UPSC/GK

  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...

  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 =...

  Action of commutator The ends of Armature winding are connected to Commutator which is connected to brushes which are then connected to Supply or Load. Commutator is also called as Slip Rings. It performs two important functions: Convert alternating quantities to direct quantities and vice versa. Keep rotor or armature MMF stationary in space. One copper ring is split into 2 parts insulated from each other and also from the shaft on which it is mounted. Coils ends are connected to copper segments on which two carbon brushes are resting. Alternating emf is generated in N-turn coil, which is converted from ac to dc by commutator. When armature conductors are rotated in the influence of Stator Magnetic Field then there is a relative motion between the conductors and field and hence there is a dynamically induced emf in the conductors. This is working principal of DC generator. The dynamic emf induced in any moving conductor is given by:             ...

  Armature reaction Due to relative motion between armature conductors and field mmf there is an emf induced in armature conductors. This emf causes flow of current in armature conductors which cause armature flux. This flux is produced as a reaction to field flux and hence is called as  Armature Reaction . Geometric Neutral Axis ( GNA ) is defined as the axis that is perpendicular to the field axis of the stator. Magnetic Neutral Axis ( MNA ) is defined as the axis perpendicular to the net flux that is flux due to field as well as armature mmf. The perpendicularity is taken in terms of electrical angle and not mechanical angle. It is well known that the brushes are placed on the MNA to collect maximum emf and also to ensure that the undergoing commutation have zero rotational voltage to prevent serious commutation problems. On no-load, the MNA coincides with the GNA because on no-load there is no armature current and armature reaction can be neglected so net flux is same as f...

  If the brushes are left on GNA, then emf collected would reduce and coil undergoing commutation would no longer have zero rotational voltage leading to serious commutation problems. The immediate solution therefore, appears to shift the brushes in the new MNA. The brush shift results into improved commutation but reduce thus resultant flux resulting into reduction in emf in generator action and increase in speed in motor action. F ar  ( demagnetizing ) = {(Z/2) / P} * {(2ꞵ elect ) / 180°} * {I a  / A} Brush shift has serious limitations. Since, shift in MNA is proportional to Armature current the brush has to be shifted in a new position every time the load changes, direction of rotation changes or mode of operation changes. Therefore, brush shift is limited to various small machines and there too the brushes are fixed at a position corresponding to expected load, direction of rotation and mode of operation. In practice the brushes are moved slightly further to counter ...

  Inter-poles Inter-poles are long but narrow poles placed in inter-polar region and has the polarity of succeeding ( incoming ) poles for generator action and preceding poles for motor action. The inter-poles winding is designed to neutralize armature MMF in inter-polar region. It has an additional duty to create an inter-polar flux density that induces a commutation voltage in the coil undergoing commutation such that it cancels reactance voltage of the coil. The inter-pole winding carries the armature current as it is connected in series with the armature winding. The presence of inter-poles ensures spark -less linear commutation. The inter-pole is kept narrow so that influence is restricted to coil undergoing commutation only and does not spread to other neighboring coils. However bar is winder at bottom to prevent saturation and improve response. Inter-poles work satisfactorily irrespective of the load, the direction of rotation and mode of operation of the machine.