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Seat No.: Enrolment
GUJARAT TECHNOLOGICAL UNIVERSITY
PDDC SEMESTER-VIII EXAMINATION WINTER 2018
Subject Code:X80902 Date: 15/11/2018
Subject Name: Electrical Machine Design-I II
Time: 02:30 PM TO 05:00 PM Total Marks: 70
Instructions:
1. Attempt all questions.
2. Make suitable assumptions wherever necessary.
3. Figures to the right indicate full marks.
Q.1

Derive the output power equation of a DC machine for designing main dimension of it.
07

Explain the design procedure for cooling system of the transformer.
07
Q.2

Explain the factors that affect the choice of specific magnetic loading in case of a induction machine.
07

Determine the D and L of a 70 Hp, 415V, 3-phase, 50-Hz, star connected, 6 pole induction motor for which ac 30000 amp.cond/m and Bav 0.51 wb/m2. Take Efficiency 90 and p.f. 0.91. Assume Pole pitch L. Estimate the number of stator conductors required for a winding in which the conductors are connected in 2-parallel paths. Choose a suitable number of conductors/ slots, so that the slot loading does not exceed 750 amp. cond.
07
OR

A 22 kW,3 phase,4 pole, 50 Hz,415V star connected induction motor has 48 slots each containing 20 conductors. Calculate the values of bar and end ring currents. The number of rotor bars is 55. The machine efficiency of 0.88 and a power factor of 0.89. The rotor mmf may be assumed of 85 percent of stator mmf. If current density is 6 A/mm2 find area of rotor bar and area of end ring.
07
Q.3

Why circular coils are preferred in transformer winding? Also explain: Window space factor Stacking factor.
07

Derive equation Et k√Q where Q kVA rating of a transformer. Explain how service condition of transformer affect the value of K.
07
OR
Q.3

How will the output and losses in transformer vary with linear dimensions?
07

Derive the expression of leakage reactance of a 3-phase core type distribution transformer.
07
Q.4

What is dispersion coefficient? Explain its effect on maximum output power factor.
07

Explain various factors affecting selection of number of poles for DC machines.
07
OR
Q.4

Explain steps to design shunt field winding of a D.C. machine.
07

Calculate the diameter and length of armature for a 7.5kW, 4 pole, 1000 r.p.m 220 V shunt motor. Given: full load efficiency= 0.83; maximum gap flux density=0.9 Wb/m2 specific electric loading=30,000 ampere conductors per meter; field form factor 0.7. Assume that the maximum efficiency occurs at full load and field current is 2.5% of rated current. The pole face is square
07
Q.5

Define SCR and its importance in designing of synchronous machine.
07

Explain the difference between turbo and hydro alternator in a point of view of design.
07
OR
Q.5

Explain the factors affecting the choice of specific Magnetic loading in case of a Synchronous machine.
07

Explain the terms "critical speed" and "run away speed" with reference to synchronous machine
07