# Why armature MMF is triangular in waveform?

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1. Jun 23, 2017

### 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.

2. Jun 28, 2017

### PF_Help_Bot

Thanks for the thread! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post? The more details the better.

3. Jun 28, 2017

### jim hardy

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 ?

4. Jul 7, 2017

5. Jul 7, 2017

### 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.

Got a picture from his book ?

Sorry,

old jim

6. Jul 8, 2017

7. Jul 8, 2017

### 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)

8. Jul 16, 2017

### Noaha

"
"
What does this line means?

9. Jul 16, 2017

### Noaha

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.

10. Jul 16, 2017

### jim hardy

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

11. Jul 16, 2017

### Noaha

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.

12. Jul 16, 2017

### Noaha

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

13. Jul 16, 2017

### jim hardy

Conventional or electron current ?

Big Ten-OOPS !

Last edited: Jul 16, 2017
14. Jul 16, 2017

### Noaha

Conventional current.

15. Jul 16, 2017

### Noaha

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

16. Jul 16, 2017

### jim hardy

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

i knew something was upside down...

17. Jul 16, 2017

### 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.

Last edited: Jul 16, 2017
18. Jul 16, 2017

### Noaha

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.

19. Jul 17, 2017

### 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.

Last edited: Jul 17, 2017
20. Jul 23, 2017

### Noaha

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.

21. Jul 23, 2017

### jim hardy

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

22. Jul 23, 2017

### Noaha

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?

23. Jul 23, 2017

### jim hardy

What's your line of thought there ?

24. Jul 23, 2017

### Noaha

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?

25. Jul 23, 2017

### jim hardy

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 .