56 ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 
LL, = length of mean turn; 
l= length of the armature core including the ventilating 
ducts ; 
z = the pole pitch, that is, the peripheral are from centre to 
centre of pole pieces; and 
k= a constant depending on the type of winding and the 
value of the terminal pressure. 
The constant for polyphase generators of low aa medium 
pressure is 1.6 to 2; for high tension machines, 2 to 2.5; 
and for single phase generators it is about 20% less. 
99.- Definition of High Voltage.—It is difficult to give a 
distinct limit where to begin calling the pressure a high 
one, but it may be said that for generators of about 100 
KVA. capacity, a tension of, say, 2,000 volts is considered 
enough to influence materially the economy of the design. 
Alternators of several hundred KV A. output may be con- 
sidered high tension machines when the voltage exceeds 
4,000 volts, and pressures above 6,000 volts are to be con- 
sidered high for large generator sets. 
300. Example 11. 
of calculating the mean length of the armature turns, the 
determination of the armature resistance and copper losses. 
A 300 KVA. three phase generator of 1900 volts terminal 
pressure has in its star connected armature 462 total turns. 
Each conductor consists of three parallel connected No, 
9 §.W.G. wires, each wire having a diameter of 0.144 inch 
and a section of 0.0163 square inch. The bore of the gen- 
erator is 51 inches; width of core, 10.5 inches; and the num- 
ber of poles is 14. Find (1) the length of mean turn, (2) 
the armature resistance; and (3) the copper losses. 
Solution: Current density: First, the copper section 
of the conductor is checked in order to see if it will carry 
the full normal current. 
On referring to Par. 63, we find that the current per phase 
