/THE INSTITUTION OF ENGINEERS, SRI LANKA

PART II EXAMINATION - JUNE 2013 2-D~ELECTRICAL MACHINES I

This paper contains eight questions answer any five (05). All questions carry equal marks

Ql.a) Prove that per-turn emf induced in a transformer winding is given by

E=4.44fcPm [4 ]

(State any assumption that you make)b) Certain 800 VA, 230 V 1110 V, 50 Hz single-phase transformer has following data:

Weight of the core : 8 kgCross -sectional area : 25xlO-4 m2

Mean length of the magnetic path : 0.4 mMaximum flux density (Bmax) : 1.2 TMagnetic flux intensity (H) : 200 NmSpecific core loss : 0.85 W/kgWinding resistance primary and secondary sides (respectively) : 1.2 nand 0.25 nLeakage reactance primary and secondary sides (respectively) : 2.8 n and 0.63 n

Calculate:I. number of turns at the primary and secondary sides [2]

II. No-load current when the primary side of the transformer is supplied with the ratedvalue [6]

III. Magnetizing reactance and core resistance [3]

c) Determine efficiency and voltage regulation when the transformer delivers 800 VA at0.85 power factor lagging [5]

Q2a) Explain briefly ONAN/OFAF cooling methods of transformers. What method of cooling

is used for the transformers with higher capacity? [4]b) Draw sketches and show two different methods of connecting the primary in delta and

secondary in star of three-phase transformers. Draw the phasor diagram for each.[6]

c) What are the conditions that should be satisfied to connect two transformers in parallel[3]

d) Two three-phase 400 kVA, 11 kV/400 V, 50 Hz transformers share a common load of600 kVA at 0.85 power factor lagging. Both transformers have on load taps and one ofthe transformers is on the nominal tap, while other is on the +2.5 %. Each transformer has

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leakage impedance of 6+j 12 n referred to the high voltage side. If the common outputvoltage is 11200 V calculate the load shared by each transformer. [7]

Q3a) Draw the per-phase equivalent circuit of a three-phase induction motor and briefly

explain what each component represent [5]b) List the methods of controlling speed of an induction motor [5]c) Explain why the magnetic loss of the rotor core is negligible [2]d) 30 kW, 380 V, 50 Hz, four pole induction motor is operating at full load at 0.9 power

factor lag. The speed of the motor at full load equals to 1450 rpm. Stator and rotationallosses are 1.1 kW and 0.8 kW respectively.

CalculateI. Slip at full loadII. Rotor copper loss

III. Input powerIV. Current drawn to the motorV. Efficiency at full load [8 ]

Q4a) Sketch the circuit diagrams for the following types of DC machines

I. Separately exited de machineII. Series wound de machine [4]

b) A 300 V de shunt wound motor takes supply current of 45 A at a certain load. Armatureand field winding resistances are 0.25 n and 1450 respectively. Frictional and fieldwinding loss is 950 W. Calculate winding losses in both armature and field winding andefficiency of the motor [6]

c) A 400 V de shunt motor has armature current of 18 A when it runs at the speed of 1400rpm. The resistance of the armature equals to 0.5 n

I. Determine the external resistance that need to be connected in series with thearmature to reduce the speed up-to 750 rpm with same armature current [6]

II. What would be the new speed of the motor (with the external resistance as above)at the armature current of 15 A [4]

Q5.a) Explain why the alternators are rated in kVA and not in kW [3]b) What do you understand by "synchronous reactance"? [2]c) What is meant by "infinite bus"? [2]d) A three-phase 12.5 kV, 2500 kVA star connected synchronous generator is connected to

an infinite bus and delivers rated power at 0.85 power factor lagging. Synchronousreactance of the machine equals to 4.8 O.

I. Calculate EMF of the generator [5]II If the generator is increased by 20% with increase of excitation, determine new

load current and power factor. (armature resistance of the machine can beneglected) [8]

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Q6a) 10 MVA, 14 kV star connected synchronous machine is subjected to the open circuit and

short circuit tests. Per-phase resistance of the armature windings equals to 0.65 Q.Results of the tests are shown in the table below:

h(A) 100 150 200 250 300 350occ (kV) 9 12 14 15.3 15.9 16.4SCC (A) 490Air gap 18line

I. Calculate unsaturated and saturated synchronous reactance of the machine [6]II. Determine required field current to deliver rated load at 0.9 power factor lagging to an

infinite bus [6]b) Certain 1.5 MW, 6.6 kV, 10 pole, 50 Hz, wye connected synchronous motor draws

current of 100 A at 0.9 power factor leading. Synchronous reactance of the machineequals to 12.5 n (per-phase). Determine the torque developed by the motor [8]

Q7a) State the conditions that are to be satisfied before connecting two alternators in parallel

[5]b) List the factors that affect load sharing in parallel operating generators [5]c) What steps are to be taken before disconnecting one alternator from parallel operation ?

[4]d) Two identical 3 MVA synchronous generators share a common load of 3. 5 MVA. The

frequencies of first and second machines drops from 50 Hz to 48 Hz and 50 Hz to 48.25 onfull load respectively. Determine the load on each machine [6]

Q8a) What are the advantages of short-pitch winding over full-pitch winding? [4 ]b) Explain why an induction motor cannot operate at synchronous speed [4]c) What types of synchronous generators are used in hydro and steam power plants? Give

reasons [4]d) Explain following two theories with which the principle of single phase induction motor is

explained:I. Double revolving field theory

II. Cross filed theory[4]

e) List the advantages of slip ring induction motor over squirrel-cage induction motor[4]

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