CHAPTER XIV
THE DYNAMO: CURRENT COMMUTATION
=How the Dynamo Produces Direct Current: The Commutator.=--The essential difference between an alternator and a dynamo is that the alternator delivers alternating current to the external circuit while the dynamo delivers direct current. In both machines, as before stated, alternating currents are induced in the armature, but the kind of current delivered to the external circuit depends on the manner in which the armature currents are collected.
In the case of an alternator, the method is quite simple. As previously explained, each end of the loop is connected with an insulated collector ring carried by the shaft, the current being collected by means of brushes which bear against the rings. This principle, rather than the actual construction, is shown in the preceding illustrations. Its important point, as distinguished from other methods of collecting the current, is that _each end of the loop is always in connection with the same brush_.
=Ques. How is direct current obtained in a dynamo?=
Ans. A form of switch called the _commutator_ is placed between the armature and the external circuit and so arranged that it will reverse the connections with the external circuit at the instant of each reversal of current in the armature.
=Ques. How is a commutator constructed?=
Ans. It consists of a series of copper bars or segments arranged side by side forming a cylinder, and insulated from each other by sheets of mica or other insulating material.
=Ques. Where is the commutator placed?=
Ans. It is attached to the shaft at the front end of the armature.
=Ques. What are inductors?=
Ans. The insulated wires wound on the armature core, and in which the electric current is induced.
=Ques. How are the inductors connected to the commutator?=
Ans. The ends of each conducting loop or coil must be connected with the commutator segments in a certain order to correspond with the type of winding.
=Ques. Explain in detail how direct current is obtained in a dynamo.=
Ans. It will be easily seen by the aid of a series of illustrations just how the alternating armature currents are transformed into direct current. Figs. 174 to 178 show, in several positions, a single loop of wire with its ends joined to a commutator; the latter has only two segments, one for each end of the loop. In fig. 174 the loop is shown in the vertical position, and it should be noted that the division between the two segments forming the commutator is in the same plane as the loop. When the loop is in the vertical position, as shown in fig. 174, brush M is in contact with segment F, and S with G. As the armature rotates, the current flows for one half revolution in the direction A B, through segment F and out to the external circuit through brush M as shown in figs. 174 and 175, returning through brush S and segment G. At the beginning of the second half of the revolution, fig. 176, the current in the loop reverses and flows in the opposite direction B A as indicated by the arrows. At this instant, however, the brushes M and S pass out of contact with segments F and G, and come into contact with G and F respectively; that is, M leaves F and contacts with G, while S leaves G and contacts with F. The effect of this is _to reverse the connections with the external circuit at the instant the alternation or reversal of current in the armature takes place_, thus keeping the current in the external circuit in the same direction.
=Ques. How is this indicated by the sine curve?=
Ans. The sine curve, instead of falling below the axis, as in figs. 169 to 173, again rises as in the first half of the period, that is G′H′I′ is identical with E′F′G′.
=Ques. Is the direct current indicated by the sine curve in figs. 174 to 178 continuous?=
Ans. _No_; it is properly described as a _pulsating current_, or one, constant in direction, but periodically varying in intensity so as to progress in a series of throbbings or pulsations instead of with uniform strength.
=Ques. What is generally understood by the word “continuous” as applied to the current obtained from a dynamo?=
Ans. It is usually accepted as meaning a steady or non-pulsating direct current; one that has a uniform pressure and constant direction of flow as opposed to an alternating current.
=Ques. Is a continuous current ever obtained with a dynamo?=
Ans. _No._
It should be clearly understood at the outset that it is impossible to obtain a continuous current with a dynamo. The so-called continuous current which it is said to produce is in reality a pulsating current, but with pulsations so minute and following each other with such rapidity that the current is practically continuous, and as such is generally called continuous.
=Ques. How is the so-called continuous current produced by a dynamo?=
Ans. In order to obtain a large number of small pulsations per revolution of the armature instead of two large pulsations, as with the single loop armature, the latter must be replaced by one having a great number of loops properly connected to commutator segments and so arranged that the successive loops begin the cycle progressively.
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The difficulties encountered in connecting up numerous loops were overcome by Gramme, who, in 1871 invented a “ring” armature. His method consists in winding a ring with a continuous coil of wire, connections being made at suitable intervals with the commutator.
In order to understand the action of such an arrangement, it will be well to first consider four separate coils wound on a ring as shown in fig. 184. These coils are all similar, but at the moment occupy different magnetic positions on the ring. The rotation being clockwise, 1 is about to enter the field adjacent to the north pole, while 2 is emerging from the field in the region of the south pole. Again, 3 is approaching the south pole and 4 receding from the north pole.
=Ques. Describe in detail the action of the four coils wound around the ring as in fig. 184.=
Ans. According to the laws of electromagnetic induction, pressures are set up at the ends of the coils such as tend to produce currents in the directions indicated by the arrows. Now, assuming the electromotive forces in coils 1 and 2 to be equal, if the adjacent ends be joined, no flow of current will take place, but the junction will be at a higher pressure than the loose ends of the coils and if a wire be attached to this junction, and the necessary circuits completed, a current will flow along the wire outward from the junction. Similarly, if the adjacent ends of coils 3 and 4 be joined, there will be no flow of current, but the junction will be at a lower pressure than the loose ends, and if a wire be attached to the junction and the necessary circuits completed, current will flow from the junction around the coils.
=Ques. What may be said with respect to the four coil Gramme ring armature shown in fig. 185?=
Ans. According to the laws of electromagnetic induction, with the north pole of the field at the left and clockwise rotation, the induced currents flow _upward_ on both sides of the ring, hence, _the electromotive forces oppose each other at only two of the junctions, namely: at the one connected to brush M where the pressures on either side are both directed toward the junction and the other at the junction connected to brush S, at which the pressures are both directed from the junction._
It is evident, then, that the pressure at M is higher than at S; that is, M is positive and S negative; consequently, the current flows from M to the external circuit and returns through S.
=Ques. In what other way may the four coils of the armature in fig. 185 be regarded?=
Ans. They may be considered as two pairs A A′ and B B′, the action of either pair being identical with the two coil armature shown in fig. 183; this, in turn, produces the same effect as the one coil armature of fig. 182, with the exception that the amplitude of the current generated with two coils is twice as great as that with one coil of the same number of turns.
Again considering the action of the four ring coil shown in fig. 185, and starting at the beginning of the revolution, the variation of electromotive force induced in coils AA′ is indicated by the dotted sine curve 1, and of BB′ by dotted curve 2. It will be seen that 1 begins at the axis or line of no pressure, and 2 at maximum pressure.
The two curves overlap each other, and in order to determine the effect of this it is necessary to trace the resultant curve, 3. This is easily done, as the resultant electromotive force induced at any point in the revolution of the armature is equal to the sum of the pressures induced in AA′ and BB′. Thus, at the beginning of the revolution the pressure induced in AA′ is at zero point, and in BB′ at its maximum J, hence, the resultant curve begins at the point J. Again, for any point in the revolution, as N, the height of the resultant curve is equal to NP + NT = NV. For 45° or 1/8 revolution, the resultant curve reaches its amplitude, which is equal to 2 × RZ = RW, and at 90° it again reaches its minimum, XY.
=Ques. State the conditions upon which the steadiness of the current depends.=
Ans. _It depends on the number of coils and the manner in which they are connected._
Comparing curves 1 and 3, in fig. 185, it will be noted that with four coils the variation of pressure or amplitude of the pulsations is less than half that obtained with two; moreover, with four coils the number of pulsations per cycle is doubled.
In order to further observe the approach to continuous current obtained by increasing the number of coils, the effect of a six coil armature is shown in fig. 186, the resultant curve being obtained in the same manner as just explained. For comparison, the curves for the three cases of two, four, and six coils are reproduced under each other in fig. 187.
As the number of coils is further increased, the amplitude of the pulsations decreases so that the resultant curve approaches nearer the form of a straight line.
In the actual dynamo there are a great many coils, hence the amplitude of the pulsations is exceedingly small; accordingly, it is customary to speak of the current as “continuous,” although as previously mentioned such is not the case.