higher or lower kv

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higher or lower kv

Post by bionicon » Sep 30 2016 6:56am

I wondered what is best for middrive motor higher kv and bigger reduction or lower kv and less reduction? I have in mind a fast acceleration :D

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Re: higher or lower kv

Post by boisrondevens » Sep 30 2016 9:07pm

The equation will result in the same in terms of performance if you are playing with reductions. More reduction will give you more torque ( acceleration) but poor top speed.
Higher KV need more reduction and more moving parts. More moving parts means more friction, less efficient and reliability.
My two cents: chose the right motor for the application and avoid playing with reduction.

Hope that help

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Re: higher or lower kv

Post by John in CR » Oct 01 2016 4:18am

The relationship related to your question is the most misunderstood concept in ebiking. Assuming the same basic motor wound to different Kv's (same copper fill % assumed), they are only equal if the motors are spun to the same rpm. Otherwise the higher Kv motor wins every time, because it's spun to a higher rpm. Power = Torque X rpm, and maximum torque potential has nothing to do with how a motor is wound. Torque and max rpm are determined by the magnetic circuit and the physical limitations of its structure. The Kv of a given motor structure only determines the pack voltage needed to get the motor up to it's maximum potential power.

With small motors such as RC type outrunners, the difference is less obvious, because it's so easy to run pack voltages to get the motors up to their rpm limits regardless of the winding. With hubmotors there's a stark difference that very few really get. That's because their rpm limit isn't achievable, and varying gearing to take the most advantage of high Kv winds can only be done by reducing the wheel size.

To specifically answer your question I have to assume that you're talking about identical motors with different Kv's, and since you said you'd need a greater gear reduction with the higher Kv motor, that means it's spinning up to higher rpm. That means the higher Kv motor will absolutely result in greater power, performance, and acceleration.

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Last edited by John in CR on Oct 01 2016 12:34pm, edited 1 time in total.

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Re: higher or lower kv

Post by bionicon » Oct 01 2016 4:59am

Thanks guys ,its indeed the same motor with diffrent kv, so i wil choose the higher kv motor and make the reduction bigger :)

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Re: higher or lower kv

Post by nutnspecial » Oct 01 2016 1:10pm

Hmmm. Yep imo if it's the same motor with same copper iron mass etc etc, kv means nothing in terms of power. . .

The saturation point will be the same, when appropriate 'gearing' (voltage, sprockets, wheel size, etc) is used.

So, I would first pick a motor and voltage range to run, and consider how many amps you plan to use. From there you can figure what reductions make sense for your ride, and lastly find the most appropriately wound motor to fit.


__bionicon_____specific middrive example, single reduction: the 11lb bht or bigblock variations. Roughly same capability to handle roughly the same voltage and amperage. Keep in mind the motors are somewhat amp limited (to *reasonably maybe about 100a on stock wiring and winds), so to get to the 6-8kw 'happy place' for the motor, you run 16-20s. Workable kV for reasonably sized reductions would be 30-75kV.
I like the 30kV. I can run up to 20s and still keep speed under 40mph, without a big 100t rear sprocket. Otherwise (70kV) you would run more like <16s and have to get specialized for higher amperage to make up the difference, if you want the same power/speed.

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Re: higher or lower kv

Post by John in CR » Oct 01 2016 5:31pm

Etrike, while wind doesn't change a motor's capability, the facts of given situations do change results, and running same capacity motors of different winds in a manner that one will be higher rpm means that it will have higher power with the needed reduction in gearing producing more torque at the wheel (more acceleration for the OP) at the same heat generated in the windings. With hubmotors the effective limitation is with voltage, so we never reach the rpm limits of the motor, so that means higher Kv winds are always capable of more power for a given level of heat (the other motor limitation).

In the OP's case it sounds like he's at a fixed voltage, since he plans a greater gear reduction with the higher Kv motor. It goes without saying that he'll need to deliver more current for more power, but if planned correctly the reduction in gearing will allow him to enjoy more power and torque at the wheel, while running the motor at lower heat stress levels. That's exactly the approach I employ by running the smallest diameter wheel I can live with. The only way the lower Kv motor could be on par for the OP would be if he ran it at greater voltage to get to the same rpm, but then he wouldn't be talking about a different gear reduction.

Justin at Ebikes.ca , Liveforphysics, Miles, and others who truly understand these motor performance relationships will absolutely support my statements, because they're facts not conclusions. They did so my similar disagreement with Kingfish, who left the forum like a baby once he finally understood he was wrong. Yes, otherwise identical motors of different Kv have identical potential, but as soon as any item in the equation other than controller current becomes fixed, the higher Kv motor will win, and if you you combine that with more reduction the win becomes a slaughter.

I've used these facts on my ebike builds (primarily hubmotor driven) for nearly a decade, and that's why the fat guy living in the mountains in the tropics enjoys the highest performance without ever melting a motor. I never even bothered with a heat sensor until I pushed my primary bike up to 28kw peak input, but even then my motor cooling mods make it one of the coolest running motors on the forum...so cool that I only bother to turn the thermometer on for the hardest riding. My motor has never seen higher than 103°C, and that was pushing an over 400lb all up load up a multi-kilometer continuous 20% grade constructed to get wind turbine parts to the ridge of a nearby mountain, accelerating to up to 80kph after numerous low speed switchback curves.

The only drawback of using higher Kv for more performance is the greater current capacity controller and wiring required, but the difference is negligible compared to the enhanced performance.

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Re: higher or lower kv

Post by John in CR » Oct 02 2016 3:27am

eTrike wrote:None of that is disputed. Can you provide an example in the sim? That's where I got the data I mentioned.
Compare a MXUS higher Kv 3 turn motor and the kiddie power 6 turn motor, and put the 3 turn in a smaller wheel to gear it down. To make heat generation comparable don't forget to feed the low Kv motor with half the current, since it's windings have roughly 4 times the resistance.

Don't let yourself get lost in the simulations though. The concept is simple. If voltage and current are infinitely variable then Kv of a given motor doesn't make a hill of beans difference, because it has the same stator steel and magnetic circuit (which determine it's ability to make torque) along with the same physical construction (that sets the rpm limits and ability to dissipate heat). In the real world, voltage is limited with readily available controllers, but thanks to the popularity of big high power electric scooters in China we can get controllers capable of any current levels our motors can handle. That's why a high Kv hubbie will always kick the crap out of a slower low Kv hubbie, but to take real advantage of the difference you have to be willing to go with lower gearing (a smaller wheel).

RC type motors are different. They're built for maximum power and low weight, so their rpm limits come into play as a real limit, and controllers are readily available for voltage capable of getting them there regardless of Kv. If nothing is purchased yet then there is no difference in gear reduction, and you just vary voltage and current for identical results. As a practical matter, I'd still go with higher Kv of the same motor, because lower voltage is more convenient (less cells in the series string). I couldn't care less if I have to carry a bit heavier copper wiring.

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Re: higher or lower kv

Post by bionicon » Oct 02 2016 6:08am

WOW can you understand that i don't see the wood between the trees :)
The motor I consider to buy is the HackerQ150.45 (dont have any details of this motor yet just from his little brother the Q150.25)see post https://endless-sphere.com/forums/viewt ... 30&t=82367
Now the reason I want to know the kv is because i want to run this motor with the hacker HST350 controller (delivers 350 amps continue and 580 peak)but the voltage is limited at 60 volts(14S)
So with the voltage limits, the kv is more important( i think) I would run this motor on a light motoped with a max speed arround 70 km/H, maybe with a higher kv motor i have more playroom in the reduction and choise of topspeed and acceleration?

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Re: higher or lower kv

Post by recumpence » Oct 02 2016 6:56am

I completely agree with John. I go through this all the time with my builds. Higher KV with greater reduction will result in more acceleration. I tried this on my mountaincycle build recently. I ran a 3 turn motor and a 6 turn motor. The 6 turn was quite anemic (pathetic actually) while the 3 turn is explosively powerful.

That being said, it also results in higher power pulled from the pack.

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Re: higher or lower kv

Post by nutnspecial » Oct 02 2016 9:09am

Ok, so how bout this: whichever kV works best on your setup . . . that you can pull the most power with.
It really comes down to component choice and setup, yes?

For instance, going to the 6 turn doesn't mean you should have to draw less power, making it doggish. If the 6t can't handle as many amps, you must balance the equation with voltage, yes? And voltage is the side of equation that doesn't directly cook wires and windings and fets, so as high as reasonable for safety/controller/battery is the best way to get @ the watts for most people. That would generally mean a lower kV might be more favorable, unless you want to deal with the extra reduction(s).

For a general kit buyer/user, they want a low kv that still hits about 105% of their max desired speed. A 'tweaker' might choose to up the power with gearing, amperage, and/or voltage, so at that point the kV could go either way to match the setup, though it seems the favorite is higher for many.

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Re: higher or lower kv

Post by recumpence » Oct 02 2016 9:57am

nutspecial wrote:Ok, so how bout this: whichever kV works best on your setup . . . that you can pull the most power with.
It really comes down to component choice and setup, yes?

For instance, going to the 6 turn doesn't mean you should have to draw less power, making it doggish. If the 6t can't handle as many amps, you must balance the equation with voltage, yes? And voltage is the side of equation that doesn't directly cook wires and windings and fets, so as high as reasonable for safety/controller/battery is the best way to get @ the watts for most people. That would generally mean a lower kV might be more favorable, unless you want to deal with the extra reduction(s).

For a general kit buyer/user, they want a low kv that still hits about 105% of their max desired speed. A 'tweaker' might choose to up the power with gearing, amperage, and/or voltage, so at that point the kV could go either way to match the setup, though it seems the favorite is higher for many.
Yes, good point. In my case the bike used the same battery, the same motor style, same controller, same everything, but altering the gearing for the 50% difference in KV. That meant I had 50% less torque but also less current draw.

Each setup has various items to consider. In my case the difference was huge.
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Re: higher or lower kv

Post by John in CR » Oct 02 2016 3:38pm

eTrike wrote:The lower Kv wins acceleration, hill climbing, efficiency, etc. and can be faster under extreme loading.
These statements are patently incorrect. In fact, in the real world the opposite is true, and I prove it every day I ride. I only wish the factory I get my high efficiency motors from had even higher Kv options, so I could run them at even more extreme power through higher rpm (same torque). The alternative is economical high voltage controllers capable of real pack voltages of 150-200V, which don't yet exist, though I don't like that route. I'd rather use lower voltages than the 30s packs I'm forced to use now to get to the power levels I demand.

As a community we should find a way to penalize those who propagate bad information.


Bionicon,

Since you're voltage limited more than you are current limited, then you definitely want the higher Kv version of the same motor as long as it doesn't push it past the physical rpm limits. I couldn't find the motor dimensions, but if the 150 is the stator diameter and 45 the width (both in mm), then you should have plenty of motor to accomplish what you want on your moped.

I always recommend the most motor possible to be able to run it at low stress (the secret to absolute dependability), and gearing to a speed 10%-20% higher than the max speed you want. While it's a bit wasteful gearing that way due to greater controller losses running more at partial duty, the benefit is greater acceleration through the mid-range where it's the most fun and give you better passing acceleration as well. This is due to the shape of the torque and power curves of all electric motors. Above 50% of no-load speed torque and power fall off rapidly, and that's right in the range where acceleration is the most fun and useful. From a stop, where torque is highest, having torque that can easily flip you on your back is all but useless not to mention dangerous. Giving some of that low speed torque in favor of more torque, acceleration, and power in the upper mid-range is always a good trade off for a street machine. To me it's an absurd notion that any vehicle be ridden normally at wide open throttle. It's an unacceptable level of performance for any car or motorcycle sold in the world, so it's definitely unacceptable for your own DIY high performance build. So what if it requires some throttle restraint, I rarely ride my 100+mph capable ebike at more than 70mph, and 50+ is just on the highway.

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Re: higher or lower kv

Post by Punx0r » Oct 03 2016 4:32am

Perhaps instead you could share the results of some of the simulations you've already run?

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Re: higher or lower kv

Post by John in CR » Oct 03 2016 6:07am

Etrike,

Don't try to drag Justin into your misuse of his simulator. He wouldn't disagree with a word I've stated. While his hubmotor simulator is a great tool, like with any computer program or simulation, garbage in means garbage out.

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Re: higher or lower kv

Post by recumpence » Oct 03 2016 8:03am

General rule of thumb, for acceleration power spin up and gear down. It's as simple as that. No need to complicate things too far.
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Re: higher or lower kv

Post by John in CR » Oct 03 2016 10:44pm

I don't need to prove anything. This has all been hashed out numerous times, with the most comprehensive discussion being in the "Myth" thread in the ebike technical section. Figure out for yourself where your simulations were apples and oranges comparisons.

Recumpence summarized it nicely with a simple factual statement.

Don't worry, once you understand the relationship between differences in Kv and changes in Rm (copper resistance) the light bulb will go off for you. Our primary concern is heat created in the copper quantified as current squared X resistance. At first glance that might make it seem than high Kv makes more heat (less efficient), since it requires more current. However, to double Kv not only is the copper half as long (half the turns), but it's twice as thick, so resistance is 1/4th, which puts us back in the same position for the same torque, ie same heat (efficiency). The missing part is our voltage limitation, so unless you're already at max rpm, the higher Kv motor will always be capable of more power (same torque for same heat, but higher rpm), and then you gear it down and it's best of both worlds, because your power increase comes with greater efficiency and greater torque at the wheel.

Note that I've oversimplified a bit by leaving out iron core losses that increase with rpm, but because our variable speed systems are heavily skewed toward copper losses, it's really only those with long constant speed cruising conditions who need to factor iron losses heavily in their decision tree. Most of us can just look at no-load current X voltage, to see the maximum iron loss as a hard number, and if it's too high to live with at cruise, then get a better more efficient motor.

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Re: higher or lower kv

Post by John in CR » Oct 06 2016 7:45am

eTrike wrote:Again, I understand all of that and am not disputing it.

Can you use the sim to compare two winds of the same motor to help illustrate your point or does the sim not reflect the real world?
It's a simple matter to vary voltage and current to sim virtually identical results with different winds, and that's been shared on the forum plenty of times. It's also easy to demonstrate the higher power and performance potential of higher Kv motors run to higher rpm with reduced gearing. Things that throw you off are non-comparable resistance...unequal total strand counts on the windings, and non-comparable resistance in the batteries and controller.

Sims are just a tool to help make decisions, but reflect little about quality other than shown by peak efficiency. The biggest shortcoming is they can't simulate they can't simulate an entire ride due to variable load and throttle position during the course of any ride. All that's a moot point though, because due primarily to misconceptions and prejudices about wheel size, the "ah I got it" moment doesn't occur until you throw your leg over an ebike that's been set up for equal motor torque with the motor taken to higher rpm and geared down. We're so attuned to the sensations of acceleration that even relatively small differences are felt. Beware, it's addictive, but our ebikes are such incredible machines that's it's relatively easy to get to amusement park ride performance for pennies a ride and no lines.

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Re: higher or lower kv

Post by nutnspecial » Oct 06 2016 11:33am

amusement park ride performance for pennies a ride and no lines
Ahh yes, good description!

I'm probably least educated among you posters so I'm chasing understanding. My bike has worked really well as a high torque little machine, but I'm aware Bzh who runs a similar motor and speed and prefers a similar motor with double the kV.
I run 30kV, and have run 16-20s, 60-100a battery, and 30-40mph max speed.
With more volts I get more watts for smallish 11lb motor that's somewhat amp limited.
(100a and 100v VS 60v and 166a)
So 100v volts can get me up to a great 30kV motor rpm, w/ reduction of 1: 4.5 for ~45mph.

If I ran 70kV, my reduction would be less managable at a size of 1: 10, because my motor would spin 2.5x. But are you saying 100a 100v 45mph 70kV beats 100a 100v 45mph 30kV in terms of torque?
A measurable amount, or just a noticable amount? Thanks!

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Re: higher or lower kv

Post by bionicon » Oct 06 2016 3:25pm

So afther all this discution i have the option to go for a kv of 48 or 71 for what kv should i go? :roll:

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Re: higher or lower kv

Post by Punx0r » Oct 07 2016 3:12am

Comparing two motors of different Kv with the same voltage/current input is not an apples-to-apples comparison. That's like comparing a petrol and diesel engine by insisting they run on the same fuel...

John is correct - hub motors are almost always torque-limited in their performance. If you want more power you've got to spin it faster and gear down. That means higher Kv unless you want voltages that are difficult to manage.

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Re: higher or lower kv

Post by John in CR » Oct 08 2016 7:50am

eTrike wrote:...In some cases, slow winds are faster than fast winds.
Yes, like when the high Kv motor is turned off (and other non-comparable cases).

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Re: higher or lower kv

Post by sn0wchyld » Oct 12 2016 7:57pm

Ok, this has been done before, but lets do it again...

find a motor on the sim with a kv of x.
find the same motor but with a kv of 2x.

Feed motor x with voltage 2v and current limit a
feed motor 2x with voltage v and current limit 2a. (current limits should be for phase windings, but iirc the ebikes.ca sim does an ok job of this anyway using battery currents)

you'll see identical (or close enough too) torque and power curves for a given wheel size, as well as time to overheat, and top speed. because each motor is identical in terms of how much torque it can produce at a given rpm. Just one motor needs 2a and 1v and the other 1a and 2v. VA=P/rpm=T after all. And A^2R=P(heat), but R(2x)=0.25R(x), so P(heat 2x)=P(heat x) for any given Pout at a given rpm (ie any given torque output).

Note - if the torque/power curves aren't very close then this would highlight a limitation of the sim, or potentially your comparing 2 motors where the copper fill of a particular winding is worse than the other.

Now, lets run motor x in a wheel size 2d, and motor 2x in a wheel size of d, but with their voltages equalized.
Motor 2x can still handle 2x the current of motor x, but its also got double the reduction. because its also got 2x the voltage, it can also still reach the same speed, but with its extra reduction its now producing 2x the torque at the wheels, for a given Pheat produced. Remember that we haven't changed the current levels, so each motor will still overheat in approximately the same amount of time. It will naturally require more power from the batteries to produce that extra torque - but thats the price of more acceleration. Another way of looking at it is that it can produce the same torque for half the heat produced, so it can do the same acceleration for longer... and have a much higher top speed (and thus mid-high range acceleration) to boot.

Put simply, other than practicalities around the amount of reduction you can get in a particular space/efficiency of each stage (or wheel size) and rpm limits of a given controller+/motor, your generally going to be better off with a higher kv and more reduction.

We could 'try' the same with motor x, by doubling the pack capacity and by extension the currents driving the motor, but increasing current may mean more torque, but it will also mean more heat (4x the heat for the same torque as motor 2x). The other way is to double the voltage again to 4v and halving the wheel dia to d as with motor 2x, but this has the drawback of (generally in ebike terms) needing larger voltages that make controller choice limited, and battery management more difficult, with the only advantage being smaller phase/battery wires, and (very slightly) lower end turn losses. a 80v pack is much safer to deal with than a 160v pack too.

Its a bit differnt for RC motors, as generally they're designed for voltages <50v, to attain their max speed before losses become too high (something most hubs never get close too). So a lower KV RC motor (such as the REVOLT motors) is not such a bad idea, given that running '2 times' the voltage (to pick a value) is still pretty easy, without needing very high voltages.

For the OP
Probably best to go for the higher kv motor. Id recommend sticking below 1:3 or so per stage, any more becomes very inefficient, ie a single 1:9 reduction will usually be less efficient than 2 1:3 reductions. The only time you'd want a lower kv is if after whatever reductions you plan your motor still is capable of about 20% more speed than you want. the lower KV motor will be easier on your controller (though minor benefit if your not right at the ragged edge of its capability). Neither motor will be 'better' than the other in performance, assuming you can find a controller/battery combo to provide it with the appropriate voltage and current (ie more current but less voltage for the higher kv motor, and inversely for the lower kv).

eTrike wrote:Thanks John.
I shared before that I used identical power levels to make that spreadsheet that shows the results from the sim.
The sim does report acceleration in the bottom right corner but I've found few people pay attention to that or their power curve when deciding which motor to use. Many end up with a lot of potential performance (acceleration) left untouched because they choose a fast wind in a large wheel.
Thus the results from the sim show clearly that gear reduction (smaller tire) is warranted for fast winds especially at high speeds. I know I'm not describing anything new to you, but for future readers it helps to illustrate the point.

So, assuming you have a working ebike and only want to change your motor with a different Kv motor and wheel, you can compare those apples-to-apples using the same power settings to confirm the results I've gathered from the sim.
I also shared that I tried varied inputs to suit the motor with high Kv motor double current and gave the low Kv double voltage (which would require swapping battery and controller so this isn't a very likely scenario) in order to gauge the results. You didn't reply when I shared this and asked for clarification for inputs to try in the sim.
If you prefer apples-to-oranges I will concur with what I have not disputed-- the higher Kv motor can take more power. But for apples to apples, I think the sim results ought to be favored over subjective experience (though your experience ought to be verifiable and reflected in the sim, which you've yet to be able to provide despite multiple polite requests). It is certainly good for more than just efficiency.

My roller coaster experience and results are verifiable in the sim. I've used a high Kv 9C in 26", then swapped to a low Kv 9C in 26", then swapped to a high Kv MXUS in a tiny 16" bike rim. With the exact same power system (battery and controller) used between all three. This is why I made the spreadsheet, to compare real-life results. My experience directly reflects what the sim shows.

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Re: higher or lower kv

Post by toolman2 » Oct 12 2016 11:27pm

sn0wchyld wrote:Ok, this has been done before, but lets do it again...

find a motor on the sim with a kv of x.
find the same motor but with a kv of 2x.

Now, lets run motor x in a wheel size 2d, and motor 2x in a wheel size of d, but with their voltages equalized.
Motor 2x can still handle 2x the current of motor x, but its also got double the reduction. because its also got 2x the voltage, it can also still reach the same speed, but with its extra reduction its now producing 2x the torque at the wheels, for a given Pheat produced. Remember that we haven't changed the current levels, so each motor will still overheat in approximately the same amount of time. It will naturally require more power from the batteries to produce that extra torque - but thats the price of more acceleration. Another way of looking at it is that it can produce the same torque for half the heat produced, etc
All good snowy, only that the double kv motor geared 2:1 can produce the same torque at the wheel for a QUARTER the heat produced not just half, as ir losses are squared. :wink:

I think the anomalies eTrike has highlighted and the very big call of "low kv motors sometimes faster than high kv motors" are simply caused by starving the high kv motor of the required phase current -ie the settings or size of the controller are not sufficient to make everything equal (and for battery draw) the way it should, and i believe Justins sim has an adjustable parameter for the controller of (.2 ohms default ?) that would account for the poor simulated result for a high kv motor. :?:

John is correct though (dont worry id be prepared to say so if i didn't agree :) ), and maby something that has not been mentioned is that a motor with higher kv requires a bigger controller, even for the same battery current or total power drawn off the pack.

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Re: higher or lower kv

Post by sn0wchyld » Oct 13 2016 2:34am

eTrike wrote:Cheers snowchyld, I understand that and I've done exactly as you've described to come to the conclusions I shared, first using Clyte H3525 vs H3540, then again with MXUS 4503/4504/4506 and the results repeated themselves. I tried to use a common setup like we'd expect with bikes, but also tried manipulating the numbers as you perfectly described. Can you point out anything that looks off? https://endless-sphere.com/forums/viewt ... 30&t=67183

My main interest stems from what I shared in that link, but if you were to take a random sampling of MXUS3k users and their power level I suspect you would find a trend of large wheels (near 26"OD) and high voltage (72+V) with the highest Kv motor(4503). Large wheels are common for looks and ease of build but this is the worst combination for this motor as it leaves a lot of performance on the table. In a 20" wheel it would leave the 26" in the dust off the line and could pull heavier loads.
sorry mate dont have excell on this pc (new computer) - can you upload some screens and i'll talk you through it. It may simply be that the ebikes sim doesn't account for phase currents, which is what really matters.

ok so i went to the simulator and found some motors myself:
motor comparison.PNG
motor comparison.PNG (113.4 KiB) Viewed 1638 times
as you can see, ive used a 408 and a 404 - two motors with a kv ratio of exactly 2. I've uses some custom batties as they were throwing off the results (different resistances meaning different voltage drops) - though i haven't bothered with controllers since i cant manipulate phase amps, and the remaining difference is minor anyhow. What you can see is two motors, with the 2:1 ratio of kv, V and A, with the same torque, top speed, and overheat time (the missing value is 3.2 min, so not quite spot on but close enough to make the point, in real life it'd be even closer than this, and this difference may be down to copper fill etc). The other value thats missing is WH/km - which is exactly equal. Acceleration (and thus, force, and thus, torque) is almost exactly the same too, as shown by the corresponding lines.
motor comparison2.PNG
motor comparison2.PNG (102.95 KiB) Viewed 1638 times
now heres my 2nd case. I've had to play with the nubmers on controllers here to get the same phase amps (ie equal stall torque) Ive also used V and 1/2V rather than V/2V to keep the numbers legible, but the effect is the same. Aas demonstrated both motors have the same torque otuput (and thus ratio of phase amps) and by extension, the same time to overheat as the last example. The faster wind however has a much larger performance envelope. So both motors in this setup will climb at the same speed while producing the same heat.... but only one has a respectable top speed.

I could do one showing different wheel sizes, but i figure you get the point, and you have the settings now to do your own playing.

And i want to stress, higher kv ~= better performance in most cases, but hubs and other slow spinning (<1krpm) are generally being underutilised, as they dont have the pole count to run near their peak potential at the RPM's they typically run at. There's a thread on here somewhere where some of the guru's were designing a better 'hub' motor, with about 50 odd pole pairs (about double typical hubs) that had far more potential than most hubs we see in bikes... simply because it was optimised for such a low output rpm (by among other things having a much larger pole/tooth count).
toolman2 wrote:
sn0wchyld wrote:Ok, this has been done before, but lets do it again...

find a motor on the sim with a kv of x.
find the same motor but with a kv of 2x.

Now, lets run motor x in a wheel size 2d, and motor 2x in a wheel size of d, but with their voltages equalized.
Motor 2x can still handle 2x the current of motor x, but its also got double the reduction. because its also got 2x the voltage, it can also still reach the same speed, but with its extra reduction its now producing 2x the torque at the wheels, for a given Pheat produced. Remember that we haven't changed the current levels, so each motor will still overheat in approximately the same amount of time. It will naturally require more power from the batteries to produce that extra torque - but thats the price of more acceleration. Another way of looking at it is that it can produce the same torque for half the heat produced, etc
All good snowy, only that the double kv motor geared 2:1 can produce the same torque at the wheel for a QUARTER the heat produced not just half, as ir losses are squared. :wink:

I think the anomalies eTrike has highlighted and the very big call of "low kv motors sometimes faster than high kv motors" are simply caused by starving the high kv motor of the required phase current -ie the settings or size of the controller are not sufficient to make everything equal (and for battery draw) the way it should, and i believe Justins sim has an adjustable parameter for the controller of (.2 ohms default ?) that would account for the poor simulated result for a high kv motor. :?:

John is correct though (dont worry id be prepared to say so if i didn't agree :) ), and maby something that has not been mentioned is that a motor with higher kv requires a bigger controller, even for the same battery current or total power drawn off the pack.
yea i knew i'd screw something up hahah. cost of writing stuff up at work while waiting for matlab to build new models...

regarding the controller size - it depends. the highest power density fets are for votlages up to about 90V, so a kv optimised to utilise a 80-90V pack will typically be the smallest controller for a given power output, though i stress 'typically' as there's 15 million other variables that go into a controller. look at the adaptto's for one anecdotal example - they're tiny for their power capability, the new ones will do 20kw peak from what i've read, in something smaller than a infineon 18fet, which would manage about half that...

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Emoto   10 kW

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Re: higher or lower kv

Post by Emoto » Oct 14 2016 10:30am

John in cr / toolman2 / sn0wchyld . Great reading This kv info couldnt have come at a better time for me :idea:

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