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Old Feb 11, 2002, 08:52 AM
Electrics rule
risto's Avatar
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Brushless motor "cogging". What is it?

Like the thread says - what means motor cogging when talking about brushless motors.

There is said that Kontronik Tango and Samba motors are non-cogging

Risto
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Old Feb 11, 2002, 10:52 AM
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Cogging means that when you turn the motor by hand it jumps from one magnet pole to the next and feels bumpy when you turn it. A non cogging motor turns freely.

I think that whether it happens or not is due to the materials of construction of the motor.

By itself, it has no detrimental effect on performance.
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Old Feb 11, 2002, 11:39 AM
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What Dave said but I think the way the motor is constructed affects cogging - if the magnets and coils are all straight (front to back of motor) you will have a big cogging effect, if the windings are laid with a twist it won't be so noticeable.

Brian
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Old Feb 11, 2002, 12:29 PM
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Actually the cogging effect (technically known as detent)
is due to pole construction. The windings have no effect.

I used to design motor drives for large (300+ ft-lb torque)
DC brushless motors.

The cog or detent is the point at which the center of the magnets
perfectly line up with the ideal magnetic path through
the poles.

This detent can be sharp or soft depending on pole constrution.

A straght pole (no twist) along it's length will have
a sharp detent.

A twisted pole will have less. This is because for different
points along the rotor the magnetic "perfect" point is at a different
point in the rotation of the rotor.

Pole shape also has a factor in the detent amount as does air gap between the rotor and poles.

The drawback is loss of torque, high detent motors have higher
peak torque and better efficency than low detent motors.

The positives are smoother operation.
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Old Feb 11, 2002, 12:31 PM
Electrics rule
risto's Avatar
Bielefeld, Germany
Joined Jun 2000
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Thanks for the replies.

> if the magnets and coils are all straight
> (front to back of motor)
Looking on the Kontronik catalog at Tango motor, it has magnets straight from front to back and also the windings (not in an angle).
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Old Feb 11, 2002, 07:00 PM
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There are motors that have no turning resistance at all. The Lehner range of brushless motors for one. I don't know about the Kontronic.

If there is no magnetic material in the case or windings the motor won't cog. Is that not right?
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Old Feb 11, 2002, 07:49 PM
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That is right Dave. Aveox, Mega and Jeti motors have magnetic laminations which the copper is wound through (iron or steel), Kontronik, Hacker and probably Lehner do not - it is just a basket of copper would around nothing. The copper becomes magnetic when energized but when the power is off there is nothing for the poles on the rotor to "grab at".

According to Steve Nue theoretically the laminations give stronger magnetism but the other way packs in considerably more copper so there is little to choose between them. The Kontronik and Hackers do seem to have lower Io's for a given stator resistance which makes them more efficient at lower power levels while still maintaining the ability to give up lots of power at higher amps. Just my observations...
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Old Feb 11, 2002, 07:53 PM
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The motor will not detent/cog at zero current if
there is no magnets in the design. No magnetic
force = no cogging.

With current there are still highs and lows in the
torque waveform due to finite number of poles/circuits
in the design. A motor with infinite number of
poles/circuits (which is impossible) would have zero
detent/cogging. This type of motor can be a variable
reluctance motor or a couple of other types that
the name of escapes me at the moment.

It has been a long time since I did a design for a variable
reluctance motor but as I remember they can me made
very linear. The problem with them is high pole
count is difficult due to complexity of design. That is why
most electric brushless motors have magnets. Most of
the designs I have worked on like this were 2 pole limited
travel (40 degs or so) designs.
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Old Feb 11, 2002, 09:05 PM
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Quote:
Originally posted by The Other Dave
The motor will not detent/cog at zero current if
there is no magnets in the design. No magnetic
force = no cogging.
No magnets = No turning round
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Old Feb 11, 2002, 09:12 PM
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Hacker, Lehner, Kontronik and probably a few others have no cogging effect at all. The iron has no slots and this results in no cogging. The particular Kontronik pictured above has a slotless backiron that spins with the shaft and magnets. There is in fact back iron for windings in all of these motors. The windings are placed inside a "tube" of iron. There are benefits and drawbacks to both designs standard slotted iron, and the newer slotless type. The slotless type is new seemingly and not yet accepted by industry. I have a very new book about motors and of the 1500 pages only about 4 lines describe slotless brushless types.

Cogging has little effect on our use. In some motors it can take a substantial amount of current to begin rotation, but once turning the effect is negligible. We don't run the motors at an RPM that cogging is an issue.

BTW our motors are regular drushless DC three phase types. Variable reluctance or stepper motors are not applicable to modeling.

Greg
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Old Feb 11, 2002, 10:54 PM
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Ah so they are slotless types.

That would explain the no cogging.

I have never done a design for a no slot motor but I
have looked at them and sat in on design reviews for a couple.

I allways thought that they were only good
for low flux density/high rpm designs because the effective
air gap was so high. I would have thought they would
be too ineficient at partial throttle.

The other problem with them was high heating of the stator
iron due to eddy currents, unless they were using laminated stacks which that motor (in the picture) is not.

I was at a design conference one time and I remember that the
disk drive folks were crazy about the slotless motors.

HMMM the other thing about that motor. Moving stator = high
inertia = broken props if you hit one. Not to mention much
bigger bearing requirements.
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Old Feb 12, 2002, 12:17 AM
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Most slotless motors do use laminations. The one in the picture above does not because it probably wouldn't stay together. The magnets never move relative to the iron. If you spin the shaft of this motor by hand it spins for almost a minute. There is virtually no loss in the iron. The only thing stopping it are the bearings and air friction. Of course there are drawbacks of relatively high resistance windings due to the number of turns required to get the flux density. There must be some eddy current loss in the iron when the coils are actually switching though. Overall it ends up very good.

Greg
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Old Feb 12, 2002, 05:05 PM
homo ludens modellisticus
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The Netherlands, GE, Nijmegen
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iron-less LRK-torquemax??




Could a do-it-yourself brushless outrunner like the ones above, be turned into a iron-less design? More information on these motors in my signature.
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Old Oct 11, 2009, 08:52 AM
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Joined Apr 2009
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Quote:
Originally Posted by The Other Dave
Actually the cogging effect (technically known as detent)
is due to pole construction. The windings have no effect.

I used to design motor drives for large (300+ ft-lb torque)
DC brushless motors.

The cog or detent is the point at which the center of the magnets
perfectly line up with the ideal magnetic path through
the poles.

This detent can be sharp or soft depending on pole constrution.

A straght pole (no twist) along it's length will have
a sharp detent.

A twisted pole will have less. This is because for different
points along the rotor the magnetic "perfect" point is at a different
point in the rotation of the rotor.

Pole shape also has a factor in the detent amount as does air gap between the rotor and poles.

The drawback is loss of torque, high detent motors have higher
peak torque and better efficency than low detent motors.

The positives are smoother operation.
Quote:
Originally Posted by Ron van Sommeren


And anyone who thinks more cogging indicates higher efficiency is a ______ in physics and electrics.

Vriendelijk groeten Ron
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Old Oct 11, 2009, 10:17 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
The Netherlands, GE, Nijmegen
Joined Feb 2001
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Please, don't report Bob's post.

Vriendelijke groeten Ron
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