Monday, 21 March 2016

Interview Questions for Fresh Electrical Engineer

SESSION:1

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1)  In the motors starting purpose why the armature rheostats at maximum position while the field rheostat is kept in minimum position?
                At the time of starting a motor, the field rheostat is kept in minimum state for the high            starting torque.

          At the time of starting a motor, the armature rheostats are used to control or reduce               the high starting current. This high starting current can damage the motors starting               systems.
2)    Name power losses in rotating electrical machines.
            Power losses in rotating electrical machines are Copper losses, core losses,                           mechanical losses and stray losses.
3)    What is the significance of vector grouping in Power Transformers?
  Every power transformer carries a vector group mentioned by its manufacturer. Fundamentally it tells us the information about how the windings are connected (delta or wye) and the phase difference between the current and voltage. For example, DYN11 means Delta primary, Wye Secondary and the current is at 11 o clock referred to the voltage.
4)    Why link is provided in neutral of an AC circuit and fuse in phase of ac circuit?
  Link is provided at a Neutral common point in the circuit from which various connections are taken for the individual control circuit and so it is given in a link form to withstand high Amps. But in the case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is calculated for the particular circuit (i.e. load) only. So if any malfunction happens the fuse connected in the particular control circuit alone will blow off.
5)    What is ACSR cable and where we use it?
  ACSR means Aluminium conductor steel reinforced, this conductor is used in transmission & distribution.
6)    What does the KVAR means?
  The KVAR indicates the electrical power. KVAR means “Kilo Volt Amperes with Reactive components”.
7)    Why use the VCB at High Transmission System? Why can't use ACB?
    Actually the thing is vacuum has high arc quenching property compare to air because in VCB, the dielectric strengths equal to 8 times of air. That y always vacuum used as in HT breaker and air used as in LT.
8)    What is the difference between MCB & MCCB, Where it can be used?
     MCB is Miniature Circuit Breaker which is thermal operated and use for short circuit protection in small current rating circuit. MCCB is moulded case circuit breaker and is thermal operated for over load current and magnetic operation for instant trip in short circuit condition. Under voltage and under frequency may be inbuilt. Normally it is used where normal current is more than 100A.
9)  What is the diff. between Electronic regulator and ordinary rheostat regulator for fans?
   The difference between the electronic and ordinary regulator is that in electronic regulator, power losses tend to be less because as we minimize the speed the electronic regulator give the power necessary for that particular speed but in case of ordinary rheostat type reg. the power wastage is same for every speed and no power is saved. In electronic regulator TRIAC is employed for speed control. By varying the firing angle speed is controlled but in RHEOSTATIC control resistance is decreased by steps to achieve speed control.
10) What is two phase motor?
     A two phase motor is often a motor with the starting winding and the running winding has a phase split. For example, ac servo motor where the auxiliary winding and the control winding have a phase split of 90 degree.
11) What does quality factor depend on in resonance?
     Quality factor Q depends on frequency and bandwidth.
12) What are the types of power in electrical power?
     There are normally three types of power are counted in electrical power. They are,
·         Apparent power
·         Active power
·         Reactive power
13) What are the advantages of VSCF wind electrical system?
                  Advantages of VSCF wind electrical system are:

·         No complex pitch changing mechanism is needed.
·         Aero turbine always keeps going at maximum efficiency point.
·         Extra energy in the high wind speed region of the speed - duration curve can be extracted
·         Significant reduction in aerodynamic stresses, which are associated with constant - speed operation.
14) How can you start-up the 40w tube lite with 230v AC/DC without using any  choke/Coil?
      It's possible with Electronic choke. Otherwise it's not possible to ionize the particles    in Tube Light, with normal voltage.
15) What are the types of domain in Laplace transforms? What behaviour can Laplace transform predict about how the system works?
     All types of domain of Laplace transforms is s-domain, Laplace transforms provide a method to find position, acceleration or voltage the system will have.
16) In the magnetic fluxes, what is the role of armature reaction?
    The armature flux has an important role for the running condition. This armature flux can oppose the main flux or it may support the main flux for better running condition. This effect of supporting and opposing of main flux to armature flux is called armature reaction.
17) Explain thin film resistors and wire-wound resistors.
    Thin film resistors- It is constructed as a thin film of resistive material is deposited on an insulating substrate. Desired results are obtained by either trimming the layer thickness or by cutting helical grooves of suitable pitch along its length. During this process, the value of the resistance is monitored closely and cutting of grooves is stopped as soon as the desired value of resistance is obtained.
    Wire wound resistors – length of wire wound around an insulating cylindrical core are known as wire wound resistors. These wires are made of materials such as Constantan and Manganin because of their high resistivity, and low temperature coefficients. The complete wire wound resistor is coated with an insulating material such as baked enamel.
18)  Why star delta starter is preferred with induction motor?
a)   Star delta starter is preferred with induction motor due to following reasons:
Starting current is reduced 3-4 times of the direct current due to which voltage drops and hence it causes less losses.
b)   Star delta starter circuit comes in circuit first during starting of motor, which reduces voltage 3 times, that is why current also reduces up to 3 times and hence less motor burning is caused. In addition, starting torque is increased and it prevents the damage of motor winding.
19)  State the difference between dc generator and alternator?
 Generator and alternator are two devices, which converts mechanical energy into electrical energy. Both have the same principle of electromagnetic induction, the only difference is that their construction. Generator persists stationary magnetic field and rotating conductor which rolls on the armature with slip rings and brushes riding against each other, hence it converts the induced emf into dc current for external load whereas an alternator has a stationary armature and rotating magnetic field for high voltages but for low voltage output rotating armature and stationary magnetic field is used.
20) Why AC systems are preferred over DC systems?
Due to following reasons, AC systems are preferred over DC systems:
a)   It is easy to maintain and change the voltage of AC electricity for transmission and distribution.
b)   Plant cost for AC transmission (circuit breakers, transformers etc) is much lower than the equivalent DC transmission
c)   From power stations, AC is produced so it is better to use AC then DC instead of converting it.
d)   When a large fault occurs in a network, it is easier to interrupt in an AC system, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.
21) How can you relate power engineering with electrical engineering?
  Power engineering is a sub division of electrical engineering. It deals with generation, transmission and distribution of energy in electrical form. Design of all power equipments also comes under power engineering. Power engineers may work on the design and maintenance of the power grid i.e. called on grid systems and they might work on off grid systems that are not connected to the system.
22) What are the various kind of cables used for transmission?
Cables, which are used for transmitting power, can be categorized in three forms:
a)    Low-tension cables, which can transmit voltage upto 1000 volts.
b)   High-tension cables can transmit voltage upto 23000 volts.
c)   Super tension cables can transmit voltage 66 kV to 132 kV.
23) Why back emf used for a dc motor? Highlight its significance.
The induced emf developed when the rotating conductors of the armature between the poles of magnet, in a DC motor, cut the magnetic flux, opposes the current flowing through the conductor, when the armature rotates, is called back emf. Its value depends upon the speed of rotation of the armature conductors. In starting, the value of back emf is zero.
24) What is slip in an induction motor?
Slip can be defined as the difference between the flux speed (Ns) and the rotor speed (N). Speed of the rotor of an induction motor is always less than its synchronous speed. It is usually expressed as a percentage of synchronous speed (Ns) and represented by the symbol ‘S’.
25) Explain the application of storage batteries.
Storage batteries are used for various purposes, some of the applications are mentioned below:
a)    For the operation of protective devices and for emergency lighting at generating stations and substations.
b)    For starting, ignition and lighting of automobiles, aircrafts etc.
c)    For lighting on steam and diesel railways trains.
d)    As a supply power source in telephone exchange, laboratories and broad casting stations.
e)    For emergency lighting at hospitals, banks, rural areas where electricity supplies are not possible.

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