Motor constant Km
04-21-2022, 10:34 PM
#1
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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?!
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?!
04-22-2022, 11:55 AM
#2
homo ludens modelisticus
Join Date: Jul 2005
Location: near Nijmegen, Nederland
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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, newton
meter 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.
I've never seen Km specified in RC land, not even the torque konstant Kt (= 1/Kv, conversion in SI units, newton
meter 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.
04-23-2022, 04:00 AM
#3
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Originally Posted by ron_van_sommeren
A dear, an EE
Originally Posted by ron_van_sommeren
I've never seen Km specified in RC land (...)
I'll check out that link - thank you! - trying to put some science around design & purchasing before the fact.
04-23-2022, 04:39 PM
#4
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Originally Posted by banaari
Ha! No, just spent the last few months grappling with similar issues with an e-Scooter ...
www.endless-sphere.com
Originally Posted by banaari
... moving on to "sky"; same tech different element ...
...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
While an absolutely critical part of the system ...
... Kv is actually the item one should choose last.
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
... Kv is actually the item one should choose last.
- Decide your peak power requirement based on the weight of the model and how you want to fly it:
Magic numbers for modellers. - Pick a preferred cell count (voltage) and pack capacity for how to deliver the power.
- 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.
- 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.
- 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.
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.
04-23-2022, 04:39 PM
#5
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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.
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.
- Want
Kv and voltage determine how fast motor wants/tries to run.
(rpm_noload = voltage × kv, or, in other words, kv = rpm_noload / voltage.) - 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
). - 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.
04-24-2022, 01:07 AM
#6
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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.
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.
04-24-2022, 12:07 PM
#7
homo ludens modelisticus
Join Date: Jul 2005
Location: near Nijmegen, Nederland
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Originally Posted by banaari
... Although I see Cobra at least publish resistance values.
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
04-24-2022, 01:19 PM
#8
homo ludens modelisticus
Join Date: Jul 2005
Location: near Nijmegen, Nederland
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Originally Posted by banaari
... Although I see Cobra at least publish resistance values.
www.bavaria-direct.co.za, RCG user skylar a.k.a. Christo v.d. Merwe
→ Motor Constants Kv, Rm, Io
→ motor winding resistance (Rm)
