Selected 5 Interview Questions for Electrical Engineers asked for PSUs

Q1. For a 100 kW generator, only 50 kW of load is connected. Will the generator generate only 50 kW or 100 kW?

A1. Under steady state conditions, the (mechanical input - rotational losses) will be equal to the electrical power output. Hence, the generator will generate only 50 kW, even though its full capacity is 100kW.

Q2. What happens if we connect more than 100 kW of load?

A2.  If we connect more than 100kW of load, there will be flow of more current in the lines than their capacity and if present, the fuses shall blow off. If that doesn't happen then the wires will get overheated.

Q3. What is the difference between "earth" and "neutral"?

A3. The "neutral" refers to the star point in a star connection. The star connection of the 3 phase windings of an alternator, transformer or a 3 phase motor has one end of each phase-winding connected together, forming the neutral point or the star point. Neutral is not present in a delta connection.

"earth" refers to the zero potential level ( the ground) to which an electrical equipment is connected for personnel safety. The neutral of star connection can be earth to protect the 3 phase machine for large short circuit currents.

Q4. At what condition, a synchronous motor acts as a synchronous condenser?

A4.  When a synchronous motor is run unloaded, its primary function becomes improving the pf pf the system, by generating or absorbing reactive power. As we know that increasing the If changes its pf from leading to lagging(inverted-V curve). Thus a synchronous condenser is a synchronous motor designed with large field current. There is no shaft extension. Larger If requires greater amount of copper and large area of cross section of field poles as compared to ordinary synchronous motor of same rating.

Q5. How is a 100 MVA transformer cooled?

A5.  80 MVA -> ONAN (Oil-Natural Air-Natural)

       120 MVA -> ONAF (Oil-Natural Air-Forced)

       200 MVA -> OFAF (Oil-Forced Air-Forced)

       250 MVA -> OPAF (Oil-Pumped Air-Forced)

Hence a 100 MVA transformer has to be ONAF(Oil-Natural Air-Forced) cooled.  

"Soon I will be ready with the next five. If you want explanation on any of the above, or want to get your own question answered,or want to give a better answer to these questions, post them in the  comments section below."