Top Electrical Interview Questions and Answers?

Electrical Engineering Interview Questions and Answers

Question 1.Why back emf used for a DC motor ?

Answer 1 -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.

Question 2.What is Slip in an induction motor?

Answer 2 -Slip can be defined as the difference between the flux speed and the rotor speed.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 and represented by the symbol “S”.

Question 3.Explain different losses in a transformer ?

Answer 3-There are two types of losses occurring in transformer

Constant losses - The losses that occur in the core are known as core losses or iron losses. Two types of iron losses are - Eddy current loss Hysteresis loss

These losses depends upon the supply voltage,frequency,core material and its construction.As long as supply voltage and frequency is constant,these losses remain the same whether the transformer is loaded or not.These are also known as constant loss.

Variable loss or Copper Loss - When the transformer is loaded current flows in primary and secondary winding,there is loss of electrical energy due to resistance of primary winding and secondary winding and they are called variable losses.These losses depend upon the loading conditions of the transformers.Therefor these losses are also called as variable losses.

Question 4. Define following electrical Terms Reliability,Maximum Demand,Reverse Generating Capacity,Availability Answer 4. Reliability - It is the capacity of the power system to serve all power demands without failure over long periods.

Maximum demand - It is maximum load demand required in a power station during a given period.

Reserve Generating Capacity - Extra generation capacity installed to meet the need of scheduled downtime for preventive maintenance is called reserve-generating capacity.

Availability - As the percentage of the time a unit is available to produce power whether needed by the system or not.

Question 5.Define Real power,Apparent power,Reactive power

Answer 5.

Real power - It is the product of voltage current and power factor.

Apparent power - It is the product of voltage and current.

Reactive power - It is the product of voltage,current and sine of angle between the voltage and current

  1. Why star delta beginner is recommended with induction motor?

An induction motor is referred with star delta starter for the following reason:

• The power is saved because of voltage drops as the starting current is reduced 3-4 times.

• The motor is saved because of the star delta circuit comes along with circuit during start of motor and thus voltage is reduced by three units.

• Damage of motor winding is prevented because of the increase in starting torque.

  1. Comparison between generator and alternator

Generator and alternator are two devices, which converts mechanical power into electrical power. The basic principle followed by both of them is electro-magnetic induction, the only distinction is their construction. A fixed magnetic field and a rotating conductor is available on the generator that comes on the armature connected with slip rings and brushes colliding against each other, hence it transforms the emf induced into the dc current for external load whereas an alternator has a fixed armature and a rotating magnetic field for higher voltages but for reduced voltage outcome both rotating armature and stationary magnetic field is used.

  1. Benefits of AC systems over DC systems

Due to the following factors, AC systems are recommended over DC systems:

a. Maintenance is easy and can change the voltage of AC power for transmission and distribution.

b. Plant cost for AC transmission (circuit breakers, transformers etc) is much lower than the equivalent DC transmission

c. AC is produced from the power stations so AC is better for usage than DC instead of converting it.

d. If a big fault occurs in a network, it is simpler to get rid of in an AC system, as the sine wave current will automatically become zero at some point making the current interruption easier.

  1. How can you compare power power engineering with electrical engineering?

Power engineering is a sub department of electrical engineering. It deals with generation, transmission and distribution of power in electric form. Design of all power accessories also comes under power engineering. Power engineers focus on maintenance and design of the power grid i.e. called on grid systems and they might perform on off grid systems that are disconnected to the program.

  1. What are the various kind of wires used for transmission?

Cables, which are used for power transmission, can be classified in three forms:

• Low-tension wires, which can transfer voltage up to 1000 v.

• High-tension wires can transmit voltage up to 23000 v.

• Super tension wires can transmit voltage 66 kV to 132 kV.

  1. Why back emf used for a dc motor? emphasize its importance.

The emf which is induced developed when the rotating conductors of the armature between the magnetic poles, in a DC motor, cut the magnetic flux, is a contradiction to the current flowing through the conductor, when the armature is rotating, is known as back emf. Its value depends upon the rotational speed of the armature conductors. The initial value of back emf is zero.

  1. 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 indicated as a percentage of synchronous speed (Ns) and showed by the symbol ‘S’.

  1. Explain the application of storage battery power.

Storage batteries power are used for various purposes, some of the uses are mentioned below:

• For the working of protective devices and for emergency lighting at power generating stations and substations.

• For ignition and automobile lighting, airplanes etc.

• For steam lighting and diesel engines.

• As a supply source of power in telephone exchange, labs and broad casting channels.

• Many medical centres, banks, non-urban areas where power supplies are not available, emergency lighting can be the solution.

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