Brushless Motor Construction Discuss design and construction of custom Brushless motors

Motor constant Km

Old 04-21-2022, 10:34 PM
  #1  
banaari
New Member
Thread Starter
 
Join Date: Apr 2022
Posts: 5
Default Motor constant Km

Second question for the day...

Having been scratching head trying to get a grip on what the various BLDC offerings are capable of, in terms of power output vs RPM.

They all seem to quote the Kv (good) - but none that I can find quote either the Km or the motor resistance.
Instead there are various more-or-less fuzzy values around maximum continuous or peak power or current; and vague suggestions around what propeller sizes might be suitable.

One even quotes the no-load current, which in my understanding is backwards - I'd have expected the _stall_ current instead.

Am I missing something?!
banaari is offline  
Old 04-22-2022, 11:55 AM
  #2  
ron_van_sommeren
homo ludens modelisticus
 
ron_van_sommeren's Avatar
 
Join Date: Jul 2005
Location: near Nijmegen, Nederland
Posts: 1,421
Default

A dear, an EE

I've never seen Km specified in RC land, not even the torque konstant Kt (= 1/Kv, conversion in SI units, newtonmeter per ampère).
Maybe this newish thread will help, Louis Fourdan (retired radar designer) and Ralp Okon (a.k.a. www.Powercroco.de) are very well versed in Km
BLDC-PMSM principles and tendencies - RCG



A dear, an EE part II.

Note that 'our' RC brushless ESC's are not VFD's (variable frequency drive, it's the motor 'tells' the ESC when to commutate/switch.
The ESC is just an electronic commutator instead of the mechanical commutator on brushed motors. Same motor model, same model parameters, and same formula for rpm.

Last edited by ron_van_sommeren; 04-23-2022 at 09:47 PM.
ron_van_sommeren is offline  
Old 04-23-2022, 04:00 AM
  #3  
banaari
New Member
Thread Starter
 
Join Date: Apr 2022
Posts: 5
Default

Originally Posted by ron_van_sommeren View Post
A dear, an EE
Ha! No, just spent the last few months grappling with similar issues with an e-Scooter and having got "land" sorted, moving on to "sky"; same tech different element

Originally Posted by ron_van_sommeren View Post
I've never seen Km specified in RC land (...)
So it's not just me
I'll check out that link - thank you! - trying to put some science around design & purchasing before the fact.
banaari is offline  
Old 04-23-2022, 04:39 PM
  #4  
ron_van_sommeren
homo ludens modelisticus
 
ron_van_sommeren's Avatar
 
Join Date: Jul 2005
Location: near Nijmegen, Nederland
Posts: 1,421
Default

Originally Posted by banaari View Post
Ha! No, just spent the last few months grappling with similar issues with an e-Scooter ...
An e-traction forum:
www.endless-sphere.com

Originally Posted by banaari View Post
... moving on to "sky"; same tech different element ...
Wheels need high torque, and therefore high current (torque = current × Kt = current / Kv, in SI units) at zero/low rpm.
Whereas propellers don't need (high) torque and and current at startup, torque gradually increases with rpm squared.


About motor selection and powersystem misconceptions.
An excellent quote from brushless motors Kv?.
Originally Posted by scirocco View Post
While an absolutely critical part of the system ...
... Kv is actually the item one should choose last.
  1. Decide your peak power requirement based on the weight of the model and how you want to fly it:
    Magic numbers for modellers.
  2. Pick a preferred cell count (voltage) and pack capacity for how to deliver the power.
  3. Pick a prop that will a) fit on the model and b) fly the model how you want - often as big as will fit is a good choice, but if high speed is the goal, a smaller diameter higher pitch prop will be more appropriate.
  4. Look for a size class of motors that will handle the peak power - a very conservative guide is to allow 1 gram motor weight for every 3 watts peak power.
  5. Then, look for a motor in that weight range that has the Kv to achieve the power desired with the props you can use - a calculator such as eCalc allows very quick trial and error zooming in on a decent choice. For a desired power and prop, you'd need higher Kv if using a 3 cell pack compared to a 4 cell pack. Or for a desired power and cell count, you'd need higher Kv if driving a smaller diameter high speed prop compared to a larger prop for a slow model.
The reason I suggest picking Kv last, is that prop choices have bounds - the diameter that will physically fit and the minimum size that can absorb the power you want. On the other hand, combinations of voltage and Kv are much less constrained - at least before you purchase the components.

So Kv is not a figure of merit, in that higher or lower is better, it is simply a motor characteristic that you exploit to make your power system do what you want, within the constraints you have, e.g. limited prop diameter, if it's a pusher configuration, or if you already have a bunch of 3S packs and don't want to buy more, and so on.

Minor lay-out changes by RvS

Last edited by ron_van_sommeren; 04-23-2022 at 09:46 PM.
ron_van_sommeren is offline  
Old 04-23-2022, 04:39 PM
  #5  
ron_van_sommeren
homo ludens modelisticus
 
ron_van_sommeren's Avatar
 
Join Date: Jul 2005
Location: near Nijmegen, Nederland
Posts: 1,421
Default

A bit more background information.

IC-engines and e-motors have fundamentally different behaviour.
An ic-engine tries to keep a constant torque.
As a result, as propeller load goes up, rpm must go down.

An e-motor tries to keep a constant rpm, no matter prop-pitch and -diameter. It does not want to slow down as you prop up.
As a result, as load goes up, torque goes up, to keep rpm the same.
Mechanical power is proportional to torque, therefore electrical power has to go up.
Electrical power is proportional to voltage and and proportional to current. Voltage has not changed, therefore current has to go up.

Even if this would lead to the fiery demise of motor and/or controller.


Kv is not related to maximum motor power.
Note that the velocity Konstant kv says very little about max.power capacity, nor about max. current a motor can handle, nor about max. torque, nor propsize, nor rpm, nor rpm range.
A 1:1 train motor and the motor in your toothbrush or in a bedroom appliance can have the same kv = 1000pm/volt.
Kv is the velocity Konstant, a physical property (like mass, length and volume), expressed/measured in the physical unit rpm/volt (like kg, meter and m³).


Kv is not a rating, not a figure of merit.
Not something the motor delivers like power and torque (explanation).
A kv=1000pm/volt motor on 10volt and a kv=2000rpm/volt on 5volt will give same no_load speed of 10,000rpm.


Kv, what the motor wants to do, versus max. power, what it can do.
Kv matches desired rpm and battery voltage, there's nothing more to it.
  1. Want
    Kv and voltage determine how fast motor wants/tries to run.
    (rpm_noload = voltage × kv, or, in other words, kv = rpm_noload / voltage.)
  2. Current ~ torque
    RPM and prop determine torque needed, which in turn determines current drawn.
    Torque and current are proportional, in the same way that rpm is proportional to voltage.
    (torque = current × kt, where kt = 1/kv (in SI units!), or proportional to kv³ ).
  3. Can
    A motor's max.current and max.power ability determine whether that battery/motor/rpm/prop combo can run without going up in smoke.

Last edited by ron_van_sommeren; 04-23-2022 at 09:43 PM.
ron_van_sommeren is offline  
Old 04-24-2022, 01:07 AM
  #6  
banaari
New Member
Thread Starter
 
Join Date: Apr 2022
Posts: 5
Default

Thanks Ron.
TBH the issues I've got aren't with the physics or the electronics - just that one crucial parameter annoyingly absent from (most of) the manufacturers' published specs.
Although I see Cobra at least publish resistance values.
banaari is offline  
Old 04-24-2022, 12:07 PM
  #7  
ron_van_sommeren
homo ludens modelisticus
 
ron_van_sommeren's Avatar
 
Join Date: Jul 2005
Location: near Nijmegen, Nederland
Posts: 1,421
Default

Originally Posted by banaari View Post
... Although I see Cobra at least publish resistance values.
A.k.a. Lucien Miller
www.innov8tivedesigns.com
&
www.cobramotorsusa.com

Lucien knows what he is talking about, good tech advice, measures the motors himself, true/real prop and performance numbers, propcharts, outstanding advice.
Sterling reputation by what I have read.

His excellent motor, esc, soldering, bearing, shaft, connector, ... instruction videos
www.youtube.com/c/Innov8tive8/videos
ron_van_sommeren is offline  
Old 04-24-2022, 01:19 PM
  #8  
ron_van_sommeren
homo ludens modelisticus
 
ron_van_sommeren's Avatar
 
Join Date: Jul 2005
Location: near Nijmegen, Nederland
Posts: 1,421
Default

Originally Posted by banaari View Post
... Although I see Cobra at least publish resistance values.
You could use a milliOhm meter, but they tend to be expensive. Fortunately there are other methods for determining motor winding resistance (Rm):
www.bavaria-direct.co.za, RCG user skylar a.k.a. Christo v.d. Merwe
→ Motor Constants Kv, Rm, Io
motor winding resistance (Rm)
ron_van_sommeren is offline  

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright © 2021 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.

Page generated in 0.08110 seconds with 10 queries