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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 END SEMESTER EXAMINATION
(REVISED CREDIT SYSTEM: 2007)
14 November 2009
ELECTRICAL MACHINERY - I (ELE 205)
Time: 3 hours
Note
1A. 1B. 1C. 2A. 2B. 2C. 3A. 3B. 4A. 4B. :
Max. Marks: 50
Answer any FIVE full questions.
Missing data, if any, may be suitably assumed.
A Scott connected transformer set supplies two 1‐phase furnaces each at 120V from a 3‐phase
6.6kV system. If the furnace connected to the teaser transformer consumes 300kW at upf while the other furnace connected to the main transformer consumes 495kW at 0.8 pf lag, determine the 3‐phase line currents. Determine the equivalent circuit parameters referred to LV side for a 1‐phase, 50Hz, 200/2000V, 30kVA transformer with the following test data: OC Test : 200V, 6.2A, 360W SC Test : 75V, 18A, 600W Derive the equivalent circuit diagram of a three‐winding transformer.
A 3‐phase, step down transformer draws 15A from the supply when connected across 4.4kV mains. If the turns ratio is 10, determine the secondary line voltage, secondary line current and power output if the transformer windings are connected in (a) star‐delta (b) delta‐star. Two transformers A and B are connected in parallel across a load of (2+j1.5)Ω. Their secondary impedances are ZA=(0.15+j0.5)Ω and ZB=(0.1+j0.6)Ω. If the no‐load terminal voltages are EA=207∠0°V and EB=205∠0°V. Determine a) current delivered by each transformer (b) terminal voltage and (c) pf of each transformer. Draw the complete phasor diagrams for a 1‐pahse transformer for RL load.
At standstill, the equivalent impedance of inner and outer cages of a double cage induction motor are (0.3+j0.4)Ω and (0.1+j1.5)Ω respectively. Calculate the ratio of the torques produced by the two cages at (i) standstill and (ii) 5% slip. Neglect stator impedance. A 230V, 50Hz, 4 pole single phase induction motor has the following equivalent circuit impedances: R1=2.2Ω, R2'=4.5Ω, X1=3.1Ω, X2'=2.6Ω, Xm=80Ω. Friction, windage and core losses=40W. Calculate input current, power factor, power developed, output power and efficiency for a slip of 3%. The O.C.C. of a d.c. shunt generator driven at rated speed is as follows:
Field current(Amps) 0.5
1
1.5
2
2.5
3
3.5 Induced emf (Volts) 60
120
138
145
149
151 152 (i)
If the field resistance is adjusted to 53Ω, determine the voltage to which the machine will build up at speed (ii)
Calculate the armature current when the terminal voltage is 100V at rated speed. Assume an armature resistance is 0.3Ω and neglect armature reaction. Find the demagnetizing ampere‐turns per pole and cross magnetizing ampere‐turns per pole for a 120kW, 600V, 8 pole Wave connected d.c. shunt generator with 540 armature conductors. The brushes are given a lead of 4 mechanical degrees. Resistance of the shunt field winding is 150Ω. Also derive the expressions used. (04)
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Department of Electrical and Electronics Engineering
5A. 5B. 5C. 6A. 6B. 6C. Calculate the reduction in starting current and starting torque when the supply voltage to a cage rotor motor is 75% instead of 100%. A 3 phase star connected 400V, 4 pole, 50 Hz induction motor has the following parameters. Xm=26.3Ω, X1=1.11Ω, X2'=0.46Ω, R1=0.64Ω, R2'=0.33Ω. If the mechanical losses are 1.1kW with negligible core losses, for a slip of 2.2% determine (a) stator current (b) power factor (c) gross power output (d) Shaft Torque (e) Efficiency using the exact equivalent circuit diagram. Discuss the merits, demerits and applications of 3 phase induction generator. A 3 phase, 4 pole, 1440RPM, 50Hz induction motor has star connected rotor winding having a resistance of 0.2Ω per phase and a standstill leakage reactance of 1Ω per phase. When the stator is energized at rated voltage and frequency, the rotor induced emf at standstill is 120V per phase. Calculate the rotor current and rotor power factor both at starting and full load. A 4 pole 50Hz slip ring induction motor runs at 1440RPM on full load. The rotor resistance per phase is 0.3Ω. Calculate the additional resistance per phase to be inserted in the rotor circuit if the speed is to be reduced to 1340RPM for full load torque. A 25kVA, 2000/200V, two‐winding transformer is to be used as an auto‐transformer with a constant source voltage of 2000V and both windings in series opposition. At full‐load upf, determine the power output and power transformed. If the efficiency of the two‐winding transformer at 0.8 pf is 95%, determine the efficiency of the auto‐transformer. (02)
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