electrialstandards

Synchronous machines construction depends upon type of prime mover used in machines. There are following types of prime movers used in power generation:- 1.        Steam turbines 2.        Hydraulic Turbines 3.        Diesel Engines Let’s discuss them :- 1.      Steam turbine:- These types of synchronous machines have high speed.  Generators driven by steam turbine are also called Turbogenerators. Maximum speed of Turbogenerators is 3000 RPM as per formula Frequency= PN/ 120; Where P is no. of poles N is no. of revolutions In 2 Pole machine at 50 HZ frequency Speed of Turbogenerator comes out to be= 50X120/ 2= 3000 RPM With Such high speed lower value of armature diameter is to be designed. Lower diameter is selected to limit the centrifugal forces which have very much influence on the generator design. Peripheral Speed of a Machine is given by formula as below:- Peripheral Speed(V)= πDn  m/Sec Where D is diameter of rotor in meters n=speed of rotor in revolutions per second As we hav

In case of synchronous machines  Armature is always placed on stator and field winding on armature as there are following advantages:- 1.      Better economy:- It is always economical to place armature winding on Stator and field winding on rotor in synchronous machines as this can be illustrated by example as below:- Consider a 3-Phase Star connected 500 MVA, 11KV synchronous machine, now its line current will be = 500X10 3                                11 X Which Will be = 26244 A Now If we place armature winding on rotor than we will required 3 slip rings each will be capable of handling such a high current of 26244A. Further Slip rings needed to be insulated from shaft for a voltage= 11/  = 6.35 KV. Also Star point needed to be brought out by using fourth slip ring which will be grounded through a resistance. Now consider a case where we will place field winding on rotor and armature winding on  Stator, in that case field winding need to handle lower current and voltages.  Now con

In electrical systems its usual that cable may get faulty there are following methods of checking the type of fault in cable:- 1.      Using Meggar for LT Cables:- Meggar is most commonly used method for checking the fault in cables. If Meggar is used at 500V/ 1000V DC then cable shows infinite value If cable is Ok.  if value lies above 100 Mohm then cable is also acceptable it indicates only that there cable is having some moisture which will be dried out when cable is put on load but if value falls below this value upto 50 Mohm then it will be used where there were light loads. If there is value is near to zero than cable should not be used as if cable insulation level between two leads is zero this means cable is short circuited. If Cable insulation with respect their leads is ok but between lead and earth is zero than this indicates cable get short circuited with armored. In Meggar we can check resistance value also in this 2 leads one is placed on resistance and other is placed on