What should a brushless feel like?

[Pete B]

Hi All,

Have come to a temporary halt to the renovation here in France due to an unfortunate collision between a disc cutter and my groin yesterday (stitched but I'm still alive and shaking.......)

You may recall I recently posted some newbie questions, kindly addressed by Timbo and others, about the great electrickery mystery. I took the advice and made the recommended purchases so thanks to all.

I now have the perfect excuse, for a few days, to turn my attention to the accumulation of bits and kits I've acquired, at frightening expense(!), with a view to converting the Nebula to brushless, assembling an Easyglider Electric with an inrunner replacement for the stock brushed motor (see source of info here under 'Flight Tests' ) and eventually, tackling a CMPRO Discus. Yes, I know the link is to a French site but the numbers are European and you'll recognise most of the nouns!

To the issue, at last. 

I have purchased two brushless outrunners, a 200w and 300w, and I'm surprised at the variation in effort required to turn the motors by hand. One turns over quite easily whilst the other has considerable resistance, clunking from one sector to the next. Being out in the sticks, my only knowledge reference point, apart from your good selves, is the rest of the web. I have found this short video where the chap is demonstration one motor against another, claiming one is defective. The turning resistance on my two motors seems to fall somewhere between that shown in the video but not far off the extremes. Unfortunately he does not state which one he claims is defective.......

Having never handled such a beast before, I have no experience of what should be the correct 'feel' for a motor. I haven't yet wired them up to test them in case one needs returning. Do motors vary in their turning resistance? Is the defective motor the looser or tighter example? Does it matter?

You know, I've just realised that ignorance isn't a problem until it becomes a problem.........

 Pete

[Tim Mackey]

Motor resistance and what is often described as "notchiness" varies a lot across models and brands - affected by amongst other things the strength, number, and spacing of the magnets. Unless you are feeling actual friction, or fouling then I wuld just hook 'em up and gently run them before opening the tap fully. There is a slight chance that a magnet has become unseated...but frankly its pretty unlikely, and as for bearing or shaft misalignment, with todays CNC production methods etc, I would be very surprised if it was anything like that.

The video of the two motors is less than rivetting - as for which is the duffer ( if any ) I would say that the easier turning one has weak or incorrrectly aligned magnets, assuming they are both of the same model of motor of course, otherwise he is simply not comparing apples with apples

[Ron van Sommeren]

Cogging, or the lack of it, is not a good indicator for quality, power, efficiency, torque, magnet stength etc. Too many parameters effect cogging, Timbo already mentioned a few. Also statortooth geometry, ironquality (magnetic), flux ring iron quality and thickness, magnet shape, gap between magnet and stator, gap between magnets themselves, magnet mis-alignemnts (radial and axial, skewed) and ratio of #statorpoles <> #magnetpoles. The closer this ratio is to 1, the lower the cogging (3slot/2magnet versus 60slot/62magnets). However, a ratio of 1 will give excessive cogging. That's no problem because a with an equal number of magnetpoles and statorpoles motor would never start by itself.

Cogging is not effected by number of winds and Kv, it's a magnetic effect, not an electric effect. The rotor wants to take a (locally) position were the energy in the system is the lowest (and resistance against flux, magnetic resistance a.k.a reluctance is the lowest too).

When shorting the motorleads you will feel more 'cogging'. This is not cogging but the effect of the motor operating as a shorted generator.

Vriendelijke groeten Ron

[Ron van Sommeren]

See the pictures here, 'rasten' = 'cogging' :

http://www.powercroco.de/rasten.html

[Pete B]

Thanks, both, I'm reassured. There is certainly no roughness and I understand that the magnets will have a 'resistive' effect when the motor is turned by hand. I shall proceed gently.......

I must add, by the way,  that I've been very pleased with the quality of products and mail order service from BRC, TJD and Modelfixings - all have been prompt, helpful and a pleasure to deal with!

Pete

ps Babelfish did very well with the translation, Ron, but didn't even try 'Flusschwankungen'!!! 

[Ron van Sommeren]

'flus' = flux

'schwankug(en) = change(s), variation(s)

[Erfolg]

I found your explanation interesting and enlightening.

Regards

Erfolg

[Ron van Sommeren]

More cogging, user 'Martin_G' ("Götzenberger") designs the statorshapes for Scorpion:

http://www.rcgroups.com/forums/showthread.php?t=859562

Vriendelijke groeten Ron

[Ron van Sommeren]

Ron van Sommeren wrote (see)

When shorting the motorleads you will feel more 'cogging'. This is not cogging but the effect of the motor operating as a shorted generator.

Start your car engine and let it tick over stationary. Now turn on the headlights and rear-window de-fogging. You will hear the motor sagging a bit because now it's more difficult to crank the alternator.

 Vriendelijke groeten Ron