DESIGN OF ELECTRICAL MACHINES IMPORTANT QUETIONS

EE2355 DESIGN OF ELECTRICAL MACHINES

                              

UNIT – I

1. A laminated steel tooth of armature of a d.c. machine is 30mm long' and has a.taper such the maximum width is 1.4. times the minimum. Estimate the mmf required for a mean flux density of 1.9 wb/m2 in the tooth. B-H characteristics of steel is given below:

B wb/m2        1.6      1.8       1.9       2.0       2.1       2.2      2.3

H A/m      8700  10000  17000  27000  41000  70000  10900

2. Determine the apparent flux density in the teeth of a d.c. machine when the real flux density is 2.15wb/m2. Slot pitch is 28 mm, slot width is 10 mm and the gross core length 0.35 metre. The number of ventilating ducts is 4. Each duct is 10 mm wide. The magnetizing force for a flux density of 2.15 wb/m2,is 55000 H/m. The iron stacking factor is 0.9.

3. Compute the main dimensions of a 2500 KVA, 187.5 rpm, 50 Hz. Three phase, 3 KV salient pole synchronous generator. The specific magnetic loading is 0.6 wb/m2 and the specific electric loading is 3400 ac/m. The ratio of core length to pole pitch= 0.65.

4. State and explain the main factors which influence the choice of specific magnetic loading and specific electric loading in a synchronous machine. Explain the role of digital computes in the design of electrical machines.

5. A 15 kW, 230 V, 4 pole d.c machine has the following data:

    Armature diameter = 0.25 m ; armature core length = 0.125 m ; length of air gap at pole centre =

    2.5 mm ; flux per pole = 11.7 x 10-3 Wb, ratio Polearc/ pole pitch=0.66. Calculate the mmf

    required for air gap.

6. Derive the expression for the specific permeance of slots with double layer windings and for

    special purpose induction motors.

7. Compute apparent magnetic flux density in the teeth of a dc machine when the real flux density is       

    2.15 Wb/m2. Slot pitch is 28 mm. Slot width is 10mm and the gross core length is 0.35 metre. The

    number of ventilating ducts is 4, each 10 mm wide. The magnetizing force for a flux density of

    2.15 Wb/m2 is 55000 A/m. The iron stacking factor is 0.9.

UNIT –II

1. Derive output equation of a d.c. machine and point out its salient features.

2. State and explain the factors which governs the choice of spe ific magnetic loading in a

    d.c. machine.

3. A 5 KW, 250 volts and 4 pole, 1500 rpm d.c. shunt generator is designed to have a square pole

    face. The average magnetic flux density in the air gap is 0.42 wb/m2 and ampere conductors per

    metre = 15000. Compute the main dimensions of the machine. Assyme full load efficiency = 87%.

    The ratio of pole arc to pole pitch = 0.06.

4. Find the main dimensions of .a 200 kW, 250 volts, 6 pole, 1000, rpm DC generator. The maximum

    value of flux density in the air gap is 0.87 wb/m2 and the ampere conductors per metre length of

    armature periphery are 31000; The ratio of pole arc to pole pitch is 0.67 and the efficiency is 91

    percent. Assume that the ratio of length of core to pole pitch = 0.75.

5. A rectangular field coil of a dc machine is to produce an mmf of 7500 ampere turns when

    dissipating 220 watts at a, temperature of 60°C The inner dimensions of the coil are: length = 0.24

    metre. Width == 0.1 metre. Height of the coil = 0.15 metre. The heat dissipation is 30 w/m2/oC

    from the outer surface neglecting the top and bottom surfaces of the coil. The temperature of the

    ambient air is 20°C. Compute the thickness of the coil. Resistivity of copper is 0.02 Ω/m and mm2 .

6. A 250 kW, 500 volt, 600 rpm. d.c. generator is built with an armature diameter of 0.75 m and a

    core length of 0.3 m. The lap connected armature has 720 conductors. Using the data obtained

    from this machine, determine the armature diameter core length, number of armature slots,

    armature conductors and commutator segments for 350 kW, 440 volt, 720 rpm. 6 pole d.c.

    generator. Assume a square pole face with ratio of pole are to pole pitch equal to 0.66.The 'full

    load efficiency is 0.91 and the internal voltage drop is 4 percent of rated voltage.The diameter of

    commutator is 0.7 of armature diameter. The pitch of commutator segments should not be less than

    4mm. The voltage between adjacent segments should not exceed 15 V at no load.

7. Explain the design procedure for the shunt field winding of DC machine.

8. Compute suitable dimensions of armature core of a d.c. generator which is rated 50 kW. P = 4, N =

    600 rpm. Full load terminal voltage is 220 volts. Maximum gap flux density is 0.83 Wb/ m2 and

    specific electric loading is 30,000. ampere conductors/metre. Full load armature voltage drop is 3

    percent of rated terminal voltage. Field current is 1 percent of full load current Ratio of pole arc to

    pole pitch is 0.,67 pole face is a square.

9. A 4 pole 50 HP de shunt motor operates with rated voltages of 480 volts at rated speed of 600 rpm.

    It has wave wound armature with 770 conductors. The leakage factor of the poles is 1.2 . The poles

    are of circular cross section. The flux density in the poles is 1.5 Wb/ m2. Compute diameter of

    each pole.

UNIT –III

1. Determine the dimensions of core and yoke for a 200 KVA, 50 Hz single phase core type

    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 of 14 volts, maximum flux density of 1.1

    wb/m2, window space face of 0.32, current density of 3 A/mm2 and stacking factor equal to 0.9. the

    net iron area is 0.56 d2 wher d is diameter of circumscribing circle. Width of the large-stamping is

    0.85d.

2. A 250 KVA, 6600/400 volts, 3 phase core type transformer has a total loss of 4800 watts at full

    load. The transformer tank is 1.25 metre in height and 1 m x 0.5 m in plan. Design a suitable

    scheme of tubes if the average temperature rise is to be limited to 35 oC. The diameter of each tube

    is 50 mm 'and the tubes are spaced 75 mm from each other due to radiation and convection is

    respectively 6 and. 6.5 W/m2 oC. Assume that convection is improved by 35 percent due to

    provision of tubes.

3. A three A three phase 50 Hz core type transformer has the following data:

    Width of H.V. winding = 25mm

    Width of L.V. winding = 16 mm

    Height of the coils =0.5 metre

    H.V. winding turns = 830.

    Width of duct between HV and LV windings. = 15mm, Complete leakage reactance of the   

    transformer referred to H.V. side. The transformer ratings are 300KVA, 6600/400 volts,

    Delta/Star.

4. Determine the main dimensions of the core of a 5 KVA, 11000/1400 volts, 50 Hz, single phase

    core type distribution transformer having the following data: The net conductor area in the window

    is 0.6 times the net cross sectional area of iron in the core. The core is of square cross section,

    maximum flux density is 1 wb/m2. Current density is 1.4 A/mm2. Window space' factor is 0.2.

    Height of the window is 3 times its width.

5. Derive output equation of a three phase transformer.

6. State different methods of cooling the transformers and explain each method with relevant

    diagrams. State merits and limitations of each method.

7. Calculate approximate overall dimensions for a 200 KVA, 6600/440 V, 50 Hz, 3 phase core type

    transformer. The following data may be assumed: emf per turn = 10 V, Maximum-flux density =

    1/3 Wb/m2 , currentdensity = 2.5 A/mm2, window space factor = 0.3, overall height = overall

    width, stacking factor = 0.9. Use a 3 stepped core. For a three stepped core, width of largest

    stamping = 0.9 d and net iron area == 0.6 d2 where d is a diameter of circumscribing circle.

8. A 250 KVA 6600/400, 3 phase core type transformer has a total loss of 4800 W at full load. The

    transformer tank is l.25 m in height and 1m x 0.5m in plan. Design a suitable scheme for tubes if

    the average temperature rise is to be limited to 35 oC. The diameter of tubes is 50mm and are   

    spaced 75 mm from each other. The average height of tubes is1.05m.

9. Calculate the main dimensions of core of 100 KVA, 2000/400 Volts, 50 Hz, single phase shell

    type transformer. Voltage per turn = 10 volts. Peak flux density in the core is 1.1 Wb/ m2. Window

    space factor is 0.33. Ratio of core depth to width of central limb = 2.5. Ratio of window height to

    window width = 3.0 current density in the winding is 2 A/mm2, Stacking factor = 0.9.

10. A 250 KVA, 6600/400 volts, three phase core type transformer has a total loss of 4800 watts at

     full load. The transformer tank is 1.25 m in height and 1m x 0.5m in plan. Design a suitable

     scheme for tubes if the average temperature rise is to be limited to 35°C. The diameter of tubes is

     50 mm and are spaced 75 mm from each other. The average, height of tubes is 1.05 m. Specific

     heat dissipation due to radiation and convection is respectively 6 and 6.5 W/ m2 / oC. Assume that

     convection is improved by 35 percent due to provision of tubes.

UNIT – 4

1. Derive the output equation of a three phase induction motor.

2. State and explain factors governing the choice of ampere conductors per metre in the design of a

    three phase induction motor.

3. Compute the' main dimensions of a 15 KW, three phase, 400 volts, 50 Hz, 2810 rpm squirrel cage

    Im having efficiency of 88 percent and full load power factor of 0.9.Assume specific magnetic

    loading equal to 0.5 Wb/m2 and specific electric loading equal to 25,000 A/m. The rotor peripheral

    speed' may be approximately 20 m/sec at' synchronous speed.

4. Determined diameter and length of the stator core for a 11 kW, 400 V, 3 phase, 4- pole,1425 rpm

    induction motor. Specific magnetic loading is 0.45 wb/m2 and specific electric loading is 23000

    ac/m. Full load. efficiency is 0.85 and full load power factor is 0.88. The ratio of core length to

    pole pitch = 1

5. A 90 kW, 500 volts, 50 Hz; three phase, 8 pole slip-ring induction motor has star connected stator

    accommodating 6 conductor per slot. The number of stator slots = 63. If the slip ring voltage on

    open circuit is to be about 400 volts, find the number of rotor slots and the number of conductors

    in each rotor slot.

6. Write notes on:

          i. Design of rotor bars and slots.

         ii. Design of end rings.

7. Find the values of diameter and length of stator core of a. 7.5 kW. 220 V, 50 Hz. 4 pole. 3 phase

    induction motor for best power factor.

8. Find the main dimensions of a 15 kW, three phase, 400 volts, 50 Hz, 2810 rpm squirrel cage

    induction. motor having all efficiency of 88 percent and full load power factor of 0.9. Specific

    magnetic loading is 0.5 Wb/ m2. Specific electric loading = 25000 A/m. Take rotor peripheral

    speed 'as approximately 20 m/sec synchronous speed.

9. A 11 kW, three phase 6 pole, 50 Hz; 220 volts star connected induction motor has 54 stator slots,

    each containing 9 conductors. Calculate the value of bar and end ring currents. The number of

    rotor bars is 64. The machine has an efficiency of 8.6 percent and a powerfactor of 0.85. The rotor

    MMF may be assumed to be 85 percent of stator MM F. Also find the bar 'and the end ring 

    sections if the current density is 5 A/mm2

UNIT – V

1. Compute the main dimensions of a 1000 KVA, 50 Hz, three phase, 375 rpm alternator. The

    average air gap flux density is 0.55 Wb/m2 and ampere conductors per metre are 28000. Use

    rectangular poles. 'Assume the ratio of arc length to pole pitch equal to 2. Maximum permissible

    peripheral speed is 50 m/sec. The runaway speed is 1.8 times the synchronous speed.

2. The field coils of a salient pole alternator are wound with a single layer winding of bare copper

    strip 30 mm drop, with separating insulation 0.15 mm thick. Compute thickness of the conductor,

    number of turns and height of the winding to develop an mmf of 12000 ampere turns with a

    potential difference of 5 volts per coil and a loss of 1200 watts/m2 of coil surface area. Mean

    length of turn is 1.2 metre. Resistivity of copper is 0.021.Ω/m/mm2.

3. Calculate the .mmf required for the air gap of a salient pole synchronous machine having core

    length of 0.32 metre including 4 ducts of 10 mm each; pole arc = 0.19 metre. Slot pitch = 65.4

    mm, Slot opening = 5 mm. Air gap length = 5 mm. Flux per pole = 52 mwb.

4. Explain the role of digital computers in the design of electrical machine.

5. Describe the construction of Turbo – alternators with sketch.

6. Explain the design of field winding of alternator.

7. State and explain advantages of hydrogen cooling as applied to turbo alternator.

8. Draw cross sectional view of rotor slot and explain the method involved in direct hydrogen cooling

    of rotor of turbo alternator.

9. Derive the output equation of a synchronous machine.

10. State and explain the salint features of Computer Aided design of electrical apparatus.

11. Determine the main dimensions of a 75000 KVA, 13.8 kV, 50 Hz, 62.5 rpm, three phase star

      connected alternator. The peripheral speed of rotor should be about 40 m/sec. Assume average

      gap density equal to 0.65 Wb/ m2, ampere conductors per metre equal to 40,000 and current

      density =4 A/ mm2. Assume Kw = 0.955.

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