UPSC/GK

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  What is Magnetic Material ? The materials which get magnetized in the presence of magnetic field is called  Magnetic materials. Non Magnetic materials get magnetized in presence of magnetic field but they exhibit weak magnetization. Classification of Magnetic Materials On the basis of magnetic behaviour, material may be classified as Diamagnetic Paramagnetic Ferromagnetic Diamagnetic materials It is a weak form of magnetism that is non-persistent and persists only when an external field is applied. Due to an applied magnetic field a change is occur in the orbital motor of electrons, due to this change a magnetic moment is induced in this materials. The magnitude of induced magnetic moment is extremely small and in a direction opposite to that of applied magnetic field. Diamagnetic materials are repelled by magnetic field. Some of the materials that exhibit diamagnetism are  Cu, Au, Ge, Si, Diamond, Al 2 O 3 , NaCl  etc. Paramagnetic Materials When Paramagnetic Mate...

  Applications of DC Generators Separately-excited  DC generator are rarely used in practical applications because of the additional expenses to be limit for the separate excitation supply. DC shunt  generator can be used for general purpose  lighting and low-voltage DC supply system. DC series  generators do not find much application because of their rising voltage characteristics at higher loads, except these are sometimes  used as boosters to compensate excessive voltage drops  that can take place in a long DC feeder. Flat compound  DC generators are most commonly  used for low-voltage DC distribution systems.  Flat compound generators can also be  used for charging of batteries  since they can give fairly constant output terminal voltage, irrespective of load current. Over-compound  type DC generators can be  used for lighting and general power supply applications  because over-compounding can compensate...

  Hopkinson's Method  In this method, two identical DC machines are both mechanically and electrical coupled, and are tested simultaneously. One of the machines is run as a motor, whereas the other as a generator. the connection diagram of Hopkinson's test is given below:- For performing the test, machine I is started as a DC shunt motor and brought to rated speed with switch 'S' open. Both machines run at same speed as both are mechanically coupled. The field current of the generator (machine II) is so adjusted that its output terminal voltage changes and becomes equal to that supplied to the motor (machine I) terminals.  At this time voltmeter  V 2   reads zero voltage. the switch 'S' is closed at that instant. Under this condition, the generator will neither taking nor giving current to the supply. After this state is achieved, any desired load can then be put on the generator by controlling the induced EMF of the machines. I f2   >   I...

  Construction Details of DC Machines DC Machine has two parts Stator and Rotor. The Field Winding if DC Machines is wound on the Stator and Armature Winding is wound on the Rotor. Different parts of Stator and Rotor have been explained below: Yoke It provides path for pole flux ⲫ and carries half of it ⲫ/2. It provides Mechanical support to whole machine. Cast iron is used for small DC machines and fabricated steel for large DC machines. If DC machines is operated through power electronic converter then yoke is laminated else not. Field Poles It consists of pole core and pole shoe. Pole core is made of cast steel but pole shoe is laminated and fixed to pole core appropriately. At present both pole shoe and pole core is laminated. Field Winding The pole excited by a winding wound around pole core. The winding is made from copper. Number of terns and cross-section of field winding depend on type of DC machine.  →  For DC shunt machine, large number of turns and small cross...

  Armature Winding Armature winding is always closed and double layer type closed means all winding are connected in series to form a closed circuit. the junctions of two coils terminated on copper segment called as  commutator segments. A coil has two sides occupying distinct specified slots. To maximum induced emf, the spacing between two ends should be kept at 180° electrical. it means if one side is under North Pole then other should be under South Pole. Coil span spacing between the two sides of coil. The spacing is expressed in terms of number of slots between the sides. if S is the total number of slots and P is the total no. of poles then coil span is S/P E.g.: For 20 slots, 4 poles, coil span=5, if one side of a coil is placed in slots 3, then other end must occupy slot (3+5=8). A double layer winding means that each slot has two coil sides (belonging to 2 different coils). one coil is placed in lower portion of slot and other above it. if S=20, P=4, coil span=5, if a...

  Lap winding we know that, coil span = S/P where, S=Number of slots             P=Number of poles Assuming we want to design a lap winding for 4 pole DC machine having a total number slots, S=16 Coil span = 16/4 = 4 we also know that Commutator pitch for lap winding, Y c  = ± 1 The upper coil side present in slit number 1 is shown by firm line and named 1 while lower coil side is shown by a dashed line and named as '1' . Since, coil span = 4, the first coil has sides 1 and 5 and coil can be identified as (1-5'). If we terminate coil 1 on commutator segment 1, so where to terminate coil side 5'. Since commutator pitch is ±1 , 5' should be terminate on commutator segment 2(=Y c  + 1) .    DC armature winding, all coils are to be connected in series. So naturally next coil (2-6') should start from 2 and end in slot 6. Coil (2-6') lies in the lap of (1-5'), hence winding is called lap winding. the winding proceeds from left to rig...

  Wave winding We know that, Y c  is commutator pitch Here Y c  ≠  1 but Y c ≈  2S/P Assume S=16 and P=4. Coil span = S/P =16/4 =4, Y c  = 8 The first coil is (1-5') and terminated on commutator 1 and 9. The second coil (9-13') to be connected in series with the first and to be terminated on commutator segment (9 + 8 = 17). Since there are only 16 commutator segments so 17 is identical to 1. Hence, we terminate where we started and cannot connect any more coils in series. Our inability to complete the winding, will persist till 2S is a multiple of P. So, we modify the expression for Y c  = 2(S ± 1)/P No. of poles, P = 4 No. of slots, S = 17 Winding pitch, Y c  = 2(S+1)/P choosing +1 for progressive winding Y c  = 2(17+1)/4 = 9 Coil span = S/P = 4 First segment (1-5') starts from 1 and ends at 20, where second coil starts and ends on commutator segment-2 Between any two consecutive commutator segments (P/2) coils will be present winding progre...

  Torque production in DC machine Rotating machines require a steady torque for rotation. All Rotating Machines have two field one due to Armature winding and other due to Field winding. The necessary condition for steady torque production is that both fields must be stationery with respect to each other. If there is relative motion between the two fields then the torque produced is pulsating in nature and it has the frequency corresponding to the relative speed between the two fields. The steady torque produced in any rotating machine is :- T ∝ sin α  , where α is electrical angle between the two fields. So, for maximum Torque production the angle between the two fields must be kept 90°. Developed torque :- Developed power, P a  = E a. I a P a  = TꞶ m E a. I a  = TꞶ m T =E a. I a  / Ꞷ m  = KⲫI a  , ( we know that,  E a =  KⲫꞶ m  ,  where  K=PZ / 2𝜋A) T = KⲫI a Where,  T= Developed torque P a = Developed torque E a =...

  Commutation process Just before the armature coil reaches the brush, it carries current (= I a  / A ) in one direction after coil has traversed brush width, current gets reversed to ( -I a  / A ). This reversal of current is called  Commutation . Here, A represents number of parallel paths. Good commutation means no sparking at brushes and commutator surface remains unaffected. So, current in coil reduces from I c  to zero and then increase in negative direction to I c  again. Under commutation : T commutation  > T c Over commutation : T commutation  < T c Commutation period, T c  = Brush width / Commutator peripheral speed Resistance Commutation :- R c  = coil resistance r 1  = Resistance between bar1 and brush r 2  = Resistance between bar2 and brush If no emf is induced in commutated coil, then applying KVL in brush bar1 and bar2 (2I c  - I 2 )r 1  + (I c  - I 2 )R c  - I 2. r 2  = 0 I 2...