Example 6.3a
A 3-phase star-connected, 5 kVA, 208 V, 4-pole, 60 Hz, power factor of 0.8 lagging
synchronous machine has a synchronous reactance of 8 Ω/phase. The machine is connected to an
infinite bus system, operating as a motor. The synchronous resistance is neglected.
(i)
Determine the excitation voltage, Ef and the power angle, δ when the machine is
drawing 3 kW at power factor of 0.80 lagging to the infinite bus bar. Sketch the
phasor diagram.
(ii)
The field excitation current is increased by 10 percent (relative to that in (i)) and the
shaft load power is the same as in (i). Determine the current and the power angle in
which the machine is operated at this condition.
(iii) If the shaft load power is now increased by 20 percent with the field excitation
current held constant as in (i), find the stator current, power factor, and reactive kVA
drawn by the machine.