pedal "assist"

[Lanchon]

hi stevo,

> 1) Re your note "the proportionality constant k1 is a parameter of the motor", is there any way to modify k1 for a motor?

only by physically altering the motor. for example removing loops from the windings, using weaker magnets, and removing some of the magnets, all would increase k1 (make it go faster on same voltage). but of course there's a reason people take the trouble of using these strong magnets made of rare materials: if you do any of this, you'd be killing torque (you'd be diminishing k2). like most engineering tasks, motor design involve trade-offs.

> For that matter, is there any way for me to find out what the k1 is for a particular motor?

with a test bank, but usually the manufacturer provides these basic constants. GM does better, it provides the output of a real life test run (for the HBS-36, see http://www.goldenmotor.com/hbs-36.pdf). please reread my long post, it tells you how to obtain the constants from this graph.

> 2) Re your notes about un-intended regen when w>k1*vbat, assuming I am pedaling fast enough such that w>k1*vbat, if I wanted to "limit" the un-intended regen, would it be better to keep the throttle on full rather than turning the throttle off?

in theory there should be no difference. in practice, zeroing the throttle could be marginally better (less regen). (let me say this without proof, you don't need it anyway.)

> 3) re your note "when entering this 'mode' there's really no current limiting in force", I find it odd that controllers are not made to be able to limit current in "regen" mode (even if they aren't regen controllers)

regen controllers should and probably do limit the current in regen mode, but this is not regen mode we're talking here, this is a parasitic and unwanted regen functionality that is a side effect of the power stage being the way it is. it's difficult to eliminate this parasitic function. this is a guess but probably all e-bike controllers display this property.

> because I'm sure it's quite common for riders to exceed k1*vbat e.g. on steep downhills.

actually they won't exceed it by much, their speed will be limited and excess "accelerating" force will be charging their batts instead of accelerating them. this happens regardless of whether they use a regen or non-regen controller, and regardless of whether they are aware of this. (btw, I've never seen this issue discussed anywhere before so most people must be unaware of it.)

> Is the risk of "short-circuiting the power input to the controller when the wheel is turning fast" a risk that people should be concerned about?

I really can't say. people are usually concerned about not missing "big brother". when you drive your car around town, should you be concerned that you're sitting on 60 liters of liquid high-yield explosive? should you be concerned that leaders of the most powerful armies of the world routinely invoke god to explain their actions? I don't know about you, but I'm not interested in starting any public awareness campaigns. and I guess people are already avoiding short circuits on the battery lines for other reasons anyway.

[Lanchon]

you seem a bit hung up on the w>k1*vbat thing. why don't you try it yourself? disconnect the batt, wait a bit, short the power input to the controller, lift the bike, and turn the wheel; it's entirely safe.

given that equation, breaking should start at w>0. however, there will be a (slow) threshold speed below which no breaking will occur due to diode voltage drops.

[Lanchon]

in 2006 I modeled the HBS-36 before buying it. I best-fitted the numeric data in the pdf into my model, so I should have all the constants laying around somewhere. do you really need this?

[Lanchon]

some graphs of the HBS-36. first is a torque graph; the circles are Philip's samples taken from the pdf and the continuous lines come from the model. it's a good fit but the model is more complex including aero drag from the freely rotating wheel for instance (regressed from the data). second graph is torque vs. speed extrapolated to zero speed.

(both assume full throttle with a 36V non-regen controller (max imotor 23.81A) fed by a 36.167 V source with source resistance of 0.024 Ohm (this is the real source used by Philip, regressed from the pdf).)

[stevo]

Smeee,
To your point, I was a bit overwhelmed by the equations at first but I don't really mind them.  I took engineering in University so I recognize all of the terms and if I think about them hard enough, I can understand them.

However, its also nice to have simple real world experience ie your point "i can over double my distance with a little pedal on the take off"

[stevo]

Thanks Lanchon.

> if you do any of this, you'd be killing torque
So would it be fair to say "if I modified a motor to be able to achieve a higher top speed, I would have to sacrifice torque which would mean that my hill climbing ability would be less and my overall acceleration (flats and hills) would be slower"?

> I'm not interested in starting any public awareness campaigns
I'm on the same wavelength as you.  I just wanted to make sure that the short-circuiting scenario you described wasn't something that happens frequently and would therefore be a safety concern.

Re the w>k1*vbat thing, based on your explanations, I'm okay with it now.
Re the graphs, thanks.

BTW, after all this, I ended up buying a Crystalyte 408 w/36V 20A controller and Lithium Polymer 36V 10.5 Ah batt. (since this is a Golden Motor forum, my apologies to any die-hard GM'ers)

[Lanchon]

> So would it be fair to say "if I modified a motor to be able to achieve a higher top speed, I would have to sacrifice torque which would mean that my hill climbing ability would be less and my overall acceleration (flats and hills) would be slower"?

not exactly; I said higher k1, not higher top speed. a fair amount of torque is needed when traveling at top bike speed. a higher k1 and lower k2 could just as easily increase or decrease top speed, depending on the torque vs. speed characteristic of the load. (one wants to imagine that these hubs are more or less tuned to a typical ebike load.)

> I ended up buying a Crystalyte 408 w/36V 20A controller and Lithium Polymer 36V 10.5 Ah batt

could you detail the costs? btw, I guess you are aware that the cobalt chemistry typically used in li-polys is quite dangerous. won't you consider using lifepo4?

[Leslie]

However, its also nice to have simple real world experience ie your point "i can over double my distance with a little pedal on the take off"

I studied electronics under some army guy in oz about 23 years ago and I could calculate the beta of a transistor as well as any one back then...

I Started to get back into it when I bought some evs.

My guess is that the ligthter batteries unlike mine :| wont advantage from the pedal power the same as I see the weight playing a big fat drain on my batteries, I do have 15 kgs of lead packed on my frame....  The advantage of the big batteries is to get more distance and the pedal power is used to off set the disadvantage of the weight.  Ive done over 80 kms in a single day with a few short charges between a few short trips and a long one.

I cant see 4X 12ah LiFePO4 doing the what my 3X18 ah sla's with pedals but I think it would be pretty even with out the pedals.

[stevo]

Lanchon,
I paid $465.95 CAD for the wheel/motor/controller and $535.00 CAD for the battery. 

I considered lifepo4 but its more expensive.  And my understanding is that the battery that I bought (from Its Electric) are Li(Ni-Mn-Co)O2 which are safer than pure Cobalt-based Lithium batteries due to the Manganese.

[Lanchon]

that's true, spinel is much safer. but lifepo4 is much safer than spinel in turn. however I think you ought to know that spinel batts have a terrible shelf life spec. apparently the manganese reacts with and kills the other electrode very quickly.

the problem is serious. thundersky made the LMP cells based on manganese; here are some comments from endless-sphere:


LMP - 40 Ah
There's a reason that the LMP's are so cheap. Their lifecycles are simply terrible. No way would you want to only get 300 cycles for this kind of money.

Apparently "LMP" is an acronym for "Limping" as in:
"I was riding my bike when the cells cut out and I had to limp home."

You are talking about the old "LMP" cells. Those were junk (you had to "limp" your way home) and were rated at only 300 cycles, but the new ones are called "LFP" and are now made with LiFePO4 and are rated at 2000 cycles.


note that these 300 cycles are spec'ed at only 80% DoD. but never mind the cycles, the problem is shelf life. the manufacturer test cycle life by cycling back-to-back in a short period of time. but these cells will probably corrode in a matter of months, even if you don't cycle them.

I'd strongly advice that you cancel the batt order if possible; strongly. btw, 500 USD is not cheap, you can buy trusty lifepo4s for less. see http://cgi.ebay.com/Ping-Battery-48V-12AH-LiFePO4-Electric-Scooter-E-Bike_W0QQitemZ220276121942QQihZ012QQcategoryZ11332QQcmdZViewItem#ShippingPayment. that seller and product seem very trustworthy. research it on endless-sphere.

[stevo]

crap.  too late.  I've already received the kit and battery (and started using it).  Oh well. 

[Lanchon]

let's hope your batt does well. when it finally dies it's to be hoped that you'll be able to get very cheap and proven lifepo4s.

[stevo]

Hi Lanchon,
If you're still available, I have some questions about one of your posts.  You wrote:

there's a rotational speed target "wt" that linearly depends on battery voltage "vbat" and modulation index "x" (0 to 1, controlled by throttle position):

wt=k1*(x*vbat)

(the proportionality constant k1 is a parameter of the motor.) think of it as if the throttle regulated the batt voltage from 0 to max batt voltage. so the throttle regulates target speed from 0 to max target speed (if vbat is constant).

My understanding is that one of the implications of this is that the power delivered by the motor peaks at a little over half of wt and then drops to 0 at wt. 

This seems like a very unintuitive design to me. I don't understand the purpose of having a rotational speed target "wt"  ie why aren't controllers designed such that they regulate the battery voltage from 0 to max battery voltage (and/or regulate amperage from 0 to max amperage) based linearly on the modulation index "x".  This way, the more you turn the throttle, the more power you would get.  So at full throttle, you would get full power from the battery/motor.

As an aside, does a controller actually regulate the battery voltage? or does it regulate the amperage? or a combination of both?