ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 5D. 
96. 
97. 
98. 
than the average commercial efficiency, or when the per- 
missible temperature rise is lower than usual, then it is 
necessary to keep the armature current density below 
2,000 amperes. 
Loss in Windings.—The loss in the armature windings 
is equal to the square of the current multiplied by the 
armature resistance, that is: 
WwW, = C’R, BES PR a ee eS (10) 
where C is the armature eurrent per phase, and R, the 
warm resistance of all phases in ohms. 
Resistance in Windings.—The ohmic resistarice of the 
armature winding is expressed thus: 
Be me Societe (11) 
| qd, 2 
where L, is the mean length of an armature turn in inches; 
72, the sectional area of the conductor in square inches; m, 
the number of phases; Z, the number of conductors per 
armature phase; and @ the specific resistance of the 
copper, which, for an average temperature may be 
| 0.8 
k ee 
taken as 10° 
Length of Mean Turn.—tThe sectional area of the copper, 
the value of the current, and the density are already fixed, 
which leaves only the mean length of a turn to be ealeu- 
lated. 
An empirical formula may be used for the preliminary de- 
termination of the lengfh of mean turn, but the proper 
way of obtaining the exact length, is to make a sketch from 
the actual dimensions of the armature. The empirical 
formula referred to above is only an approximate one and 
must be used with the necessary precautions. It is as fol- 
lows: | 
Length of mean turn = L, = 2 (l+k 7)..(12) 
