increase power is to widen the power by using gears.
If there are no power limits then the easiest thing to do is just produce more power and then you can compensate for the inherent losses that exist with a motors powerband.
The main point is that even a motor running at 89% peak efficiency can't equal it's geared equivalent... there's still room to improve even when you are starting with 89% as your best "peak" efficiency value because all the improvement is in your midrange performance.
Obviously a NuVinci hub would probably explode with that much power, but you get the idea... a CVT does improve things a bit. Actually I did see one guy with a nice electric motorcycle using a CVT and his performance specs were a little better than the others. If I can find it again I'll post it here.
Found it:
http://www.evalbum.com/716
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