(i) MMF For Airgap (Atg):-

MMF for the Airgap can be calculated by

𝑨𝑻𝒈 =800000 BgKgLg

(ii) MMF For Armature Teeth (Att):-

Generally Parallel Sided slots are used in the

synchronous machines and therefore the

teeth are tapered.

We know that the mmf for teeth can be

calculated by finding flux density Bt1/3 at 1/3

height from the narrow end.

From the B-at Curve , the value of mmf permetre at1 can be find out corresponding to Bt1/3.

The length of flux path in the teeth is equal tothe depth of the slot ds.

(iii) MMF For Core (Atc):-

Corresponding to this flux density Bc, the mmfper metre atc for the core is found from B-atcurve for dynamo steel sheet.

Total mmf for core Atc = atc lc

And the length of flux path in the core is takenequal to one half of the pole pith on the meandiameter or

lc=ᴨ (𝐷+2𝑑𝑠+𝑑𝑐)

2𝑝

(iv) MMF For Poles (Atp):-

The flux in the poles is equal to the useful flux

which crosses the air gap and entres the

armature plus the leakage flux.

Actually the flux in the pole is not uniform

throught its radial length owing to different

values of leakage flux from the pole shoes and

pole bodies.

It is assumed for calculations that the top 2/3

length of pole carries the useful flux plus leakage

flux from both pole shoes and pole bodies.

Thus the flux at the pole top is minimum while at

the bottom it is maximum or

Minimum flux in the poles = ϕp(min)=ϕ+ ϕsl

And maximum flux in the poles = ϕp(max)=ϕ+ ϕsl+ ϕpl

The value of ϕsl and ϕpl are given by,

Axial length of body Lp=axial length of pole

shoe Ls

And Atl=Atg + Att + Atc

From the above it follows that:

Maximum flux density in the pole body

Bp(max) = ϕp(max)/Ap

And minimum flux density in the pole body

Bp(min) = ϕp(min)/Ap

The mmf per metre corresponding to Bp(max) and

Bp(min) are found from the B-at Curve.

And let them be atp(max) and atp(min)

Therefore total mmf for body is,

ATP= atp(max) hpl/3+atp(min) 2hpl/3

Continue…..

(v) MMF For Yoke (Aty):-

Flux in the yoke ϕy = ϕ + ϕsl + ϕpl

Area of yoke Ay = Length of yoke ⨯ depth of yoke

Flux density in yoke,

By= ϕy /Ay = ϕ + ϕsl + ϕpl/2Ldy

Corresponding to this flux density and the materialof yoke , the mmf per metre aty for the yoke is takenfrom the following fig.1 depending upon the type ofmaterial used

MMF for yoke Aty= atyly

Where ly= path of magnetic flux through the

yoke which is taken one half of the pole pitch

on the mean diameter of the yoke,

= ᴨ (𝐷𝑟−2ℎ𝑝𝑙−𝑑𝑦)

2𝑝

Total field mmf required at no load,

Atfo= Atg + Att + Atc + Atp + Aty

A Course in electrical Machine Design- By

A.K. Sawhney

Query ???

Thank you !