Alternator Additional Review Problems
1. A 50kVA, 550V single phase alternator draws a field current 10A at rated load.
With the same field current, the open circuit emf and short circuit current are
300V and 150A. The ohmic resistance of the armature between terminals is 0.18
ohm. The ratio of effective to ohmic resistance maybe taken as 1.5. Determine the
regulation at 0.8 power factor lagging.
A. 27.8% C. 25.5%
B. 26.4% D. 24.9%
2. Determine the efficiency of a 1500kVA, 2300V, 3 phase, Y-connected alternator,
which operates at rated output with a power factor of 80%. The DC armature
resistance at 70oC between terminals is 0.08 ohm. The field takes 70A at 120V
from the exciter equipment. Friction and windages losses are 15kW, iron loss is
35kW and stray load losses is 1.5kW. Assume the effective armature winding
resistance is 1.5 times the DC value.
A. 94.56% C. 93.35%
B. 90.31% D. 92.62%
3. An 11,000 volts three phase wye-connected turbo alternator has a synchronous
reactance of 6 ohms and a negligible resistance per phase. When the field current
is 8A, the open circuit voltage is 12,000 volts. Determine the armature current
when alternator developed maximum power.
A. 1456A C. 1643A
B. 1712A D. 1566A
4. Semirara Mines, has two 3-phase, 60Hz AC generators operating in parallel. The
first unit has a capacity of 1000kVA and the second unit has a capacity of
1500kVA. The first is driven by a prime mover so adjusted that the frequency fall
from 61Hz at no-load to 59.6Hz at full-load. The second has a different speed-
load characteristics, the frequency fall from 61.4Hz at no-load to 59.2Hz at full
load. When these alternators are jointly delivering 2000kW, what is the load of
each generator?
A. 884kW, 1116kW C. 915kW, 1085kW
B. 867kW, 1133kW D. 804kW, 1196kW
5. There are two alternators, 100kW, 3-phase in parallel are driven by shunt motors
whose speed-load characteristics are as follows: Alternator No.1, no-load speed
600rpm and full-load speed 530 rpm, while alternator No.2, no-load speed 590
rpm and full load speed 550rpm. For what load will the alternator divide the load
equally?
A. 62.45kW C. 67.54kW
B. 68.78kW D. 64.67kW
