Brushless vs brushed DC motors (again)

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Eposter - 03 Jul 2005 19:01 GMT

I read the "Electric Motor Handbook" from Astroflight.
The book explains the basic physics of hobby-motor
operation, but there is only a 1 page discussion on
brushless motors. This left me with many questions.

According to the book, brushed-motors have greater
efficiency at lower (high-load) rpms, whereas brushless
motors have greater efficiency at high (low/no-load) rpms.
Is this still true?

The book doesn't talk about magnets (gauss) and their
effect on motor performance. All things being equal,
does a stronger (cobalt) magnet increase efficiency, or does it
just increase the power-dissipation through the windings?
And what is a "wet" magnet (I see that term in advertisements
for RC/Car stock-motors)?

My last question is more of a motor-design question
than anything else. The book's graphs show various
plots for Astroflight's product-line (circa 1994.) The
common trend was smaller-sized motors had *higher* no-load
RPM than the larger-sized motors. Is this a freely-made
decision, or was there a conscious trade-off/compromise
involved in choosing the Kv constant?

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kenji - 04 Jul 2005 05:08 GMT

> I read the "Electric Motor Handbook" from Astroflight.
> The book explains the basic physics of hobby-motor
[quoted text clipped - 20 lines]
> decision, or was there a conscious trade-off/compromise
> involved in choosing the Kv constant?

Jeeze, and I thought *I* was a geek.

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The Natural Philosopher - 04 Jul 2005 10:20 GMT

> I read the "Electric Motor Handbook" from Astroflight.
> The book explains the basic physics of hobby-motor
[quoted text clipped - 5 lines]
> motors have greater efficiency at high (low/no-load) rpms.
> Is this still true?

Not intrinsically, no.

Brushes don't work well at high RPM's thats all. But if they did, in the
end there is little difference in efficiency.

> The book doesn't talk about magnets (gauss) and their
> effect on motor performance. All things being equal,
> does a stronger (cobalt) magnet increase efficiency, or does it
> just increase the power-dissipation through the windings?
> And what is a "wet" magnet (I see that term in advertisements
> for RC/Car stock-motors)?

What a stronger magnet does is bring the max efficiency point down in
RPM. This minimises frictiobnal and orther 'iron' losses and leads to a
better efficiency oiverall: But an air beraiing motor with quality iron
doing 250K RPM might be as effecient with just a teeny magnet..

there are three magnetic materiasl in basic type: ferrite - cheap and
fairly tough. Cobalt, stroinger, but uits real advantage is teh huge
amount of temeperature it can take before de-magging, and neodymium,
extermely powerful magnets but fragile under high temepartures. Never
riun a neo magnet motor hoter than you can touch.

I have no idea what wet mag means. Let me know - its a term from silly
car racing boys who think that alcohol engines should be called 'nitro'

> My last question is more of a motor-design question
> than anything else. The book's graphs show various
[quoted text clipped - 3 lines]
> decision, or was there a conscious trade-off/compromise
> involved in choosing the Kv constant?

In general the higher RPM you can go on a brushed motor the better the
power and efficiency. Hiowever brush gear limits that - but smaller
motors can rev higher han larger ones before it gets to be an issue.

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kenji - 04 Jul 2005 16:30 GMT

> alcohol engines should be called 'nitro'

it definitely sounds kewler

...but we also run off road and on road "flashlights".

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Model Forum / Radio Controlled / Land Models / July 2005

Brushless vs brushed DC motors (again)