CHAPTER III 

 ALTERNATORS AND WAVE FORMS 



1. Required the frequency in periods per second of a cur 

 given by an 8-pole alternator running 900 r.p.m.; also of a 28-pole 

 machine running 180 r.p.m. (1 min.) 



2. A generator is to be driven by an engine running 120 r.p.m.; 

 how many poles must it have to give a frequency of 60? A 40- 

 period generator has 16 poles; at what speed does it run? (/ mm.) 



3. What is the speed of a two-pole turbo-generator for a 60- 

 period system? Also for a 25-period system? (1 min.) 



4. Find a number which when divided by the number of poles 

 of any 60-cycle generator or motor will give the synchronous speed 

 in r.p.m. Also for a 25-cycle machine. (/ min.) 



5. Construct a table giving the speeds for machines from 2 to 

 36 poles for 60 and for 25 cycles. (10 min.) 



6. What are the kilowatt capacities of a 100-kv-a. generator 

 with currents lagging 30, 45 and 60 degrees? (1 min.) 



7. An engine builder whose engine runs at 650 r.p.m. demands 

 a 60-cycle alternator for direct coupling. What can be done for 

 him? (2 min.) 



8. By the rotation of a vector and taking its projections, plot 

 the sine wave of current, i = 100 sin a. (5 min.) 



9. By division into narrow strips and measuring the r. 

 ordinates obtain the average ordinate for a half cycle. (10 min.) 



10. Construct the curves of squared ordinates and by measun - 

 ment of the surface obtain the mean ordinate, and hence the square 

 root of the mean square ordinate. (15 min.) 



11. Construct the following curves and explain which might 

 be the wave form of an alternator and why: 



e = 100 sin a + 20 sin 2 a, 



e = 100 sin a + 20 sin (3 a - 30). (40 min.) 



12. Find the effective value of a wave made up of straight lines 

 rising from zero to a 100-volt maximum and falling to zero again, 



53 



