Worked Examples, Chapter 6
Worked Example 1
Q
A 4-pole d.c. shunt generator has an armature resistance of 0.018 . The armature is lap-wound with
520 conductors. When driven at 750 rev/min the machine produces a total armature current of 400 A
at a terminal voltage of 200 V. Calculate the useful flux/pole.
A
p 2; a 4; Ra 0.018 ; z 520; n 750
12.5 rev/s
60
I a 400 A; V 200 V
E V I aRa volt 200 ( 400 0.018 )
E 207.2 V
2 pzn
volt
E
a
Ea
207.2 4
so, weber 2 pzn
4 520 12.5
31.9 mWb Ans
Worked Example 2
Q
A short-shunt compound generator has armature, shunt field and series field resistances of 0.5 100 , and 0.3 respectively. When supplying a load of 8 kW at a terminal voltage of 250 V the
input power supplied by the driving motor is 10.4 kW. Calculate (a) the generated emf, (b) the
efficiency, (c) the iron, friction and windage loss, and (d) the total fixed losses.
A
Ra 0.5 ; Rf 100 ; RSe 0.3 ; Po 8000 W; V 250 V; Pi 10 400 W
RSe
Ia
Ra
IL
0.3 Ω
0.5 Ω
If
Pi
E
Rf
100 Ω
V 250 V
80 kW Po
10.4 kW
(a)
IL PO
8000
amp 32 A
VL
250
Vf V I LRSe volt 250 (32 0.3)
Vf 259.6 V
If 259.6
Vf
amp 2.6 A
100
Rf
I a I L I f amp 32 2.6 34.6 A
E Vf I aRa volt 259.6 (34.6 0.5)
E 276.9 V Ans
167
168
Supplementary Worked Examples
(b) Po
8
100% 100%
Pi
10 .4
76.92% Ans
(c) From the power flow diagram
PFe Pi E I a watt 10 400 (276.9 34.6 )
PFe 819.26 W Ans
(d) total fixed losses PFe I f2Rf watt
819.26 (2.6 100 )
total fix ed losses 1.08 kW Ans