Important Electrical Interview Questions Part – 2
01: What is Ferranti effect?
Output voltage is greater than the input voltage or receiving end voltage is greater than the sending end voltage.
02: Why the capacitors work on ac only?
Generally capacitor gives infinite resistance to dc components (i.e., block the dc components). It allows the ac components to pass through.
03: 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 die electric strength is equal to 8 times of air. That always vacuum used as in HT breaker and air used as in LT.
04: Enlist types of dc generator?
D.C. Generators are classified into two types
1) Separately excited D.C. generator
2) Self excited D.C. generator,
Which is further classified into:
Series
Shunt and
Compound
(Which is further classified into cumulative and differential).
05: What happen if we give 220 volts dc supply to a bulb or tube light?
Bulbs (devices) for AC are designed to operate such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so high that it may damage the bulb element.
06: What is “pu” in electrical engineering?
Pu stands for per unit and this will be used in power system single line diagram there it is like a huge electrical circuit with no of components (generators, transformers, loads) with different ratings (in MVA and KV). To bring all the ratings into common platform we use pu concept in which, in general largest MVA and KV ratings of the component is considered as base values, then all other component ratings will get back into this basis. Those values are called as pu values. (p.u=actual value/base value).
07: How electrical power is generated by an A.C Generator?
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.
08: What is the difference between earth resistance and earth electrode resistance?
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 the earth, which is far away.
The resistance of the electrode has the following components:
The resistance of the metal and that of the connection to it.
The contact resistance of the surrounding earth to the electrode.
09: Two bulbs of 100 W and 40 W respectively connected in series across a 230 V supply which bulb will glow bright and why?
Since two bulbs are in series they will get equal amount of electrical current but as the supply voltage is constant across the bulb (P=V2/R).So the resistance of 40 W bulb is greater and voltage across 40 W is more (V=IR) so 40 W bulb will glow brighter.
10: If we give 2334 A, 540 V on Primary side of 1.125 MVA step up transformer, then what will be the Secondary Current, If Secondary Voltage = 11 KV?
As we know the Voltage & current relation for transformer- V1/V2 = I2/I1
We Know, VI= 540 V; V2=11 KV or 11000 V; I1= 2334 Amps.
By putting these value on Relation-
540/11000= I2/2334
So, I2 = 114.5 Amps
11: Why field rheostat is kept in minimum position while armature rheostat at maximum position?
In motors at the time of starting the armature resistance is introduced to reduce the high starting current and the field resistance is kept minimum to have high starting
torque.
12: Why transformer ratings are in kVA?
Since the power factor of transformer is dependent on load we only define VA rating and does not include power factor .In case of motors, power factor depend on construction and hence rating of motors is in KW (Kilo-Watts) and include power factor.
13: 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.
14: Why star delta starter is preferred with induction motor?
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.
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.
15: Explain advantages of storage batteries
Few advantages of storage batteries are mentioned below:
Most efficient form of storing energy portably.
Stored energy is available immediately because there is no lag of time for delivering the stored energy.
Reliable source for supply of energy.
The energy can be drawn at a fairly constant rate.
16: State Thevenin’s Theorem:
According to Thevenin’s theorem, the current flowing through a load resistance
Connected across any two terminals of a linear active bilateral network is the ratio open circuit voltage (i.e. the voltage across the two terminals when RL is removed) and sum of load resistance and internal resistance of the network. It is given by Voc / (Ri + RL).
17: Explain different types of D.C motors? Give their applications
Different type of DC motors and their applications are as follows:-
Shunt motors: It has a constant speed though its starting torque is not very high. Therefore, it is suitable for constant speed drive, where high starting torque is not required such as pumps, blowers, fan, lathe machines, tools, belt or chain conveyor etc.
Service motors: It has high starting torque & its speed is inversely proportional to the loading conditions i.e. when lightly loaded, the speed is high and when heavily loaded, it is low. Therefore, motor is used in lifts, cranes, traction work, coal loader and coal cutter in coal mines etc.
Compound motors: It also has high starting torque and variable speed. Its advantage is, it can run at NIL loads without any danger. This motor will therefore find its application in loads having high inertia load or requiring high intermittent torque such as elevators, conveyor, rolling mill, planes, presses, shears and punches, coal cutter and winding machines etc.
18: What is 2 phase motor?
A two phase motor is a motor with the starting winding and the running winding have a phase split. E.g. AC servo motor,
where the auxiliary winding and the control winding have a phase split of 90 degree.
19: 1 ton is equal to how many watts?
1 ton = 12000 BTU/hr and to convert BTU/hr to horsepower, 12,000 × 0.0003929 = 4.715 hp therefore 1 ton = 4.715 × 0.746 = 3.5 KW.
20: What is ACSR cable and where we use it?
ACSR means Aluminum conductor steel reinforced, this conductor is used in transmission & distribution.
21: What does quality factor depend on in resonance?
Quality factor q depends on frequency and bandwidth.
22: What does the KVAR means?
The KVAR indicates the electrical power. KVAR means “Kilo Volt Amperes with Reactive components”
23: What’s the other name for short circuit?
Fault.
24: 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.
25: 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.