Demagnetizing and Cross-Magnetizing Conductors
With the brushes in the G.N.A. position, there is only cross-magnetizing effect of armature reaction. However, when the brushes are shifted from the G.N.A. position, the armature reaction will have both demagnetizing and crossmagnetizing effects. Consider a 2-pole generator with brushes shifted (lead) θm mechanical degrees from G.N.A. We shall identify the armature conductors that produce demagnetizing effect and those that produce cross-magnetizing effect.
(i) The armature conductors θ°m on either side of G.N.A. produce flux in direct opposition to main flux as shown in Fig. (2.4) (i). Thus the conductors lying within angles AOC = BOD = 2θm at the top and bottom of
the armature produce demagnetizing effect. These are called demagnetizing armature conductors and constitute the demagnetizing ampere-turns of armature reaction (Remember two conductors constitute a turn).
ii) The axis of magnetization of the remaining armature conductors lying between angles AOD and COB is at right angles to the main flux as shown in Fig. (2.4) (ii). These conductors produce the cross-magnetizing (or
distorting) effect i.e., they produce uneven flux distribution on each pole. Therefore, they are called cross-magnetizing conductors and constitute the cross-magnetizing ampere-turns of armature reaction.
Calculation of Demagnetizing Ampere-Turns Per Pole (ATd/Pole)
It is sometimes desirable to neutralize the demagnetizing ampere-turns of armature reaction. This is achieved by adding extra ampere-turns to the main field winding. We shall now calculate the demagnetizing ampere-turns per pole (ATd/pole).
Let Z = total number of armature conductors
I = current in each armature conductor
= Ia/2 … for simplex wave winding
= Ia/P … for simplex lap winding
θm = forward lead in mechanical degrees
Referring to Fig. (2.4) (i) above, we have,
Total demagnetizing armature conductors= Conductors in angles AOC and BOD = 4θm/360 × Z
Since two conductors constitute one turn,
Therefore Total demagnetizing ampere-turns =
Note. When a conductor passes a pair of poles, one cycle of voltage is generated. We say one cycle contains 360 electrical degrees. Suppose there are P poles in a generator. In one revolution, there are 360 mechanical degrees and 360 ×P/2 electrical degrees.
Cross-Magnetizing Ampere-Turns Per Pole (ATc/Pole)
We now calculate the cross-magnetizing ampere-turns per pole (ATc/pole).
Total armature reaction ampere-turns per pole
Demagnetizing ampere-turns per pole is given by;
(since two conductors make one turn)
Demagnetizing ampere-turns per pole is given by;
Therefore Cross-magnetizing ampere-turns/pole are
Written by arjun on May 10th, 2009 with
2 comments.
Read more articles on Armature Reaction and Commutation and Direct Current Machines and Electrical Machines.
naresh
#1. August 17th, 2009, at 11:27 PM.
excellant topic