Should identical motors "cog" the same? - RC Groups
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Apr 17, 2017, 11:07 PM
Missileer Extraordinaire
Mel Duval's Avatar
Discussion

Should identical motors "cog" the same?


I have two Eflite 60 400kv motors. One is right out of the box and cogs strongly. One I got used and has only a small/moderate amount of clogging. Is the used motor bad? Iavenot run it yet and don't have a good way right off. Any ideas?
Thx,
Mel D.
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Apr 18, 2017, 12:44 AM
Registered User
I don't think you can use cog feel to determine very much.
Apr 18, 2017, 02:11 AM
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Skylar's Avatar
You may be onto something here. Two identical motors should have the same amount of cogging. Cogging is determined by magnet strength, magnet size, hammerhead shape, air gap, stator slot & pole geometry and skew angle - all of which should be identical in identical motors.

It's possible that the used motor have weaker magnets. You can verify that by measuring the Kv, because weaker magnets result in higher Kv. It is not difficult to measure Kv yourself.

The bearings in motors are usually tighter until run-in. Make sure you take that into account when comparing 'cogging'.
Apr 18, 2017, 09:21 AM
Registered User
One thing to check. Make sure the wires aren't touching each other. If they are, when you spin the motor the moving magnets create a current in the coils and it will resist wanting to turn. That could easily be mistaken for strong cogging.
Apr 18, 2017, 10:45 AM
Suspended Account
I read here years ago that magnets can be partially demagnetized if the motor overheated.
Apr 18, 2017, 12:28 PM
Registered User
Skylar's Avatar
Quote:
Originally Posted by Zerts
I read here years ago that magnets can be partially demagnetized if the motor overheated.
You are quite correct, Zerts.

And when that happens, the Kv goes up. And by measuring the Kv, and comparing it to stock Kv, one can determine if the magnets got demagnetized - which would also explain the weaker cogging.
Apr 18, 2017, 02:58 PM
Suspended Account
Not to say all motors should cog strongly. Some don't and for some reason unknown to me, that's fine ( I have a Thrust/Scorpion motor that hardly cogs at all).
Apr 18, 2017, 03:59 PM
Registered User
In most applications, cogging doesn't matter. It really only matters in slow speed or positioning applications. One of the biggest factors in cogging is the number of poles and the number of slots. Certain combinations cog more and certain combinations cog less.
Apr 18, 2017, 05:52 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
All of the above.

Could be a slightly misaligned magnet, no problem.

Do both motors have the same number of slots/teeth, and same number of magnetpoles? (I would not put it past them )

Disconnect motors completely from controller, and don't let the connectors touch/short. Crank both motors by hand, one could have excess/more cogging (magnetic bumps). That makes it more difficult for controller to start the motor. Not a big deal.

Cogging, the magnetic bumps you feel when cranking the shaft by hand, or the lack of cogging, says nothing about motor power, efficiency, iron type/grade, quality, magnet strength or quality, max.current, max. power, Kv, Rm, Io, Kt. Too many motor design variables effect cogging.
Cogging depends a.o. on number of magnetpoles versus statorpoles, magnet strength, distance between magnets, magnet (mis)alignment/placement, magnetshape etc.
This effect can be put to good use:
en.wikipedia.org/wiki/Reluctance_motor

Don't forget to disconnect motor(s) from ESC when checking for magnetic cogging, and don't let the connectors touch. Otherwise the ESC may short the motor wires (depending on ESC type/make, yes/no power), thus causing the motor to act as a shorted generator → brake action (e.g. when you want to stop a prop in a glider).
Crank the motorshaft with your fingers with wires connected and motorwires shorted, you'll feel the difference.

Vriendelijke groeten Ron
Apr 19, 2017, 01:49 PM
AMA 754
BobRCnut's Avatar
Many times, I've seen two identical motors, brand new, never been connected to anything, and fresh out of the box they have different 'cogginess' but perform identically. No relationship between price/quality either... same situation with a $15 Suppo or a $60 Scorpion.
Apr 19, 2017, 07:13 PM
Registered User
No knowledge of the various ..Theories.. re cogging causes .
Do know though that in taking a previously cogging less motor and rewinding it with thicker wire and slightly fewer turns has resulted in quite strong coggings.
This has happened several times.. So hardly a chance result.
One motor I had to Re - rewind with more turns /thinner wires.
So the ESC could actually manage startups.

'Splain that Ricky
Apr 20, 2017, 08:37 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
How did you notice the cogging?

Vriendelijke groeten Ron
Apr 20, 2017, 09:58 AM
Missileer Extraordinaire
Mel Duval's Avatar
Thanks to all.
1. Motors are physically identical as close as I can tell.
2. Motors are disconnected from ESCs.
3. They are mounted on a twin I am finishing up. I just grabbed each motor in turn and said WHOA!!! They don't match....
4. I have no idea how the low cog motor was treated. Got it used with a Phoenix 80.
Apr 20, 2017, 10:24 AM
Suspended Account
Check both motors running props with a wattmeter and rpm meter?
Apr 20, 2017, 04:17 PM
Registered User
The winding doesn't matter for cogging. It has no influence on cogging at all. It's purely the design of the magnets and steel. Unless, of course, the lead wires are shorted together.


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