Construction of Unipolar Dynamos 



THE direct-current dynamo, as pic- 

 tured by almost everyone, is a 

 complicated machine having many 

 poles and an iron armature which is 

 wrapped up with many turns of copper 

 wire and which has at one end a huge 

 copper commutator on which copper or 

 carbon brushes bear gently, to conduct 

 the energy to distributing wires and ca- 

 bles. Very few, however, realize that 

 there is another type of direct-current 

 machine which, although suitable, as yet, 

 only for some special uses, may eventual- 

 ly earn an important place for itself. This 

 machine is the unipolar dynamo. 



In the old style dynamo, the current 



Fig. 1. Barlow wheel Fig. 2. Two wheels 

 acting as a current that revolve in op- 

 ^ generator posite directions. 



set up in the armature windings is alter- 

 nating, because the conductors, as they 

 revolve, pass successively under a mag- 

 netic north pole and then under a south 

 pole. In order that direct current may 

 be delivered to the line, an expensive and 

 delicate commutator is required, which 

 reverses the connections with the line 

 every time the current begins to flow in 

 a direction opposite to that in which it 

 was flowing before. 



If arranged so that the armature con- 

 ductors, as they revolve, cut across a 

 magnetic field always in the same direc- 

 tion, the current generated will always 

 flow in the same direction, and no com- 

 mutator will be required. This arrange- 

 ment has received the name of unipolar 

 dynamo. 



The most practical form of unipolar 

 generator in use is, to a certain extent, 

 a reproduction of the apparatus known 

 as the Barlow \\'heel (Fig. i). It 

 consists of a metal disk mounted so 

 that it projects between the poles of a 



magnet. Connections are made to the 

 shaft of the wheel and to the periphery 

 of the disk by means of sliding-contacts. 

 These contacts can be compared, in some 

 respects, with the brushes of multi- 

 polar dynamos. If, now, the disk is 

 rotated, the lines of force passing 

 through it from pole to pole will be cut, 

 and if the sliding-contacts are connected 

 together, an electric current will flow in 

 the circuit so formed. The disk is 

 equivalent, electrically, to a large num- 

 ber of radial conductors connected in 

 parallel, and hence, the voltage of the 

 machine is the same as that obtained 

 from a single conductor only; however, 

 on account of the very large cross-sec- 

 tion of the disk, the machine can supply 

 a very large amount of current. It is 

 evident that in the construction just de- 

 scribed, the disk always cuts the lines of 

 force of the magnet in the same di- 

 rection, and hence the current supplied 

 by the machine is direct and absolutely 

 continuous, showing no pulsating effects. 

 It is known that in order to induce a 

 tension of one volt in a conductor mov- 

 ing across a magnetic field, the conductor 

 must cut one hundred 

 million lines of force 

 per second, and from 

 this, it is evident that in 

 order to have a unipo- 

 lar dynamo delivering 

 current at a high ten- 

 sion, it is necessary ei- 

 ther to use a very large 

 disk and magnet, or to 

 rotate the disk at an ab- 

 normally high speed. 

 Two or more disks, con- 

 nected in series, can be 

 used also, but in that 

 case, adjacent disks must either be ro- 

 tated in opposite directions, as shown in 

 Fig. 2, or insulated from the shaft and 

 connected by means of sliding-contacts. 

 Adjacent disks may also be . connected 

 with the shaft, revolved in opposite mag- 

 netic fields (Fig. 5), and connected to- 

 gether by sliding-contacts on their pe- 

 riphery ; for, if the conductors connect- 

 ing the disks were revolved with them, 

 an electric force would be induced in 



Fig. 3. Uni- 

 polar dynamo 

 with one disk 



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