Motor comparison spreadsheet
- Thread starter Miles
- Start date
crossbreak
1 MW
ok this is reasonable. for a 20" wheel, you need only 77Nm@ the wheel for the 20% hill climb, but you have the Km² of 29.3. Guess you will be able to climb the 20% hills in the upper 80s then
too lazy to calc
need to go further with the simulator...
too lazy to calc
need to go further with the simulator...
crossbreak
1 MW
At 100kg you could even be in the 90s. Will be fun to simulate
Interesting mathematical example there, Crossbeak
I think John is upset because his Hubmonster is clearly a powerful beast, but this example of 20% incline at 14kph in a 26" wheel does not suit it well at all. I think if the example was the same hill, a 20" wheel and at 100kph his motor would be the only one capable (the smaller motors having to spin so fast with such high gear reductions the core losses would be huge).
I think Km becomes confusing when we include a gear reduction. Otherwise any motor with infinite gear reduction has infinite Km, but in reality has zero power output and can do no work (is useless). If this is the sort of think John talks about as a criticism of Km as unit of relative merit then I understand his concern.
Edit, nearly forgot to praise the awesome calculated figures for the MilesMotor!
I think John is upset because his Hubmonster is clearly a powerful beast, but this example of 20% incline at 14kph in a 26" wheel does not suit it well at all. I think if the example was the same hill, a 20" wheel and at 100kph his motor would be the only one capable (the smaller motors having to spin so fast with such high gear reductions the core losses would be huge).
I think Km becomes confusing when we include a gear reduction. Otherwise any motor with infinite gear reduction has infinite Km, but in reality has zero power output and can do no work (is useless). If this is the sort of think John talks about as a criticism of Km as unit of relative merit then I understand his concern.
Edit, nearly forgot to praise the awesome calculated figures for the MilesMotor!
John in CR
100 TW
crossbreak said:
Absolutely ridiculous conditions, because only those who don't know better would use a high speed high power hubmotor in such a manner, forcing it to draw 191A at such low sustained speed. Your example Is like me forcing your pair of BPMs to use a 1:1 reduction for the same climb. The difference is that even a pair of BPMs couldn't do it and would quickly burn up, while Hubmonster could handle your ridiculous conditions without overheating.
We agree that mid-drive is the way to go for off-road, and when I build one for myself it will get a HubMonster, MidMonster, or MiniMonster as a mid-drive. Which one will depend on the load and performance I'm after.
Comparisons based on any manipulation of Km are invalid, because Km is a meaningless number. The proof is your insistence the 2 BPMs is more motor than a single HubMonster. Put BPM's numbers in the sheet so I can prove it, but of course we already know that since there's 0% chance you could run them at 14kw peak input into each. If the conditions were running just 1200W at the wheel, then a much smaller and lighter motor than even a single BPM would be a better tool for the job.
crossbreak
1 MW
I chose this example since it is a real world example. Two BPM are indeed capable of spreading ~250W of loss each. Even justins simulator tells that it wont overheat. One alone could do the job, too, with the right reduction. I tried that. But you need to spin it so fast that eddy current loss becomes a problem (around 2000rpm at the rotor around 9:1 reduction). Efficiency does not get into the 80s. I'd say that 1200W output is the max the BPM can do with acceptable efficiency. All figures you need to calc are given in the spreadsheet i uploaded above. Besides eddy current figures, had no time to dig them out yet. Not too interesting anyway.
Proof? Proof is, that in this condition, the 2 BPM win. They get better efficiency. Even with core loss, i bet. I really dont get what you wanna tell me. I never questioned that the Hubmonster is capable of a lot more power. Just wanted to explain the formulas using example that has results that are not so obvious if someone is not familiar with them
Dont question that the Hubmonster wont overheat from 600W of loss. I see people who try climbing technically passages with the big DD hubs in 26" rims. They even get worse Km² figures then the Hubmonster, like the Cromotor. Still people do so
Punx0r, you got it. Sadly reduction ratios much larger than 5:1 become impractical in a single stage. i still think that Km² is very good tool for choosing gear reduction. I could also have said a Km² of 60 is a good number. But it isn't since for the motors that people use for such purposes on this forum, the core loss would become a problem at these gearing ratios. The truth is anywhere in between that 30 and maybe 60.
Sure a bike using the Hubmonster in a 26" wheel just shall not be used in the above manner. But i dont want a bike that shall not be used in some manner, it shall do the most possible. This is why i can only support miles in his trying to establish drives that can go fast with minor core loss while going up steep hills at low i²R loss. DD cant do either thing.
The Nissan leaf has a reduction ratio of 7.9377:1. The leaf motor does 280Nm, this is 2222Nm at the wheel. Just enough to make the tires slip. the Km² becomes 1738. Tire circumference is ~1985mm, close to a 26" wheel at ~2100mm.
The leaf weight is about 10 times the weight of a 150kg bike. So it needs round about 1000Nm at the wheel to climb that 20% hill. I²R loss is I²R = (T/(Kt*reduciton))² * Rm = (1000/ (.525*7.9377))²*.01Ω = 575W loss
The wheel Kv is 18.2/7.9377=2.3, at a battery voltage of 360V, this is only 828rpm no-load at the wheel. Enough to get 99.3 kph no load speed. not enough for 140kph. Guess this is where field weakening starts. Odd numbers, still they need to be compared somehow.
John in CR
100 TW
Those generic chinese motors with ribbons of stator steel laminates that too commonly fail don't have the efficiency to come close to what HubMonster is capable. Looking at Rm after factoring in Kv/Kt will tell you exactly that...HubMonster makes less heat, and 600W of heat certainly won't burn it up, even at low rpm in stock sealed form. Of course that's only half the story, because the better design and materials gives HubMonster an even bigger advantage at the high speed end of operation. You're so consumed by the meaningless number, Km, that you simply can't see it. Torque divided by the sqrt of watts is just a mathematical abortion developed by a manufacturer to make their motors appear better.
We have precious few premium motors available to us, and even fewer available for reasonable prices, so if you're going to hold yourself out as some kind of expert you need to be able to tell the difference between a premium motor and mass produced motors made with cheap parts.
You're the one who said dual BPMs are more motor that HubMonsters, and that was and is an absurd statement.
We have precious few premium motors available to us, and even fewer available for reasonable prices, so if you're going to hold yourself out as some kind of expert you need to be able to tell the difference between a premium motor and mass produced motors made with cheap parts.
You're the one who said dual BPMs are more motor that HubMonsters, and that was and is an absurd statement.
Come on John, accept the possibility that Hubmonster is a poor choice for usage in the given low-speed/big wheel example From my own reading on the subject I can tell you Km isn't marketing nonsense. It's used by an awful lot of people smarter than you or I who design or specify motors. They probably also know it's limits and how to interpret its results.
One thing that ought to be added in any example featuring a gear reduction is the efficiency of the reduction. Let's say 10% per stage. This added inefficiency isn't heat load the motor must shed, but it's still energy wasted as heat.
Currently available Chinese bicycle hub motors may not be able to do that, but there's no reason why they can't be designed to do so. DD is the ultimate solution for efficiency, simplicity and reliability. It's something that has been discussed on this forum before in some detail (large radius, high tooth count).
From my own reading on the subject I can tell you Km isn't marketing nonsense. It's used by an awful lot of people smarter than you or I who design or specify motors. They probably also know it's limits and how to interpret its results.One thing that ought to be added in any example featuring a gear reduction is the efficiency of the reduction. Let's say 10% per stage. This added inefficiency isn't heat load the motor must shed, but it's still energy wasted as heat.
crossbreak said:
Currently available Chinese bicycle hub motors may not be able to do that, but there's no reason why they can't be designed to do so. DD is the ultimate solution for efficiency, simplicity and reliability. It's something that has been discussed on this forum before in some detail (large radius, high tooth count).
crossbreak
1 MW
your wrong. maybe you get it or not. Tried to tell you how the formulas work, if you dont want to understand, then dontJohn in CR said:
marketing "tricks"...
i would be so glad if all manufacturers would at least state Rm and Kv. Most dont do neither
speedmd
10 MW
Thought I was getting this all before I saw this chart. Now, totally confused. :? Are we deriving a constant from a curve?
crossbreak
1 MW
No. This charts just tells you how a motor behaves at full throttle and a certain battery voltage.
You can derive such a curve for any motor in the spreadsheet as long as you have Rm,Kv and two no load figure for two different rpm.
You can derive such a curve for any motor in the spreadsheet as long as you have Rm,Kv and two no load figure for two different rpm.
John in CR
100 TW
crossbreak said:your wrong. maybe you get it or not. Tried to tell you how the formulas work, if you dont want to understand, then dontJohn in CR said:
marketing "tricks"...
No, you were and are still wrong. I obviously understand more about motors than you think. "More motor" is not the same as "a better choice for use in it's designed form to climb steep hills at less than 10mph". The fact that you're still claiming dual BPMs is more motor than HubMonster exposes your lack of real understanding. You obviously know some numbers, but you simply don't understand them well enough to see that Km squared is not a valid number upon which to base such a claim.
To make matters worse you tried to hide the error in your claim with impossible conditions. A 26" can't be installed on HubMonster. I run a 19.25" OD wheel, which would put your efficiency comparison right on par. Of course you neglected to include iron losses, which would be far higher with the BPMs, as well as the losses in the gearing, which are substantial in geared hubbies. You also ignore that a gear reduction can be used on any motor. Setting conditions to make your claim appear correct is disingenuous at best. The truth of the matter is that the BPM's power is so limited by low efficiency, that there's no test you can come up with that's survivable by a pair of BPMs that I can't set Hubmonster up to not only exceed, but have higher efficiency than your BPMs' peak efficiency under optimum conditions on a flat road. This of course excludes any advantage 2wd may have on handling.
You started this crap with your ridiculous statement, but I'm going to finish it, so set your BPMs up however you want and come up with your virtual test. I'll set HubMonster up and prove how wrong you are with the numbers you erroneously think I don't understand. I typically let my motors speak for themselves with real world results like my run up a 20% grade, but your petty attempts at insult pissed me off enough to make an exception now that you're starting to remind me of Kingfish.
2 BPMs are not "more motor" than a single HubMonster, and it's not even close, period. That means Km² isn't the number you purport it to be. It may be closer to a good number than Km, but there's obviously something still missing.
"Marketing tricks"...that's a good one. I only make motors available as a favor to forum members. If there was profit motive, I'd simply double the price.
crossbreak
1 MW
period, i dont get what you wanna tell me.crossbreak said:
I made an example, where reduction ratio comes into play. I compared two motors with almost similar pole count, a tiny with a lame specific sKm of 0.2 Nm/kg with a HUGE motor with a much much better sKm of 1.1 (which was just the best one i found in the list, my friend
) You just shit around, wasting my time. If you think, comparing a bare BPM w/o any reduction with hubmonster w/o any reduction would be a "fair" comparison, then you are very very far away from sanity. Still dont get what you wanna tell me, and i just dont wanna hear anymore
John in CR
100 TW
It's good that you chose HubMonster, because it proved that Km² isn't the awesome number for direct motor comparisons that you thought it was.
crossbreak
1 MW
it's the number to compare i²R loss. not more, not less. we all agree that we need to take core loss into account. Let's work further on this
speedmd
10 MW
crossbreak said:
Thanks. Makes a bit more sense with that. Will need to work out the numbers a bit on the spread sheet to get more comfy with the whole topic. The whole idea of torque sensitivity and constants is a very interesting way to look at motors. One thing that still makes me dig into a motors suitability for a app is not only its torque output / efficiency at low RPM, but if it can spin extremely high RPMs efficiently also. Truly one with a larger/ broader power / efficiency band. Thinking core and winding losses can be possibly compared via the slopes of their respective loss curves.
speedmd
10 MW
Looks like a great trade off going to the 30 mm stack. Big jump in Kt percentage wise compared to mass increase. Lower / very good Kv on it also. I want one! Keep me on that build list.
Yes, you gain from the less than proportional increase in mass and also the less than proportional increase in Rm.... There's probably a certain point beyond which it's better to step up in diameter, though.speedmd said:
Anyway, it's useful to have the spreadsheet for this kind of comparison...
John in CR
100 TW
Once we start adding a gear reduction, does the new no-load current info tell us almost everything, or will a significant portion of the added parasitic losses be torque dependent as well as rpm dependent that measurements of no-load will have little meaning?
Good question.. No doubt there's a torque dependent factor but what effect that'll have.... Most reductions increase in efficiency with load. Non-positive ones, like the Nu Vinci, might decrease, at some point?John in CR said:
Quick Google:
Rest of the article, including formulae for estimating load-dependent losses: http://webcache.googleusercontent.com/search?q=cache:QUCMoSQdxKkJ:engj.org/index.php/ej/article/download/316/270+&cd=4&hl=en&ct=clnk&gl=uk
(Analytical and Experimental Investigation of Parameters Affecting Sliding Loss in a Spur Gear Pair
Chakrit Yenti, Surin Phongsupasamit, and Chanat Ratanasumawong)
IMO the geartrain should be included in the no-load loss measurements. However, I still have the feeling that gear reductions should not be included - they mess up the Km figures and confuse the issue of a database comparing *motors*. The motor data is useful for selecting any reduction someone might wish to use.
Rest of the article, including formulae for estimating load-dependent losses: http://webcache.googleusercontent.com/search?q=cache:QUCMoSQdxKkJ:engj.org/index.php/ej/article/download/316/270+&cd=4&hl=en&ct=clnk&gl=uk
(Analytical and Experimental Investigation of Parameters Affecting Sliding Loss in a Spur Gear Pair
Chakrit Yenti, Surin Phongsupasamit, and Chanat Ratanasumawong)
IMO the geartrain should be included in the no-load loss measurements. However, I still have the feeling that gear reductions should not be included - they mess up the Km figures and confuse the issue of a database comparing *motors*. The motor data is useful for selecting any reduction someone might wish to use.
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