. A text-book of electrical engineering;. e field under this pole-tip will be too weak, or mayeven become negative, making sparkless commutation impossible. As arough approximation, it may be assumed that a flux density of 1500 linesper sq. cm is necessary in the case of drums and 2500 in the case of ring-armatures to ensure sparkless commutation. The fact that a stronger field is required for the commutation of a ring-armature does not indicate that the cross-magnetisation is greater in thiscase. The reason is to be found in the greater self-induction of the ring-winding. To overcome this-gre
. A text-book of electrical engineering;. e field under this pole-tip will be too weak, or mayeven become negative, making sparkless commutation impossible. As arough approximation, it may be assumed that a flux density of 1500 linesper sq. cm is necessary in the case of drums and 2500 in the case of ring-armatures to ensure sparkless commutation. The fact that a stronger field is required for the commutation of a ring-armature does not indicate that the cross-magnetisation is greater in thiscase. The reason is to be found in the greater self-induction of the ring-winding. To overcome this-greater self-induction during the interval of 54- Armature Reaction 151 short-circuit the ring-armature requires a stronger commutating field thanthe drum-armature. ■ If Bg represents the mean flux-density in the air-gap and B„ the flux-density under the pole-tips due to the cross-magnetisation, then the resultantflux-density under the commutating tip is B = B,- B,. Now, the ampere-turns of cross-magnetisation are X -^-P ^« 2 2a .{85)-. The principal reluctance in the path of the cross ampere-turns is thatdue to the air-gap, and, neglecting the reluctance of the iron, we have fromequation (43) on page 61 ^.^^ ^^ Bc= —7 -, where Ig is double the air-gap in this with the equation 0-477 ■ Xg B„ = h we have for the actual strength of the commutating fieldB = Bg-B, = o-4r. ^~^ The empirical conditions for sparkless commutation, which were men-tioned above, may therefore be expressed as follows: X — X 0-477. —^-= ° i 1500 for drum-armatures ■9 .(86), 0-477. X„ — X, : 1:500 for ring-armatures (87). 152 Electrical Engineering We see from these equations that the influence of the cross ampere-turnsincreases with the armature current, and that we have here a hmit to theoverload capacity of the dynamo. This Umit is not merely due to the factthat very large loads would cause a prohibitive temperature rise in thearmature, but is fixed by the number o
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