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
