Using RC motors on E-bikes [Archive]

[Miles]

So, is 6kW continuous the target?

 

[johnrobholmes]

For e-bike purposes ultra fine laminations are not so critical as for RC ones.
We don't want rpm in 20k+ or 30k+ range.

Speak for yourself please, I will take a motor that spins as fast as possible with thin lamination's that have less iron loss. 30k rpm is nothing a gear can't take care of.


Personally I am not a fan of outrunners. I just use them for convenience and price. Try to switch them at too high of a frequency, spin them very fast, or use them in bumpy situations and they fail. Efficiency is lower than an inrunner too.

 

[johnrobholmes]

Do we try to match Kv values?

Are you speaking of two motors vs. one motor, or the two smaller motors in relation to each other?

 

[Miles]

Two motors matching the Kv of a single larger motor, in this case.

 

[Jeremy Harris]

I was thinking of around 6kW to 7kW as the target power, and making a compound motor that would deliver around 5kW fairly continuously.

Looking at the budget motors on the Hobby City site, it seems that several are potential contenders for a cheap multi-motor set up in this sort of power range. The cheapest is a 2800W max, 215kV motor for only around $45 or so. Derating this to about 1800 watts and coupling four of them together, would give the sort of power I'm looking for at a very low price. Even the weight is competitive with big (non-RC) motors, as I reckon a 4 motor compound setup could weigh less than 4.5 kg (~10lbs), including all the metalwork to bolt the motors together. This compares pretty well to something like an Etek, which would be more than double the weight and cost.

I've no doubt that those cheap motors might develop bearing problems or other reliability issues over time, but they would provide a useful proof-of-concept model at a reasonable price.

I know that expensive controllers are generally a good idea, but I'm tempted to experiment with some cheap controllers for the proof-of-concept model. Has anyone any thoughts on these really cheap ones for a low voltage test setup: http://tinyurl.com/5ngdsy? I realise they may not be exactly wonderful, but am looking for a way to test these motors on the bench without spending loads of money on something that may not work out. My intention is to test at 24V maximum.

Jeremy

 

[johnrobholmes]

If you want the same speed at the same voltage and geardown, yes select the same KV.

Small differences in KV can be adjusted for with gearing, since more than likely you won't want to change voltage.






One thing that hasn't been mentioned is what happens when one system fails. If both motors are driving the same gear and one system fails, generally the other system fails too. ESC #1 frys, motor locks down, system #2 gets overloaded in a few seconds and the motor gets fried. Another issue is when one system "hiccups" and causes an out of synch moment. It can cause undue stress on the other system and fry the ESC.

 

[johnrobholmes]

The "super simple" ESC is made by hobbywing, same as the turnigy stuff. I have a few ground versions of the SS and they don't have good low speed control and they don't keep sync well. You are just asking for big trouble tying a few systems together.

But for $30 you could blow up $120 and still be under the cost of one nice ESC.

 

[eP]

For e-bike purposes ultra fine laminations are not so critical as for RC ones.
We don't want rpm in 20k+ or 30k+ range.

Speak for yourself please, I will take a motor that spins as fast as possible with thin lamination's that have less iron loss. 30k rpm is nothing a gear can't take care of.


Personally I am not a fan of outrunners. I just use them for convenience and price. Try to switch them at too high of a frequency, spin them very fast, or use them in bumpy situations and they fail. Efficiency is lower than an inrunner too.

I don't know why inrunners are better then outrunners in bumpy environment.
But i'm afraid high gear ratio gearbox for inruners will not be very efficient
too.
If i'm wrong let me know such efficient reducer from 20k+ rpm to 200 rpm.

So, is 6kW continuous the target?

It depent on battery prices i suppose.

Keep in mind the ESC software don't depend on kW number. Software and brand are the main ECS price componetnts. Hardware are relatively cheap now.
So ESC price don't depend on kilowatts (power out) the same way as motors price do.

 

[Miles]

One thing that hasn't been mentioned is what happens when one system fails. If both motors are driving the same gear and one system fails, generally the other system fails too. ESC #1 frys, motor locks down, system #2 gets overloaded in a few seconds and the motor gets fried. Another issue is when one system "hiccups" and causes an out of synch moment. It can cause undue stress on the other system and fry the ESC.

I'd assumed we'd connect them via overrunning clutch bearings.

 

[johnrobholmes]

I don't know why inrunners are better then outrunners in bumpy environment.
But i'm afraid high gear ratio gearbox for inruners will not be very efficient
too.
If i'm wrong let me know such efficient reducer from 20k+ rpm to 200 rpm.

Outrunners cannot be supported externally without an extra housing, there is only face mount available with exception to a few small motors. The front end of the bell is generally not supported either and will deflect when jostled. Some outrunners are available with a 4th ring bearing to keep the bell from flexing but it adds quite a bit of drag and limits the motor to under 8k or so. Flexing of the bell is generally the worst problem, and most manufacturers get around this by making a huge magnetic gap. I have destroyed so many outrunners from bell flexing it isn't funny, they are designed for use in airplanes and just don't take a bumpy environment.

A three stage gearbox is enough. The 3% loss in one gear stage is worth it to me. It more than makes up the efficiency difference between an AXI and a Nuemotor. Check out my build thread in the bicycle section for a rough sketch of what is being made.

 

[johnrobholmes]

I'd assumed we'd connect them via overrunning clutch bearings.

That would certainly remedy the issue. No regenerative braking for you!

I prefer freewheels over regen. I like going downhill fast.

 

[Miles]

I'd assumed we'd connect them via overrunning clutch bearings.

That would certainly remedy the issue. No regenerative braking for you!

I prefer freewheels over regen. I like going downhill fast.

I want both, but it's not an immediate priority

 

[Jeremy Harris]

Thanks for the tip about using over-run clutches, I hadn't really thought about the "hiccup" problem. It should be easy enough to fit clutch bearings into the pulleys, in fact in might simplify things a bit, mechanically.

I might just risk those cheap controllers for a bench test setup - if I blow one or two up it'll be no great loss.

Jeremy

 

[johnrobholmes]

Bench test you are fine. Real running they have horrible low speed resolution and won't accelerate very fast without losing sync.


Example- running a Hobbywing ESC I felt very unsafe and couldn't goose my bike too much or ask it to go offroad. Constant garbling from the motor. Running a Castle ESC I get nothing but a slipping belt and mad dash acceleration when I need it. Plus low speed resolution is 10x better, so is low speed control.

I would say that a Hobbywing ESC is fine for light power assist and nothing more. Once you want real power or acceleration it just won't deliver.

 

[recumpence]

I totally agree that a bike could be built for far less money than mine. I have $1100 in my original bike, then roughly $2200 more in e-bike equipment. However, my motor was $1,000. You could get by with a $150 too if you don't mind sacrificing a bit. The Plettenberg uses a can skirt support bearing and a pretty exotic layout (interesting wind, high quality materials, etc), but it is a boutique, niche motor. I like super high-end, so that is what I got. :wink:

I think I could replicate my system for half the cost or even less and sacrifice about 30% of the power and efficiency. So, my cost per unit of useability is not the best. I was willing to pay alot more for a little more performance. I like seeing just how far a system can be taken.

Oh, also, I think the point was made similar to this, but I just thought;

Let's assume a given motor was capable of driving a bike, but was running too hard and not efficient because of that. If you went with two motors, and could target them into their best efficiency range, yes, in that case I could see a huge benefit to this. You have to remember, I never ran motors that were too small. I always started with a motor that was adequate, but I wanted more. So, when I doulbed up the motors, I may have been running each below their ideal efficiency range.

That is one likely scinerio.

Matt

 

[Tiberius]

I was thinking of around 6kW to 7kW as the target power, and making a compound motor that would deliver around 5kW fairly continuously.

Hi Jeremy, hi everyone,

This is a very interesting topic, but 6 kW is serious power, is it not? I've just built a bike with 2 kW peak and that's fast. 6 kW opens up other possibilities. Some of you may remember a thread ages ago on electric hovercraft. Jeremy, are you thinking along those lines, or perhaps a flying machine? Or is this still for the motorcycle?

Nick

 

[eP]

You have to remember, I never ran motors that were too small. I always started with a motor that was adequate, but I wanted more. So, when I doulbed up the motors, I may have been running each below their ideal efficiency range.

That is one likely scinerio.

So it is case B as i've supposed.

But for that case you still can run them in the right way by lowering gear ratio.
As a result you get higher speed range for the same voltage and higher total load adequate for pair of them (to heavy for single one BTW)

 

[johnrobholmes]

Ok, how about this. I will test this out with two systems on my bike when the gearboxes are done. First test with one motor. Second test with both motors- same gearing. Third test with taller gearing to accommodate the extra available power. Then we shall see what happens. I see no reason why taller gearing wouldn't allow for using all of the available power.

 

[eP]

Outrunners cannot be supported externally without an extra housing, there is only face mount available with exception to a few small motors. The front end of the bell is generally not supported either and will deflect when jostled. Some outrunners are available with a 4th ring bearing to keep the bell from flexing but it adds quite a bit of drag and limits the motor to under 8k or so. Flexing of the bell is generally the worst problem, and most manufacturers get around this by making a huge magnetic gap. I have destroyed so many outrunners from bell flexing it isn't funny, they are designed for use in airplanes and just don't take a bumpy environment.

Ok. It could be an issue. But once again 8k rpm limit is still no problem for me. Maybe heavier (rigid) bell could adress the issue ?

A three stage gearbox is enough. The 3% loss in one gear stage is worth it to me. It more than makes up the efficiency difference between an AXI and a Nuemotor. Check out my build thread in the bicycle section for a rough sketch of what is being made.

Are you sure you get the same loss at each stage ?
I'm afraid you get higher loss at first (top rpm) stage.

 

[Fumesucker]

Since my funds are quite limited at the moment I'm considering an incremental build, start with an "adequate" motor/controller and add another and possibly another as funds become available.

A Castle Creations Thunderbird 54 is good for 54 amps continuous at 10 cells, which of course works out to around 500 watts or so.

I could run one on a single 12 V lead acid just to get a feel for things and my feet wet so to speak.

Playing with Scorpion's motor calc, it looks like a 3026-8 would be the most likely candidate from their line for my build.

I can get the two items on ebay for just a hair over $100 shipped.

Any thoughts?

 

[eP]

A Castle Creations Thunderbird 54 is good for 54 amps continuous at 10 cells, which of course works out to around 500 watts or so.

It should works out to around 1500 watts i hope :wink:

 

[Fumesucker]

A Castle Creations Thunderbird 54 is good for 54 amps continuous at 10 cells, which of course works out to around 500 watts or so.

It should works out to around 1500 watts i hope :wink:

No, those are NiCd or Nimh cells, I'm not used to thinking in Li cells.

 

[Jeremy Harris]

Hi Jeremy, hi everyone,

This is a very interesting topic, but 6 kW is serious power, is it not? I've just built a bike with 2 kW peak and that's fast. 6 kW opens up other possibilities. Some of you may remember a thread ages ago on electric hovercraft. Jeremy, are you thinking along those lines, or perhaps a flying machine? Or is this still for the motorcycle?

Nick

At the risk of going off topic (as this thread is supposed to be ebike related) I'm not thinking of using this motor on either my Tongxin-motored recumbent or as a replacement for the Mars motor in the Yamaha project. I can't yet reveal what the final application may be, as I need to test a few more things out first of all.

Suffice to say that if I can get a cheap, light and powerful multiple motor proof-of-concept going OK, then I will probably be aiming to build something a bit bigger than 6kW.............

The key to this is avoiding the use of more than 75V DC, 50V AC, to appease the safety mafia who may start citing compliance with the Low Voltage Directive), whilst still getting good efficiency. Multiple, high efficiency, low voltage motors is a way of still getting high power by using a "distributed architecture" model to avoid having very high currents anywhere in the power train (the plan is to use separate battery packs for each motor).

Jeremy

 

[johnrobholmes]

I don't have a dyno to test of course, so finding out exact numbers of an efficiency loss in drivetrain will be difficult. Best I could do is a no load test and compare it to the no load amperage of the motor. Right now my drivetrain really sucks, so it will be a large improvement for me.


I should say that I also want a lot of geardown range so that I can take my bike offroad. 19:1 is plenty fast for commuting but 120:1 with a motor spinning 35k is best for offroad. I can only get this with three stages. I am also shying away from outrunners because I have already busted three riding around on the roads. I will try another one or two before discarding the idea fully, but rubbing magnets is very familiar to me. I probably don't ride near as smooth as recumbence, as he said "this is just my experience in the matter".


Fumesucker- As long as you are expecting light assist the 3025 will work well for you. I have quite a few here and they are well built. You won't be able to hold it full throttle up a hill without gearing it 15mph or below, so be careful on throttle use. As for the longevity around bumps, that model does not have a ring bearing so keep an eye and ear out for scraping and wobbly cans.

 

[Miles]

Suffice to say that if I can get a cheap, light and powerful multiple motor proof-of-concept going OK, then I will probably be aiming to build something a bit bigger than 6kW.............

With or without powered wheels?