Brushless vs brushed DC motors (again)

Jul 03, 2005 2 Replies
E

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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K

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

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T

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.

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'

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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