Power devices and Electrical Machines (INDSC5C) — DU B.Sc. (Hons) Instrumentation Previous Year Question Papers

•'(c) Explain different type of losses that occur in a transformer. (3) 5 (a) Differentiate between on line, Offline, and Line-interactive ups systems. (8) (b) Draw the block diagram of regulated power supply and explain the function of each block. (7) (c) What is a flyback converter? How does it work in an SMPS? (3) 6 (a) What is a series inverter? Explain its working principle using scRs. (7) (b) Explain the working of Permanent-magnet Stepper motor. (7) (c) Why all-day efficiency is always less than ordinary efficiency in transformer. (4) 7 (a) Compare the characteristics of a DC series motor and a DC shunt motor. (8) (b) Explain the working of different types of compound motor. A 250-V, 4-pole, wave.-wound d.c. series motor has 782 conductors on its armature. It has armature and series field resistance of 0.75 ohm. The motor takes a current of 40 A. Estimate its speed and gross torque developed if it has a flux per pole of 25 mwb. (7) Uniq.ue paper code : 2512043503 1. (a) DiffereJntiate betwe..en controlled and uncontrolled rectifiers (3) (b) Explain how a DIAC is different in operation from an SCR. (3) (c) A single-phase transformer has 400 primary and 1000 secondary -turns. The net cross-sectional area of the core is 60 cm2; If the primary.winding be connected to a 50-Hz supply at _520 V, calculate `(i) the peak value of flux density in the core (ii) the voltage induced in the secondary winding. (3 ) (d) What happens during a power failure i'n an on line UPS vs offline ups? (3) (e) Define step angle in stepper motor. A stepper motor has a step angle of 2.5 degree. Determine resolution and number of steps required for the shaft to make 25 revolutions. t (3) 2 (a.) Draw the turn-off characteristics of SCR and explain the mechanism of turn-off. (8) (b) Which type of triggering circuit is used for an SCR to achieve a firing angle greater than .90 degrees? Explain with the help of a diagram. (7) (c) Explain the need for snubber circuits in SCR. (3) 3. (a) What is the necessity of starter. in dc motor? Explain the function and working of a 3-point starter for a Dc motor. (7) (b) How does rotor rotate in an induction motor?.A 4-pole, 3-phase induction motor operates from a supply whose frequency is 50 Hz. Calculate: (7) (i) Speed of the rotor when the slip is 0.04 (ii) Speed at which the magnetic field of the stator is rotating. (iii) Frequency of the rotor current when the slip is (c) A single-stack, 3-phase Variable Reluc.tance (VR) motor has a step angle of l5°9 (4) Find the number of its rotor and stator poles. (i) When Ns > Nr and 4 (a) Explain how a single winding transformer can be used for copper saving? Why we.use single winding transformer in comparison of two windring transformer. (8) (b) Find all day efficiency of a transformer havirtg maximum-efficiency of 98 °/o at 15 kvA at unity power factor and loaded as follows : (7) 12 hours -2 kw at 0.5 p.f. lag 6 hours' - 12 kw at 0.8 p.f. lag 6 hours - at no load