Why armature MMF is triangular in waveform?

[Noaha]

Why armature mmf is triangular in waveform? According to Faraday's law, the mmf should be rectangular. But I read some where that the total mmf of all the coils sums up to produce triangular waveform. I don't understand how that happen.

 

[jim hardy]

But I read some where that the total mmf of all the coils sums up to produce triangular waveform. I don't understand how that happen.

Without knowing what that author was describing , how would anybody venture even a guess ?

What kind of machine is it you describe that has triangular armature mmf ?

 

[Noaha]

Without knowing what that author was describing , how would anybody venture even a guess ?

What kind of machine is it you describe that has triangular armature mmf ?

Sorry about that. I am asking for a DC machine.

 

[jim hardy]

The phrase "Triangular Armature MMF" makes no sense to me

Armature MMF in a DC machine adds (by vectors) to the field MMF , shifting it off axis. It's not time variant unless armature current is also time variant.

I can't fathom what your author had in mind.

Got a picture from his book ?

Sorry,

old jim

 

[cnh1995]

The phrase "Triangular Armature MMF" makes no sense to me

Armature MMF in a DC machine adds (by vectors) to the field MMF , shifting it off axis. It's not time variant unless armature current is also time variant.

I can't fathom what your author had in mind.

Got a picture from his book ?

Sorry,

old jim

I think he's talking about this.

 

[jim hardy]

Ahhh uniform air gap,
that step wave says it all,
each step includes another armature turn
smooth out the steps and you have your triangle wave.

I never thought of it that way.

Picture is worth a thousand words again ?

thanks cnh !

here's another picture
use your right hand rule (my 'hand' came out a bit small)

 

[Noaha]

"

each step includes another armature turn

"
What does this line means?

 

[Noaha]

Ahhh uniform air gap,
that step wave says it all,
each step includes another armature turn
smooth out the steps and you have your triangle wave.

I never thought of it that way.

Picture is worth a thousand words again ?

thanks cnh !

here's another picture
use your right hand rule (my 'hand' came out a bit small)
View attachment 206779

Can you please explain how step wave is obtained due to uniform air gap?

Also, I am really sorry to take your time but I didn't understand what the above picture wants to say.

 

[jim hardy]

What does this line means?

a picture is worth a thousand words..

The steps i took as author's attempt to convey idea that total mmf is sum of mmf's from alll the individual conductors.

I was taught to regard armature current as a sheet so just drew one vector for MMF.

So to me this was a new idea. Old guys don't handle new ideas so well as you younger folks with your more facile brains.

old jim

 

[Noaha]

a picture is worth a thousand words..

View attachment 207273

The steps i took as author's attempt to convey idea that total mmf is sum of mmf's from alll the individual conductors.

I was taught to regard armature current as a sheet so just drew one vector for MMF.

So to me this was a new idea. Old guys don't handle new ideas so well as you younger folks with your more facile brains.

old jim

I applied right hand rule on the pair of conductors as you showed and the direction of flux was downwards. I still don't understand how it forms a step wave.

 

[Noaha]

I was taught to regard armature current as a sheet so just drew one vector for MMF

I am not able to imagine how this thing will look like.

 

[jim hardy]

I applied right hand rule on the pair of conductors as you showed and the direction of flux was downwards.

Conventional or electron current ?

Big Ten-OOPS !

 

[Noaha]

Conventional or electron current ?

Conventional current.

 

[Noaha]

Conventional or electron current ?https://www.physicsforums.com/attachments/207274

In this image, the cross represents outward electron current and inward conventional current. Am I right?

 

[jim hardy]

In this image, the cross represents outward electron current and inward conventional current. Am I right?

You're right i drew 'em backwards ! Will fix it

i knew something was upside down...

 

[jim hardy]

I'm not sure what author is portraying there.
At first glance it looks as if he may have drawn mmf upside down too,

What's known is mmf's add up. I think that's what he is attempting to convey.

 

[Noaha]

What's known is mmf's add up

The author is smoothing/adding the step waves to obtain the triangular wave as you said in one of the comments. But how the step wave is obtained on the first place? I am not able to figure this out.

 

[jim hardy]

Every amp-turn adds its MMF to the total.
Every conductor is the boundary of a current loop carrying some number of amp-turns.
So at each conductor is an increment in MMF., or a decrement depending which direction you are traversing, equal to the amp-turns in that loop.
That his steps are all the same size suggests the condustors he showed are in series.

 

[Noaha]

Every amp-turn adds its MMF to the total.
Every conductor is the boundary of a current loop carrying some number of amp-turns.
So at each conductor is an increment in MMF., or a decrement depending which direction you are traversing, equal to the amp-turns in that loop.
That his steps are all the same size suggests the condustors he showed are in series.

This is informative. But I got one more doubt here. According to this picture ,

The MMF should increase in direction of red to light violet, but the armature MMF is shown to be decreasing. please correct me where my understanding is going wrong.

 

[jim hardy]

The MMF should increase in direction of red to light violet, but the armature MMF is shown to be decreasing. please correct me where my understanding is going wrong.

Red encircles only one pair .

Each pair produces its own MMF which adds to the total MMF.

Step wave represents total not individual mmfs.

I think the author drew either
the current dots and crosses backward
or
the MMF step wave upside down,

see post # 17

Most likely it was a grad student who drew the image or an undergrad student better at CAD drafting than magnetics
and editors missed that detail.

That's what i think.

What do your basics tell you ?

old jim

 

[Noaha]

the MMF step wave upside down,

Yes, even I was thinking that mmf should be upside down with the shown current direction.

If we consider this equation "MMF = flux*reluctance", can we deduce something?

 

[jim hardy]

If we consider this equation "MMF = flux*reluctance", can we deduce something?

What's your line of thought there ?

 

[Noaha]

What's your line of thought there ?

In this image, armature flux is increasing under the north pole after the north pole axis. It can only increase if the armature mmf is increasing because under the poles, the reluctance is constant.
The same goes for under south pole before the south pole axis.

I think taking this thing into account, the author arrived at this graph.

But I am not able to figure out what is happening in the large air gap region. There, the reluctance is very high and hence flux reduces rapidly. But why still mmf is increasing?

 

[jim hardy]

But why still mmf is increasing?

Armature MMF is amp-turns from the armature. No reluctance in that identity.

Flux = MMF/ Reluctance and observe it DOES decrease where reluctance is high.

...

a thought about that upside-down-ness ----------------------------------------------------
We don't know from where @cnh1995 got that drawing.
Most textbooks today use conventional current.
Before WW2 (and even today in vocational textbooks) it was(is) not uncommon to find authors using negative (electron) current.

If his image came from one of those books it explains the mystery .

I worked with many exceedingly capable former Navy technicians who were trained using electron current.
One really needs to be fluent in both conventional and negative current to succeed in industry .

 

[cnh1995]

We don't know from where @cnh1995 got that drawing.

I googled it.

Actually, I have seen that diagram in a standard machinery reference book, whose auther is Indian. I was planning to take a picture of that page and upload it here, but I couldn't find the book in our library.
So I just googled OP's title and found this relevant image.