what is cogging torque?

Electric Motors and Controllers
Punx0r   100 GW

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Re: what is cogging torque?

Post by Punx0r » Feb 13 2020 4:28am

Regarding what magnets can and cannot do in general google "can magnets do work" and read around a bit. physics.stackexchange is usually pretty good for these sorts of questions. If you have the basics in mind it's less tempting to speculate on what they might be doing in a motor.

Cogging is cogging
Iron losses are iron losses

If you've compared a skateboard with a motor that exhibits cogging with a skateboard with no motor then you can't really conclude anything about the relationship between cogging and drag because you can't separate out the different sources of drag in a motor. All you can safely conclude is: the draggy thing has more drag.

Speaking of drag, kinetic energy is also non-linear with speed (E = 1/2 m v^2). So at higher speed you'd need much more motor drag to slow you down at the same rate as at lower speeds. So when you talk about drag from iron losses being small at very low speeds, the amount of drag required to slow down your skateboard is also small.

Hummina Shadeeba   1 MW

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Re: what is cogging torque?

Post by Hummina Shadeeba » Feb 13 2020 12:33pm

Punx0r wrote:
Feb 13 2020 4:28am
Regarding what magnets can and cannot do in general google "can magnets do work" and read around a bit. physics.stackexchange is usually pretty good for these sorts of questions. If you have the basics in mind it's less tempting to speculate on what they might be doing in a motor.
as I wrote above magnets can’t do work alone

Cogging is cogging
which is a loss of momentum which seems to possibly be an ANOMALOUS form of eddies according to the paper I posted
Iron losses are iron losses

If you've compared a skateboard with a motor that exhibits cogging with a skateboard with no motor then you can't really conclude anything about the relationship between cogging and drag because you can't separate out the different sources of drag in a motor. i don’t think “drag” in the formal sense relating to traveling through a fluid is what is slowing inside the motor All you can safely conclude is: the draggy thing has more drag.

Speaking of drag, kinetic energy is also non-linear with speed (E = 1/2 m v^2). So at higher speed you'd need much more motor drag to slow you down at the same rate as at lower speeds. So when you talk about drag from iron losses being small at very low speeds, the amount of drag required to slow down your skateboard is also small.
The drag of wind resistance is gunna make all comparisons impossible and think would need to map the unpowered motor resistance at all rpm with a dyno. Since all the measurements are of a motor unpowered could just couple two motor shafts after getting the no load currents at all speeds from the driving motor, then do it all again w the tested motor attached

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spinningmagnets   100 GW

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Re: what is cogging torque?

Post by spinningmagnets » Feb 22 2020 5:19pm

The discussion about "magnet motors" is different enough from the subject of cogging forces in PMDC motors like those found on power-boards and ebikes that it is off-topic here. It has been given its own thread.

"Permanent magnet motors, Bedini, Perendev, etc"
viewtopic.php?f=26&t=104793

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Re: what is cogging torque?

Post by Ianhill » Feb 23 2020 12:09pm

I appreciate the effort 👍, the bit that strikes me is ES is no place to talk about the combustion engine, are threads going to be EV releated only i see some are evem abouy diet so where does that fit in ?

Hummina Shadeeba   1 MW

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Re: what is cogging torque?

Post by Hummina Shadeeba » Feb 23 2020 12:32pm

It was a diversion off the topic and worth separating I thought but I wish the magnet motor thread would stay open. I spent 2 hours this morning searching a new lead with Dr Kozeka and supposed evidence of different pull forces when magnets are arranged at an angle! And he died at a young age before finally building the machine based on his discovery. Something for everyone.

Considering how many people believe both magnet motors are real and there’s a conspiracy to suppress the knowledge by oil companies I think a great topic to pursue especially these days. Classic toxic thread material.

I don’t know what he did before but hope u don’t boot motormoder and think the discussion is beneficial for all of us in some way.

I still believe a magnet motor could maybe be made and like to learn more.
Last edited by Hummina Shadeeba on Feb 23 2020 1:04pm, edited 3 times in total.

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spinningmagnets   100 GW

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Re: what is cogging torque?

Post by spinningmagnets » Feb 23 2020 12:40pm

Every good discussion occasionally has a little "thread drift", and most members don't mind as long as it doesn't get too far off topic, or it quickly comes back to the thread subject.

We also have a "General Discussion" section, and a "Toxic" section.

In theory, discussions about diet and any other thing under the sun can be discussed in the General section (not to be confused with "Ebike General, or EV General, etc)

viewforum.php?f=1

The magnet motor thread is unlocked, but will remain in Toxic section unless the majority of the membership requests it be allowed in Alternative energy.

Motor Modder: He broke rules, he was banned.

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Re: what is cogging torque?

Post by Ianhill » Feb 23 2020 1:34pm

I see thanks for clarifying,
The magnet motor thread can go to hell and free up some memory for all I care about free energy mumbo jumbo even self sustained is just crazy talk but that's all they can cling to when it comes to real science they have none.

Hummina Shadeeba   1 MW

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Re: what is cogging torque?

Post by Hummina Shadeeba » Feb 23 2020 2:27pm

Why would there be distinction between free energy and self sustaining. It’s just degrees of efficiency. To be only just self sustaining and perpetual would be a lucky outcome and I’d assume if u could reach that point of sustaining then free energy is a milliwatt away

john61ct   1.21 GW

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Re: what is cogging torque?

Post by john61ct » Feb 23 2020 2:53pm

last two posts should be in the thread specifically split off for that purpose

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Re: what is cogging torque?

Post by Ianhill » Feb 23 2020 3:07pm

John I think you need to chop the legs a bit on your horse mate as this is a discussion a little off topic that's going to return to the thread title I've no time for bickering, misleading, any of it many are fed up with not just electric god but you too filling threads with negativity.

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Re: what is cogging torque?

Post by Ianhill » Feb 24 2020 9:19pm

Let's get back to topic I have my greatness like doc brown when my brain is on the shitter !

How can we picture cogging to see and understand a bit better for us novices.

When we drop a magnet down a copper tube it doesn't drop with gravity as it induces a current in the tube that sets up it's own magnetic field that opposes the magnet and slows it down smaller the air gap stronger the field created slower the magnet falls.

This only happens in a complete circuit so a tube is complete and sets up a field in a dual bubble circulating motion around the magnets N/S pushing and pulling on the magnet as it falls, but a wound coil top to bottom in open circuit will have no slowing effect until the top and bottom are connected and a circuit is made but with larger resistance than the tube in its complete circuit the magnet will fall faster in the coil and as it falls the opposing magnetic field is set up in the coil top to bottom so is weaker but larger the whole coil length from the moment the magnet is dropped vs the tube that focus a field close to the magnet that diminishes with range as the magnet drops down the tube from the start point.

With me so far 🤯 im not but what I think we have emulated is a slotless motor there's no iron circuit if we put a iron pipe on the outside of the copper coil/pipe we need to imagine it as the motor layed out from the inside so there's tooth slots so the iron is segmented along the pipe.

Now as we drop the magnet it will have two speeds and vary between them as it induces more where there iron is behind the copper it will slow more but as it passes it there's hyterisis in the core and the tooth will have a magnetic field left that will only go over time or when there's power applied to the coil in an opposing direction, so as the magnet passes there's a latching effect as the tooth tip trys to grab on and it's this torque we overcome as we turn the motor.

A slotless motor company in US stated that cogging torque effects grow with rpm not decrease I beleive this is true as a motor gets to a point at rpm it gathers heat fast even under low current conditions I've found this under testing myself, I can only hypothesis on what is happening by here when I think it's as the speed increases the speed at which the core is flipping is increasing and the hysteresis is not being cleared but the opposing field as it's timing becomes less and less and the rpm get greater and greater giving less time for hysteresis and flipping of the core on the tooth tip, I'd love to see a 1000fps video of the magnetic field setting up and collapsing and the core reacting but till then I'm blind feeling round for some Braille.

Alot to be learned here for myself and I'm sure current designs are improving as they get released the general public can become more clued up but slotless is where it's at for gathering the right tools to working it out.

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Re: what is cogging torque?

Post by Hummina Shadeeba » Feb 26 2020 10:26am

Be nice to at least see evidence of what relation cogging has to speed if u have any or find any

This tool for just an hour would be nice
https://m.youtube.com/watch?v=MhoneulduAU

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spinningmagnets   100 GW

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Re: what is cogging torque?

Post by spinningmagnets » Feb 26 2020 12:15pm

A coil of copper wire with nothing in the center makes an air-core solenoid, useful for some high frequency electronics. If you add steel to the center, it becomes a very powerful electromagnet. If you use laminated steel sheets to makes the core (instead of a block) then doing that reduces eddy current waste heat.

The stator electromagnets can be turned on and off, but even when they are off, the magnets on the rotor are mildly attracted to the steel laminated cores of the electromagnets. That is cogging, the drag of the permanent magnets trying to hold a position over the stator teeth.

A while ago, there used to be hubmotors with "skewed" stator teeth faces, somewhat like a helical gear as opposed to a common spur gear. Skewed teeth virtually eliminated all cogging, because one stator tooth-face overlapped the next stator tooth-face in relation to the square faces of the permanent magnets on the rotor.

This immediately raises the question, why is that no longer done? If you have two identical motors, with the exception of squared or skewed stator teeth-faces...the skewed teeth will not be quite as powerful, but will have no cogging.

Common squared-off stator teeth have some cogging because of the air-gap between the adjacent teeth faces, but they also have more peak power per a given input watt.

Crystalyte 5404, doctor bass

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Hummina Shadeeba   1 MW

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Re: what is cogging torque?

Post by Hummina Shadeeba » Feb 26 2020 12:35pm

At the beginning of this thread, I think, I posted a paper on skewed magnets which has a similar result to skewed teeth. Interesting was the greatly reduced cogging but slightly increased iron losses making it seem cogging is not related to eddies r hysteresis..in their normal form, but it does bring up the term “anomalous eddies”. Which I’ve never seen before. And the graphing of iron losses is confusing besides.

Have to join some more private libraries to see the numerous studies out there on cogging and get somewhere.

Skewing takes a bit more work to make but more so the down-side is reduced performance

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Re: what is cogging torque?

Post by Ianhill » Feb 26 2020 12:56pm

When the coils in a motor are open it turns fairly easy as we short the coils to eachother it locks up and needs more effort to turn, the iron is similar it's a complete circuit and so it induces a current the stator is segmented so any build up is isolated and smaller than one large eddy so if the core is made to keep eddys small as possible cogging torque will reduce i would imagine.

Skewed teeth I've not seen on any of my stuff and I've not had a really high rpm motor, I imagine skewed stuff is for high rpm, there's a motor firm I was reading into and they stating the effects of cogging torque amd why they use a slotless motor but I can't find the video link.

John in CR   100 GW

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Re: what is cogging torque?

Post by John in CR » Feb 26 2020 11:17pm

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Re: what is cogging torque?

Post by Ianhill » Feb 26 2020 11:59pm

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Re: what is cogging torque?

Post by Ianhill » Feb 27 2020 12:21am

I guess your gonna say it's called lenz's law and this has all been figured out in the days of faraday, but there's no harm in familiarizing ourselfs with these basics even if they apply differently from a linear tube to a centrifugal motor with cogging.

We are not all on the same GOD level as you John did u want EG gone to take the name for yourself you act like your one so enlighten us almighty one with your great wisdom on the cogging conspiracy.

I'm I too far off topic for the thread title ? Have I give whacky way inaccurate facts no just talking on a forum with fellow members if you want to be a dick go take your shit elsewhere no bitching or moaning to mods from me I'm telling you myself grow up engage positively or frock OFF the forum please.

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Re: what is cogging torque?

Post by John in CR » Feb 27 2020 12:09pm

Actually Ian, the dead horse reference wasn't meant for you, sorry I didn't include a quote so you'd know. If I thought you could take a joke as intended I'd respond to your witty retort, but I'll bow out instead and leave you and Hummina to hash it out.

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spinningmagnets   100 GW

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Re: what is cogging torque?

Post by spinningmagnets » Feb 27 2020 5:35pm

I apologize for going so far back into the basics, but it didn't seem like Hummina Shadeeba had gotten an answer, and I thought I'd state the foundation so I could be corrected, or have more info added on top of the basics.

A lot of newbies also read these threads, and "what is cogging torque" is a question that might bring googlers here.

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Re: what is cogging torque?

Post by john61ct » Feb 27 2020 8:51pm

Has it been answered so a noob would understand it?

or at least well enough to decide if it's worth learning more about?

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Re: what is cogging torque?

Post by Ianhill » Feb 27 2020 9:01pm

Easiest way to invision it I beleive is to imagine the magnet as a tube north one end south the other, and drop a steel ball in there it will pull to the centre and require a torque being applied by the hand to break it free that is a simple form of cogging torque, in the motor it's happening between the stator teeth and magnets so multiple times, And i notice there not an even magnet ratio to stator teeth so the pulling effect will vary as the motor turns and it's this we feel as the variance in torque required to hand crank the motor.

I think if we emptied a motor of its copper and spun the rotor inside at a fast enough rpm the steel will be overcome by the induced current and get to a point it's gathering quicker than dissipating and meltdown the core.

This is not really relevant to a motor as the core would have melted long before I'm normal powered use becuase the coils will introduce heat to but I'm trying to invision the induction happening aside to single out cogging.

From what my narrow knowledge tells me if examine torque ripple on the current scale you will get an idea of the force being applied by the motor to overcome the cogging, monitor it at differing rpm and bingo you got your data.

Hummina Shadeeba   1 MW

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Re: what is cogging torque?

Post by Hummina Shadeeba » Feb 28 2020 12:21pm

What Seems safe to assume:

No windings in motor same cogging

Many teeth and one magnet is lots of cogging loss
Many teeth and many magnets is low cogging loss
(assuming good ratio and high lcd)

Iron losses will be more with the many magnets vs few
(Another example of how iron losses in the typical sense aren’t responsible for cogging)

If it were “anomalous eddies” they should be increasing with a square relation to erpm as regular eddies do

“Stray losses” encompass a bunch of unknowns.



The pull force of a pm is the same while coming or going past ferromagnetic material right?
But this seems maybe more than can be assumed and if not the case would give an easy explanation of what causes cogging. As if maybe when coming together there is a delay in the pm field connecting with the tooth, but when departing it has all field lines connected and a delay in the field disconnecting from
Ferromagnetic material

This could be shown by finding the maximum distance possible of any pull force while the pm is approaching tooth and seeing if that distance is increased after passing the tooth.

At least that seems easy enough to test using magnets on a low friction track passing the tooth and comparing the distance at which the magnet has the strength to pull towards the tooth alone, vs the distance at which the magnet has lost the pull force enough to be able to be stationary after passing the tooth.

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