Dynamo Electric Machinery: Its Construction, Design and Operation ... by Samuel Sheldon Hobart Mason

Author:Samuel Sheldon , Hobart Mason
Language: eng
Format: epub
Publisher: Crosby, Lockwood & Son
Published: 1901-03-25T05:00:00+00:00

Pi«. 176.

Represent by ^ 4> 4> K and /, the length in centimeters of the parts marked in Fig. 176. From the assumed values of the flux density, and from the magnetization curves of the metals from which the various parts of the magnetic circuit are constructed, one can get the respective permeabilities. The reluctance may then be calculated

as follows:

7

Reluctance of the pole (R- =

/V^;

<(

«

i(

of \ section of field frame (Hy = —^.

7 of the air gap (H- =

2/j/7r^(2.54)* of -J- section of the armature core,

THE DESIGN OF MACHINES.

243

To determine the reluctance offered by the teeth and winding-slots, it is convenient to assume that the total flux is carried by the teeth alone. Owing to the fringing of the field at the pole tips, not merely the teeth immediately under the pole face carry the flux from that pole, but, with very short air gaps, an extra tooth takes part in the trans-

TABLE OF TOOTH-DENSITY CORRECTIONS.

DYNAMO ELECTRIC MACHINERY.

mission. With large air gaps two or three extra teeth may take part. The value of the permeability obtained from the flux density which is calculated upon the above assumption would be too small The value of the reluctance based uix>ii it would in consequence be too large. The flux density arrived at will have to be corrected by reference to the table on page 243.

The permeability, ^ corresponding to the corrected dens-

lS

ityand to be obtained from Fig. 178, should be inserted in

the formula for the reluctance of the teeth.

where A^ is the net iron cross-section of the teeth under one pole corrected for fringing. The reluctance, the flux through which must be maintained by the field-windings on one pole, is made up of a bi-parallel path in the armature and a bi-parallel path in the field frame, both arranged in series with the pole, the gap, and the tooth reluctances. This reluctance is equal to

-•^+(R^ + (R, + (R, + ^. 2 ^ ^^ 2

XV. Alagneto-motive force. The ampere turns per pole nl^ necessary to produce the flux <^^ in the armature at no load is equal to

47r L " 2 2 J

These ampere turns are furnished by the shunt coil on one pole.

XVI. Shunt Coils, Assuming that 7^^ volts are consumed in the field regulating rheostat.

i?.* =

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