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SET - 2 CODE: GR11A2016 GR 11 II B Tech I Semester Regular Examinations,GR11D5104 Nov, 2013 Electrical Machines-I GR11D5104 GR11D5104 (Electrical and Electronics Engineering ) GR11D5104 Max Marks: 75 GR11D5104 Time: 3 hours 1). a b Answer any FIVE questions All questions carry equal marks ***** With the help of sketches, describe constructional features of a DC generator. An 8 pole DC generator running at 1200 rpm with a flux of 25 mWb per pole generates 440 V. Calculate the number of conductors, if the armature is i) lap wound ii) wave wound. [8] [7] 2). a b Derive an expression for the torque developed by a DC motor. A 4 pole, 220 V shunt motor has 540 lap wound conductors. It takes 32 A from the supply mains and develops output power of 5.595 kW. The field winding takes 1 A. The armature resistance is 0.9 ohms and the flux per pole is 30 mWb. Calculate i) speed ii) armature torque iii) shaft torque. [8] [7] 3). a Describe the Hopkinson’s test for obtaining the efficiency of two similar shunt motors. Two identical DC machines when tested by Hopkinson’s method gave the following test results: Field currents are 2.5 A and 2 A. Line voltage is 220 V. Line current including both the field currents is 10 A. Motor armature current is 73 A. The armature resistance of each machine is 0.05 ohms. Calculate the efficiency of both the machines. [8] From the fundamentals, derive the expression for the EMF equation of a single phase transformer. A transformer on no load takes 1.5 A at a power factor of 0.2 lagging when connected across 230 V, 50Hz supply. The ratio between the primary and secondary turns is 3.Calculate the value of primary current when secondary is supplying a current of 40 A at a power factor of 0.8 lagging. Neglect the voltage drop in the winding. Draw the relevant phasor diagram. [8] Derive the expression for saving in copper of auto transformer compared to two winding transformer. A 600 KVA, single phase transformer has an efficiency of 92% both at full load and half load at unity power factor. Determine its efficiency at 60% of full load at 0.8 power factor lag. [8] Why a starter is necessary for DC motor and explain the working principle of a 3-point starter? Describe a suitable method for determining the efficiency of two similar DC series machines. [8] Explain the effect of variations of frequency and supply voltage on iron losses. Discuss various possible poly phase connections in transformers. [8] [7] b 4). a b 5). a b 6). a b 7). a b ***** [7] [7] [7] [7]