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]

Post by 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?

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

[The Natural Philosopher]

Post by 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?

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.

Post by Eposter
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'

Post by Eposter
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]

Post by The Natural Philosopher
alcohol engines should be called 'nitro'

it definitely sounds kewler

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

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