1. A 3-phase star connected alter no open circuit is required to generate a
line voltage of 3600 V a 50 Hz when driven at 500 r.p.m. The stator
has 3 slots per pole per phase and 10 conductors per slot. Calculate the
i) number of poles and ii) useful flux per pole. Assume all conductors
per phase to be connected in series and the coils to be full pitch.
Ans: P =12, ϕ = 0.0543 Wb
2. A 1000 kVA, 2300 V, 3-phase, star connected alternator has a
resistance of 0.309 om/phase and a synchronous reactance of 3.34
ohm/phase. Calculate the change of line voltage when the rated output
of 1000 kVA at a power factor of 0.8 lagging is switched off. Assume
the speed and the exciting current to remain unaltered.
Ans: 1141 V
3. A 1200 kVA, 3300 V, 50 Hz, three phase, star connected alternator has
armature resistance of 0.25 ohm/phase. A field current of 40 A
produces a short circuit current of 200 A and an open circuit e.m.f of
1100 V line to line. Find the voltage regulation on i) full load 0.8 p.f.
lagging ii) full load 0.8 p.f. leading.
Ans: i) 25.9% ii) -13.54 %
4. A 2000 kVA, 11000 V, 3-phase star connected alternator running on
constant voltage, constant frequency has a resistance of 0.06 ohm/phase
and a synchronous reactance of 1.8 ohm/phase. If excited to give a
terminal voltage of 13000 V on open circuit, find its maximum possible
output power.
Ans: 77103 kW
5. A 3-phase star connected salient pole synchronous generator is driven
at a speed near synchronism with field circuit open and the stator is
supplied from a balanced 3-phase supply. Voltmeter connected across
the lines gave minimum and maximum reading of 2800 V and 2820 V.
The line current fluctuates between 360 A and 275 A. Find the direct
and quadrature axis synchronous reactances per phase. Neglect
armature resistance.
Ans: 5.92 ohm, 4.5 ohm
6. A 400 V, 10 h.p. (input), 3-phase synchronous motor has negligible
armature resistance and a synchronous reactance of 10 ohm/phase.
Determine the minimum armature current and the corresponding e.m.f
for full load conditions. Assume an efficiency of 85 %.
Ans: 263.4 V
7. Consider a 3300 V delta connected synchronous motor having a
synchronous reactance per phase of 18 ohm. It operates at a leading p.f.
of 0.707 when drawing 800 kW from mains. Calculate its excitation
e.m.f and load angle.
Ans: 4973 V, 17o