ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 5 
66. 
Give the formula expressing the output (1) of the delta, and 
(2) star connected three phase alternator. 
State the voltage and armature current per phase of a 420 
KVA. alternator when working, (1) as a single phase 
generator; (2) as a two phase generator; and (3) as a 
three phase generator, with a line voltage of 600 volts. 
What are the guiding factors in choosing the speed of an 
alternator ? 
Why should the application of an odd number of pole pairs 
be avoided ? 
A 12-pole direct coupled alternator has to produce a current 
having a frequency of 60 cycles; what should be the speed 
of the prime mover ? 
What difficulties are presented in designing low and high 
frequency alternators? 
State the different constant and variable losses of an alterna- 
tor. 
An alternator has to be designed with the object of possessing 
a nearly constant efficiency for a wide range of external 
load. In such a case what constant and variable losses are 
to be taken ? 
Give the formula expressing the efficiency of an alternator. 
In a 400 KVA. three phase alternator the following losses 
have been measured : 
Copper loss in the armature................ 5,000 watts 
ER SS iy erittel SN aes CL ae: 9,000 * 
Friction and ventilation losses............... 6,000 ‘ 
Field copper losses at full load..... ae TART E000 5. 
Field copper losses at half load.............. 4500 ‘* 
What is the efficiency of this alternator (1) at full and 
(2) at half-load ? 
Give the average friction and ventilation losses of an 800 
KVA. alternator. 
