Few interview type questions with answers

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.
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).
· wide variation of electrical parameters gets eliminated by p.u. representation
· Since the p.u. impedance 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?
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)