electricalengineeringquestionanswer

Electrical Machines Viva Questions and Answers [1] How to reverse the direction of rotation of dc motor? To reverse direction of rotation of dc motor, either direction of main field produced by the field winding is reversed or direction of the current passing through the armature is reversed. By changing the direction of current passing through the field winding,the direction of the main field can be reversed. [2] What are the functions of yoke? what is the choice material for the yoke? It serves the purpose of outer most cover of the dc machine. so that the insulating material get protected from the harmful atmospheric environmental like noise, dust and various gases like SO2, acidic fumes etc. It provides mechanical support to the poles. It forms a part of the magnetic circuit. It provides a path of low reluctance for magnetic flux. It is prepared by using cast iron. For large machines, rolled steel, cast steel, silicon steel is used which provides high permeability. [3] Why generato...

Q: Why synchronous motor is not self starting? Synchronous motor is a doubly excited machine i.e two electrical inputs are provided to it. It’s stator winding which consists of a 3 phase winding is provided with 3 phase supply and rotor is provided with DC supply. The 3 phase stator winding carrying 3 phase currents produces 3 phase rotating magnetic flux. The rotor carrying DC supply also produces a constant flux. At a particular instant rotor and stator poles might be of same polarity (N-N or S-S) causing repulsive force on rotor and the very next second it will be N-S causing attractive force. But due to inertia of the rotor, it is unable to rotate in any direction due to attractive or repulsive force and remain in standstill condition. Hence it is not self starting. Q: What are the methods of starting synchronous motor? 1.     Pony motor starting( using ac or dc motor) 2.     Starting as squirrel cage Induction motor or using damper winding 3.  ...

Q: Write the most important mode of operation of power system and mention the major problems encountered with it.  Symmetrical steady state is the most important mode of operation of power system. Three major problems are encountered in this mode of operation. They are,  1) Load flow problem  2)  Optimal load scheduling problem 3) Systems control problem Q: Why power flow analysis is made? Power flow analysis is performed to calculate the magnitude and phase angle of voltages at the buses and also the active power and reactive voltamperes flow for the given terminal or bus conditions. The variables associated with each bus or node are, a. Magnitude of voltage |V|  b. Phase angle of voltage δ  c. Active power, P d. Reactive voltamperes, Q Q: What is power flow study or load flow study?  The study of various methods of solution to power system network is referred to as load study. The solution provides the voltages at various buses, power flowing in Vari...

Q: Why 3 phase power is used and not 6 phase or 12 phase? There are two reasons: (1) using three phases results in the lowest conductor cost/watt to transmit power; (2) using three phases displaced in space around a cylinder by 120 degrees results in a rotating magnetic field when the three phases are excited by three voltage sources displaced in time by 120 electrical degrees - this is a fundamental energy conversion principle for the operation of AC motors and generators. Q:What is transmission line? Transmission line is the long conductor with special design (bundled) to carry bulk amount of generated power at very high voltage from one station to another as per variation of the voltage level. Q:What are the types of transmission line? Short Transmission Line Medium Transmission Line long Transmission Line Q: What is short transmission line? Length is about 50 km. Voltage level is up to 20 kV Capacitance effect is negligible Only resistance and inductance are taken in calculation. Q...

Electrical Circuit Theorems: To solve problems in electrical networks, there are number circuit theorems have been developed like (1) Kirchhoff's laws (2) Superposition theorem (3) Thevenin theorem (4) Norton Theorem (5) The maximum power transfer theorem In this post we have listed all these theorems. We believe that this post will serve as a ready reference for you. Kirchhoff's laws: (a) Current Law:  At any junction in an electric circuit the total current flowing towards that junction is equal to the total current flowing away from the junction, i.e. I =0 (b) Voltage Law:  In any closed loop in a network, the algebraic sum of the voltage drops (i.e.products of current and resistance) taken around the loop is equal to the resultant e.m.f. acting in that loop. Superposition Theorem: In any network made up of linear resistances and containing more than one source of EMF, the resultant current flowing in any branch is the algebraic sum of the currents that would flow in that ...

Q.Define combinational logic  When  logic  gates  are  connected  together  to  produce a  specified  output  for  certain  specified combinations of  input  variables,  with no  storage  involved, the  resulting  circuit is  called combinational logic.  Q. Define Half adder and full adder  The logic circuit that performs the addition of two bits is a half adder. The circuit that  performs the addition of three bits is a full adder.  Q. Define Decoder  A decoder is a multiple - input multiple output logic circuit that converts coded  inputs into coded outputs where the input and output codes are different.  Q. What is binary decoder?   A  decoder  is  a  combinational  circuit  that  converts binary  information  from  n  input  lines to a maximum of  2 n   out puts line...