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Electrical Science - II (ECE260)

Assignment-2NOTE: Deadline for submissions is Thursday (1st November) at 5 P.M.

1. The following data applies to a single phase transformer:

(a) Peak flux density in the core = 1.41 T

(b) Net core area = 0.01 m2

(c) Current density in the conductors = 2.55 MA/m2

(d) Conductor diameter = 2.0 mm

(e) Primary supply voltage (sinusoidal) = 240 V, 50 Hz

Calculate the rating (in kVA) of the transformer and the number of turns on the primary wind-ing. (5 marks)

2. The primary of a certain transformer takes 1 A at a power factor of 0.4 when connected across 240 V,50 Hz supply and the secondary is on open circuit. The number of turns on the primary is twice that onthe secondary. A load taking 50 A at a lagging power factor 0.8 is now connected across the secondary.Sketch and explain briefly the phasor diagram for this condition, neglecting the voltage drops in the trans-

former (i.e., neglect leakage reactances and windind resistances). What is now the value of the primarycurrent? (5 marks)

3. A 50 kVA, 6360/240 V transformer is tested on open and short circuit to obtain its efficiency, the resultsofthe test being as follows:

(a) Open Circuit Test: primary voltage = 6360 V; primary current = 1 A power input = 2 kW

(b) Short Circuit Test: primary voltage = 180 V; seondary current = 175 A; power input = 2 kW. powerinput = 2 kW

Draw the equivalent circuit diagram of the transformer under both tests.

Find out all the parameters of the transformer.

The efficiency of the transformer when supplying full load at a power factor of 0.8 lagging. Phasor diagram for the above condition.

(10 marks)

4. The armature resistance of a 30 kW, 440 V, shunt generator is 0.1 . Its shunt field resistance is 200 .Find

(a) The electrical power developed by armature at the rated load.

(b) The load, field, and the armature currents.

(c) The electric power loss.

(5 marks)

5. Find the efficiency of a long shunt compound generator rated at 250 kW, 250 V when supplying 78% ofrated load at rated voltage. The resistance of armature and series field are 0.009 and 0.001 . The shuntfield current is 20 A. Take rotational losses (core or iron losses + mechanical losses) to be 3600 W.

(Refer Fig. 1 of Tutorial Problem Sheet - 3 for circuit diagram of long shunt compound genera-tor) (5 marks)

CONTINUED

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2 Electrical Science - II (ECE260)

6. Draw a labelled diagram of the cross-section of a four pole DC shunt generator. What are the functions ofthe field coils, armature, commutator, and brushes.

The E.M.F generated by a four-pole DC generator is 400 V when the armature is driven at 1000 rpm.Calcultae the flux per pole if the wave-wound armature has 39 slots with 16 conductors per slot. (Note: Fora wave wound armature number of parallel paths is 2) (10 marks)