Engineering B.E. (Electrical Engineering) Sem 3 December 2018 2018 Electrial Machine I Question Paper - Mumbai University | munotes

Questions asked in this paper

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1. Q2 Attempt any three from the remaining Questions No.2 to No.6
2. Q3 Illustrate answer with diagrams wherever necessary
3. Q1 Attempt any four : 20 marks
   • a) Explain the use commutator and brushes in DC machine
   • b) Explain in brief the principle of Electromechanical Energy conversion
   • c) Draw and explain Torque-stepping rate characteristics of stepper motor
   • d) How back emf makes DC motor as a self-regulating machine?
   • e) What is the necessity of starter in DC motor?
4. Q2 a) Aring has diameter of 21 cm and cross sectional area of 10 cm*. The ring is made up of semicircular sessions of cast iron and cast steel, with each joint having reluctance equal to an air gap of 0.2 mm. Find the ampere turns required to flux of 8 x Wb. The relative permeability of cast steel and cast iron are 800 and 166 respectively. Neglect fringing and leakage effect 10 marks
   • b) Derive the expression for torque developed in doubly excited magnetic field. 10
5. Q3 a) Explain the armature reaction in DC generator and hence explain how to minimize 10 marks
   • b) A220 V,970 RPM, 100 A DC separately excited motor has an armature resistance of 0.05 Q. It is braked by plugging from an speed of 1000 RPM Calculate : 1) Resistance to be placed in armature circuit to limit braking current to twice the full load value, ii) Braking torque, iii) Torque when speed has fallen to zero 10
6. Q4 a) Explain the process of commutation and hence mention the methods to improve 10 marks
   • b) Two identical DC shunt machines when tested by Hopkinson's test gave the following Field currents are 2.5A Line voltage is 220V Line current (including both the field currents) is 10 A Motor armature current is 73A The armature resistance of each machine is 0.05 ohms Calculate the efficiency of both the machines Explain the Variable Reluctance Stepper Motor (VRSM). (10) 10
   • b) A 200 V, 2000 rpm DC shunt motor has the armature and field resistances of 0.5 Q and 400 respectively. It drives a load whose torque is constant at rated motor torque. Calculate the motor speed if the source voltage drops to 175 V 10
7. Q6 Explain retardation test for determination of moment of inertia of DC motor. 10 marks
   • b) Derive the torque speed relation and explain speed-torque characteristic for DC series and shunt motor 10

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