higher or lower kv

Discussions related to motors other than hub motors.
This includes R/C motors, botttom bracket, roller and geared drives.
Balmorhea   10 kW

10 kW
Posts: 546
Joined: Oct 01 2019 8:30pm

Re: higher or lower kv

Post by Balmorhea » Nov 20 2019 2:36am

danielrlee wrote:
Nov 20 2019 2:22am
Not quite. Assuming both motors have the same copper fill....

You're forgetting the fact that if you also proportionally reduce the voltage supplied to the higher kV motor at the same time as increasing the current, you get the same heat generated while producing the same torque from the same wattage at the same speed, both running at the same efficiency.
The problem that hurts efficiency more than that is when you use a high kV motor and then run in a lower fraction of its RPM range. Motor efficiency increases with speed, up to a peak of about 80 percent of the motor's free speed. With a slow wind motor, you can often cruise at that speed. With a fast motor, you top out at something less than the motor's most efficient speed. Especially when your typical operating speed can be attained by the slower wind motor.

So it really doesn't matter what the fast wind motor's peak efficiency is, because you never reach it.

User avatar
danielrlee   10 kW

10 kW
Posts: 502
Joined: Mar 29 2013 8:30am
Location: Wiltshire, UK
Contact:

Re: higher or lower kv

Post by danielrlee » Nov 20 2019 3:00am

Balmorhea wrote:
danielrlee wrote:
Nov 20 2019 2:22am
Not quite. Assuming both motors have the same copper fill....

You're forgetting the fact that if you also proportionally reduce the voltage supplied to the higher kV motor at the same time as increasing the current, you get the same heat generated while producing the same torque from the same wattage at the same speed, both running at the same efficiency.
The problem that hurts efficiency more than that is when you use a high kV motor and then run in a lower fraction of its RPM range. Motor efficiency increases with speed, up to a peak of about 80 percent of the motor's free speed. With a slow wind motor, you can often cruise at that speed. With a fast motor, you top out at something less than the motor's most efficient speed. Especially when your typical operating speed can be attained by the slower wind motor.

So it really doesn't matter what the fast wind motor's peak efficiency is, because you never reach it.
That's not correct. Since you're not adjusting any other system parameters to account for the differing winding speeds, you're making an 'apples & oranges' comparison.

If you proportionally adjust the voltage by the same amount as the current, both motors will run with the same RPM range and therefore at the same efficiency.
https://TorqueTech.co.uk : Electric bike spares, parts & accessories : 10% discount code ’ENDLESS10

Balmorhea   10 kW

10 kW
Posts: 546
Joined: Oct 01 2019 8:30pm

Re: higher or lower kv

Post by Balmorhea » Nov 20 2019 12:29pm

danielrlee wrote:
Nov 20 2019 3:00am
If you proportionally adjust the voltage by the same amount as the current, both motors will run with the same RPM range and therefore at the same efficiency.
That's not how my batteries work. I suppose I could design and build e.g. a 17V battery to suit a needlessly fast wind motor, but it's easier to use a widely supported voltage and get a motor with the right kV.

Because using a widely supported battery voltage is what most people do regardless what winding they use, fast windings are more often than not a mistake.

larsb   1 MW

1 MW
Posts: 1743
Joined: Dec 10 2014 5:12am
Location: Gothenburg, Sweden

Re: higher or lower kv

Post by larsb » Nov 20 2019 12:38pm

Last time i checked you could buy or build any type of battery. There are available batteries between 10s and 24s at least and the question was if low or high kv makes a difference in motor torque.

No it doesn’t - stop spreading misguided info. people coming here for help might pick it up.
Last edited by larsb on Nov 20 2019 3:08pm, edited 1 time in total.
Ride on!

Balmorhea   10 kW

10 kW
Posts: 546
Joined: Oct 01 2019 8:30pm

Re: higher or lower kv

Post by Balmorhea » Nov 20 2019 2:33pm

larsb wrote:
Nov 20 2019 12:38pm
[...]the question was if low or high kv makes a difference in motor torque.

No it doesn’t -
At the same power and different RPM, motor winding absolutely does make a difference in torque. At the same power and the same RPM (and the same efficiency), then by definition it does not.

If you're trying to make the most torque from a given controller with a fixed amp limit, or from a given battery with a limited current capability, high count motor winding is how you do that. You trade off motor RPM for more torque.

larsb   1 MW

1 MW
Posts: 1743
Joined: Dec 10 2014 5:12am
Location: Gothenburg, Sweden

Re: higher or lower kv

Post by larsb » Nov 20 2019 2:43pm

True with modification, if motor is saturated at max amp/lower kV it won’t create more torque.

I find that general statements like “low kV motors are..” aren’t very meaningful. People who don’t know the limitations get mislead and the rest don’t need them.
Last edited by larsb on Nov 20 2019 3:36pm, edited 1 time in total.
Ride on!

sleepy_tired   100 W

100 W
Posts: 102
Joined: Jul 02 2019 11:46am

Re: higher or lower kv

Post by sleepy_tired » Nov 20 2019 3:33pm

If you're trying to make the most torque from a given controller with a fixed amp limit, or from a given battery with a limited current capability, high count motor winding is how you do that. You trade off motor RPM for more torque.
With a the same limit on voltage and amperage you get less torque at higher RPM and more torque at lower RPM with a low-KV motor versus a high-kv motor.

Here is a simulation of two DD hub motors climbing a 7% grade.

https://www.ebikes.ca/tools/simulator.h ... &grade_b=7

The M2705 has a 12.5 Kv
The M2707 has a 9 Kv.


You can see that in the simulation both motors climb at the same rate, overheat at the same time, and have pretty much the same torque at that RPM. The lower KV motor has a slight efficiency advantage, but it's not something that will be noticeable. If you do that same simulation with a 40 amp controller instead of the 25 amp one then the lower-KV motor can actually climb faster because it has more torque at the relevant RPM.

Kinda funny how it works out.

The difference is that the high KV motor has higher phase amps then the lower KV motor.

So, yeah, while it's technically true that given the amperage going into the motor (not the controller) the lower KV motor does produce more torque... that's not really what anybody actually cares about. .

If somebody wants to have a easier time during burn out sand you were to tell them that the 2707 motor would be better, you would be right.

If somebody has 5-15% grades in their hometown and you were to tell them that a 2707 motor would provide better hill climbing performance.. you would be wrong. Except in the case were they are starting out from being stopped on a hill.

larsb   1 MW

1 MW
Posts: 1743
Joined: Dec 10 2014 5:12am
Location: Gothenburg, Sweden

Re: higher or lower kv

Post by larsb » Nov 20 2019 3:43pm

you compare apples to oranges, if you want to prove a statement like “low kV MOTORS are..”

This is what should be compared
1xkV 1xamps 1xvoltage
vs
2xkV 2xamps 0.5xvoltage

They are the same power, torque and rpm.
—>The system just needs to be matched to the motor for good performance.
—> The motor needs to reach max rpm and max torque for max performance, doesn’t matter if it’s through low kV/low current/high voltage or high kV/high current/low voltage
Ride on!

User avatar
minde28383   10 kW

10 kW
Posts: 549
Joined: Apr 02 2010 7:11am

Re: higher or lower kv

Post by minde28383 » Nov 20 2019 4:37pm

Speaking of 205 3kw hub in rear wheel.
The reality is that lower kv pulls better of the line and following is simple example, explanation why people are saying so.
People are having controllers like Sabvoton or Kelly or some Infineon etc. Controller is very important factor in how much thrust you will have with higher kv motor.
Say you need 150a for slow kv and you need 230a for high kv to have the same thrust from stand still than you have to understand second thing: where from you will get that additional 80a? Hence lower kv has better torque. Even If you have these additional 80a than good for you. You can use higher kv motor but even than does not guarantee you that you will have the same thrust as with lower amps and lower kv motor.
For high kv hub, controller must be able to deliver that current with high rampage, not like Kelly. Otherwise even lots of amps controller and high kv motor will feel dull from stand still.

You can't set any amps, also battery is not limitless. So true world true outcome is that lower kv pulls better of the line and is better in forest, trails and similar slower applications. That is why people say lower kv has better torque, thrust, because it does better accelerate.
If you ask theory guy what motor is better for you will get one answer and if you will ask another guy one who rides and had several bikes you might get different answer (or maybe the same, who knows) or better get few bikes with different setups and make a test and present results.
ebike build in progress
CN 5.5kw scooter (on sale)
E Mindless Maverick Talisman longboard, 2.2KW motor, 150Amps controller (on sale)

larsb   1 MW

1 MW
Posts: 1743
Joined: Dec 10 2014 5:12am
Location: Gothenburg, Sweden

Re: higher or lower kv

Post by larsb » Nov 20 2019 5:01pm

You buy a votol em72300 with 800 phase amps, get a turnigy lipo battery. Boom! Motor is way into saturation, no limits on torque, same cost as a sabvoton. It can be done (but not if you don’t know..)
where from you will get that additional 80a
I run 470phase amps controller so i don’t see the issue and it costs about the same as a kelly kls7230 :D

But i get what you mean, in real use there can be limits. Standstill is a true limit as most controllers don’t output full phase current at zero rpm.

“low kV motors are..” will still not be true as it has more to do with controller limits than the motor.
Ride on!

Balmorhea   10 kW

10 kW
Posts: 546
Joined: Oct 01 2019 8:30pm

Re: higher or lower kv

Post by Balmorhea » Nov 21 2019 1:17am

larsb wrote:
Nov 20 2019 5:01pm
[...]get a turnigy lipo battery. Boom!
Yeah, that's why most of us don't use janky model airplane batteries. We like our bikes and our houses un-boomed.

Post Reply