7th Sem

Electrical Engineering Department

Batch – B1 (2014 Batch)

Abhishek Choksi

(140120109005)

Design of AC Machines (2170909)

ALA Presentation

On

“Design of Magnetic Circuit”

Prepared By: Guided By:

Prof. Purv Mistry

Gandhinagar Institute Of Technology

Magnetic Circuit Design

(i) MMF For Airgap (Atg):-MMF for the Airgap can be calculatedby

• 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 per metre at1 can be find out correspondingto Bt1/3.

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

• (iii) MMF For Core (Atc):-Corresponding to this flux density Bc, the mmf per metre atc for thecore is found from B-at curve for dynamo steel sheet.

Total mmf for core Atc= atc lc

And the length of flux path in the core is taken equal to one half of thepole pith on the mean diameter or

• (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 andpole 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

• (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 material of yoke , the mmf permetre aty for the yoke is taken from the following fig.1 depending uponthe type of material 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 theyoke ,

• Total field mmf required at no load,

Atfo= Atg + Att + Atc + Atp + Aty

References

• Wikipedia

• A Course in electrical Machine Design - By A.K. Sawhney