EE1352 ELECTRICAL MACHINE DESIGN 2 MARKS QUESTIONS AND ANSWERS PREPARED BY Sr.ANNAI THERESA St.ANNE’S COLLEGE OF ENGGINEERING & TECHNOLOGY ANGUCHETTY PALAYAM PALAYAM PANRUTI-607110 UNIT – I FUNDAMENTAL CONCEPTS 1. What are the main dimensions of a rotating machine? Armature or rotor diameter Stator core length 2. Define specific magnetic loading It is defined as the ratio of total flux around the air gap and the area of flux path at the air gap. 3. Define specific electric loading. It is defined as the ratio of total number of ampere conductors and the armature periphery at the air gap. 4. What is magnetic circuit? It is the path of magnetic flux. The mmf of the circuit creates flux in the path by overcoming the reluctance of the path. 5. What is leakage flux? It is the flux passing through unwanted path. The leakage flux will not help either for transfer or conversion of energy. 6. What is leakage coefficient? It is defined as the ratio of total flux to useful flux. 7. What is fringing flux? The bulging of magnetic path at the air gap is called fringing. The fluxes in the bulged portion are called fringing effect. 8. What are the factors that modify the reluctance of air gap. It is modified by slots, radial ventilating ducts and non uniform air gaps. 9. Define gap contraction contraction factor for slots. It is defined as the ratio of reluctance of air gap of slotted armature to reluctance of air gap of smooth armature. 10. Define gap contraction factor for ducts . It is defined as the ratio of reluctance of air gap with ducts to reluctance of air gap without ducts. 11. Define total gap contraction contraction factor. It is defined as the ratio of reluctance of slotted armature with ducts to reluctance of smooth armature without ducts. 12. Define field form factor. It is the ratio of average a verage gap density over the pole pitch to maximum flux density in the air gap. 13. List the methods used for estimating the mmf for teeth.
i) Graphical method ii) Three ordinate method iii) Bt 13 13 method 14. What is real and apparent flux density? The real flux density is due to the actual flux through a tooth. The apparent flux density is due to total flux that has to be passing through the tooth. Since some of the fluxes pass through slot so that the real flux density is always less than the apparent flux density. 15. Define specific permeance. It is defined as the permeance per unit length of field. 16. What factor decides the no. of turns in a winding? The emf per turn and flux density and the emf per turn depend on the type of insulation employed. 17. How the area of cross section of a conductor is estimated? The area of cross section of a conductor is estimated based on temperature rise, resistivity and cooling methods. 18. What are the factors to be considered for estimating the length of air gap in dc machines? Armature reaction, cooling, iron losses, distortion of field form and noise. 19. What is the fundamental requirement of a good insulating material? High dielectric strength, high insulating resistance with low dielectric loss, good mechanical strength, good thermal conductivity and high degree of thermal stability. 20. Why silicon content in electrical sheet steel is limited to four to five percent? If silicon content in electrical sheet steel exceeds five percent then it acts brittle and creates difficulties in punching. 21. Why short time rating of an electrical machine is much higher than the continuous rating? To reach the maximum permissible temperature rise in a short duration, the machine can be loaded to higher than the continuous ratings 22. What are standard periods for short time rating of a machine? 10, 30,60 and 90 minutes. 23. Why large size machines have large rating time constant? As the size of the machine increases, the volume and hence weight increases in proportion to third power of linear dimension and surface area in proportion to second power giving large heating time constant for large size machine. 24. State any two methods of cooling turbo alternators. Methods of cooling turbo alternators: a. Air Cooled Turbo - Alternators (Small units) 1. One sided axial ventilation 2. Two sided axial ventilation 3. Multiple inlet system b. 1) Hydrogen cooled turbo-alternators 2) Direct cooled turbo-alternators 25. Distinguish between continuous continuous rating and short time rating of an electrical machine. Continuous Rating (Duty types S1):
The continuous rating of a motor may be defined as the load that may be carried by the machine for an indefinite time without the temperature rise of any part exceeding the maximum permissible value. E.g., Continuously running fans, pumps and other equipment which operate for several hours and even days at a time. Short time rating (Duty type S2): The short time rating of a motor may be defined as its output at which it may be operated for a certain specified time without exceeding the maximum permissible value of temperature rise. Standard short time ratings are 10, 30, 60 and 90 minutes. E.g., Pump sets and grinders, etc. 26. What is the difference between real and apparent magnetic flux densities in rotating machines?
27. List the advantages if direct cooling of rotating electrical machines. 1. Natural cooling 2. No investment 28. What are the assumptions made to calculate tire slot leakage? Current in the conductors of a slot is uniformly distributed over the cross section. Leakage path is straight across the slot and a nd around the iron at the bottom Presence of air is only considered reluctance iron path is assumed as zero. 29. How is leakage magnetic flux different from useful magnetic flux? The leakage flux is not useful for energy transfer or conversion. But the fringing flux is useful flux. The leakage flux is the unwanted path. But the fringing flux flows in the magnetic path. The effect of leakage flux on machine performance is accounted by leakage reactance. The fringing flux increases the slot reactance. 30. How is heat produced in a rotating electrical machine? i) Over loaded ii)Poor ventilation iii) Continuous duty 31.What is is carter’s coefficient?
The carter’s coefficient(Kcs) is the ratio of slot width to gap length. Kcs= 1/(1+5lg/Ws) For parallel sided open type of slot (Kcs)=2/ᴨ )=2/ᴨ[tan-1y-1/y log√ (1+y (1+y2); where y=Ws/2lg 32.What is gap contraction factor for slots? KgsYs/Ys) 33.Define field form factor. factor. Kf is defined as Kf =average =average gap density over the pole pitch/maximum flux density in the gap=Bav/Bg Bav=flux per pole/area per pole =/Ʈl=ᶲ l=ᶲ/(ᴨ /(ᴨDL/P) ᶲ =/Ʈ 34.Define stacking factor. I t is defined as the ratio of actual length of iron in a stack of assembled core to total axial length of stack. 35.What is meant by rating of machines? It refers to the whole of the numerical values of electrical and mechanical quantities with their duration and sequence assigned to the machines by their manufacturer and stated on the rating plate, the machine complying with the specified conditions.s 36.Define rating. Rating of a motor is the power output or the designated operating power limit based upon certain definit conditions assigned to it by the manufacturer. 37.Define continuous rating. It is defined as the load that may be carried by the machine for an indefinite time without the temperature rise of any part exceeding the maximum permissible value. 38.Define short time rating The of motor may be defined as it’s output at which it may be operated for a certain specified time without exceeding the maximum permissible value value of temperature rise. 39.Define duty factor/cyclic duration factor. It is defined as the ratio of the heating period to the period of whole cycle. Duty factor=t h/(t h+) Where t h=Heating period, t e=Period of rest. 40.What is meant by? The intermittent rating of motor applies to an operating condition during which short time.Load periods alternate with period of rest or no load without the motor reaching the thermal equillibirium and without the maximum temperature rising rising above the maximum permissible value. 41.List the method used for determining the motor rating for variable load drives. i)Method of average losses ii)Equivalent current methods iii)Equivalent torque method iv)Equivalent power method. UNIT –II D.C MACHINES
1. What are the factors to be considered for the selection of no. of poles in a dc machine? Frequency of flux reversal, current per brush arm and armature mmf per pole. 2. What are the parameters that are affected by no. of poles? Weight of iron and cupper, length of commutator, dimension of brushes. 3. What is the range of specific magnetic loading in dc machine? 0.45 to o.75 wb/m2 or tesla. 4. What is the range of specific electric loading in dc machine? 15000 to 50000 ac/m 5. List the advantages of large no. of poles. Weight of armature core and yoke, cost of armature and field conductors, length of commutators. 6. List the disadvantages of large no. of poles. Frequency of flux reversal, labour charges, possibility of flash over between brush arms. 7. Why square pole is preferred? To reduce copper requirements. 8. What factor decides the minimum no. of armature coils? The maximum voltage between the adjacent commutator segments. 9. State the difference between armature winding of dc machine and stator winding of ac machine. The armature winding of dc machine has closed coils but the stator winding of ac machine has open coils. 10. Define commutator pitch. It is defined as distance between be tween two commutator segments to which the two ends of coils are connected. 11. What is equalizer connection? They are low resistance copper conductors employed In lap winding to equalize the induced emf in parallel path. 12. What are the effects of armature reaction? Reduction in induced emf, Increase in iron losses, Delayed Dela yed commutation, sparking and ring firing. 13. How the polarities of are inter pole decided? The polarity of interpole must be that of main pole just ahead for the generator and just behind for a motor. 14. What is the effect of inter pole on main pole? In case of generator, the inter pole will magnetise the leading edge and demagnetize the trailing edge of main pole. In case of motor, the inter pole will demagnetize the leading edge and magnetize the trailing edge of main pole. 15. Why equalizer connection is necessary for the armature winding of a dc machine with lap winding? The equalizer connections help in bypassing the circulating current through the winding and avoid over heating. 16. In a dc machine what are the limiting value of armature peripheral speed? 30 m/s. 17. In a dc machine what are the maximum voltage between commutator segments?
30 volts. 18. What is output equation? The equation describing the relation between the output and main dimensions, specific loadings and speed of the machine is known as output equation. 19. State the relationship between number of armature coils and number of commutator segments in d.c. machine. Relationship between number of armature coils and number of commutator segments in a d.c. machine where βC is commutator segment pitch C is Number of coils DC is Diameter of commutator 20 State different losses in a d.c. generator. Losses in a d.c. generator 1) Copper losses i) Armature copper loss = Ia 2Ra. ii) Field copper loss = (I2shRsh, I2seRse) 2) Magnetic losses (iron (or) copper loss) i) Hysteresis loss, Wh B1.6 maxf ii) Eddy current loss We B2max f iii) Mechanical loss 21. State different losses in a machine. The losses in a d.c. machine can be classified into two general types. They are 1. Rotational losses i. frictional and windage losses ii. Iron losses 2. Copper loss 22. What are the main parts of a DC generator? The DC generator essentially have three major parts i. Field system ii. Armature iii. Commutator The field system includes main poles, interpoles and frame. 23. Write the guiding factors for choice of number of armature slots of D. C. Machine. 1.Slot 1. Slot width 2.Cooling 2. Cooling of armature conductor 3.Flux 3. Flux pulsation 4.Commutation 4. Commutation 5.Cost 5. Cost 24. Define brush contact loss. The losses at the commutator are the brush contact losses and the brush friction losses. The brush contact loss depends on the material condition and quality of commutation obtained. 25. Give the main parts of a d. c. motor. i. Field system (stator) ii) Armature (Rotor) iii) Commutator
iv) Main poles v) Inter poles and vi) Frame 26. State the relation between the number of commutator segments and number of armature coils in a d.c, generator. The number of commutator segment is equal to the number of coils in a d.c. generator, sis Commutator segment pitch C is No. of coils Dc is Diameter of commutator 27.State the use of yoke in DC machine. The yoke serves as a path for flux in DC machine and it also serves as an enclosure for the machine. 28.Give methods adopted to reduce the effect of armature reaction in a DC machine. i) By increasing the length of air gap at pole tips ii) By increasing the reluctance at pole tips iii) Providing compensating winding and interpoles. 29.List the advantages of lap winding in a DC machine. i) ii) iii)
Reduction of weight of armature core Cost of armature and field conductors Overall length and diameter of machine.
30. State the relationship between number of armature coils and number of commutator segments in a dc machine. The relationship between number of armature coils and number of commutator segments in a dc machine is
where - commutator segment pitch C- no. of
coils D- diameter of commutator 31.State 31. State different losses in a dc generator. a) Copper losses: (i) armature copper loss (Ia2 Ra) ii) field copper loss (I 2snRsh, I2seRse) b) Magnetic losses ( iron or core loss): i) hysteresis loss (Whα B 1.6maxf ) ii) eddy current loss(Weα B2maxf ) iii) mechanical losses 32.What 32. What are the main dimensions of a rotating machine? The main dimensions of a rotating machine are armature diameter and stator core length. 33.Define 33. Define specific magnetic loading. It is defined as the average flux density over the air gap of the machine. Specific magnetic loading =
34.Define 34. Define specific electric loading.
It is defined as the no of armature conductors per meter of armature periphery at the air gap. Specific electric loading =
35.What 35. What is output equation? The equation describing the relation between the output and main dimensions, specific loadings and speed of a machine is called is known as output equation. 36.Write 36. Write the expressions for output co-efficient. For dc machine output co- efficient is C 0 =∏2Bavg ac *10-3 (kw/m3-rps) -3 3 For ac machine output co- efficient is C 0 =∏2Bavg ac k ws ws*10 (kva/m -rps) 37.List 37. List the factors that influence the separation of D and L of a dc machine. i) Pole proportions ii) Moment of inertia iii) Peripheral speed iv) Voltage between adjacent segments 38.What 38. What is square pole criterion? The square pole criterion states that for a given flux and cross section area of pole, the length of mean turn of field winding will be less, when the periphery forms a square. This implies that the length must be approximately equal to pole arc. 39. In a dc machine what are the limiting values of armature peripheral speed and voltage between adjacent commutator segments. Maximum armature peripheral speed = Vamax=30 m/s Maximum voltage between commutator segments Eem= 30 v 40. List the various L/τ used for separation of D and L in induction motor. For minimum cost 1.5-2 for good power factor 1-1.25 for good efficiency 1.5 for overall length 1. 41.What 41. What are the factors that affect the size of the rating machines? The factors affecting the size of the rating machine are: speed, specific electric and magnetic loading. 42.What 42. What are the factors that decide the choice of specific magnetic loading? i) Maximum flux density in iron parts of a machine ii) magnetizing current iii) core loss 43.What 43. What are the factors that decide the choice of specific electric loading? i. Permissible temperature rise ii. Voltage rating of machine iii. Size of the machine iv. Current density 44.What 44. What are the factors to be considered for the selection of number of poles in a dc machine?
The factors to be considered for the selection of number of poles in a dc machine are frequency of flux reversals, weight of iron parts, length of commutator, labour charges, weight of copper, current per brush arm and armature mmf per pole 45. List the advantages and disadvantages of large number of poles. Advantages: i) weight of armature core and yoke ii) cost of armature and field conductors iii) overall length and diameter of machine iv) length of co mmutator v) distortion of field form under load conditions. Disadvantages: i) frequency of flux reversals ii) labour charges iii) possibility of flash over between brush arms. 46. Why square poles preferred? If the cross section of the pole body is square, then the length of the mean turn of field winding is minimum. Hence to reduce the copper requirement a square cross section is preferred for the poles of a dc machine. 47. What is the effect of interpole on main pole? In case of generator the interpole will magnetize the leading edge and demagnetize the trailing edge of main pole. In case of motor the interpole will demagnetize the leading edge and magnetize the trailing edge of main pole. 48.Give 48. Give the main parts of a d.c motor. Field system (stator), Armature (Rotor ),Commutator,Main poles, Inter poles and Frame. 49. Write the guiding factors for choice of number of armature slots of D.C machine. i. Slot width ii. Cooling of armature conductor iii. Flux pulsation iv. Commutation and v. Cost 50.Define 50. Define brush contact loss. The losses at the commutator are the brush contact losses and the brush friction losses. The brush contact loss depends on the material condition and quality of commutation obtained. 51.What 51. What are the main parts of a DC generator? The DC generator essentially has three major parts. i) Field system (includes main poles, inter poles and frame) ii) Armature iii) Commutator 52. Distinguish between lap and wave winding in a D.C.machine. S.No Lap Wave 1 Parallel paths = no.of poles Parallel paths =2 2 Current through conductor=I a/p Current through conductor=Ia/2 3 Winding have large no. of conductors with Winding have less no. of conductors with Smaller area of C.S Larger area of C.S 53. What are dummy coils? coils?
The coils which are placed in armature slot for mechanical balance but not connected electrically to the armature winding are called dummy coils 54. What is meant by equalizer connections? In lap winding, due to difference in the induced emf in various parallel paths , there may be circulating currents in brushes and winding. The connections that are made to equalize the difference in induced emf and to avoid circulating currents through brushes are called equalizer rings. UNIT –III TRANSFORMERS 1. Why the area of yoke of a transformer is usually kept 15 to 20 % more than that of core? The flux density in the yoke is reduced resulting in to reduction in iron losses for yoke. 2. What is the cause of noise in transformer? It is because of magnetostriction effect and also losing of stampings and mechanical forces produces during working. 3. What are the important properties of transformer steel? High permeability, high resistivity, low coercive force 4. Why stepped core are generally used for transformer? LV & HV coils are circular, for better bette r utilization of space, for reducing the mean length of LV & HV turns, resulting in saving of copper material. 5. What are the advantages of using higher flux density in the core? Reduced overall size and weight of transformer 6. List the disadvantages of using higher flux density in the core? Increased magnetizing current and iron losses, saturation of magnetic material, lower efficiency. 7. Why the cross section of yoke is taken greater than cross section? In order to reduce flux density in the yoke, thereby reducing iron losses and no load current in yoke section. 8. What are the types of windings commonly used for LV winding? Cylindrical winding with rectangular conductors and helical winding. 9. What are the drawbacks of sandwich winding? Requires more labour in its maintenance, more difficult to insulate different coils from each other and from yoke. 10. Name few insulating materials used in transformer. Press board, cable paper, varnished silk, transformer oil, porcelain, insulating warmish. 11. How iron losses occur in transformer minimized? By laminating magnetic cores and yokes. 12. Why the efficiency of transformer is so high? Mechanical losses zero and iron losses are comparatively less. 13. Mention clearly the condition for maximum efficiency? Efficiency is maximum at a load at which copper losses are equal to iron losses. 14. Mention the main function of cooling medium used in transformer. i) To transfer heat from convection from the heated surface to tank surface. ii) To create good level of insulation between various conducting parts. 15. What is window space factor in design of transformer?
It is defined as the ratio of copper area in window to total area of window. 16. What are the different losses in a transformer? Losses in a transformer: a) Core (or) iron loss. b) Copper loss 17. State merits of three phase transformer over single phase transformers. Merits of three phase transformers: a) A three phase transformer occupies less space for same rating compared to a bank of three single phase transformers. b) It is less weight. c) Cost is less. d) The core will be smaller size and the material required for the loss is less. 18. Why is the core of the transformer laminated? The cores of transformer are laminate in order to reduce the eddy current losses. The eddy current loss is proportional to the square of the thickness of laminations. This apparently implies that the thickness of the laminations should be extremely small in order to reduce the eddy current losses to a minimum. 19. What are the advantages of three phase transformers over single phase transformers? i. A three phase transformers is lighter, occupies lesser space, cheaper and more efficient than a bank of single phase transformers. ii. In case of three phase transformers than is only one unit to install and operate. Hence the installation and operational costs are smaller for three phase units. 20. Write the relation between core area and with of iron and copper for a single phase transformer.. Ac=Tp δp+ Ts δs 21. Differentiate core and shell type transformers. Core type Shell type Easy in design and construction Has low mechanical strength due to nonbracing of windings Comparatively complex High mechanical strength 22. State different losses in a transformer. i) Iron losses (or) Core losses ii) Copper losses 23.What is the range of efficiency of T/F? It will be in the range of 94% to 99%. 24.What are the different types of high voltage windings? i)cylindrical winding with circular conductors ii)cross over winding with either circular or small rectangular conductors iii)continuous disk type winding with rectangular conductors. 25.Define copper space factor. For a T/F it is the ratio of conductor area and window area. 26.Name various types of cross section used for core type T/F. Square,rectangular,cruciform and multi stepped cores. 27.What is window space vector?
I t is defined as the t he ratio of copper area in window to total window area. 28.How is the value of window window factor estimated? It depends upon the relative amount of insulation and copper provided which in turn depends upon the voltage rating and output of transformers.It transformers.It is given by Kw=10/(10+KV) Where KV is the voltage of HV winding in kilovolt. 29.How is the area of window calculated? Area of window(Aw)=Height of window(Hw) x width of window(Ww) 30.Define copper space factor. For a T/F it is the ratio of conductor area and window area. 31.List the assumptions made ofr calculation of leakage flux and leakage reactance. The primary and secondary windings have an equal axilal length. The flux paths are parallel to the windings along the axial height. Primary winding MMF I pTp is equal to secondary winding MMF I sTs. Half of the leakage flux in the duct links with each winding. The length of the mean turn of the windings are equal. The reluctance of flux path through yoke is negligible. 32.Name the factors to be considered to choose the type of winding for a core type T/F. i)current density ii)short circuit current iii)surge voltage iv)impedance v)temperature rise vi)transport facilities. 33.Why stepped cores are used in T/F? When stepped core are used the dia of circumscribing circle is minimum for a given area of the core, which helps in reducing the length of mean turn of the winding with consequent reduction in both cost of copper and copper loss. 34.List the different methods of cooling of T/F. i)air natural ii)oil natural iii)oil natural –air forced iv)oil natural water forced v)forced circulation of oil vi)oil forced-air natural vii)oil forced-air forced viii)oil forced-water forced. 35.Mention the factors to be considered for selecting the cooling method of a T/F. The choice of cooling method depends on KVA rating of transformer.size,application and the site condition where it has to be installed. 36.Mention the different characteristics desirable in T/F oil. Electric strength Resistance to emulsion Viscosity Purity Flash point Sludge formation 37.State merits three phase T/F over single phase T/F. Less cost Requires less space Less weight. Higher efficiency compared to 1 phase T/F. UNIT – IV THREE PHASE INDUCTION MOTORS 1. What is rotating transformer?
The principle of operation of induction motor is similar to that a transformer. The stator winding is equivalent to primary of a transformer and the rotor winding is equivalent to short circuited secondary of a transformer. In transformer the secondary is fixed but in induction motor it is allowed to rotate. 2. Why wound rotor construction is adopted? The wound rotor has the facility of increasing the rotor resistance through slip rings. Higher values of rotor resistance are needed during starting to get a high value of starting torque. 3. What are the main dimensions of induction motor? The main dimensions of induction motor are stator core internal diameter and stator core length. 4. What are the different types of induction motor? How they differ from each other? The two different types of induction motor are squirrel cage and slip ring type. The stator is identical for both types but they differ in the construction co nstruction of rotor. 5. How the slip-ring motor is started? The slip-ring motor is started by using rotor resistance starter. The st arter arter consists consists of star connected variable resistances and protection circuits. circuits. The resistances resistances are connected to sliprings. While starting the full full resistance is included included in the rotor circuit to get high starting s tarting torque. Once t he he rot or or starts rot ating, ating, the resistances are gradually reduced in steps. At runn unniing conditio condition slip-rings are shorted and so it is equivalent to squirrel squirre l cage rotor. rotor. 6. What type of starter cannot be used for squirre squirrel-cage l-cage motors motors? The starter which cannot be used for squirrel cage ca ge motor is rotor resistance resistance starter. 7. What type of connectio connecti on is pref erred f or or stator of induction inducti on motor? Under running condition the stator of inducti inductio on mo mot or is normally connected connected in delta delta.. (In (In delta connection the torque developed will be higher than t he he star connection). connection). But for reducing the starting current, the the sta stato torr can be connec connect t ed ed in star while starting and then changed to delta. delta. 8. Write the expression for output equati on and and output coef f f icient icient of induction of induction motor. The equation for input KVA is considered as output equation in induction motor. The input KVA, Q=CoD2Lns in KVA Output coefficient, Co=11KwsBavac x 10-3 in KVA/m3-rps 9. What are the materials used for slip-rings and brushes in induction motor? The slip-rings are made of brass or phosphor bronze. The brushes are made of metal graphite which is an alloy of copper and carbon. 10. What are the ranges of speci spe cific fic magn magnetic etic loading in induction motor? Specific magnetic loading = 0. 0.3 to 0.6 Wb/m2 Specific electric loading = 5000 to 45000 amp a mp .cond /m. /m. 11. What are are t he ranges of efficiency and power factor in induction motor? Squirrel cage cage motors motors Efficiency = 0.72 to 0.91 Power factor = 0.66 to 0.9 Slip ring motors Efficiency = 0. 0.84 to 0. 0.91 Power factor = 0.7 to 0.92
The ISI specification says that the product of efficiency and power factor shall be in the range of 0.83 to 0. 0.88. 12. What are the factors to be considered for ' the choice of specific electric loading? The choice of specific electric loading depends on copper loss, tempe temperrature ri rise, se, voltage rating and overload capacity. 13. How the indu ind uction motor can be designed for best power factor? For best power factor the pole pitch, is chosen such that. 14. What What are are the factors to be considered for the choice of specific magnetic loadin g? The choice of specific magnetic loading depends on power factor, iron loss and over load capacity. 15. What types of slots are preferred in induction motor? Semi enclosed slots are preferred for induction motor. It results in less a gap contraction factor giving a small value of magnetizing currents, low tool too l pulsation loss and much quieter operation (less noise). noise). 16. What is full pitch and short pitch or chording? When the coil span is equal to pole pitch (180 180o oe) e).. The winding is called f ull ull pitched winding.. winding When the coil span is less than the pole pitch (180 180o oe). The winding is called short pitched or Chorded. 17. Why short chorded windings are employed in induction i nduction motor? For short chorded windings the length of mean turn will be lesser than t he he full pitch coils. Hence it results in reduction of copper. Also the shor hort t cho chorrded winding eliminates certain harmonic magnetic fields. 18. What is integral slot winding and f ractional slot winding? In int egral egral slot slot winding winding the the total number of slots is chosen such th tha at t he slots per pole are an integer. integer. The integer should also be a multiple of num numb ber of phases. In f ract ional slot winding the t otal otal number of slots is chosen such that per pole is an integer. 19. What are the different types of stator windings in induction motor? The different types of stator windings are mush winding, winding, lap winding winding and wave winding winding.. 20. List the unde undessirable effects produced by certain combina combi nat t ion ion of rotor r otor and stator slots. The follo following problems may develop in induct ion ion mo moto torr wit h certain combination of rotor and stator slots. 1. The motor may refuse to start , (clogging (clogging)) 2. The motor may run at sub synchronous speed (crawling) 3. Severe vibration may develop and the noise wi willl be excessive. 21. What is i s slot space factor? The slot s slot space factor is the ratio of conductor (or copper) copper) area per slot slot and slot area. area. It gives an indication of the space occupied by the conduc cond uct t ors and and the space available for insulation. The sl slot space factor for indu inducti ctio on mo mot t or varies from 0.25 to 0. 0.4. 22. What are the factors to .be co considered nsidered fo for r est imating the leng lengt h of air-gap of air-gap in ind in duction motor? The following factors are to be considered for estimating the length of air-gap air-gap.. 1.Power factor 2.Overload capacity
3.Pulsation loss 4.Unbalanced magnetic pull 5.Cooling 6.Noise 23. Write the expression for length of mean turn of stator windin g. Length of mean turn of stat or, Lm Lmts ts= = 2L + 2. 2.3τ + 0.2 0.24 4 24. Which part of induction mot mo t or or ha has maximum flux densit densit y? y? What t is is the max maximum value of flu flux density density in that part ? The teeth of t of t he he stator stator and rot or or core will will have have maxi maximum flux flux den denssit y. The maximum va valu lue e of flu of flux x densi ensit t y in in t he teet h is 1.7 Wb/m2 Wb/m2.. 25. What What are are the advantages advantages and disad disa dvantages of large air-gap length length,, in induct induct ion ion motor? Advantages Ad vantages A large airair-gap le length res results in hig higher over overlload capacity acity,, better coo cooling ling,, red redu uction in noise noise and reduct ion in unbalance unbalanced d mag magn netic pull. Disadvantages Disa dvantages The Th e disa disadv dva antage of large large ai airr-gap lengt lengt h is is th that at it res results in high value of magnetizing of magnetizing current . 26. What is skewing? Skewing is t wisting wisting either the stator or rotor rotor core core.. The motor motor noise noise,, vibratio vibrations ns,, clogging and sy synchronous cusps ca can n be be reduced or even entirel tirely y elimin eliminated ated by sk ewing. In order order to eliminate the effect of any harmo harmonic, th the e ro rotor bars sh should be ske skewe wed d through an angle angle so so that tha t the the bars bars lie under under al alternate harmon harmonic ic poles of of the the sa sam me polarit y or or in othe other words words t he he barss must bar ust b be skewed skewed through two pitches itches.. 27. Explain the phenomena of clogging. When Wh en the number of stator and rotor slot s are equa equal, th the e speed speeds of of a all th the e harmo harmonic nicss produced by stator slotting coinci coincide wi with th the speed of corr correspond ndiing ng,, rot or harmonics harmonics.. Thus harmonics harmonics of ev ever ery y order order wou would ld try to exert synchr synch ronou nouss tor torq ques at at their their correspo corresp ond ndiing syn synch chrronou onouss sp spee eeds ds an and d the machine wo would refuse to start. This This is known as clogging. clogging. 28. What are the methods to redu ce harmonic harmonic torques? torques? The methods methods used for for reduc ducti tion on or elimi mination nation of harm harmonic torque torques are ch chording ording,, integral slot lot w wind indiing ng,, skewing skewing and increasing the length of airai r-g gap. 29. What is crawling and clogging cloggin g? Crawling Crawlin g is a ph phenom nome enon in in which t he in induct duct ion ion motor runs runs at a spee speed le lesser sser than sub synchronous spee speed. Clogging Cloggin g is a phenome phenomen non in whi which t he indu inducti ctio on motor refuses to st st art. art. 30. What is harmonic induc i nduct t ion ion t orque orque and har harmonic synch synchro ronous nous torq torqu ue? Harmonic induction torque is the torque produced by by harmonic fields fields due due t o stat or or winding and slots. Harmoniic sy Harmon synchronous torques are torques produced by the comb combined effect effect of of sa same order of stator and rotor harmonic f ield ieldss. Due to both the harmonic torques the th e machine may crawl, bu but t here here will will be difference in the crawling speed speeds. In case of harmo harmonic induc inducti tion on t orq rque ue cra crawl wling ing speed is slightly lesser than the sub sub synchronous speed. speed. But But in in case of of harmonic harmonic sy synchronous torqu torque e the crawli crawling ng spe sp eed is sam same as that that of of ssub synchronous speed and this is also called call ed synchron synchrono ous cusps.
31. State the main constructional differences between cage induction motor and slip ring induction motor. The squirrel cage motor has the following advantages a s compared with wound rotor machine. They are a. No slip rings, brush gear, short circuiting devices, rotor terminals for starting rheostats are required. The star delta starter is sufficient for starting. b. It has slightly higher efficiency. c. It is cheaper and rugged in construction. d. It has better spce factor for rotor slots, a short overhang and consequently a smaller copper loss. e. It has bare and rings, a larger space for fans and thus the cooling conditions are better. f. It has a smaller rotor overhang leakage which gives a better power factor and a greater pull out torque and overload capacity. Disadvantages: The greatest disadvantage of squirrel cage rotor is that t hat it is not possible to insert resistances in the rotor circuit for the purpose of increasing the starting torque. The cage motor has a smaller starting torque and larger starting current as compared with wound rotor motor. 32. What are the different losses in an induction motor? The various power losses in an induction motor can be classified as b. Constant loss: These can be further classified as a s core losses and mechanical losses. c. Variable loss: This includes the copper losses in stator and rotor winding due to current flowing in the winding. 33. What are the merits of slip-ring induction motor over cage-induction motor? 1. It is possible to insert resistances in the rotor circuit for the purpose of increasing the starting torque. 2. The starting current is low when compared to the cage induction motor. 34. How does the external resistance of slip-ring slip -ring induction motor influence the motor performance? The starting torque of a slip ring motor is increased by improving its power factor by adding external resistance in the rotor circuit from the star connected rheostat resistance being progressively cut out as the motor gathers speed. Addition of external resistance, however increases the rotor impedances and so reduces the rotor current. 35. State the effect of change of air gap length in a 3 phase Induction motor -. i. The length of the air gap determines the magnetizing current. ii. Greater the length 'Of the air gap, greater will be the over load capacity. 36. Define unbalanced magnetic pull: Unbalanced magnetic pull is the radial force acting on the rotor due to non uniform air gap around armature periphery. 37. List the main parts of a slip-ring Induction motor i) Rotor core ii) Rotor winding iii) Slip rings
iv) Frame v) Stator core vi) Stator winding 38. What is the function of end rings ill the rotor of a cage induction motor? End rings are provided to short circuit the rotor bars at both the ends. 39.What are the main dimensions of induction motor? Stator core internal diameter Stator core length. 40.Why induction motor is called as rotating T/F? The principle of operation of I.M is similar to that of a T/F.The stator winding is equivalent to primary of T/F.the rotor winding is equivalent to short circuited secondary of T/F.I T/F thje secondary is fixed but in I.M it is allowed to rotate. 40.What type connection is preferred for stator of I.M? Under running condition the stator of I.M is normally connected in delta.(In delta connection,the torque developed will be higher than star connection.)But for reducing the starting current the stator can be connected in star while starting sta rting and then changed to delta. 41.List the advantages of using open slots. The winding coils can be formed and fully insulated before installing and also it is easier to replace the individual coils. I avoids excessive slot leakage thereby reducing the leakage reactance. 42.Give the advantages of using semi-enclosed stator slots. The advantages are less air gap contraction factor giving a small value of magnetizing current ,low tooth pulsation loss and much quiter operation.(less noise).Semi enclosed slots are mostly preferred for I.M 43.Name the factors to be considered for choice specific electric loading. i)Copper loss ii)Temperature rise iii)Voltage rating iv)Over load capacity. 44. Name the factors to be considered for choice specific magnetic loading. i)power factor ii)Iron loss iii)Over load capacity. 44.How the I.M can be designed for best power factor?
For best power, factor the pole pitch is chosen such that √0.18L 45.What are the problems that occur in I.M due to certain combination combination of stator and rotor slots? The motor may refuse to start The motor may crawl at some subsynchronous speed. Severe vibrations are developed and so the noise will be executive. 46.What is full pitch and short pitch or chording? When the coil span is equal to pole pitch (180 deg electrical) the winding is called full pitched winding.When the coli span is less than pole pitch (180 deg electrical) the winding is called short pitched or chorded. 47. What is integral slot winding and fractional slot winding? In integral slot winding the total no of slots is chosen such that slots per pole is an integer,which should be multiple of no of phases.I fractional slot winding the total no of slots is chosen such that the slots per pole is not an integer. 48. What is slot space factor?
It is the ratio of conductioor area per slot and slot area.It gives an indication of the space occupied by the conductors and the space available for insulation.The slot space factor for I.M varies from 0.25 to 0.4 49. Name the methods used for reducing harmonic torques. i) chording ii) integral slot winding iii) skewing iv) increasing the length of airgap. 50. What is skewing? Skewiung is either twisting the stator or rotor core.The motor noise,vibrations,cogging and synchronous cusps can be reduced or even entirely eliminated by skewing either the stator or the rotor. UNIT – V SYNCHRONOUS MACHINES 1. Name the two types of synchronous machines. machines. . Based on construction the synchronous machines may be classified classified as, as, 1. Salient pole machines. 2. Cylindrical rotor machines. machines. 2. What are the factors to be considered for the choice of f specific specific magnetic magnetic loading? The factors to be considered for the choice of specific magnetic loading are 1. Iron loss 2. Voltage rating 3. Transient short circuit current 4. Stability 5. Parallel operation 3. What is runaway speed? The runaway speed is defined as the speed which the prime mover should have, if it is suddenly unloaded, unloaded, when working at its rated load. load . 4. What are the two tw o types of poles used in sa s alient pole machines? The two types of poles used in salient sa lient pole machines are Round poles and Rectangular poles.. poles 5. Wh What at are the fact ors to be considered for the choice of specific electric l oading? The factors to be considered for the choice of specific electric loading are Copper loss Temperature rise Voltage rating Synchronous reactance Stray load losses 6. What is short circuit Ratio (SCR)? The Short Circuit Ratio (SCR) is defined as the ratio of field current required to produce rated voltage on open circuit to field current required to circulate rated current at short circuit . It is also given by the reciprocal of synchronous reactance, Xd in p.u (per unit). For turbo - alternators SCR is normally between 0.5 to 0.7. 0.7. For salient pole alternator SCR varies from 1. 1.0 to 1. 1.5. 7. Write the expressions for length of air-gap air-gap.. in salient pole synchronous machine? Length of air-gap,
8.What is salient pole rotor? What is Alternator? What are the advantages of large 8.What air-gap in synchronous machine? The advantages of large air-gap are Reduction in armature reaction Small value of regulation Higher value of stability A higher synchronizing power which makes the machine less sensitive to load variation.. variation Better cooling Lower tooth pulsation loss Loss noise Smaller unbalanced magnetic pull. 9. What is the limiting factor for the diameter of synchronous synchro nous machine? The limiting factor for the diameter of synchronous machine is the peripheral speed. The limiting value of peripheral speed is 175 m/sec for cylindrical rotor machines and 80 m/sec for salient pole machines. 10. Write the expression for air-gap length in cylindrical rotor machine. machine .
11. 11. What are the constructional constructional differences between salient pole type alternator and cylindrical rotor type alternator? Salient pole Alternator: The term salient pole means projected pole. This type of rotor is used for low and medium speed machines. The prime mover used is water turbine which gives low speed 50 to 500 rpm. In order to get standard frequency 50 Hz, the number of poles lies in the range 12 to 120. Because of low speed such machines are characterized by large diameter and small length. Since water turbine is used' as a s a prime mover type of alternator is also a lso called hydro electric generator. 12. Define short circuit ratio of a synchronous gemerator. The short circuit ratio (SCR) of a synchromous machine is define as the ratio of field current required to produce rated voltage or open circuit to field current required to circulate rated current at short circuit. 13. State merits of Computer Aided Design of electrical machines. It is possible to select an optimized design with a reduction in cost and improvement in performance. Reduces the probability of error with the result likely accurate a nd reliable.
All simple arithmetic operations are performed at a high speed and makes possible to provide design in a short time. 14. State the merits of computer aided design of electrical machines. (i) Easy to access (ii) Time consumption (iii) Accuracy 15. What is the use of-damper winding? To prevent hunting
In synchronous motor, it’s function is to provide starting torque and to develop damping power when the machine starts hunting In synchronous generators, it is provided to suppress negative sequence field and to damp the oscillations when the machine starts hunting. 16. How is cylindrical pole different from salient pole in a synchronous machine? i) Cylindrical pole are non projecting pole whereas the salient pole machines are projecting pole. ii) Cylindrical rotor construction is used for turbo alternators which are driven by high speed steam or gas turbines where as salient pole construction is used for generators driven by hydraulic turbine since these turbines 'operate at relatively low speeds. 17. How is computer aided design different from f rom conventional design in the case of electrical apparatus? i) Easy to access ii) Time consumption iii) Accuracy 18. State merits of computer aided design of an electrical machine. Merits of Computer Aided design: b. It makes it possible to select an optimized design with a reduction in cost and improvement in performance. c. It has capabilities to store amount a mount of data, count integers, round off results down to integer and refer to tables, graphs and other data in advance. d. A large number of loops can be incorporated in the design programme and therefore it makes it easier to compare different designs out of which the best suited can be selected. 19.Mention the factors to be considered for the selection of no of armature slots. i)Balance windings ii)cost iii)Hot spot temperature in winding iv)Leakage reaction v)tooth losses vi)tooth flux density. 20.What are the types of colis employed by the salient pole machines? i)single turn ii)multi turn. 20.How is the design process of a machine classified in computer aided design. i)Electromagnetic ii)Mechanical design iii)Thermal design 21.Name the two acceptable approaches to machine design. i)Analysis method ii)Synthesis method. 21.Mention the advantages of synthesis method. It is fairly easy to program to use and to under stand. Results in considerable time saving therby giving quck returns of the investment made.
The programs based upon analysis methods are simple but they become the foundations for later day larger and sophisticated programs. The results are highly acceptable by ndesigns.
