Brushless vs brushed DC motors (again)

[Eposter]

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?

[kenji]

In article <zJVxe.70$6%2.31@newssvr21.news.prodigy.com>,
"Eposter" <eposter@nowhere.com> wrote:

> 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?

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

[The Natural Philosopher]

Eposter wrote:

> 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?
>

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
> 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?
>

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.

>

[kenji]

In article <1120469230.55416.0@dyke.uk.clara.net>,
The Natural Philosopher <A@b.c> wrote:

> alcohol engines should be called 'nitro'

it definitely sounds kewler

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