ECE 441
ELECTROMECHANICAL ENERGY CONVERSION
TAKE – HOME EXAM # 2
DUE DECEMBER 9, 2011
NAME:__________________________________________________________
PROBLEM #1
A 4000-hp, 13,200 V, 60-Hz, two-pole, three-phase, wye-connected cylindrical-rotor
synchronous motor, operating at rated load and 0.84 power factor leading, has an
efficiency (excluding field losses and armature resistance losses) of 96.5 %. The
synchronous reactance per phase is 49.33 Ω. Sketch the phasor diagram and
determine (a) rated torque, (b) armature current, (c) excitation voltage, (d) power angle,
(e) pull-out torque.
PROBLEM # 2
A 200-hp, 460-V, four-pole, 60-Hz, three-phase, wye-connected synchronous motor,
operating at rated load, has an efficiency of 94 % and a power factor of 80 % leading.
Assuming the synchronous reactance is 1.16 Ω/phase, determine (a) the excitation
voltage and power angle, (b) the pull-out torque, (c) the pull-out torque if the excitation
voltage is cut in half.
PROBLEM # 3
A three-phase, 60-Hz, 460-V system supplies an induction motor that draws 50 kVA at
0.84 power factor, a 30-kW incandescent lighting load, and a 125-hp, six-pole, wyeconnected synchronous motor, with a synchronous impedance of 1.45 Ω/phase, that
draws 80 kW. Determine (a) the synchronous motor power factor required to operate
the system at unity power factor, (b) the excitation voltage required for the conditions in
(a), (c) the synchronous motor power angle for the conditions in (a).