[Set 2]Electrical Engineering Interview Questions and Answers|Electrical Engineer Interview Questions and Answers

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Electrical Engineering Interview Questions Answers

Electrical Engineering Interview Questions and Answers: Here is the Set 2 of Electrical Engineering Interview Questions and Answers.

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What is the difference between MCB & MCCB, Where it can be used?


Answer: MCB is miniature circuit breaker which is thermal operated and use for short
circuit protection in small current rating circuit.

MCCB moulded case circuit breaker and is thermal operated for over load current and magnetic operation for instant trip in shortcircuit condition.

Under voltage and under frequency may be inbuilt. Normally it is used where normal current is more than 100A.

Read this Book on Electrical Engineering 1000 Questions & Answers

What is use of lockout relay in ht voltage?


Answer: A lock-out relay is generally placed in line before or after the e-stop switch so the
power can be shut off at one central location.

This relay is powered by the same electrical source as the control power and is operated by a key lock switch.

The relay itself may have up to 24 contact points within the unit itself.

This allows the control power for multiple machines to be locked out by the turn of a single key switch.

What is the difference between earth resistance and earth electrode resistance?


Answer: Only one of the terminals is evident in the earth resistance.

In order to find the second terminal we should recourse to its definition: Earth Resistance is the resistance existing between the electrically accessible part of a buried electrode and another point of he earth, which is far away.

The resistance of the electrode has the following components :

( A) The resistance of the metal and that of the connection to it.

(B) The contact resistance of the surrounding earth to the electrode.

Which power plant has high load factor?

All base load power plants have a high load factor.

If we use high efficiency power plants to supply the base load, we can reduce the cost of generation.

Hydro power plants have a higher efficiency than thermal & nuclear power plants.

Read Also: Wind Energy Power Plants|Wind Power Generation|Wind Mill Working

Why an ac solenoid valve attract the plunger even though we interchanges the terminal?
Will the poles changes?


Answer: Yes because the poles changes for every half-cycle of ac voltage so the polarity of AC
voltage is continuously changing for every half cycle. So, interchanging of terminals in ac system does not show any difference.

That’s why the ac solenoid attract the plunger even though it’s terminals are interchanged.

Define IDMT relay?


Answer: It is an inverse definite minimum time relay.

In IDMT relay its operating is inversely proportional and also a characteristic of minimum time after which this relay operates.

It is inverse in the sense, the tripping time will decrease as the magnitude of fault current increase.

What are the transformer losses?


Answer: TRANSFORMER LOSSES – Transformer losses have two sources-copper loss and magnetic loss. Copper losses are caused by the resistance of the wire (I2R).

Magnetic losses are caused by eddy currents and hysteresis in the core. Copper loss is a constant after the coil has been wound and therefore a measurable loss.

Hysteresis is loss is constant for a particular voltage and current.


Eddy-current loss, however, is different for each frequency passed through the transformer.

Read Also: Magnetic materials: Dia, Para,Ferro,Ferri and Antiferro magnetic materials

What is meant by regenerative braking?


Answer: When the supply is cut off for a running motor, it still continue running due to inertia. In
order to stop it quickly we place a load (resistor) across the armature winding and the motor
should have maintained continuous field supply.

So that back emf voltage is made to apply across the resistor and due to load the motor stops quickly. This type of breaking is called as “Regenerative Breaking”.

Why is the starting current high in a DC motor?


Answer: In DC motors, Voltage equation is V=Eb-IaRa (V = Terminal voltage, Eb = Back emf in
Motor, Ia = Armature current, Ra = Armature resistance).

At starting, Eb is zero. Therefore, V=IaRa, Ia = V/Ra ,where Ra is very less like 0.01ohm.i.e, Ia will become enormously increased.

What are the advantages of star-delta starter with induction motor?


Answer :(1). The main advantage of using the star delta starter is reduction of current during the starting of the motor. Starting current is reduced to 3-4 times Of current of Direct online
starting.

(2). Hence the starting current is reduced , the voltage drops during the starting of motor
in systems are reduced.

Why Delta Star Transformers are used for Lighting Loads?

Answer: For lighting loads, neutral conductor is must and hence the secondary must be star
winding and this lighting load is always unbalanced in all three phases.

To minimize the current unbalance in the primary we use delta winding in the primary.

So delta / star transformer is used for lighting loads.

Why in a three pin plug the earth pin is thicker and longer than the other pins?


Answer: It depends upon R=rho l/a where area(a) is inversely proportional to resistance(R), so if (a) increases, R decreases & if R is less the leakage current will take low resistance path so the earth pin should be thicker.

It is longer because the First to make the connection and last to disconnect should be earth Pin.

This assures Safety for the person who uses the electrical instrument.

Why series motor cannot be started on no-load?


Answer: Series motor cannot be started without load because of high starting torque. Series
motor are used in Trains, Crane etc.

Why ELCB can’t work if N input of ELCB do not connect to ground?


Answer: ELCB is used to detect earth leakage fault. Once the phase and neutral are connected in an ELCB, the current will flow through phase and that much current will have to return neutral so resultant current is zero.

Once there is a ground fault in the load side, current from phase will directly pass through earth and it will not return through neutral through ELCB.

That means once side current is going and not returning and hence because of this difference in current ELCB will trip and it will safe guard the other circuits from faulty loads.

If the neutral is not grounded, fault current will definitely high and that full fault current will come back through ELCB, and there will be no difference in current.

Read also: What is NGR? Neutral Grounding Resistor

How electrical power is generated by an A.C Generator?


Answer: For the generation of elect power we need a prime mover which supplies mechanical
power input to the alternator, can be steam turbines, or hydro turbines.

When poles of the rotor moves under the armature conductors which are placed on the stator ,field flux cut the armature conductor ,therefore voltage is generated and is of sinusoidal in nature…due to polarity change of rotor poles(i,e) N-S-N-S.

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