CUSAT Previous Year Question Papers BE 5th Semester
Electrical and Electronics Engineering
Electrical Machines-II Dec 2004
EE 504 ELECTRICAL MACHINES – II
I. a) Compare cylindrical rotor and salient pole synchronous machines.
(b) What are the measures to be taken for minimizing harmonics from alternator waveforms? Briefly explain.
(c) Why even harmonics don’t appear in alternator output waveforms?
II. (a) Calculate the induced emf per phase of a 10 pole, 3 phase, 50HZ alternator with 2 slots per phase and 4 conductors per slot in two layers. The coil span is 150°. The flux per pole has a fundamental component of 0.12 Wb and a 20% third component. Also calculate the line voltage for Y connection.
(b) Draw and explain the waveforms for induced emf over a double pole pitch and flux linked by conductors for (i) sinusoidal flux density distribution and (ii) rectangular flux density distribution.
III. (a) A 20 MVA, 3 phase, 50HZ, Y connected, 11 K.V salient pole synchronous machine is run at 1490 rpm by a prime mover with the rotor open and stator excited by 50Hz. The maximum and minimum armature voltages per phase are 30V and 25V respectively. The corresponding currents are 10A and 6.5 A. Calculate the excitation voltage and the reluctance power developed by the machine.
(b) With vector diagram, explain Blondels two reaction theory of salient pole synchronous alternator.
IV. (a) The occ of a 3 phase, 50Hz synchronous machine is given by the following test data :
If (A) – 15 30 50 75 90 120 160
VOT (line) (V) – 600 1200 2000 2900 3300 3700 4000 Under short circuit conditions a field current of 40A gives the full load stator current. The armature resistance and reactance per phase are 0.01 pu and 0.12 pu when the machine is operating as a motor drawing full load current at rated terminal voltage of 3.3 fCV and 0.8 pf leading, calculate the filed current required.
(b) Briefly explain the action of synchronous generator for different power factors with vector diagrams.
V. (a) Two parallel running alternators have emf s of 1 KV per phase. The alternators are star connected and have phase synchronous impedances of (0.1 + j2)i2 and (0.2 + j3.2)£2. The system feeds the following loads:
Load 1 : a Y connected balanced load of (2+j 1) Q per phase.
Load 2 : a a connected balanced load of (4+j3) per phase.
Calculate the terminal voltage, total line current, total power output and no load circulating current for a phase divergence of 10° (electrical).
(b) What are the conditions to be fulfilled before synchronizing an alternator to an infinite bus? ,
(c) Explain the method of starting of a synchronous motor by induction principle.
VI. (a) Briefly explain the effect of change in excitation and change in mechanical power input to one of the machines of a multimachine system.
(b) The speed regulations of two 800KW alternators A and B running in parallel are 100% to ‘ 104% and 100% to 105% from full load to no load respectively. How will the two alternators share a load of 100KW? Also calculate the maximum power the system can deliver.
(c) Compare synchronous motor and dc shunt motor.
VII. (a) An ideal single phase alternator is feeding a 612 resistance at 30V. Calculate the mechanical power output of the prime mover. Also calculate the terminal voltage and power delivered/absorbed by each component in the following cases :
(i) Mechanical power increased by 10%.
(ii) With mechanical power at the above value, another resistance of value 3Q is connected in parallel with 6£2,
(iii) An infinite single phase bus of 100V is connected to the system. What mechanical power input to the alternator will make the power to or from the bus zero?
(b) Derive the power Vs power angle relation of a salient pole synchronous machine. Also draw and explain the curve.
VIII. (a) The loads on a power system are the following :
(i) 200 KVA, 0.8 pf lagging
(ii) 50 KVAR lagging
(iii) 300 K W at 0.9 pf lagging
(iv) 20 KW at 0.2 pf leading
Calculate the overall pf. To improve the pf to 0.95 lagging, a synchronous condenser is used. What is the reactive power supplied by the condenser if the supply voltage is 440V, what will be the line current before and after pf improvement.
(b) An alternator connected to an infinite bus delivers 1 pu current at 0.8 pf lagging and 1 pu voltage. If the synchronous reactance of the alternator is 1.2 pu, determine the active power output and the steady state power limit. What critical excitation corresponds to steady state stability limit if the active power is fixed at the above value?
(c) What is meant by an infinite bus?
IX. (a) With neat diagrams, explain different excitation methods of a synchronous generator.
(b) What is meant by hunting in synchronous machines? How this can be avoided?
X. (a) Briefly explain the sequence of events when an alternator is suddenly short circuited. What factors affect the maximum short circuit current? Draw and explain the current waveform for symmetrical and asymmetrical short circuits,
(b) Define transient and sub transient reactance’s.