Field Weakening VS Gear Box VS Higher voltage battery

[Miles]

The paper referenced by Miles on YASA design needs clarification.

Did I

Table III referencing rotor eddy current losses does not show one important variable, the effect of the number of magnetic poles that the motor could have/should have, etc. The eddy current losses would be expected to be less if the face area of the magnets is less. Thus, if you increase the number of poles by a factor of two or more, the eddy current losses would also decrease.

Clearly, they're optimising a specific design. There are many factors that affect the level of the magnet losses, starting with the specific slot/pole combination.

 

[macribs]

Looking at the prius chart, makes me wonder why would you not want to try just a low/high gear setup in addition to the FW controller. At the higher rpms you still have the same power, understood, but torque drops off drastically. You have 190 Nm at 3200 rpms and it drops to what looks like 40 Nm at 10000 rpms.

I think you may be overlooking the marginal gains you get from a multi-speed transmission unless you really need the low end torque. Lets consider an example:

if you did install a transmission with a 10:1 reduction and a second gear with a 5:1 reduction.
in the 10:1 (1st gear) you would get 2,000Nm at 100RPM, 790Nm at 600RPM and 295Nm at 1,200RPM
in the 5:1 (2nd gear) you would only get 1,000Nm at 100RPM, 940Nm at 600RPM and 395Nm at 1,200RPM

For anyone used to analyzing transmission torque-speed curves, it makes sense that you would shift to 2nd gear above 500RPM, or about 1/6th the top speed because that's where you have more torque available. Let's assume that the top speed of this machine is 120MPH (like liveforphysics' wicked quick electric bike). This means you should shift gears at just north of 20MPH.

Here's the real question, do you NEED the extra torque provided by the low gear? Personally for sizing the gear ratio of my electric conversion, I realized that I could spin the wheels at low speeds using a single ratio, and still hit a top speed of 127MPH, which is likely quicker than I ever will get it especially considering it's been sitting for a year... but I digress. Maybe I'm biased, but in my case, I had enough torque to overcome the traction of the rear wheels and I didn't need more than that.

If you can find a use for 2000N-m, then I can see the transmission being useful, or if the sweet spot of efficiency was more towards the low speed range, like it is for most engines.

This is all new to me, but I try to read up and get a clear understanding of the subject. Unfortunately sometimes technical english or phrases used makes that hard for an oldtimer like me who only learned conversational english.

To better grasp what you are saying here, what would a traditional 6 speed gearbox setup look like in your chart? If ruled too heavy and complexed then try use a custom 3 speed with a little smaller gap in between gears then what you used 10:1 and 5:1.

Will that even remotely change the chart and how effective the gearbox would be?

Also how to determent if one"do need extra torque"? There can be several limitations I can think of that leads one to opt for gears rather then a bigger motor. First of it might be cheaper to do a dog shift then to do a real high power motor.
Heat build up can be a problem. If riding nasty hills frequently causes heat build up you only have two choices. Bigger motor or gearbox.

What kind of motor/wheelsize/reduction do you use that tops out at 127 mph yet still have enough torque on low RPM to avoid melt down? Or do you live in Ohia or nothern part of Indiana maybe where that are hardly any hills?

I am heaving a really hard time understanding this - as a life long motor head has taught me low gears get you anywhere.
And in the world of electric motor there is a one motor does it all solution? Is your setup too expensive for common people or am I biased and stubborn using references to the aging world of ICE entertainment and transport?

Great if I am wrong and we don't need gearboxes. Less complicated builds, less weight and most likely more room for batteries.

A visual example with a different chart with more gears and closer ratios would go a long way to understand this.

 

[halcyon_m]

I hope this helps. This shows a 5-speed transmission. I hope it's clear that the gain in torque capability in the middle of the speed range is always highest with the 5:1 ratio after 600RPM, and the 10:1 ratio is the most capable until 500RPM. From this perspective, you could use a 2-speed transmission if you need both 2000N-m of torque down low and a top speed of 2600RPM.

In my case, the motors are capable of around 125HP each (using two), so you can see that with that power rating, it is not surprising to imagine that I might not need more torque at lower speeds when using a single ratio to give me higher speeds. Therefore, I don't ever perceive the need to need a lower gear. I'm also not saying that this is possible in every application, but for a motor with good field weakening performance, it is often possible to simply use one gear ratio and not multiple.

 

[speedmd]

Hi halcyon_m

Great topic, thanks again for the posts. One area I see the need possibly for a simple two or three speed gear box is in a farm tractor application. You have need for extremely low tilling or planting ratios with tremendous torque requirements, medium speed towing /hauling also with moderate /high torque and also some light road/ rural route low torque transport ratios.

Same for foresting equipment with power take off winches and the like. Something more like a 20 to 1, 10 to 1 and a 5 to 1 box should easily replace the current extremely inefficient hydro setups or the conventional 8 or 12 speed crash boxes.

In your chart, would the addition of something like a 2.5 to 1 be to tall a gear to allow efficient cruse speed and still allow for some hill climbing ability.

 

[halcyon_m]

Hi halcyon_m
In your chart, would the addition of something like a 2.5 to 1 be to tall a gear to allow efficient cruse speed and still allow for some hill climbing ability.

It all comes down to application. Can you give an example? I think then we can use fact-based solutions to determine what would be best versus 'inefficient' and 'horrible' as qualitative terms.

BTW, I like how someone mentioned "you can't recharge time, but you can re-charge batteries." I like this because it's true, but I'll take a more efficient design over a less efficient one, regardless. The only exception is if there is some requirement to burn a certain amount of energy. Otherwise, good design practices reduce weight, cost, and improve 'efficiency' of travel.

 

[zombiess]

The only exception is if there is some requirement to burn a certain amount of energy.

I'm doing an electric conversion of a Saturn V rocket, how many gears will I need to reach escape velocity?

 

[macribs]

Hi halcyon_m

Great topic, thanks again for the posts. One area I see the need possibly for a simple two or three speed gear box is in a farm tractor application. You have need for extremely low tilling or planting ratios with tremendous torque requirements, medium speed towing /hauling also with moderate /high torque and also some light road/ rural route low torque transport ratios.

Same for foresting equipment with power take off winches and the like. Something more like a 20 to 1, 10 to 1 and a 5 to 1 box should easily replace the current extremely inefficient hydro setups or the conventional 8 or 12 speed crash boxes.

In your chart, would the addition of something like a 2.5 to 1 be to tall a gear to allow efficient cruse speed and still allow for some hill climbing ability.

Hehe growing up on a farm I can tell you that for tractors the more gears the better. 2 or 3 speed simply would not do it.
Most farmers can't afford the biggest engines, or their fields can't take the weight of those heavy high performance tractors. So they opt for a mid size mid powered tractor and that tractor must do it all. Haul heavy flatbeds filled with 40-50 tons, often in steep terrain, up dirt roads in any weather conditions. And they most do transport haul over longer distance going as fast as possible on paved roads. There is no substitute for multi gearboxes with hi/low and a broad range of ratios.

And that leads me to e-bikes. We are also constrained in many ways. We have limited space, we try to keep weight low, often limited budget and yet we need both high speed for long commutes and the ability to haul a heavier goods on a trailer or climb steep hills. We would like best possible acceleration, decent hill climbing abilities and a top speed that way faster then the EU scooter/moped @45km/h. For motors we use cryst 5403, cromotor, crown, hubmonsters or even RC motors. The best performers of these motors can put out peak performance north of 20 KW, but that might lead you onto a path of extreme cooling required.

So for an e-bike lighter or on par with the Stealth Bomber how do we manage all that? Motor wise there are not al lot to choose from south of 1.000 $ mark. And most of us do not have the cash to spend on a MC motor, nor the real estate inside the triangle without sacrifice other valuables like battery space, frame width that will allow you to pedal your bike or even weighing down the bike too much.

We know the Bomber has great speed. Decent acceleration too. But it does not climb very well without heat build up.
In the Stealths defense you have a great vbox that will allow you to help a little and you can use your legs to improve a little. But for many the pedals are more just for show then for heavy use. To avoid the police.

Is there really a way to avoid gearbox for e-bikes and if so what kind of motors will handle all that?

 

[speedmd]

Hehe growing up on a farm I can tell you that for tractors the more gears the better. 2 or 3 speed simply would not do it.
Most farmers can't afford the biggest engines, or their fields can't take the weight of those heavy high performance tractors. So they opt for a mid size mid powered tractor and that tractor must do it all. Haul heavy flatbeds filled with 40-50 tons, often in steep terrain, up dirt roads in any weather conditions. And they most do transport haul over longer distance going as fast as possible on paved roads. There is no substitute for multi gearboxes with hi/low and a broad range of ratios.

Understand the problem well. Born a farmer and still use a compact tractor on a small plot. This is a great application for electric, neighboring towns and residences. Love the little diesel but the stink I could live without. Look at the range of speed/ torque the electric yields. No need for 4 or eight low and another set in high range. The diesel has a extremely small power band in comparison to the electric in the example. 3 or worst case 4 gear steps should cover most everything better than conventional setups.

It all comes down to application. Can you give an example?

Small farm tractor (2-4 tons) as mentioned. Just add a 2.5 to 1 ratio to the above chart, we can work backwards on the suitable final drive ratios.

 

[halcyon_m]

Ok, so we are justifying a gear box by using the example case of a tractor. Does this mean we are done talking about ebikes?

If so, can anyone say what the necessary load profile is of one tractor use case is? I fear we are just going to find that with an extremely tall gear like 100:1 you get tons of torque and a 4MPH top speed.

You could always stop to shift in and out of this gear and its largely useless on road.

Its fairly clear that a tractor that needs to drive on-road is one application where the two use cases justify a larger range than a single ratio will allow. Is that what we are looking to prove?

 

[speedmd]

Tractor is the extreme, understood. 3- 4 MPH max in low gear. But the overlap into bikes/quads is still there. The example of a trials bike is a good one which should be covered in a 2 speed setup well. Freeride- enduro/rally machine that requires a very wide range of torque availability. Interested in what the torque curve looks like on the same chart with the three 50% increases in reduction steps 10:1, 5:1, and 2.5:1 Agree, smaller steps do not look to be at all justified which is a huge improvement to the current ICE setups.

 

[macribs]

I hope this helps. This shows a 5-speed transmission. I hope it's clear that the gain in torque capability in the middle of the speed range is always highest with the 5:1 ratio after 600RPM, and the 10:1 ratio is the most capable until 500RPM. From this perspective, you could use a 2-speed transmission if you need both 2000N-m of torque down low and a top speed of 2600RPM.

In my case, the motors are capable of around 125HP each (using two), so you can see that with that power rating, it is not surprising to imagine that I might not need more torque at lower speeds when using a single ratio to give me higher speeds. Therefore, I don't ever perceive the need to need a lower gear. I'm also not saying that this is possible in every application, but for a motor with good field weakening performance, it is often possible to simply use one gear ratio and not multiple.

Thx for this halcyon, and for bothering putting in the effort to school a newbie.
I will take my time and look at it at length tonight. When it is more quiet around me I am sure this hard to grasp subject will make more sense. Need to be focused when reading this, toddlers running around not ready to hit the sack just yet makes it hard to focus on anything

 

[macribs]

Ok, so we are justifying a gear box by using the example case of a tractor. Does this mean we are done talking about ebikes?

If so, can anyone say what the necessary load profile is of one tractor use case is? I fear we are just going to find that with an extremely tall gear like 100:1 you get tons of torque and a 4MPH top speed.

You could always stop to shift in and out of this gear and its largely useless on road.

Its fairly clear that a tractor that needs to drive on-road is one application where the two use cases justify a larger range than a single ratio will allow. Is that what we are looking to prove?

Still on the e-bikes mate
What I tried to point out was the restraint on an e-bike build. There are so many limits we can't exceed in order to meet our overall requirements. Yes we like speed. But we also fancy acceleration. And the old mighty littered my area with hills so I can't really avoid them. I could go big n heavy with vast amount of batteries and a motors that costs and arm and a leg. But then I will not meet the overall goal.

Light - nimble - responsive - fast.

Must say I've been thinking that getting an all in one e-bike is maybe not doable. I will not cave yet tough.
Maybe I must lower my spec sheet or even look at ways of splitting the battery pack to shave some weight for trail use sacrificing range for the extra fun just in that setting. And I have not completely buried the idea of water cooling the motor so that could also help with the heat build up.

To meet those goals what motor would be best suited? A DD hub as a mid motor or a twin RC motor from recumbence?
Avoiding gearbox will free up space, weight, build time and money that could be put into a motor(s).

 

[halcyon_m]

In regard to the OP, I hope we have sufficiently explored that. If we want to start an optimization/modeling/simulating/planning for individual builds, that's probably best for a new thread. I'd be happy to contribute as I can to that. I think some of the commentary about KV and controller size become more relevant then when the 'rubber meets the road' in terms of design parameters and use requirements (hills, acceleration needs, range)

 

[macribs]

I agree moderators feel free to remove my last couple of posts here. They do kind of clutter up this thread as they stray off topic.

I have another thread about gear boxes etc. So I posted my previous post there if someone would like to join in, caugh *halcyon_m et al* caugh.

http://endless-sphere.com/forums/viewtopic.php?f=28&t=63370&p=956327#p956327

 

[liveforphysics]

Tractor is the extreme, understood. 3- 4 MPH max in low gear. But the overlap into bikes/quads is still there. The example of a trials bike is a good one which should be covered in a 2 speed setup well. Freeride- enduro/rally machine that requires a very wide range of torque availability. Interested in what the torque curve looks like on the same chart with the three 50% increases in reduction steps 10:1, 5:1, and 2.5:1 Agree, smaller steps do not look to be at all justified which is a huge improvement to the current ICE setups.

Even for tractors or trials bikes or other bizarre niche applications, you're still doing it wrong if you're adding stages of loss, inefficiency, noise, wear-parts, failure modes, back-lash etc.


Liberate your mind from the confusion of thinking in terms of motors being like gasoline engines.

Liberate your mind from thinking you need more "stuff" twirling around rubbing on itself wearing out and making noise to deliver the torque you're looking for to your wheel. It is unneeded, because the process of making torque from electricity isn't limited to this range of handicap's gasoline engines suffer.

If you wish to have 1000-ft-lbs of torque at your rear wheel, and still have a 150mph top speed, it requires only selecting the appropriate motor geometry to suit your needs. If you have some application where you desire 100,000ft-lbs of torque or something, you can still achieve this without needing any stages of loss grinding teeth against each other to waste your energy in the form of wearing itself out.


Perhaps it will help if you cease viewing the motor as the "source" of the drive energy, and start viewing the electricity form as your drive energy, and your motor as the device that can be designed around transforming this drive energy into exactly the needs of the application, always in a single stage of conversion if designed correctly. Then you can cease looking at it from the perspective of what you do to help some shitty motor choice get the wheel to turn, and instead embrace that the electricity has no non-self-imposed limitations on the motion it can be converted into, and that it will always inherently be highest efficiency to make that conversion in 1-step, as any and every series step can only add loss.

 

[Punx0r]

I hope this helps. This shows a 5-speed transmission. I hope it's clear that the gain in torque capability in the middle of the speed range is always highest with the 5:1 ratio after 600RPM, and the 10:1 ratio is the most capable until 500RPM. From this perspective, you could use a 2-speed transmission if you need both 2000N-m of torque down low and a top speed of 2600RPM.

Great example

Putting this into a scenario I could easily comprehend, I thought about my ICE car, which has a modest 200HP and approx. 200 ft.lb torque peak. On a dry road with good tyres I can put down this torque in second gear (approx. 50mph). With the final drive that's a total reduction of about 8:1, giving 1600 ft.lb at the wheels (2170 Nm).

So in this example a 125HP electric motor is giving about half the torque of the ICE. Double up the power rating for the electric motor for a fairer comparison and you have ~2000NM with a fixed 5:1 reduction and would be traction limited until about 50mph in my car - a 10:1 reduction would be useless!

 

[speedmd]

Even for tractors or trials bikes or other bizarre niche applications, you're still doing it wrong if you're adding stages of loss, inefficiency, noise, wear-parts, failure modes, back-lash etc.

Luke, I understand where your coming from and agree that simple is most always best. Understand that most all applications requiring tremendous power are using some sort of gear reduction for many different constraints. Locomotives for example.

I don't see this changing any time soon. Also used in many rock trucks. Tesla also went to final drive gear reduction. I would just expect it to follow suit in tractors as well rather than going to a overly Large motor. It may be as you say, best to go to something that does not need any extra stages.

 

[zombiess]

I don't recall Luke arguing against a single speed reduction, just multiple stages of reduction.

In that locomotive diagram it looks like the motor was made a big as they could get it without having to go to using a chain drive which would be less efficient, have more wear and higher overall costs. By using single stage gear reduction they are maximizing their power transfer an minimizing wear parts.

If they could fit a bigger motor that directly turned the wheels without any gearing it would see an increase in efficiency, but it doesn't appear practical. If those parts are to scale with each other, the motor diameter looks to be bigger than the wheel diameter.

Just look at how simple it is.

 

[speedmd]

They are scale. Yes huge. 5500 horse power. Thousands of volts also. I am told they greatly benefit from the reduction gearing for towing. I am not suggesting a gear box would improve this perfectly optimized setup. However, if more work can be done with the same unit/ setup only adding a different ratio to the other side of the motor / wheel assembly and have a sliding shuttle dog engagement setup that would engage one set of gears or the other side, it could pull heavy loads and also run high speed lines with lighter loads possibly with more efficiency. It is clear from the graph that the 10:1 is much better at pulling stumps, and the 5:1 takes over quite nicely over 500 rpms. I think the challenge is to do this in a way that is as simple and efficient as possible for something that is dual purpose and needs extraordinary torque at low speed and is still able to cruse at speed efficiently and effectively taking max advantage of the power plant. Just a thought.

 

[liveforphysics]

I don't know why this is difficult to grasp my friend. That train uses a spur gear like an inboard car motor uses an axle. Only this is being done to allow the whole axle area to be in a massive plain bearing for weight handling reasons.

If they simply needed another 2x torque, it's drastically easier than adding the complexity and new failure modes just for 2x more torque which could be made by simply growing the motor radius and/or width.

 

[zombiess]

If they simply needed another 2x torque, it's drastically easier than adding the complexity and new failure modes just for 2x more torque which could be made by simply growing the motor radius and/or width.

Or they could add a 2nd identical motor to another axle or to the same axle if the mechanics permit. One larger motor with 2x the power of a single motor will be more efficient. I bring up the 2 motor setups because I've seen dual RC and hub motor drives on here.

 

[Punx0r]

They are scale. Yes huge. 5500 horse power. Thousands of volts also. I am told they greatly benefit from the reduction gearing for towing. I am not suggesting a gear box would improve this perfectly optimized setup. However, if more work can be done with the same unit/ setup only adding a different ratio to the other side of the motor / wheel assembly and have a sliding shuttle dog engagement setup that would engage one set of gears or the other side, it could pull heavy loads and also run high speed lines with lighter loads possibly with more efficiency. It is clear from the graph that the 10:1 is much better at pulling stumps, and the 5:1 takes over quite nicely over 500 rpms. I think the challenge is to do this in a way that is as simple and efficient as possible for something that is dual purpose and needs extraordinary torque at low speed and is still able to cruse at speed efficiently and effectively taking max advantage of the power plant. Just a thought.

Going back to the example of my own car, I get about 10:1 total reduction in 1st gear. Idle speed is 800rpm and this is the slowest the car can go, which is about 5mph. The above scenario implies using 10:1 reduction at full power (which we've already established is not possible due to traction) and then shifting into 2nd gear (5:1) at 500rpm, at perhaps 3mph and then staying in that gear until top speed.

Unless you've got a double-clutch gearbox you would be slower accelerating using both gears than just pulling away in second gear (assuming you had infinite traction).

I think you're doing as I did and over-thinking the problem (hence the introduction of extremes like farm tractors). We are so used to ICE engines and inadequate electric motor drives that the idea of a gearbox is ingrained in us: How can they not be good thing?

Having just read the article on top-fuel dragsters in another thread, I was reminded that even these ICE machines are single-speed.

 

[speedmd]

If they simply needed another 2x torque, it's drastically easier than adding the complexity and new failure modes just for 2x more torque which could be made by simply growing the motor radius and/or width.

Given the speed you get out of that death bike, I am certain to adopt the mind set at some point, that a bigger hammer is the simplest and best approach. Just need to get one of those high quality motors and controllers to play with a bit.

 

[Punx0r]

I'm starting to think of a motor like a flywheel: Get the important stuff (mass for a flywheel, copper & magnets for a motor) out at as greater radius as possible. Everything in the middle is useless apart from being supporting structure for the important stuff and should be minimised as far as materials technology allows.

 

[gogo]

I don't know why this is difficult to grasp my friend. That train uses a spur gear like an inboard car motor uses an axle. Only this is being done to allow the whole axle area to be in a massive plain bearing for weight handling reasons.

I can imagine that this arrangement allows the axle to be replaced on a different schedule than the motor.