ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 37 
open windings, which in the three phase system is the 
star connection. 
67. Speed.—The normal speed of the generator is very often 
specified ; if not, the choice is left to the designer. With a 
given frequency the speed is limited beforehand to certain 
values and the choice amongst them is influenced by the 
maker of the prime mover, whose engine cannot run below 
or above a certain speed. 
In most cases the generator designer tries to push the 
speed upwards, as with higher speed the alternator be- 
comes lighter and consequently cheaper. But at the same 
time one must not forget to take into account the effi- 
ciency and paralleling question. 
Fie 23.—WiInpING DIAGRAM OF A 'THREE PHASE ALTERNATOR 
WITH AN ODD NUMBER OF POLE PAIRS. 
In the case of a generator with a large number of poles, 
it is often found advantageous to make the number of 
pole pairs an even number, so as to avoid any dissymmetry 
in one of the armature phases and also to eliminate the 
employment of a hand wound coil. 
Fig. 23 shows clearly the winding of a three phase arma- 
ture with an odd number of pole pairs. 
68. Speed Variation.—As a general rule the alternator de- 
signer expects a small variation in speed of the prime 
