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Homework answers / question archive / A 4-pole 500 KVA,6
A 4-pole 500 KVA,6.6 KV, star connected synchronous motor has synchronous reactance of (0.5 + j20) runs at a speed N rpm with all field coil in series. Determine the speed when the two halves of the field winding are connected in parallel? If an additional resistance of 20 ohms is inserted in the armature circuit, compute the motor speed and the line current in case load torque varies as the square of the speed? If the excitation is reduced to 75% of the initial value and the total torque developed by the armature remains unaltered, calculate new speed.? Also calculate the value of resistance to be inserted in series with the armature to reduce the operating speed to 800 rpm?(Assume linear sinusoidal magnetization characteristic). Also calculate the armature voltage and current at rated torque? Draw the external characteristics of the machine with the assumption of same rated terminal voltage and the same rated load current?
Q7.)
A 4-pole, unbalanced star delta connected, 50 Hz, 11 KV, 40 MVA turbo generator, with a synchronous reactance of 0.95 p.u., is connected to a power network. The steam input to prime mover is increased till synchronous machine starts operating at rated KVA. The synchronous machine is drawing 100 A at rated terminal voltage and at unity power factor at its terminal voltage and at leading power factor at its terminals. (1) Draw the phasor diagram under the conditions specified above? (2) Compute the alternator and motor excitation voltages? (3) Find thee maximum power transfer between the two machine, also find the armature current terminal voltage of the synchronous motor and its power factor under these conditions? (4) Find the load angle, armature current and power factor of the machine, when the excitation voltage is adjusted to 480 V? (5) With the excitation voltage unchanged, determine the maximum load the machine can carry and the corresponding values of load angle and the armature current? (6) What would happen if its field circuit get open-circuited accidentally and the generator keeps on supplying power to the infinite bus? (7) Calculate the effective armature reactance in per unit and also in ohms per phase at 30 degrees, also find the ratio of ac to do impedance? (8) If the voltage applied to field winding is 220 V. Calculate the alternator efficiency at 0.9 power factor at half full load and full load?
Q8.)
A generator is connected to an infinite bus through an double circuit line. The admittance and the reactance of the whole feeder are 1.1 p.u and 0.4 p.u both on 200 MVA base.
The minimum permissible outputs vars when the output powers. Find the maximum and minimum reactive power limits. The whole network consist of three elements 0-1, 1-2 and 2-0.
Determine P, Q, E and δ? . If the excitation is increased by 20%, Recalculate P,Q,E and δ? .
Q9.)
A generator operating at 50 Hz delivers 1 p.u. power to an infinite bus when a fault occurs which reduces the maximum power transferable to 0.4 p.u.
If the machine is running is running steadily at synchronous speed with constant (H) of 5KW. Station A transmits 50 MW of power to station B Through a tie line. The moment of inertia of rotor is 8000 Kgm2.
1. Determine the change in the torque angle?
2. Determine the kinetic energy stored in the generator?
3. Calculate the inertia constant of the equivalent machines?
4. Determine the steady state stability limit of the line if the sending end and receiving end voltage are held at 220 KV.
5. Plot the swing curve for the sustained fault conditions?
6. Determine the allowable sudden load that can be switched ON without loss of stability.
A 440 V, 3-phase, delta connected cylindrical synchronous motor with synchronous reactance of ?Xsm=2Ω? is connected to a turbo-generator with ?Xsg=3Ω? through transmission line of reactance ?Xtr=1.5Ω?. If its field current is adjusted to give an
Excitation voltage of 12 Kv, then Calculate:
(a) Calculate the maximum magnitude of the resultant flux emf?
(b) Calculate the magnitude of armature current and its P.F?
(c) Calculate the magnitude of the armature current in phase with stator voltage?
(d) Calculate the maximum power output from the motor?
(e) Compute the excitation voltage ?Ef? on a per unit basis, when the generator is delivering rated KVA at rated voltagee and at power factor of (i) 0.8 lagging and (ii)0.8 leading?
(f) Draw the complete phasor diagram and show that armature current leading the terminal voltage?
