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
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
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
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