A 4-pole lap wound d. c. generator delivers a full load current of 400 A. It has shunt field

current of 12 A and 123 commutator segments in the commutator ring of the machine. If the brushes are advanced by 3 commutator segments on full load, find

(a) The demagnetizing AT/pole;

(b) The cross-magnetizing AT/pole.

**Given Data**

Number of Poles (P) = 4

Load Current (IL) = 400A

shut field current (If) = 12A

Number of Commutator segments (Nc) = 123

Brush Advancement = 3

**Find**

(a) The demagnetizing AT/pole;

(b) The cross-magnetizing AT/pole.

Demagnetizing Ampere Turns /pole (ATd/P) = (ZθI)/360 (1)

Number of conductors (Z) = Number of commutator segments x number of parallel paths = 123 x 4 = 492

Angle between MNP and GNP (θ) = (360/123)x3 = 8.78 mechanical degrees

Current per parallel path (I) = Ia/A

Armature current (Ia) = Load current + shunt field current

Ia = 400 + 12 = 412 A

Current per parallel path (I) = Ia/A = (412 / 4) = 103A

Putting the Value of Z, θ, I in equation (1)

**Demagnetizing Ampere Turns /pole (ATd/P)** = (ZθI)/360 = (492×8.78×103)/360 = 1236

**ToTal Amper turns per pole** = (ZI)/(2P) = (492 * 103)/(2 * 4) = 6334.5

**cross-magnetizing AT/pole** = (ToTal Amper turns per pole )- Demagnetizing Ampere Turns /pole (ATd/P) = 6334.5 – 1236 = 5098.5