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(b) What are the guiding factors for choice of number of poles in a DC machine?.
(a) Revolving field and stationary armature in synchronous machine.
(b) Illustrate the methods used for selecting the diameter of stator bore for designing a salient pole synchronous machine.
(a) Describe the factors that influence the choice of average flux density in designing the synchronous machines.
Determine the main dimensions, turns per phase, total conductors and number of slots of a 250 H.P., 3-phase 50 Hz, 400 V, 1410 rpm slip ring induction motor. Assume Bay = 0.5 Wb/m2, ac = 3000 A/m, efficiency = 90%, power factor = 0.9, winding factor = 0.955, current density = 3.5 A/mm2. The slot space factor is 0.4 and the ratio of core length to pole pitch is 1.2. The machine is delta connected.
(b) Describe the methods used for reduction of harmonic torques in an induction motor.
(a) What are the guiding factors for selecting the rotor slots of a squirrel cage induction motor?
(b) Determine the dimensions of core and yoke for a 200 kVA, 50 Hz core type single phase transformer. A cruciform core is used with distance between adjacent limbs equal to 1.6 times the width of core laminations. Assume voltage per turn 14 V, maximum flux density 1.1 Wb/m2, current density 3 A/mm2, window space factor 0.32 and stacking factor = 0.9. The net iron area is 0.56 d2 in a cruciform core where d is the diameter of circumscribing circle. Also the width of largest stamping is 0.85 d.
(a) Derive the expression for voltage per turn for a transformer.
(b) What is the window space factor of a transformer? Derive the expressions for the window dimensions of a transformer.
(a) Describe the different methods for cooling the dry and oil immersed types of transformes.
(b) Determine the main dimensions of a 5 kW, 250 V, 4-pole, 1500 rpm DC shunt generator having a square pole face. Assume the average flux density in air gap = 0.42 We/m2, ampere conductors per meter = 15000, full load efficiency = 87% and ratio of pole arc to pole pitch = 0.66.
(a) Describe the factors which should be considered when selecting the core length are armature diameter of a DC machine?
(a) State the factors which affect the choice of ampere conductor per meter of a DC machine.
(b) Damper winding in synchronous machines.
Write the precise and complete answer of each of the following:
(a) Define two types of specific loadings used in designing of rotating electrical machines and state how they affect the size of the machines.
(b) What are the main dimensions of a DC machine? Derive the outpute quation of a DC machine in terms of its main dimensions.
(c) Write the conditions to design a transformer for minimum total cost,minimum volume, minimum weight and minimum losses.
(d) State how the power factor and the magnetizing current of an induction motor are affected by the length of its air gap.
(e) How does the short circuit ratio (SCR) of a synchronous machine affect its performance?
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