## Frequently Asked Interview Questions_Electrical Engg.

Q1. What do you mean by harmonics?

Definition for a harmonic is-a sinusoidal component of a periodic wave or quantity having a
frequency that is an integral multiple of the fundamental frequency.
The main sources of harmonic component are the phase angle controlled rectifiers and inverters
due to the fast switching action and firing scheme.
Filters are used to eliminate these harmonics as they may cause overheating of capacitors and
nearby generators and interfere with telecommunication systems.

Q2.Hunting of a synchronous motor?

When the mechanical load is constant, the rotor settles down to an absolutely constant speed with
the torque angle fixed by the particular delivered power.
When there is a change in load the rotor speed changes momentarily until the torque angle
adjusts itself to the new power.
If the load increases, the rotor slips backwards to an increased torque angle, while reduction
causes the rotor to advance a smaller torque angle.
Due to the M.I of the rotor either it overshoots or undershoots the mean position.
During this overshooting and undershooting some K.E. is stored in the rotor which causes
oscillation of the rotor periodically.
This rapid forward and backward motion of the rotor as it resolves at the average constant speed
is called HUNTING.
DISADVANTAGES:
Hunting is an objectionable characteristic of a synch motor as it produces severe mechanical
stress as well as great variation in current and power taken by the motor.
In order to prevent hunting, dampers (or) damping grids are employed. Damper windings are
short circuited; copper bars are embedded in the faces of the field poles of the motor.

Question: what is "p.u." in electrical engg and what is the advantage of p.u. quantities?

Answer: p.u. stands for per unit (p.u=actual value/base value).
Advantages:
· wide variation of electrical parameters gets eliminated by p.u. representation
· Since the p.u. impedances of any component remains the same on either side of a
transformer, a power system can be analyzed on a single voltage level as the different
voltage level vanishes.

Question: Give two basic speed control scheme of DC shunt motor?

Answer: 1. By using flux control method: in this method a rheostat is connected across the field
wdg to control the field current. So by changing the current the flux produced by the field wdg
can be changed, and since speed is inversely proportional to flux speed can be controlled.
2. Armature control method: in this method a rheostat is connected across armature wdg and by
varying the resistance the value of resistive drop (IaRa) can be varied, and since speed is directly
proportional to Eb-IaRa the speed can be controlled.

Question: what is meant by armature reaction?

Answer: The action of armature m.m.f. on the main m.m.f. is known as armature reaction.
Question: Enlist the types of dc machines?
Answer: D.C. machines are classified into two types 1) separately excited 2) self excited which
is further classified into 1)series 2)shunt and 3)compound(which is further classified into
cumulative and differential).

Question: why, when birds sit on transmission lines doesn't get shock?
Answer: It’s true that if birds touch the single one line (phase or neutral) they don't get electrical
shock... if birds touch 2 lines than the circuit is closed and they get electrical shock... so if a
human being touches single one line (phase) then he doesn't get shock .if he is standing on the
ground then touching the line (phase) he will get a shock because the ground on what we are
standing is like line (ground bed - like neutral) that means that human who touches the line
closes the circuit between phase and neutral.

Question: what is ACSR cable?
Answer: ACSR means Aluminum conductor, steel reinforced, e.g. 7/24 ACSR conductor has 7
steel strands forming a central core, around which there are two layers of aluminum strands(24).

Question: what is electric traction?

Answer: Traction means transport system i.e. transport of men and material from one place to
other. Electric traction means use of the electricity for all these .electric traction services are
broadly classified to railways, trams, trolleys, battery driven and solar powered vehicles.

Question: 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 value and
Ia becomes very high.

Question: What are the transformer losses?

Answer: TRANSFORMER LOSSES –
There are mainly two kinds of losses in a transformer namely
1. Copper loss (ohmic loss) and 2.magnetic loss (core loss).
Copper losses are caused by the resistance of the wire (I2R).
Magnetic losses are caused by eddy currents and hysteresis in the core.
In addition to the core loss and ohmic loss the following two losses are also present in the
transformer stray load loss and dielectric loss.

Question: what is Ferranti effect?

Answer: when receiving end voltage is greater than the sending end voltage due to no load or
light loading, this effect is known as Ferranti effect.

Question: types of cooling system in transformers?

Answer: 1. ONAN (oil natural, air natural)
2. ONAF (oil natural, air forced)
3. OFAF (oil forced, air forced)
4. ODWF (oil direct, water forced)
5. OFAN (oil forced, air forced)

Some Imp. Questions:-

What is thyristor?

How is a 100 MVA transformer cooled?

What is the instantaneous value of voltage in a 3-phase balanced star connected circuit?

What is the difference between regulator and stabiliser.

Whats happens when we give DC to a induction motor?

Why is the transformer rated in KVA?

What is the function of a choke in tube light?

What is the purpose of calculating knee point voltage for Current transformers?

What will happen if we apply dc volt across transformer?

Why less loss occurs when we use DC supply than AC supply?

What is induction motor?

What is a Transducer?

What is the use of SCOTT connections?

What is Synchronous condenser ?

What is the resistance between neutral and earth?

Why secondary of open delta pt (two element pt) centre tap is earthed? What happen when this earth is removed?

Alternators are invariably star connected why?

What is the difference between SSR drive& AC drive?

What will be change in programming when we use 5/3 solenoid in place of 5/2 in operating of a cylinder?

What is isolation Transformer?

Why battery is connected in series?

Why Transformer in KVA?

What is function of pump in DG?

why DG neutral is earth?

What is the principle of high speed direct drive motor ?

Can we load Power capacitors on DG sets?

What are the factors required for selection of transformer rating?

Why the frequency is 50 hz in india.can we use some other frequency.

What is the meaning of rms value?

If i want to build 230kv substation what are the equipments to be installed

How a generator is positioned in a hydroelectric power plant?

What is the generating voltage of a power station consided generally?

How to control the generator output voltage? What r the condition should satisfy for synchronization?

What is inrush current in transformer?

What r the advantage of selecting micro controller type tripping of circuit breaker than relay type tripping?

What is the meaning of BUS?

In step down transformer (415/200V),mistakenly secondary side is connected to 415V for some hours, what would be result of this condition?

If any lead acid battery is having low or high sp.gravity then what will be effect of battery and why?

What is the diff. Between two phase & three phase supply?

What is the reason for balance of voltage in magnetic balance test?

How can we measure and monitor critical performance parameters of Diesel Generator Sets, both electrical and engine ?

How can you prove that in CE configuration a transistor has 180 degree phase shifted output ?

What do you mean by 1 ton in AC`s?

Procedure for conducting vector group test on power transformers.why it is conducted? How to increase the specific gravity of DG batteries?

What is mean CT Saturation? How to calculate?

What is IGBT?Where it is used? What is the difference between IGBT and Thyristor?

When a square wave is applied to primary of a transformer then what will be output wave form of secondary

Resistance of a conductor in a close electrical ckt is practically zero why?

Why C.T secondary are rated 5A generally

Draw waveform of capacitor or inductor in a d.c ckt(during transient) with zero resistance with it(in series or parallel)also neglect resistance of wire

What is the significance of per unit impedance ?

What is the function of commutator in dc motor?

In case of delta/delta transformer if the one phase is blown out then what will be secondary line voltage ?

What is linear motor?

Why electric shock due to dc is dangerous than that of ac?

Why the generating voltage in India is 11kv or why is it be transmitted as multiples of 11?

Why the current leads voltage in the case of a capacitor?

How to select earthing conductor size of amy motor/panel etc?

What is the difference b/w winding insulation class and winding temperature class?

Is it possible for a feedback surge to travel over a wireless connection?

What is a DLC? Explain the advantage of DLC over PLC?

What are the tests to be taken on a transformer and generator?

Do you know how to measure the earth resistance? If so explain it.

Why a starter is used for a motor?

Do you know how to joint the H.T and L.T Cables?

How you take care of a motor from over loading and what are the different overloading ratings of different motors?

Do you know the faults occurs generally in motors and how to rectify them?

Do you know the alignment of a motor?

What accessories required to give the motor with given circuits and do you know the ratings of those accessories

Do you know how to connect a motor of different H.Ps?

Do you know how to maintain a generator, if so what are those maintenances?

How do you maintain the batteries for generators, what is the daily maintenance of a generator?

How do you maintain the earthing resistance?

What are the different protection systems used to protect the transformer internally and externally?

What is meant by H.R.C fuse?

Do you know how to measure the earth resistance? If so explain it

What are the different sizes of cables for H.T and L.T side of supply?

Is your generator is automated or manual if so do you know the circuits?

How do you connect the capacitors on polyphase circuits?
by:-
Namrata
M.tech (Power System)