Interview Questions for Electrical Engineers - Part 03

Question No. 01: what is meant by insulation voltage in cables? Explain it?
Answer: It is the property of a cable by virtue of it can withstand the applied voltage without rupturing it is known as insulation level of the cable.

Question No. 02What is power factor? Whether it should be high or low? Why?
Answer: Power factor should be high in order to get smooth operation of the system. Low power factor means losses will be more. It is the ratio of true power to apparent power. It has to be ideally 1. If it is too low then cable over heating &equipment overloading will occur. If it is greater than 1 then load will act as capacitor and starts feeding the source and will cause tripping.(if pf is poor ex: 0.17 to meet actual power load has to draw more current (V constant),result in more losses if pf is good ex: 0.95 to meet actual power load has to draw less current (V constant),result in less losses).

Question No. 03: What is Automatic Voltage regulator (AVR)?
Answer: AVR is an abbreviation for Automatic Voltage Regulator. It is important part in Synchronous Generators; it controls the output voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator.

Question No. 04: Give two basic speed control scheme of D.C. shunt motor?
Answer:
  1. By using flux control method: in this method a rheostat is connected across the field winding to control the field current. So by changing the current the flux produced by the field winding 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 winding 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 No. 05: Advantages of (Variable Voltage Variable Frequency (VVVF) drives over non-vvvf drives for EOT cranes?
Answer:
  1. Smooth start and stop.
  2. No jerking of load.
  3. Exact positioning
  4. Better protection for motor.
  5. High/low speed selection.
  6. Reliability of break shoe.
  7. Programmable break control.
  8. Easy circuitry.
  9. Reduction in controls
  10. Increases motor life

Question No. 06: What is MARX CIRCUIT?
Answer: It is used with generators for charging a number of capacitor in parallel and discharging them in series. It is used when voltage required for testing is higher than the available.

Question No. 07: How can you start-up the 40 W tube light with 230 V AC/DC without using any choke/Coil?
Answer: It's possible by means of Electronic choke. Otherwise it's not possible to ionise the particles in tube light, with normal voltage.

Question No. 08: What is derating? Why it is necessary, it is same for all means for drives, motors, and cables. 
Answer: The current currying of cables will change depending upon the site temperature (location of site), type of run (it will run through duct, trench, buried etc.), number of tray, depth of trench, and distance between cables. Considering this condition actual current currying capacity of cable reduce than current currying capacity (which given to cable Catalog) this is called derating.

Question No. 09: What is the power factor of an alternator at no load?
Answer: At no load Synchronous Impedance of the alternator is responsible for creating angle difference. So it should be zero lagging like inductor.

Question No. 10: What is power quality meter?
Answer: Power Quality meters are common in many industrial environments. Small units are now available for home use as well. They give operators the ability to monitor the both perturbations on the power supply, as well as power used within a building, or by a single machine or appliance. In some situations, equipment function and operation is monitored and controlled from a remote location where communication is via modem or high-speed communication lines. So we can understand the importance of power measurement through power quality meters.

Question No. 11: When we connect the large capacitor bank in series? 
Answer: we connect large capacitor bank in series to improve the voltage power supply at the load end in balanced transmission line when there is considerable voltage drop along the balanced transmission line due to high impedance of the line. So in order to bring the voltage at the load terminals within its limits i.e. (+ or - %6) of the rated high terminal voltage) the large capacitor bank is used in series.

Question No. 12: How to determine alternating current frequency?
Answer: Zero crossings of the sine wave to trigger a monostable (pulse generator) is a way to determine alternating current frequency. A fixed width pulse is generated for each cycle. Thus there are "n" pulses per second, each with a constant energy. The more pulses there are per second, the more the energy. The pulses are integrated (filtered or averaged) to get a steady DC voltage which is proportional to frequency. This voltage can then be displayed on an analogue or digital voltmeter, indicating frequency. This method is more suitable than a direct counter, as it can get good accuracy in a second or so.

Question No. 13: What type’s domain of Laplace transforms? What behavior can Laplace transform predict how the system work?
Answer:
Types domain of Laplace transforms is s-domain, Laplace transforms provide a method to find position, and acceleration or voltage the system will have.

Question No. 14: What is the voltage gain or transfer function of amplifier?
Answer:
Vout/Vin

Question No. 15: Why AC systems are preferred over DC systems?
Answer: Due to following reasons, AC systems are preferred over DC systems:
  1. It is easy to maintain and change the voltage of AC electricity for transmission and distribution.
  2. Plant cost for AC transmission (circuit breakers, transformers etc) is much lower than the equivalent DC transmission
  3. From power stations, AC is produced so it is better to use AC then DC instead of converting it.
  4. When a large fault occurs in a network, it is easier to interrupt in an AC system, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.

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