# Electrical Machine Design Question Paper

ELECTRICAL MACHINE DESIGN question paper

Time : Three hours Maximum : 100 Marks

Answer ALL question

PART A – (10*2 = 20 marks)

- List some factors providing limitation in design.
- State the relation between real and apparent magnetic flux densities in rotating machines.
- What are the advantages of a dc machine with larger number of poles?
- List the factors affecting the design of length of airgap.
- What is the condition for minimum cost in the design of transformers?
- Name the different types of windings used in transformer design.
- What is the basis for selecting the number of rotor slots in three phases induction motor?
- State the advantages of selecting semi closed type of slots in the stator design of three phase induction motor.
- What is short circuit ratio?
- What type of synchronus generators are preferred in thermal and hydel power stations?

PART B – (5*16 = 80 marks)

- (a) Explain the choice of specific electric and magnetic loadings in designing the rotating machines.

(OR)

(b) An armature tooth 50 mm in height has a width if 16 mm at the top and 12 mm at the root. The real flux density at the root is 2.15 wb/m^{2. }Calculate the mmf required to magnetise the tooth. The following B. at curve may be used.

^{B. wb/m2} | ^{1.5} | ^{1.6} | ^{1.7} | ^{1.8} | ^{1.9} | ^{2.0} | ^{2.1} | ^{2.2} |

^{at A/m} | ^{2,000} | ^{3,000} | ^{5,000} | ^{8,000} | ^{14500} | ^{24,000} | ^{4,0000} | ^{8,0000} |

^{ }

- (a) Determine the main dimensions, number of poles and the length of air gap of a 600kw, 500 V,900 rpm generator. Assume average gap density as 0.6 wb/m
^{2}and ampere conductors per meter as 35,000. The ratio of pole arc to pole pitch is 0.75 and the efficiency is 90 percent. The following are the design constraints: peripheral speed should not exceed 40 m/s, frequency of flux reversal should not exceed 50Hz, current per brush arm not > 400 A and armature mmf per pole not> 7500 A . The mmf required for air gap is 50% of armature mmf and gap contraction factor is 1.12.

(OR)

(b) The commutator of a 50 rpm machine is 0.3m in diameter. The brush friction loss is 100 w. If at full load the commutator loss is twice the brush friction loss. Calculate the length of commutator which will give a final temperature rise of 40®C . Assume that a commutator of this diameter 75mm in length running at 700rpm gives a temperature rise 40® C with a commutator loss of 300 w. The cooling coefficient is C=K/(1+0.1V_{c }) where V_{c} is the peripheral speed of commutation in m/s and k is constant.

13. (a) calculate the main dimensions and winding details of a 100 KVA, 2000/400 V, 50 Hz single phase shell type , oil immersed, self cooled transformer. Assume voltage/turn = 10V flux density in core = 1.1 wb/m^{2} : current density = 2 A/mm^{2}, window space factor = 0.33. The ratio of window height to window width and ratio of core depth to width of central limb = 2.5. The stacking factor is 0.9.

(OR)

(b) The tank of a 1250 KVA, natural oil cooled transformer has the dimensions length, width and height as 1.55 m ×0.65m × 1.85m respectively. The full load loss is 13.1 KW. Find the number of tubes for this transformer assuming; w/m^{2} -®C due to radiation = 6 and due to convection = 6.5, improvement in convection due to provision of tubes = 40%; temperature rise = 40® C; length of each tube = 1 m; diameter of tubes = 50 mm. Neglect the top and bottom surface of the tank as regards cooling. Suggest a suitable scheme of arrangement of cooling tubes.

14. (a) Determine the main dimensions, terms/phase number of slots, conductor cross section and slot area of a 250 h.p, 3phase, 50 Hz, 400 V, 1410 rpm, slip ring induction motor. Assume B_{QV} = 0.5 wb/m^{2} ac = 30,000 A/M, efficiency = 0.9 and power factor = 0.9; winding factor = 0.955, current density = 3.5 A/mm^{2.} The slot space factor is 0.4 and the ratio of core length to pole pitch is 1.2. The machine is delta connected.

(OR)

(b) A 15 KW, 3 phase, 6 pole, 50Hz.squirrel cage induction motor has the following data stator bore diameter = 0.32 m; axial length of stator core = 0.125 m; number of stator slots = 54; number of conductors per slot = 24; current in each stator conductor = 17.5 A, full load power factor = 0.85 lagging. Design a suitable cage rotor giving number of rotor slots, section of each bar and t=section of each ring. The full load speed is to be about 950 r.p.m approximately use copper for the rotor bars and end rings. Resistivity of copper is 0.02Ω and mm^{2} .

15 (a) A1250 KVA, 3 phase, 50Hz, 3300 V, 300 rpm synchronous generator with a concentric winding has the following design data

Specific magnetic loading B_{av} = 0.58 wb/m^{2},

Specific electric loading ac = 33,000 A/m

Gap length = 5.5 mm; field terms per pole = 60; short circuit ratio = 1.2. the efficiency gap area is 0.6 times the actual area. Peripheral speed is 30m/s. Find stator core length, stator bore, turns per phase mmf for airgap, armature mmf per pole and field current for no load and rated voltage.

(OR)

(b) A 1250 KVA, 3 phase, 6000 V, salient pole alternator has the following data

Air gap diameter = 1.6 m; length of core = 0.45 m number of poles = 20; armature ampere conductors per meter = 28,000; ratio of pole arc to pole pitch = 0.68; stator slot pitch = 28 mm; current density in damper bars = 3 A/mm^{2}. Design a suitable damper winding for the machine.

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