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Mar 21, 2021, 06:09 PM
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Fourdan's Avatar
Finally have you selected/bought your monsters ?
Latest blog entry: Scorpion Calc
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Mar 22, 2021, 12:38 AM
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BillzillaAus's Avatar
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Apr 19, 2021, 05:31 PM
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Thats right, ...

... wait for the hommies to whiegh in to commit the monitary fundage! Laramie.
May 18, 2021, 12:12 AM
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Skylar's Avatar
Possibly the most important question when designing a motor from scratch is: what will the Kv be? Things like stator length vs diameter ratio, number of slots, magnets and turns all play a role in yielding a very specific Kv. So the question is, is it possible to determine the Kv before the motor is built and wound?

One method is using eMetor to calculate Kv and other parameters of just about any size motor. You're welcome to try it, but I found it too daunting and gave up.

I decided to look for trends in existing motors, i.e. how Kv changes with e.g. stator size and I started making a few "rules of thumb" based on my observations. When I put these "relationships" into an Excel spreadsheet and tested it, I was surprised by how accurately I could predict Kv for just about any size motor, and how many turns were needed for a target Kv. Granted, it may not be very scientific, but it works for me. For example, I used the spreadsheet to design a 10kW paraglider motor and after building it, which included having the stator laminations laser cut, the predicted Kv was just about spot on!

So the answer to your original question is, yes, there are rules of thumb for designing and building motors. I have figured out some rules that work for me.

If you really want to go the route of designing and building your own motors, here is a head start:

Doubling the stator length will halve the Kv.
Increasing the stator diameter by 33% will halve the Kv.... (e.g. 30mm vs 40mm dia)
Last edited by Skylar; May 18, 2021 at 03:59 PM. Reason: Fixed typo, etc.
May 18, 2021, 02:22 AM
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BillzillaAus's Avatar
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Thanks for that, good to know.
May 18, 2021, 01:12 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Originally Posted by BillzillaAus
I need something about 80 kW ...
For motors in RC we use a conservative rule of thumb for continuous power: 4watt per gram motormass.
But I very much doubt if that rule is applicable for your motor.
The power will be lower because surface-to-volume ratio is lower. A mouse can shed more heat than an elephant.

Originally Posted by Skylar
... Doubling the stator length will halve the Kv. ...
That rule is the result of one-on-one relationship hard physics

Voltage induced in a wind is proportional to surface of that wind.
Doubling stator length doubles induced voltage, therefore rpm/voltage halves.

Vriendelijke groeten en wees voorzichtig, Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
E-flight calculatorswatt-metersdiy motor tips&tricksCumulus MFC
Last edited by Ron van Sommeren; May 18, 2021 at 01:24 PM.

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