DIRECT-CURRENT DYNAMOS AND MOTORS. ° 199 
necessary to produce a potential equal to that lost by 
armature and series field resistance and by armature re- 
action. Therefore, the number of shunt ampere-turns 
for a compound wound machine is the magnetizing force 
needed on open circuit, and the number of series am- 
pere-turns required to maintain constant voltage is the 
difference between the excitation needed for normal 
load and that on open circuit. The proper number of 
shunt and series ampere-turns is computed as follows: 
The flux, %,, required on open circuit is that number of 
lines of force which will produce the no-load or terminal 
E. M. F., E., of the dynamo, or, by formula (21), page 
Al: 
@ _ 6 X 10° X m X Ho, 
, N X Nn, : 
hence, the number of shunt ampere-turns needed to over- 
come, on open circuit, the reluctances of air gaps, arma- 
ture core, and magnet frame, is: 
eT ge il es aE OO an. (52) 
in which the magnetizing forces of the various portions of 
the magnetic circuit are computed with reference to the 
flux densities corresponding to the above no-load flux &,. 
The compensating ampere-turns are missing from this 
formula, because there is no armature reaction on open 
circuit, as no current flows in the armature. 
The armature flux at normal load is: 
@—-& XI xm x # 
NX, : 
where JF is the total E. M. F. of the dynamo at normal 
load. 
The total ampere-turns needed for excitation at normal 
load, therefore, are obtained by computing AZ’ from 
formula (37), page 79, using the flux densities corre- 
sponding to the normal flux ®. Since AT is supplied by 
the combined effect of the shunt and series winding, the 
