The document presents a presentation on the design of magnetic circuits for AC machines, focusing on the calculation of magnetomotive force (MMF) for various components including airgap, armature teeth, core, poles, and yoke. It elaborates on the principles of calculating MMF using flux densities and geometric relationships, emphasizing the variability of flux within poles. References include Wikipedia and a textbook on electrical machine design by A.K. Sawhney.

1. 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

3. 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.

4. From the B-at Curve , thevalue of mmf per metre at1 can be find out corresponding
to 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 the
core 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 the
pole pith on the mean diameter or

6. • (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.

7. • 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,

8.  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

9. • 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

10. • (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 per
metre aty for the yoke is taken from the following fig.1 depending upon
the type of material used
 MMF for yoke Aty = atyly

11. • 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 ,
• Total field mmf required at no load,
Atfo= Atg + Att + Atc + Atp + Aty

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