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Department of Electrical and Electronics Engineering
Reg. No. :
MANIPAL INSTITUTE OF TECHNOLOGY, MANIPAL
(A Constituent Institute of Manipal University, Manipal)
THIRD SEMESTER B.E. DEGREE MAKEUP EXAMINATION
(REVISED CREDIT SYSTEM: 2007)
29 December 2009
ELECTRICAL MACHINERY-I (ELE-205)
Time: 3 hours
Note
1A. 1B. 2A. 2B. 3A. 3B. 4A. 4B. 4C. 5A. 5B. :
Max. Marks: 50
Answer any FIVE full questions.
Missing data, if any, may be suitably assumed.
The following readings were obtained for a single phase 50Hz, 2400/120V, 50 kVA transformer with the following test data: OC Test: 120V, 9.65A, 396W SC Test: 92V, 20.8A, 810W. Determine (a) equivalent circuit parameters (b) voltage regulation at 0.8 pf lag (c) Half full load efficiency at 0.8 pf lagging. With a neat sketch, explain Sumpner’s test.
With relevant waveforms, explain the inrush current phenomena. What is the doubling effect. A 3‐phase 50Hz, 4pole induction motor has rated output of 10kW at 1425RPM and maximum torque is developed at 1200RPM. Calculate the starting torque. Neglect stator resistance and rotational losses. Draw the circle diagram for a 3 phase 6 pole 50Hz, 400V star connected induction motor from the following data: No‐load test: 400V, 10A, 1400W Short circuit test: 200V, 55A, 7000W The stator loss at standstill is 60% of the total copper losses and full load current is 30A. From the circle diagram determine:(i) power factor, slip, output, efficiency and torque at full load. (ii) maximum power factor. (iii) starting torque. (iv) maximum power input. Mention the reasons for compounding a dc generator. Neatly sketch and explain the external characteristics of dc compound generators. Two single phase transformers are connected in parallel to supply a load of (10+j5)Ω. The secondary impedance of the two transformers are ZA = (0.1 + j 1.2)Ω and ZB = (0.16 + j 1.5)Ω. The no‐load terminal voltages are EA = 2.2 kV and EB = 2.1 kV respectively. Determine (a) current delivered by each transformer (b) terminal voltage and pf of each transformer A transformer has a no load loss of 55W when the primary voltage is 250V of frequency 50Hz and the no load loss of 41W when the primary voltage is 200V of frequency 40Hz, determine the hysteresis and eddy current loss at 250V, 50Hz. A 4 pole wave wound short shunt compound generator supplies a current of 50A at 230V. The armature, series and shunt field winding resistances are 0.15Ω, 0.05Ω and 110Ω respectively. The brush contact drop is 0.9V per brush. Calculate (a) the emf generated (b) the current per conductor (c) power generated. The short circuit current of a squirrel cage induction motor on normal voltage is 3.5 times the full load current and the full load slip is 4%. Determine the percentage tapping required on an auto‐transformer starter to start the motor against 1/3rd full load torque. Neglect magnetizing current. Using double revolving field theory explain why the starting torque developed by a single phase induction motor is zero. (07) (03) (05) (05) (07) (03) (05) (02) (03) (02) (03) Page 1 of 2
Department of Electrical and Electronics Engineering
5C. 6A. 6B. 6C. The O.C.C. of a shunt generator running at 300RPM is as follows:
Field current(Amps): 0 2 3 4 5 6 7 Induced emf (Volts): 7.5 92 132 162 183 190 212 If the field resistance is 40Ω (i) determine the voltage to which the machine will build up running at 375RPM (ii) What additional resistance would have to be inserted in the field circuit to reduce the voltage to 200V at 375RPM. A 10 pole 50Hz slip ring induction motor runs at 576RPM on full load. The rotor resistance per phase is 0.25Ω. Calculate the additional resistance per phase to be inserted in the rotor circuit if the speed is to be reduced to 480RPM for full load torque. A 220V, 50Hz, 4 pole single phase induction motor has the following equivalent circuit impedances: r1=1.2Ω, r2'=1.38Ω, x1= x2'=3.96Ω, xm=61.4Ω. Friction, windage and core losses=294.4W. Calculate input current, power factor, output power and efficiency for a slip of 0.05. With a neat sketch and phasor diagram, for a star connected resistive load prove that one transformer operates at leading pf and other at lagging pf for an open delta connection (05) (03) (05) (02) Page 2 of 2