Page:Popular Science Monthly Volume 88.djvu/653

 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.

��RATING

�NORMAL

�NORMAL

�NO. OF

�LENGTH or-

�DL\M. OF

�PERIPHERAL

�R. P. M

�K.W.

�VOLTS

�AMPS.

�DISKS

�AIR-r.AP

�DISK

�SPEED

�10

�25

�400

�1

�ii"

�16"

�41,900

�10,000

�20

�50

�400

�2

�^"

�16"

�41.900

�10,0(«1

�30

�50

�600

�1

�A"

�21"

�56,100

�10,100

�100

�100

�1000

�1

�-h"

�48"

�46,500

�3,700

�200

�200

�1000

�2

� �48'

�46,500

�3 700

�450

�300

�1500

�2

�h"

�60"

�55,000

�3.500

�750

�500

�1500

�4

�\"

�60"

�45,500

�2,900

��LOW SPEED GENERATORS

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

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

��RATIN(, K.W.

�NORMAL VOLTS

�NOR.MAL AMPS.

�NO. OF DISKS

�LENCTH OF AIR-CAP

�DIAM. OF DISK

�pi;kii'|ii;r.\l

.SPEED

�K. P. M.

�10 30 100

300

�20

50

100

200

�500

600 1000 1500

�1

2 4

�5'/'

�30" 32"

w

00"

�15,700 18.200 23,KOO 18,950

�2,000 2,200 1,900 1,200

�� �