Hawkins Electrical Guide, Number One Part 24

When the loop turns out of the vertical position shown in fig.

171 the current reverses, because the movement of A B and C D is reversed; at this instant the brush M becomes negative, and S positive. This reversal of current is indicated by the curve falling _below_ the axis from G to I.

During the second half of the revolution, figs. 171 to 173, the changes that occur are the same as in the first half, with the exception that the current is in the reverse direction; these changes are as shown by the curve from G to I.

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 ill.u.s.trations. 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.

[Ill.u.s.tration: FIGS. 174 to 178.--Commutation of the current. These figures show how a dynamo transforms alternating into the so-called direct current. During the first half of the revolution the current flows in the direction A B, out through segment F of the commutator and brush M, returning through brush S and segment G, figs. 174 and 175. At the beginning of the second half of the revolution, fig. 176, the current in the armature reverses and flows around the loop in the direction B A. At this instant the brushes M and S pa.s.s the gaps between the commutator segments, thus reversing contact with the segments, and causing the current in the external circuit to remain in the same direction.]

=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.=

[Ill.u.s.tration: FIGS. 179 to 181.--Elementary dynamo armatures. Fig. 1, single turn loop; fig. 2, coil of two turns _in series_; fig. 3, coil of two turns _in parallel_. In operation the amplitude or maximum pressure induced with the two turn coil, fig. 180, is double that of a single turn loop, fig. 179. In fig. 180, the pressure is double that induced in fig.

181, while the amount of current generated with series turns, fig. 180, is only half that generated with turns in parallel fig. 181.]

Ans. It will be easily seen by the aid of a series of ill.u.s.trations 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 pa.s.s 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.

[Ill.u.s.tration: FIG. 182.--Gramme ring armature with one coil, and characteristic sine curve below. With one coil as shown, there are two pulsations of the current per revolution of the armature.]

=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._

[Ill.u.s.tration: FIG. 183.--Gramme ring armature with two coils placed 180 apart. This arrangement gives double the pressure of the one coil armature, fig. 182.]

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.

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.

[Ill.u.s.tration: FIG. 184.--Four separate coils wound on ring to ill.u.s.trate the action of a Gramme ring armature. If the ring be rotated the electromotive forces induced in adjacent coils will be equal and tend to produce currents in opposite directions; hence, if the inner ends be joined, the junctions would be at a higher potential (+ or -) than the loose ends. With proper connections current may be collected at the junctions.]

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, a.s.suming 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.

[Ill.u.s.tration: FIG. 185.--Gramme ring armature with four coils. The electromotive force induced in coils A, A' reaches the zero point at the instant that of coils B, B' is at a maximum; hence, sine curve No. 1, beginning at zero, and No. 2, at the maximum, show the pressure changes for A, A' and B, B', respectively. The summation of these curves gives _the resultant curve_ No. 3, showing changes in pressure of current delivered to the external circuit.]

=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._

[Ill.u.s.tration: FIG. 186.--Gramme ring armature with six coils. The sine curves 1, 2 and 3, represent the conditions due to coils AA', BB' and CC', respectively, and 4, the resultant pulsations.]

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.