10 questions
Q.1. (a) Define specific magnetic loading and specific electric loading of an electrical machine and explain them. [4]
(b) List and explain briefly the limitations being imposed on the design of electrical machines. [7]
(c) Write and explain Ohm's law for magnetic circuit. [5]
Q.1. (a) A 350 kW, 500 V, 450 rpm, 6 pole d.c. generator is built with an armature diameter of 0.87m and a core length of 0.32 m. The lap wound armature has 660 conductors. Calculate the values of specific magnetic loading and specific electric loadings. [8]
(b) Describe real and apparent flux density. [8]
Q.2. (a) Define cooling time constant of an electrical machine and draw its cooling time curve. [6]
(b) Derive the equation for mmf required for air gap of a rotating machine having slotted armature, what is the meaning of effective length of air gap? [10]
Q.2. (a) Derive the equation for temperature rise of an electrical machine during its heating. Draw temperature rise-time curve and define heating-time constant. [8]
(b) Write short note on 'Hydrogen cooling of turbo alternators'. [8]
Q.3. (a) Derive an equation for voltage per turn in terms of phase output of a transformer. [6]
(b) Determine the main dimensions of the core, number of turns and the area of conductors for a 5kVA, 50 Hz, 11000/400 V, single phase, core type distribution transformer. The net conductor area in the window is 60% of the net cross section (square) of the iron case. Assume a flux density of 1 Wb/m^2, a current density of 1.4 A/mm^2 and a window space factor of 0.2. The window height is 3 times its width. [10]
Q.3. (a) Explain the difference between a power and distribution transformer from the design and working principles considerations. [8]
(b) A 200 KVA, 6600/440 Volts, 3-phase, delta-star connected 50 Hz, core type transformer has the following particulars: max flux density = 1.3 Wb/m^2, current density = 2.5 Amp/mm^2, window space factor = 0.3, overall height = overall width, window area = 1.25 times core area. Determine the overall dimensions of core. [8]
Q.4. (a) Explain and derive an expression for the output coefficient of an alternator. [8]
(b) Determine the main dimensions of a 3000 kVA, 6.6 kV, 50 Hz, 187.5 rpm, 3 phase, star connected alternator. Also find the turns per phase. Given that Average flux density = 0.58 Wb/m^2, specific electrical loading = 3500 ac/mt, pole-arc to pole-pitch ratio = 0.7. [8]
Q.4. (a) Explain the term 'Short Circuit Ratio' and its effects on the performance of synchronous machines. Show that the short circuit ratio (SCR) of synchronous machines is inversely proportional to its synchronous reactance. [8]
(b) Explain the design of stator core and winding. Also describe rotor design. [8]
Q.5. (a) Deduce the output equation of a 3-phase induction motor in terms of its specific loadings. Why the length of air gap in induction motor is kept as minimum as possible? [7]
(b) Determine the main dimensions, turns per phase of a 250 hp, 3-phase, 50 Hz, 400 V, 1410 rpm, slip ring induction motor. Assume: Average flux density in air gap = 0.5 Wb/m^2, specific elect. loading = 30000 Ac/m, efficiency = 0.9, power factor = 0.9, winding = 0.955, ratio of core length to pole pitch = 1.2. The machine is delta connected. [9]
Q.5. (a) What do you mean by the phenomenon of cogging in case of squirrel cage induction motor. Why is an induction motor, the number of stator slots should never be equal to the number of rotor slots? [10]
(b) Explain the design and working of stator core with suitable diagrams. [6]