LATHA MATHAVAN ENGINEERING COLLEGE

Latha Mathavan nagar, Kidaripatti,

Madurai – 625 301

Subject Name: DESIGN OF ELECTRICAL MACHINES Branch : EEE

Subject Code: EE 2355 Semester : VI

UNIT-I

PART-A

  1. Define space factor. May/June-2007
  2. What are the major considerations to evolve a good design of electrical machine?

Nov/Dec-2011

  1. Name the types of magnetic material based on hysteresis loops. Nov/Dec-2005
  2. State the properties which determine the suitability of a material for insulating materials. Nov/Dec-2004
  3. State the electrical engineering materials used in the construction of Electrical machines.

Nov/Dec-2005

  1. Define rating of a motor.Apr/May-2005
  2. Distinguish between continuous rating and short time rating of Electrical machines.

Apr/May-2008

  1. How is heat produced in a rotating electrical machies? May/June-2009
  2. What is specific Electric loading? Apr/May-2011
  3. What is specific Magnetic loading? Apr/May-2011
  4. What are the different types of duty cycles? Apr/May-2006
  5. Define duty factor. Apr/May-2012
  6. Define heating time constant. Nov/Dec-2006
  7. Define cooling time constant.
  8. What are the choice of specific electric loadings?

PART- B

  1. Write a note on classifications of insulating materials. Nov/Dec-2012
  2. What are the different conducting materials, magnetic materials and insulating

materials used in electrical engineering? Give a brief account of the technical

properties of these materials. Nov/Dec-2011

  1. Discuss about various duties and rating of rotating machines and give their

respective temperature time curves. Apr/May-2011

  1. Derive the expression for temperature rise and cooling curve for determination of motor rating.

Apr/May-2012

  1. Describe the choice of specific magnetic loading and electric loading. May/June-2011
  2. A 400KW, 500V,450rpm,6 Pole DC motor is built with an armature diameter of 0.87m and core length of 0.32m. The lap wound armature has 660 conductors. Calculate the specific electric and specific magnetic loadings. Nov/Dec-2009
  3. A copper bar 14mm diameter is insulated with micanite tube which fits tightly around the bar into the rotor slot of an induction motor. The micanite tube is 1.5mm thick and its thermal resistivity is 10Ω. Calculate the loss that will pass from copper bar to iron if a temperature difference of 35ºC is maintained between them. The length of bar is 0.2m. Apr/May-2010

UNIT-II2 MARKS

  1. State the relation between the armature diameter and commutator diameter for various ratings of dc machines.
  2. What is the peripheral speed ?
  3. State the relation between the number of commentator segment and number of armature

Coil in a dc generator.

4. Define brush contact loss.

5. Give the significance of core length to pole arc in the design.

6. What is slot coding?

7. Name any two methods to reduce armature reaction.

8. How we can avoid the flush over between the adjacent brush arms in dc machines?

9. What is the role of using interpole?

10. Define output equation.

11. What is simplex and multiplex winding?

12. What is meant by equalizer connections?

13. Why square pole is preferred?

14. Define stacking factor (or) irons space factor.

15. Give the relation between real and apparent flux densities.

16. Explain the various steps involved in the design of armature winding of d.c machine.

17. Explain the mmf calculation of air gap.

18. Explain the design of commentator and brushes.

16 mark

  1. The stator of a machine has a smooth surface but its rotor has open type of slot width slots With slot width Ws=tooth width, Wt=14mm and length of air gap lg =3mm.Find the effective length of air gap if the carters coefficient=1/1+5(lg/Ws).there are no radial ducts.
  2. Calculate the mmf required for the air gap of a machine having core length =0.32m,including 4 duct of 10 mm each ;pole arc =0.19m;slot pitch =65.4mm;slot opening =5mm;airgap length =5mm;flux per pole =52 mWb.Given carters coefficient is 0.18 for opening/gap=1,and is 0.28 for open/gap=2. (A/M-2009,N/D-2010)
  3. A 15 KW, 230 v ,4 pole dc machine has the following data: armature diameter= 0.25m; armature Core length=0.125m,length of air gap at pole centre = 2.5mm; flux per pole = 11.7*10-3Wb, radio pole arc/polepitch =0.66.Calculate the mmf required for the air gap

i)if the armature surface is treated as smooth

ii)if the armature is slotted and the gap contraction factor is 1.18. (N/D – 2008)

  1. Calculate the mmf required for air gap of a dc machine with an axial length of 20cm (no ducts)and a pole arc of 18 cm,the slot pitch=27 mm, Slot opening = 12mm, air gaap = 6mm and the useful flux/pole = 25mWb.Take carters coefficient for slot as 0.3. (N/D - 2004)
  2. Calculate the apparent flux density at a particular section of a tooth from the following data :tooth width=14mm, Slot width =10mm;gross core length=0.32m;number of ventilating ducts=6,each 10 mmWide;real flux density =2.2Wb/m2;permeability of teeth corresponding to real flux density =31.4*10-6H/m; stacking factor=0.9.
  3. Determine the apparent flux density in the teeth of a DC machine when the real flux density is 2.15Wb/m2: slot pitch 28 mm: slot width 10 mm and the gross core length 0.35m .The number of Ventilating ducts is 4, each 10mm wide .the magnetizing force for a flux density of 2.15 Wb/m2Is 55000A/m. the iron stacking factor is 0.9
  4. Find the main dimension of a 200 kW, 250 v, 6 pole, and 1000rpm generator. The maximum value of flux density in the gap is 0.87 Wb/m2 and the ampere conductors per meter of armature Periphery are 31000.the ratio of pole are to pole pitch is 0.67 and the efficiency is 91 percent. Assume the ratio of length of core to pole pitch 0.75.
  5. Find suitable value of diameter and length of armature core for a 100kw, 250v, 900 rpm dc motor .state the assumptions made.
  6. Find the suitable diameter core length and number of poles of a 400 kW, 550v, rpm dc generator having 92% efficiency. Assume an average flux density in the air gap of about 0.6 wb/m2 and ampere conductors per meter to be 35000.
  7. Find the main dimensions, number of poles of a 37KW,230V,1400 rpm dc shunt motor, so that a square pole face is obtained. The average gap density is 0.6Wb/m2and the ampere conductors per meter are 24000. The ratio of pole arc to pole pitch 0.67 and the full load efficiency is 90 percent.
  8. Determine the diameter and length of armature core for a 55 kw, 110v,1000 rpm, 4pole Shunt generator , assuming the specific electric electric and magnetic loading s as 26000 ampere conductors per metre and 0.5 wb/m2 respectively. The pole arc should be about 70 percent of pole pitch and length of core about 1.1 times the pole arc. Allow 10 ampere for the field current and assume a voltage drop of 4 volt for the armature circuit. Specify the winding used and also determine suitable values for the number of armature conductors and number of slots.(Anna univ. NOV/DEC 2004.APRIL/MAY 2011)
  9. A 4 pole, 25 hp, 500v,600 rpm series crane motor has an efficiency of 82%. The pole face is square and ratio of pole arc to pole pitch is 0.67. Assuming an average gap density of 0.55 wb/m2 and Ampere conductor s per metre as 17000. Obtain the main dimensions of the core and particulars of a suitable armature winding.
  10. Design a suitable commutator for a 350 kW. 600rpm, 440v, 6 pole dc generator having an Armature diameter of 0.75m. The number of coils is 288. Assume suitable values wherever necessary.

UNIT-III

PART-A

  1. What is window space factor in a transformer?(Anna univ, April /May 2010,Nov/Dec 2006.May/June 2010)
  2. What are factor on which no load currant of transformer depends? (Anna univ, April /May 2010)
  3. State different losses in a transformer? (Anna univ,May/June 2009,Nov/Dec 2007.)
  4. Hwy circular coils are preferred in transformer?
  5. Name the various types of mechanical forces produced in a transformer?(Anna univ, Nov/Dec 2003)
  6. What do you mean by air blast coolling? (Anna univ,May/June 2010)
  7. What is distribution transformer ? (Anna univ, Nov/Dec 2010)
  8. What is tertiary winding? ?(Anna univ, April /May 2011)
  9. Name the various cooling schemes used with transformer? (Annauniv, April /May 2011)
  10. Name a few insulating materials that are used in transformer. ? (Anna univ,Dec 2009)
  11. Define voltage regulation. (Anna univ, Nov/Dec 2005)
  12. What is conservator? (Anna univ, April /May 2005)
  13. What is the significance of stepping the core of a transformer? (Anna univ, April /May 2011)
  14. List various advantage and disadvantage of using higher flux density in design of core. (Anna univ,Dec 2009)
  15. Mention the main function of cooling medium used in transformer.(Anna univ,Dec 2009)

PART-B

  1. Derive output equation of a three phase transformer(Anna univ, April /May 2011,April /May 2008,Nov/Dec 2004)
  2. Calculate the core and window area required for a 100 KVA, 6600/400V, 50 Hz, single phase core type Transformer. Assume a maximum flux density of 1.25 Wb/m2 and a current density of 1.25 Wb/m2 and A current density of 2.5 A/mm2 , voltage per turn = 30 V. Window space factor 0.32. (N/D – 2010)
  3. Calculate the KVA output of a single phase transformer from following data :

Core height/distance between core centres =2.8; diameter of circumscribing circle/distance between core centres=0.56; Net iron area/area of circumscribing circle=0.7,

Current density =2.3 A/mm2 , Window space factor =0.27, frequency =50 Hz, flux density of core = 1.2 Wb/m2, distance between core centres = 0.4 m.

  1. Calculate the core and window area of a 400 KVA, 50 Hz, single phase ,core type power transformer. The following data may be assumed: ratio of weight of iron to weight of copper = 4, ratio of length of mean turn of copper to length of mean flux path = 0.5, maximum flux density=1.5Wb/m2, current density=2.2A/mm2, density of copper =8.9*103Kg/m3,density of iron =7.8*103 Kg/m3, copper space factor=0.12.
  1. A 250 KVA, 6600/400 V, 3phase core type transformer has a total loss of 4800W a full

Load. The transformer tank is 1.25 m in height and 1m*0.5 m in plan. Design a suitable scheme

For tubes if the average temperature rise is to be limited to 35۫˚c . The diameter of tubes is 50 mm and spaced 75 mm from each other. The average height of tubes is 1.05m.

Specific heat dissipation due to radiation and convection is respectively 6 and 6.5 W/m2-c

Assume that convection is improved by 35 % due to provision of tubes. (N/D-2003,N/D-2004,A/M-2005,N/D-2007,A/M-2011)

  1. The tank of a 1250 KVA natural oil cooled transformer has the dimensions length, width and Height 1.55m*0.65m*1.85m respectively .the full load loss is 13.1 KW. Find the number of tube for this transformer .Assume W/m2-c due to radiation=6 and that due to convection is 6.5.Improvement in convection due to provision of tubes is 40% maximum temperature rise is 1m .Diameter of the tube is 50 mm. Neglect the top and bottom surfaces of the tank as regard to cooling. ( A/M-2010,N/D – 2009)
  2. Drive output equation of a three phase transformer. ( A/M-2011,A/M-2008)
  3. Drive out put equation of single phase and three phase transformer.(N/D-2004,M/J-2010)

9. Explain the design of oil tank and tube for a transformer.( M/J-2010)

10. Derive an expression for the number of cooling tubes needed for a transformers tank.(A/M-2011)

11. Explain the design procedure of cooling tubes for a transformer. (N/D-2004)

12. Derive the voltage per turn equation for a single phase transformer. (A/M-2004)

  1. How to design the windings of a transformer? (Anna univ, Nov/Dec 2004)
  2. Write short notes on thermal rating of transformer. ?(Anna univ, May/june 2010)
  3. Derive the condition for getting maximum efficiency in a transformer. (Anna univ, April /May 2011)
  4. Derive the voltage per turn equation for a single phase transformer. (Anna univ, April /May 2004)
  5. Calculate the main dimensions of core of 100KVA, 2000/400 volts,50hz, single phase shell type transformer. Voltage per turn=10 volts, peak flux density in the core is 1.1 wb/m2.Window space factoris 0.33. Ratio of core depth to width of central limp=2.5.Ratio of window height to window width=3.0 Current density in the winding is 2A/mm2. Stacking factor=0.9 (Anna univ, May/June 2009)
  6. The current densities in the primary and secondary windings of a transformer are 2.2 and 2.1 and the length of mean turn of the primary is 10 percent greater than that of the secondary. Calculate the resistance of the secondary winding given that primary winding resistance is 10 ohms.
  7. A 6000v, 60 Hz single transformer has a core of sheet sheet. The net iron cross-selection area is 22.6*10-3 m2, the mean length is 2.23m, and there are four lap joints. Each lap joint takes ¼ times as much reactive mmf as is required per meter of core. If bm=1.1 wb/m2, determine

(i)The number of turns on the 6600v winding and

(ii)The no load current

Assume an amplitude factor of 1.52 and that for given flux density, mmf per meter=232a/m. special loss=1.76 w/kg. Specific gravity of plates (nov/dec2011)

  1. Determine the dimensions of core and yoke for a 200kva, 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 lamination. Assume voltage per turn 14v, maximum flux density 1.1 wb/m2, window space factor 1.32, current density 3a/m/m2 stacking factor= 0.9. the net iron area is 1.56d2 in a cruciform core where d is the diameter of circumscribing circle. Also the width of largest stamping is 0.85d. (nov/dec 2003,06,07)
  2. Calculate Approximate Over all dimension for a 200 kva, 6600/440v 50hz, 3 phase core type transformer. The following data may be assumed: emf per turn =10v, maximum flux density= 1.3 wb/m2, current density= 2.5a/ mm2, window space factor=0.3, over all width, stacking factor=0.9. use a 3 sttepped core for which net iron area is 0.6 d2 and the width of largest stamping is 0.9d (nov/dec 2010,2012)

UNIT- IV

Part – A

  1. What types of slots are preferred in induction motor?
  2. What are the factors to be considered for deciding the air gap length of an induction motor?
  3. What are the function of and ring in the rotor of a cage induction motor?
  4. Compare squirrel cage and wound rotors.
  5. What are factors to be considered for the choice of ampere conductors per meter (electric loading) in induction motor?
  6. What is crawling?
  7. Why is length of air gap in an induction motor kept at minimum possible range?
  8. What are the different losses in an induction motor?
  9. List the main parts of slip ring induction motor.
  10. What are the effects of dispersion coefficient on induction motor?
  11. Write the expression for output equation and co-efficient of induction motor.
  12. Derive stator slot pitch.
  13. What are the disadvantages of a cage induction motor?
  14. List out methods to improve the factor of an induction motor.
  15. What are the merits of slip ring induction motor over cage induction motor?
  16. What are factors to be considered for choosing the specific magnetic loading?

Part-B

  1. Derive the output equation of a three phase induction motor.
  2. Discuss the step by step procedure to design the rotor of a squirrel cage induction motor.
  3. Explain the design of stator slot for an induction motor.
  4. Explain the design of induction motor using circle diagram.
  5. Write shot on

(1)Design of rotor bars and slots.

(2)Design of end rings

  1. Discuss the rule for the selection of rotor slots for a cage induction motor.
  2. Find the main dimension of a 15kw. Three phase 400 volts 50hz,2810 rpm squirrel cage induction motor having an efficiency of 88 percent and full power factor of 0.9. specific magnetic load is 0.5 wb/m2and specific electric loading =25000 a/m. take rotor peripheral speed as approximately 20m/sec at synchronous speed. (nov/dec 2009,2007,2006)
  3. Determine the dimension of a 250 h.p, 3phase 50 Hz, 400v, 1410 rpm slip ring induction motor. Assume bav =0.5 wb/m2ac=30000 a/m, efficiency=0.9 and power factor= 0.9, winding factor= 0.955, current density =3.5a/mm2.(may/june-2010)
  4. Determine the main dimension for a 15jp, 400 volt, 3 phase, 4pole, 1425 rpm induction motor. Adopt of a specific magnetic loading o f0.45 wb/m2 and a specific electric loading of230 ac/cm. assume that a full load efficiency of85% and a full load power factor of 0.88, will be obtained..(april/may-2004)
  5. Find the value of diameter and length of stator core of a 7.5kw, 220v, 50hz, 4pole, 3phase induction motor for best power factor.
  6. Specific magnetic loading=0.4wb/m2, Specific magnetic loading=22000a/m, efficiency=0.86 and power factor=0.87. Also find the main dimension if the ratio of core length to pole pitch is unity.(nov/dec-2008)
  7. Estimate the stator core dimension, number of stator slots and number of stator conductors per slot for a 100kw, 3300v, 50 Hz, 12 pole star connected slip ring induction motor. Assume: average gap density=0.4 wb/m2,conductors/meter=25000a/efficiency=0.9, power factor =0.9 and winding factor=0.96. Choose main dimension to give best power factor. The slot loading should not exceed 500 ampere conductors.(nov/dec-2008,2009,2013)
  1. Estimate the main dimension, air gap length, stotor slots, stator turns per phase and cross sectional area of stator and rotor conductors for a 3-phase, 15hp, 400v, 6-pole, 50 Hz, 975 rpm, induction motor. The motor is suitable for star delta starting.bav=0.45, p.f= 0.85,n=0.9, ac=20,000 amp. Cond/meter.

UNIT-5Part-A

  1. Why salient pole construction is rejected for high speed alternator?
  2. What is runaway speed?
  3. Define short circuit ratio (scr).
  4. What are the effects of scr on machines performance?
  5. What is critical speed of alternator?
  6. List the various methods for the elimination of harmonics from the generated voltage of synchronous machine.
  7. Write the expression for the output coefficient of synchronous machine.
  8. Define pitch factor and distribution factor.
  9. What are thefactors that influence the choice of specific magnetic loading in a synchronous machine?
  10. Define output equation of synchronous machine.

PART-B

  1. Derive the output equation of a.c machine or synchronous machine.
  2. How d and l separated for a synchronous machine(main dimensions)
  3. What are the types of synchronous machines based on rotor construction? Bring out the constructional difference between them.
  4. Discuss the factor affecting the choice of specific magnetic loading and electric loading in an alternator.
  5. Discuss the effect of short circuit ratio (scr) on the machine performance of the synchronous machine
  6. With neat sketch, indicate the location of damper winding in a synchronous machine and mention its uses.
  7. Describe the procedure for the design of field winding of alternator.
  8. Compute the main dimension of a 1000kva, 50 Hz, 3 phase, 375 rpm alternator. The average air gap density is 0.55 wb/m2and the ampere conductor per meter is 28000. Use rectangular poles. Assume the ratio of core length to pole pitch equal to2.maximum permissible peripheral speed is 50m/sec. the runway speed is 1.8 times the synchronous speed.(nov/dec 2007,2004,2005)
  1. Find main dimension of 100mva, 11kv, 50 Hz, 150rpm, three phase water wheel generator. The average gap density=0.65 wb/mand ampere conductor per meter 40000. The peripheral speed should not exceed 65m/s at normal running speed in order to limit runaway peripheral speed.
  2. Determine the main dimensions of stator core for an 8 pole alternator at 3300KVA,300V, 50HZ.