Popular Science Montldy 



625 



them, equal, and opposite to that in- 

 duced in the disks, so that the total volt- 

 age of the machine would be that of one 

 disk only. 



The development of the seam-turbine, 

 however, has opened a large field to the 

 unipolar dynamo, by providing a simple 

 means for obtaining very high rotative 

 speeds, although there is as yet one in- 

 convenience to this coupling, i. e., slid- 

 ing-contacts that will operate well at the 

 high peripheral speed of the turbine- 

 driven disk, which speed is as high as 

 80,000 feet per minute in the small- 

 sized machines direct-coupled to a De 

 Laval turbine. In connection with this, 

 it must be remarked that commutator 

 sparking is always liable to occur when 

 ordinary turbo - generators are used, 

 whereas this inconvenience is entirely 

 eliminated with the unipolar dynamo, 

 there being no commutator. 



Figs. 3, 4 and 5 show the essential 

 parts of three different types of unipolar 

 dynamos, and of these types, the first 

 and last are the most efficient, since no 

 gears are needed, the wheels being 

 keyed to the same shaft. The magnet 

 of the Barlow Wheel is displaced by 

 powerful electromagnets almost entirely 

 covering the surfaces of the disks, thus 

 creating a very large magnetic field 

 for the armature to revolve in at high 

 speed. The short arrows in these three 

 figures indicate the path followed by 

 current when the dynamos are in opera- 

 tion, while the dotted lines show the di- 

 rection of the lines of force set up by 



the large coil forming the electromagnet. 



In the construction of unipolar dyna- 

 mos, the voltage of the machine is prac- 

 tically the only electrical point to be con- 

 sidered, inasmuch as mechanical consid- 

 erations, stiffness for example, compel 

 the designer to give the disk sufficient 

 cross-section to carry a large current. 

 For instance, with a single-disk, uni- 

 polar machine, required to give 50 volts 

 at the terminals at 20,000 r.p.m., a steel 

 disk 16"' in diameter cutting across a 

 magnetic field of a density of 95,000 

 lines of force per square inch, would be 

 sufficient, and for that speed and di- 

 ameter, a disk not less than 14" thick at 

 the periphery would be required to avoid 

 its bending. Such a disk, with eight 

 sliding-contacts, can safely carry 400 am- 

 peres, yielding an output of 20 k.w. 



In unipolar dynamos, the main elec- 

 tric losses are those due to the resistance 

 of the disk and that of the magnetizing 

 coil ; for the lines of f orbe being always 

 cut in the same way, hysteresis and eddy- 

 currents are practically cast out. This 

 is a great advantage over the multi- 

 polar dynamo, since with a high speed, 

 the reversals of flux are very quick, and 

 the hysteresis losses are large. Magnetic 

 leakage is very much less important 

 with a unipolar than with a multipolar 

 generator. In fact, there is no need to 

 consider it when figuring out the mag- 

 netizing windings. 



Inasmuch as the disk-armature, if 

 made of steel, can be very accurately 

 faced and mounted, and is a good con- 



HIGH SPEED GENERATORS 



Peripheral speed between 40,000 and 60,000 feet per minute. 

 Air-Gap density, 95,000 lines per square inch. 



LOW SPEED GENERATORS 



Peripheral speed beL^een 15,000 and 25.000 feet per minute. 



Air-Gap density, 95,000 lines per square inch. 



