100 
DIRECT-CURRENT DYNAMOS AND MOTORS. 
difference between AT and AT, must be the number of — 
series-ampere turns required, or 
AT, = AT. AG oe eee 
The total E. M. F. of an ordinary compound dynamo is: 
Ei By + EB CS ha Sa (54) 
where C is the current output and 7, the armature re- 
sistance of the machine, it being here assumed as an ap- 
proximation that the series field resistance is $ of the 
armature resistance, which is an ample allowance. 
In case the machine is to be over-compounded for loss in 
the line, the percentage of drop—usually 5 per cent.—is 
to be included into the output E. M. F.; hence the total 
HK. M. F. generated at normal load, for 5 per cent. over- 
compounding, is 
HE = 1:05 x H, +125 % O Xs. AB 
After having determined the number of shunt and series 
ampere-turns giving the desired regulation, the calcula- 
tion of the compound winding merely consists in the re- 
spective application of the methods given in Pars. 72 
and 73. 
The number of turns in the series field coils is A 7’ divided 
by the normal current of the machine, which is known. 
The large conductors needed for the series coils of com- 
pound dynamos should have a cross-section of 1,000 to 
1,500 circular mils per ampere. 
In calculating the number of shunt turns, the total allow- 
able power dissipation, w,. in the magnets must be 
diminished by the power dissipated in the series winding, 
which can be easily computed by multiplying the square 
of the current by the series field resist-ace. If the shunt 
dissipation so remaining is found too small to obtain a 
practical number of shunt turns, the size of the series 
wire must be increased in ord:sr to reduce the series re- 
sistance and thus to allow more power vince, ea for the 
shunt winding. 
