higher or lower kv

Discussions related to motors other than hub motors.
This includes R/C motors, botttom bracket, roller and geared drives.
nutnspecial   1.21 GW

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Re: higher or lower kv

I think I've gotten some from this too, but at the same time am less sure of my previous understanding.

Is it not true that 10kw geared for x mph has the same torque as 10kw geared for x mph . . . no matter if 'gearing' comes from kV, pack voltage, reductions, wheel size, etc?

Provided (of course) the motors are equally capable, wasting similar amounts of energy into heat.

John in CR   100 GW

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Re: higher or lower kv

Ignoring small differences resulting from differences in copper fill, motors of different Kv are capable of exactly the same power, torque, rpm, efficiency, heat etc. The magnetic circuit and physical structure determine everything, and a factor in that is the total amount of copper that fits, not how it is wound. How it is wound (iow the Kv) only determines the combination of voltage and current to get the identical result for power, torque, rpm, efficiency, etc.

Kv and Kt (the torque constant) have a fixed inverse relationship. Kv in rpm/v = 9.549/Kt in Nm/A, so if you know a motor's rpm/volt you also know it's torque per amp. Don't let that mislead you into thinking low Kv means more torque though, because it leaves out current handling and heat (efficiency) per unit of torque. The higher Kv motor has shorter thicker copper, so you reduce voltage and increase current for an identical result.

The best example is Astro's test results for it's 3210 motor, all identical for different windings:
Astro 3210 results with diff windings jpg.JPG (48.53 KiB) Viewed 2157 times
Where the meaningful differences arise is when you're chasing performance and the motor isn't at its rpm limit. We end up limited by voltage, so the high Kv verson of the same motor wins every time, and efficiency trumps everything. The proof is in the pudding. Liveforphysics Deathbike with its high efficiency custom motor beats even Teslas at the dragstrip. Sn0wchyld stays up dreaming of a higher Kv version of the 93% peak efficiency motor he has. Toolman2 races and beats smoke spewing motos with his high efficiency fast spinning Joby motor. When Recumpence wants more get up and go on his high tech works of art he opts for the higher Kv version of his Astro motors. Despite weighing 300lbs just over a year ago my extreme efficiency hubmotor allows me to ride the fastest hubmotored ebike in the world, and because I gear it down with the smallest practical wheel I can climb any incline I dare. I can't wait for rainy season to end to bring out the new baby I built as my reward for dropping 80lbs and counting. The 20% reduction in total load is going toward a 10% bigger wheel for an even more insane top speed, and the other 10% toward even scarier acceleration. Plus the lower overall system stress will allow me to turn my controllers even higher but still retain absolute reliability. This is all possible not because I run the best hubmotor, but because I run the highest Kv hubmotor at over 18rpm/volt at the highest practical voltage, 30s.

***edited for correction of Kv to Kt relationship as pointed out by Sn0wchyld***
Last edited by John in CR on Oct 16 2016 8:05am, edited 1 time in total.

sn0wchyld   1 MW

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Re: higher or lower kv

John in CR wrote:Ignoring small differences resulting from differences in copper fill, motors of different Kv are capable of exactly the same power, torque, rpm, efficiency, heat etc. The magnetic circuit and physical structure determine everything, and a factor in that is the total amount of copper that fits, not how it is wound. How it is wound (iow the Kv) only determines the combination of voltage and current to get the identical result for power, torque, rpm, efficiency, etc.

Kv and Kt (the torque constant) have a fixed inverse relationship. Kv in rpm/v = 9.549 x Kt in Nm/A, so if you know a motor's rpm/volt you also know it's torque per amp. Don't let that mislead you into thinking low Kv means more torque though, because it leaves out current handling and heat (efficiency) per unit of torque. The higher Kv motor has shorter thicker copper, so you reduce voltage and increase current for an identical result.

The best example is Astro's test results for it's 3210 motor, all identical for different windings:
Astro 3210 results with diff windings jpg.JPG
Where the meaningful differences arise is when you're chasing performance and the motor isn't at its rpm limit. We end up limited by voltage, so the high Kv verson of the same motor wins every time, and efficiency trumps everything. The proof is in the pudding. Liveforphysics Deathbike with its high efficiency custom motor beats even Teslas at the dragstrip. Sn0wchyld stays up dreaming of a higher Kv version of the 93% peak efficiency motor he has. Toolman2 races and beats smoke spewing motos with his high efficiency fast spinning Joby motor. When Recumpence wants more get up and go on his high tech works of art he opts for the higher Kv version of his Astro motors. Despite weighing 300lbs just over a year ago my extreme efficiency hubmotor allows me to ride the fastest hubmotored ebike in the world, and because I gear it down with the smallest practical wheel I can climb any incline I dare. I can't wait for rainy season to end to bring out the new baby I built as my reward for dropping 80lbs and counting. The 20% reduction in total load is going toward a 10% bigger wheel for an even more insane top speed, and the other 10% toward even scarier acceleration. Plus the lower overall system stress will allow me to turn my controllers even higher but still retain absolute reliability. This is all possible not because I run the best hubmotor, but because I run the highest Kv hubmotor at over 18rpm/volt at the highest practical voltage, 30s.
pretty sure its actually kv=9.549/kt (else increasing kv would also increase torque per amp)... but otherwise a great example/explanation.

Its a particularly useful relation to know though... if you know the Rphase of your motor you can get a good idea of how much torque you can expect from it. My little revolt 100pro for example has a kv of 45, rphase of ~.045, and can disipate (lets say) about 200w. P=I^2 x R, so max cont. I is about 66A. From the above relation we know this motor produces about .2Nm/A, so that means a cont torque of about 13-14Nm, so at the wheels, about 80Nm with the ~6:1 ratio i have atm. The 120pro for comparison (assuming the same heat disipation) has a Rph of about 0.025, so the cont. about 18Nm. Compare that to the 160 pro (again assuming same power disipation, same KV) that cont torque nearly doubles, about 3x that of the 100 pro, for about 3x the weight. In reality the 120/160 would both have higher heat dissipation than the 100, so the difference would be even bigger, but you get the idea.

John in CR   100 GW

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Re: higher or lower kv

sn0wchyld wrote:pretty sure its actually kv=9.549/kt (else increasing kv would also increase torque per amp)... but otherwise a great example/explanation.

Its a particularly useful relation to know though... if you know the Rphase of your motor you can get a good idea of how much torque you can expect from it. My little revolt 100pro for example has a kv of 45, rphase of ~.045, and can disipate (lets say) about 200w. P=I^2 x R, so max cont. I is about 66A. From the above relation we know this motor produces about .2Nm/A, so that means a cont torque of about 13-14Nm, so at the wheels, about 80Nm with the ~6:1 ratio i have atm. The 120pro for comparison (assuming the same heat disipation) has a Rph of about 0.025, so the cont. about 18Nm. Compare that to the 160 pro (again assuming same power disipation, same KV) that cont torque nearly doubles, about 3x that of the 100 pro, for about 3x the weight. In reality the 120/160 would both have higher heat dissipation than the 100, so the difference would be even bigger, but you get the idea.
Ooops, yes corrected now for the proper Kv to Kt relationship. It's a good point to drive home too, since any motors having the same Kv make the same torque per amp. Sn0wchyld segwayed nicely into the most important limitation of all which is heat. Sadly probably the most important motor specification, phase-to-phase resistance, is typically unstated by manufacturers and sellers, yet it's simple to measure.

In addition to heat created in the copper from resistance, there's iron core losses that correlate with rpm. That portion of motor losses is easily quantified by the no-load current X voltage at the rpm you want to look at. Since it goes up with rpm, the no-load current at WOT tells you the maximum core loss of your motor. While copper losses typically dominate since we dump big current into our motors during acceleration and climbing hills, core losses can be significant at cruise.

The last part of the heat equation isn't often discussed, but it can be important. That is the fact that copper resistance goes up with temperature, approximately 0.4% per degree C. The hotter the motor gets, the more heat it makes to create the same torque. I cringe at some of the temperatures guys report running their motors. Not only do they risk heat failure, but they're flushing performance and battery capacity down the drain with 30-40% higher copper losses than when the motor was cool.

The hardest part of any motor performance prediction is heat dissipation, because it varies drastically even with identical motors. Not only can cooling airflow and motor cooling strategies make a huge difference, but our variable speeds and loads as well as simply how we ride our ebikes make it virtually impossible to nail down tightly. Heat is the enemy, and I do everything I can think of to fight it. Whatever energy we suck from the battery that isn't turned into mechanical energy turns into heat, mostly in the motor.

John in CR   100 GW

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Re: higher or lower kv

Apples and Oranges!

markz   100 GW

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Re: higher or lower kv

What sold me on the higher KV was the higher amps it can take, and the wire analogy, as highlighted below, in Johns quote.

TeslaNV did a great write up in one of the MXUS threads about how much a higher KV motor can take in terms of amperages, way over and above a lower KV motor. Link here.
According to Justin's data, Teslanv get the following phase current limits on the various windings:

MXUS XF40-45H "3000W" Direct Drive Hub Motor Series:

3T:
Max. Continuous Phase Current: 55A
Overheat in 10 Minutes: 85A
Overheat in 60 seconds: 242A

4T:
Max. Continuous Phase Current: 42.6A
Overheat in 10 minutes: 66A
Overheat in 60 seconds: 186A

5T:
Max. Continuous Phase Current: 34.9A
Overheat in 10 minutes: 51.6A
Overheat in 60 seconds: 150A

6T:
Max. Continuous Phase Current: 30.3A
Overheat in 10 minutes: 47A
Overheat in 60 seconds: 132A

Generic Winding Phase Current Limits per strand:
Max. Continuous Phase Current: 2.84A per strand
Overheat in 10 minutes: 4.4A per strand
Overheat in 60 seconds: 12.4A per strand

21X3T Winding
Phase resistance = 0.072 Ohms
RPM at 50.2V = 597, 11.89 Kv

16X4T Winding
Phase resistance = 0.110 Ohms
RPM at 50.2V = 448.2, 8.93 Kv

12X5T Winding
Phase resistance = 0.163 Ohms
RPM at 50.2V = 359, 7.15 Kv

10X6T Winding
Phase resistance = 0.225 Ohms
RPM at 50.2V = 299.4, 5.96 Kv
It comes down to whats the situation you have. Are you building an ebike from scratch, or do you already have an ebike with a battery and controller.

I stumbled on these FACT's for some time, its quite a lump of facts to wrap your head around thats for sure.
John in CR wrote: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.

minde28383   1 kW

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Re: higher or lower kv

Here is an example from real life circumstances. The same everything except kv (rpm/v).

1. QS 205 V3 kv11.89
battery 72v (200A consntant capable)
Sabvoton 72150 (settings: 150A battery, 300A phase)
rim 17''

2. QS 205 V3 kv9.03
battery 72v (200A constant capable)
Sabvoton 72150 (settings: 150A battery, 300A phase). You can lower phase amps to 250A phase. It (9kv) will be still quicker from the standstill.
rim 17''

As you can see everything is the same except kv.
Ebike with 9kv will be quicker from standstill till it reaches ~30km/h.

So don't say that higher kv is the same or better. KV matters.
If you takes both these ebikes (1,2) to very slow trails, hills, 9kv motor will win, due having better torque from a standstill.

sn0wchyld   1 MW

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Re: higher or lower kv

minde28383 wrote:
Jan 29 2018 8:39pm
Here is an example from real life circumstances. The same everything except kv (rpm/v).

1. QS 205 V3 kv11.89
battery 72v (200A consntant capable)
Sabvoton 72150 (settings: 150A battery, 300A phase)
rim 17''

2. QS 205 V3 kv9.03
battery 72v (200A constant capable)
Sabvoton 72150 (settings: 150A battery, 300A phase). You can lower phase amps to 250A phase. It (9kv) will be still quicker from the standstill.
rim 17''

As you can see everything is the same except kv.
Ebike with 9kv will be quicker from standstill till it reaches ~30km/h.

So don't say that higher kv is the same or better. KV matters.
If you takes both these ebikes (1,2) to very slow trails, hills, 9kv motor will win, due having better torque from a standstill.
no, it doesnt, not when it comes to cont. torque production. please re read the thread. even in your example, your not comparing like with like, as your amp turns are entirely different, even at 250phA. if you lowered the ph amps to 220 odd for the 9kv motor you might see what is being discussed, that the torque production will be the same, as will be the heat generated (assuming that copper fill etc is the same - ie even if you do lower the ph amps to 225, you may still not have 'everything the same except kv').

toolman2   1 kW

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Re: higher or lower kv

sn0wchyld wrote:
Jan 31 2018 4:34am
minde28383 wrote:
Jan 29 2018 8:39pm
Here is an example from real life circumstances. The same everything except kv (rpm/v).

1. QS 205 V3 kv11.89
battery 72v (200A consntant capable)
Sabvoton 72150 (settings: 150A battery, 300A phase)
rim 17''

2. QS 205 V3 kv9.03
battery 72v (200A constant capable)
Sabvoton 72150 (settings: 150A battery, 300A phase). You can lower phase amps to 250A phase. It (9kv) will be still quicker from the standstill.
rim 17''

As you can see everything is the same except kv.
Ebike with 9kv will be quicker from standstill till it reaches ~30km/h.

So don't say that higher kv is the same or better. KV matters.
If you takes both these ebikes (1,2) to very slow trails, hills, 9kv motor will win, due having better torque from a standstill.
no, it doesnt, not when it comes to cont. torque production. please re read the thread. even in your example, your not comparing like with like, as your amp turns are entirely different, even at 250phA. if you lowered the ph amps to 220 odd for the 9kv motor you might see what is being discussed, that the torque production will be the same, as will be the heat generated (assuming that copper fill etc is the same - ie even if you do lower the ph amps to 225, you may still not have 'everything the same except kv').
-Spot on.

minde28383   1 kW

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Re: higher or lower kv

How many of you say this by observation or experience rather than theory or pure logic?

People who have build over 50 ebike with similar specs say that lover kv (rmp/v) hub is better from a standstill and better up to certain speed (~30km/h, ~40km/h).

minde28383   1 kW

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Re: higher or lower kv

toolman2 wrote:
Jan 31 2018 8:34am
sn0wchyld wrote:
Jan 31 2018 4:34am
minde28383 wrote:
Jan 29 2018 8:39pm
Here is an example from real life circumstances. The same everything except kv (rpm/v).

1. QS 205 V3 kv11.89
battery 72v (200A consntant capable)
Sabvoton 72150 (settings: 150A battery, 300A phase)
rim 17''

2. QS 205 V3 kv9.03
battery 72v (200A constant capable)
Sabvoton 72150 (settings: 150A battery, 300A phase). You can lower phase amps to 250A phase. It (9kv) will be still quicker from the standstill.
rim 17''

As you can see everything is the same except kv.
Ebike with 9kv will be quicker from standstill till it reaches ~30km/h.

So don't say that higher kv is the same or better. KV matters.
If you takes both these ebikes (1,2) to very slow trails, hills, 9kv motor will win, due having better torque from a standstill.
no, it doesnt, not when it comes to cont. torque production. please re read the thread. even in your example, your not comparing like with like, as your amp turns are entirely different, even at 250phA. if you lowered the ph amps to 220 odd for the 9kv motor you might see what is being discussed, that the torque production will be the same, as will be the heat generated (assuming that copper fill etc is the same - ie even if you do lower the ph amps to 225, you may still not have 'everything the same except kv').
-Spot on.

nop,
reread what I wrote in my post.
I said till it reaches certain spead ("Ebike with 9kv will be quicker from standstill till it reaches ~30km/h."), but you are writing: "when it comes to cont. torque production".

sn0wchyld   1 MW

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Re: higher or lower kv

minde28383 wrote:
Jan 31 2018 10:37am
toolman2 wrote:
Jan 31 2018 8:34am
sn0wchyld wrote:
Jan 31 2018 4:34am
minde28383 wrote:
Jan 29 2018 8:39pm
Here is an example from real life circumstances. The same everything except kv (rpm/v).

1. QS 205 V3 kv11.89
battery 72v (200A consntant capable)
Sabvoton 72150 (settings: 150A battery, 300A phase)
rim 17''

2. QS 205 V3 kv9.03
battery 72v (200A constant capable)
Sabvoton 72150 (settings: 150A battery, 300A phase). You can lower phase amps to 250A phase. It (9kv) will be still quicker from the standstill.
rim 17''

As you can see everything is the same except kv.
Ebike with 9kv will be quicker from standstill till it reaches ~30km/h.

So don't say that higher kv is the same or better. KV matters.
If you takes both these ebikes (1,2) to very slow trails, hills, 9kv motor will win, due having better torque from a standstill.
no, it doesnt, not when it comes to cont. torque production. please re read the thread. even in your example, your not comparing like with like, as your amp turns are entirely different, even at 250phA. if you lowered the ph amps to 220 odd for the 9kv motor you might see what is being discussed, that the torque production will be the same, as will be the heat generated (assuming that copper fill etc is the same - ie even if you do lower the ph amps to 225, you may still not have 'everything the same except kv').
-Spot on.

nop,
reread what I wrote in my post.
I said till it reaches certain spead ("Ebike with 9kv will be quicker from standstill till it reaches ~30km/h."), but you are writing: "when it comes to cont. torque production".
Re read the entire premise of this topic. you are producing more torque. I'm not denying that for a second. I'm pointing out that in doing so, your overheating your motor faster, because your producing more heat in the process of producing more torque. Your 'high torque' wind isn't 'high torque' at all, not relative to any other wind (ie in any comparison worth making). Put 33% more amps into the 12kv motor and you'll get the exact same torque out of it as the 9kv one. how can you call a particular wind a 'high torque' motor if a different wind can produce the same torque, for the same length of time, at the same RPM? You saying that 'my high kv motor doesn't produce the same torque' is the equivalent of saying 'my motor produces less torque with 200phA as it does with 300phA' - well no shit Sherlock.

In a situation where your somehow limited to X phase amps and want X torque out of your motor, then yea, sure, a low kv wind might be necessary. but the fact remains its not a 'high torque' motor. Its just a different one. And given how easy it is to get pretty much any phA you want out of a controller, then such a situation doesn't really exist, certainly not on the scales we use on eBikes.

sn0wchyld   1 MW

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Re: higher or lower kv

minde28383 wrote:
Jan 31 2018 10:08am
How many of you say this by observation or experience rather than theory or pure logic?

People who have build over 50 ebike with similar specs say that lover kv (rmp/v) hub is better from a standstill and better up to certain speed (~30km/h, ~40km/h).
I say it from both. But the theory is more than enough - unless you want to somehow offer a proof that the fundamental laws of electromagnetics are wrong, then a given amount of amp turns (or turns in your motor) will produce a given magnetic field. That fields interaction with the static field of the magnets will produce a given force. That force, around an axle, is torque. These laws could indeed be wrong, but a couple of centuries of their (successful) applied use kind of suggests otherwise.

But dont take my word for it. put your motors on any dyno or whatever other test bench you want. put 33% more amps into the high kv motor. see what the torque produced is. I'll bet good money that they're near identical (and would be identical, but there's bound to be some other small factors that mean things aren't quite 100% - it'll likely be far smaller than you could notice outside a lab though)

minde28383   1 kW

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Re: higher or lower kv

I don't support saying - "Theory is more than enough". If I could support one or another I would rather support just opposite, - practice.
Theory is in the ideal of a vacuum, the practical is regarding the situation.
Whole law and physics and science and everything is just to justify real stuff.

And I'm not fighting the law of physics. The are enough people doing it so besides me. Actually, I try to present why they call it "less/more more torque hub" and as I think I understand it I want to share what thinking is behind this, but I guess you understand it anyway already.

You say:
"Put 33% more amps into the 12kv motor and you'll get the exact same torque out of it as the 9kv one."

There are recommended phase amps for certain windings for the motors and of course, the is certain limits on the controller you can use. So you not just put 33% more amps on 9kv and get the same result as 12kv motor; In theory yes, but in practice sometimes not. But just in case you have a controller where you can increase phase amps to 33% you get different efficiency and at certain rpm; and overall worse efficiency if ebike is used in more stop and go situations, but anyway, we don't argue these things here.

As I said, torque from standstill to 30/40 km/h. Not to mix with continues torque production here, again.

People don't just put more 33% amps to get the same torque not taking rest into account. They put more or less the same amps or %20 as per qs manufacturer recommendation, see table. So they put almost the same amps and do get better torque compared to what they would get with similar amps from higher kv motor. That's why people call it more torque motor and in a way, they are right because they do get more torque with the same phase Amps by using lower kv motor.

So bellow two motors' description (more/less torque) is true taking into account that Amps and V are constants and within motor manufacturer recommendation.

QS 205 9kv - more torque / less speed
QS 205 12kv - less torque / more speed

And 9kv will reach certain speed faster than 12kv motor taking into account that rest settings A, V and wheel are identical.

That's why they call it more torque motor.
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Re: higher or lower kv

Performance factors are never identical. A whole bike does make performance for the terrain and environment that it is riding, and we build for a purpose, with practical requirements and limitations.

I’d say: use the highest Kv that you can accomodate for the purpose that you are building for, and the compromises that you are willing to accept.

The art of dosing compromises to achieve an optimal goal, is the fundamental of performance design.
Make it fool-proof, and I will make a better fool.
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tangentdave   1 kW

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Re: higher or lower kv

amen brother
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TheManShaker   1 µW

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Re: higher or lower kv

Hey Guys,

So if I read this right, i should always go for the higher Kv Motor?
I am now planning my build and have decided to go with a Qs205 V3. But should I take the 3,5T or 5T Variant?
I want to use it on Mountains (real Mountain-slopes, not Hills) and it takes a lot of Power to haul my fat ass (110kg) from a stop up the Mountain, often starting from an incline.
I use 20" Motocycle rim and my Battery can only output 90 Amp cont. at 72V.
Am I wrong thinking that in my case the 5T variant (lower Kv) will work best? I do not use any gearing.
Does the winding also affect the regen. charging?

bionicon   1 kW

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Re: higher or lower kv

If you gonna use the 205 as a hubmotor then a 5t will give you the most starting-torque and the lowest speed, for hills you need torque so the 5t will be good. If you use the 205 as a middrive then you can take the 3.5t and gear it down for torque.

minde28383   1 kW

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Re: higher or lower kv

TheManShaker wrote:
Jan 30 2019 3:06pm
Hey Guys,
So if I read this right, i should always go for the higher Kv Motor?
Not always. If you want speed than go with higher Kv. If you want more speed with the same voltage. than higher Kv is option nr1.

If you want more torque from stand still - lower Kv. But you will have less speed by 20 - 25 % with the same wheel.
TheManShaker wrote:
Jan 30 2019 3:06pm
I am now planning my build and have decided to go with a Qs205 V3. But should I take the 3,5T or 5T Variant?
5T

TheManShaker wrote:
Jan 30 2019 3:06pm

I want to use it on Mountains (real Mountain-slopes, not Hills) and it takes a lot of Power to haul my fat ass (110kg) from a stop up the Mountain, often starting from an incline.

5T. Will be better. There will be difference. I would not look in high kv for you. 5T diffidently.

TheManShaker wrote:
Jan 30 2019 3:06pm

I use 20" Motocycle rim and my Battery can only output 90 Amp cont. at 72V.
Am I wrong thinking that in my case the 5T variant (lower Kv) will work best? I do not use any gearing.
Does the winding also affect the regen. charging?
20'' (16'' moto rim the same size) will have very good torque in low speeds and from stand still. You can go with 17'', 18'' or even 19'', but smaller better torque and 17'' is perfect.

With 20'' with fluffy tire (softer ride, better for rocks and roots etc) you will go up to 70km/h freshly charged without Flux. If you got Flux you can do 85km/h with less efficiency and more heat.
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minde28383   1 kW

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Re: higher or lower kv

And 90 Amp cont is not ONLY. It is a lot. You don't need more for 5T.
ebike build in progress
CN 5.5kw scooter (on sale)
E Mindless Maverick Talisman longboard, 2.2KW motor, 150Amps controller (on sale)

TheManShaker   1 µW

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Joined: Jan 30 2019 2:50pm

Re: higher or lower kv

Thanks Man! Helps a lot!

tolkaNo   100 W

Posts: 111
Joined: Dec 29 2017 4:28am
Location: South Australia

Re: higher or lower kv

Im way late to the kv discussion but

Every time i look at high vs low kv in the simulator it ends up in a situation where the low kv makes more power but overheats faster, and the high kv makes less power but overheats slower

That's why im not sure theres a one size fits all solution

danielrlee   1 kW

Posts: 482
Joined: Mar 29 2013 8:30am
Location: Wiltshire, UK
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Re: higher or lower kv

tolkaNo wrote:Im way late to the kv discussion but

Every time i look at high vs low kv in the simulator it ends up in a situation where the low kv makes more power but overheats faster, and the high kv makes less power but overheats slower

That's why im not sure theres a one size fits all solution
Assuming both motors are the same model and correctly wound (same copper fill), they will generate the same amount of torque per unit of heat. If you feed the low kV motor proportionally less current and the high kV motor proportionally less voltage, they will behave identically.
https://TorqueTech.co.uk : Electric bike spares, parts & accessories : 10% discount code ’ENDLESS10

crossbreak   100 MW

Posts: 2877
Joined: Aug 02 2011 11:20am
Location: Germany
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Re: higher or lower kv

what a discussion. minde, please read what john in CR and snowchyld wrote, there is no such thing as a "high torque" or "high speed" motor. Different kV does not mean higher or lower torque or speed capability of the motor. It is just that simple. The magnetic circuit is the same, bearings are the same etc. Very simple to understand that they must be the same.

Now to the practicle advise: Choose your controller according to your windings. When you only can effort a cheap one, then go for the low kV motor, it does not need many phase amps to reach the max torque the motor is capable of. But most likely your are then stuck to low speed. If you want to use the full potential of the motor you can

a) use a low kv motor (6 kV) and a high battery voltage (96V) to get the same top speed. You dont need a controller with high phase current (only 100 amps), but one that can work with high pack voltage (rated for 110V max)

b) use a high kv motor (12kV)and a normal battery voltage (48V for example). You need a controller that can do high phase current (200A), but it does not need a high voltage rating (55V max is sufficient)

system a) and b) will perform the same

@bionicon:
there is an rpm for every motor, where it feels healthy. This point can be calculated an thus the ideal reduction ratio for a given bike (weight, wind and tire drag)

You need:
Motor resistance
Kv
Eddy and hysteresis loss constants
dissipation ability in watts at the desired conditions (max air temperature for example 40°C, max motor temperature, for example 100°C )
plus the above mentioned values for the vehicle
Last edited by crossbreak on Feb 04 2019 6:24am, edited 1 time in total.

danielrlee   1 kW

Posts: 482
Joined: Mar 29 2013 8:30am
Location: Wiltshire, UK
Contact:

Re: higher or lower kv

crossbreak wrote:
Feb 01 2019 6:18am
a) use a high kv motor (12kV) and a high battery voltage (96V) to get the same top speed. You dont need a controller with high phase current (only 100 amps), but one that can work with high pack voltage (rated for 110V max)

b) use a low kv motor (6 kV) and a normal battery voltage (48V for example). You need a controller that can do high phase current (200A), but it does not need a high voltage rating (55V max is sufficient)
FYI, in your above 'a' & 'b' examples, you have the two motors the wrong way round!
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