44 ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 
On Variable Loads: From the above ealeulations it 
is seen, that if the constant losses are ¢% at full load, 
they are very nearly 2c% at half load and 4 c% at three 
quarter load, ete.; while if the variable loss is v% of the 
normal input, then at half load, about . % will be the re- 
sult and at three quarter load about 2u%. 
82. Example 8.—In order to illustrate the application of the 
above rules, take two points on the efficiency curve of 
the 1,600 KAVA. generator Fig. 24, namely the full load 
efficieney which is 96.1% and the efficiency at half load 
93.4%, and from these figures find the efficiency at three 
quarter load. 
Solution: From Par. 81 we have the loss 3.9% =c+v; 
and the half load loss 6.6% = 2c + 4v. From this we have: 
2X6.6—3.9 = 3c, or ¢ = 3.1%; and v= 39-31 = 047, 
In the expression of the three quarter load efficiency, Par. 
$1, the constant loss percentage enters as 4c; therefore 
4 xX3.1—41%; the variable loss is #v, from which 
3 x 0.8 = 0.6% is obtained. Therefore the losses on three 
quarter load, amount to 4.7% of the input; that is, the 
efficiency is equal to 95.3%. 
The efficiency curve of Fig. 24, indicates for three quarter 
load the same figure, namely 95.5%. Ans. 
From the above results it is seen, that if the efficiency for 
any two loads be known, the efficiency for any other load 
ean be easily calculated with satisfactory exactness. — 
83. Example 9.—Supposing the full load efficiency of a gener- 
ator is 92% and the half load efficiency is 86%, what will 
be the efficiency at two thirds load ? 
Solution: The loss in the first case is 8% —c+v; and 
in the second, 14% — 2X c+4v; from which we obtain 
28 — 8 = 3c, or c = 6.66% and v = 1.34% of the full load. 
The losses at two thirds load will then be equal to $ X 6,66 + 
ee a ae a ee ee oe ——_ 
Aine 4 
