Online electrical engineering & Electrical Interview Questions

Thursday, August 27, 2015


What is the difference between power and watts, and kva and kvar and kW ? 

Being an electrical student you should be aware of the power and the units for it. power is the amount of work done divided by time. 1 watt is 1 joule per second.

So how to define electric power ?  

Electric power is nothing but rate at which the charge moves across a potential.when there is movement of charge , it produces current.
Power (P)=  V*(Q/t) = V*I
V= Voltage or potential(Volts)
I = Current (Ampere)
Q = Charge ( Coulomb)
t = Time ( Seconds)

In case of electric power , there is not only real power(P) but there is also reactive power(Q) due to inductive nature of load. All domestic load and industrial loads are inductive in nature. Real power is the does the useful work whereas reactive power doesn't do any useful work at the load , instead it wastes energy.

Example: when a fan is running , the useful work done is the rotation of it which is done by the real power consumption and the coil of the motor consumes reactive power. The unit of the reactive power is VAR .ie) Volt-Ampere Reactive .Usually it is mentioned in kVAR .

Apparent power(S) is the magnitude of real power and reactive power. When the load is reactive either inductive or capacitve, there exists a phase angle (φ) between the voltage and current. Power factor is defined as the ratio of Real power to apparent power. The unit of apparent power is VA ie) Volt-Ampere .Usually it is represented as KVA .

S= P+jQ
S^2 = P^2 +Q^2

Power factor = cos φ . Power factor has not unit and it ranges from 0 to 1. Since the industrial loads are bulk loads, they use to have capacitor banks to improve the power factor , so that the power wastage is less and electric bills too.

Hope now it will be easy to understand the difference between KVA and KW and also the difference between KVA and KVAR

Saturday, March 22, 2014


What is wave Trap and uses of wave trap ? Here is the answer to the question from our follower.

Wave trap is a device used in the substation at the incoming lines.Wave trap is also called as line trap.As the name indicates a wave trap, is used to trap/catch the wave.But what is that wave and why it should be trapped/caught by  a wave trap ?

Have you ever heard of Power Line Carrier Communication (PLCC) ? The substations uses the power line for communication purpose also. They use high frequency waves to communicate between the substations. Communication between the substation was done using the power lines in the days where communication through the telephone/communication lines were difficult. But still PLCC is adopted in the substations as it is faster, easier,  cost effective and also very safe to transmit the messages without any interference.

Are you confused, whether it is possible to send 2 waves in a single transmission line ? Yes, it is possible the power signal will be at the frequency of 50 Hz whereas the communication signal's  frequency would be in the range of kHz.

What happens when wave trap is not used at the entrance of substation ? If the communication frequencies are not filtered, then it would act as harmonics to the electrical equipments like transformers, circuit breakers and damage the equipments.

So in order to protect the electrical equipments and for communication purpose the wave trap is used in the substation. I have attended industrial training at a substation and I have seen these wave traps directly and how the maintenance works are done at substation. I would suggest you students also to attend training like this and gain knowledge. I am sure it would be a nice fun and good practical learning experience.