Electrical Interview Questions for Freshers and Experienced - Part 02 - ObjectiveBooks

Electrical Interview Questions for Freshers and Experienced - Part 02

Question No. 01: What is the difference between electronic regulator and ordinary rheostat regulator for fans?
Answer: The difference between the electronic and ordinary regulator is that in electronic regulator power losses are less i.e. for as we decrease the speed, the electronic regulator gives the power needed for that particular speed but in case of ordinary rheostat type regulator, the power wastage is same for every speed and no power is saved. In electronic regulator TRIAC is employed for speed control by varying the firing angle speed is controlled but in rheostat control resistance is decreased by steps to achieve speed control.

Question No. 02: If one lamp connects between two phases it will glow or not?
Answer: If the voltage between the two phases is equal to the lamp voltage then the lamp will glow. When the voltage difference is big it will damage the lamp and when the difference is smaller the lamp will glow depending on the type of lamp.

Question No. 03: How to determine capacitor tolerance codes?
Answer: In electronic circuits, the capacitor tolerance can be determined by a code that appears on the casing. The code is a letter that often follows a three-digit number (such as 130Z).The first two are the 1st and 2nd significant digits and the third is a multiplier code. Most of the time the last digit tells you how many zeros to write after the first two digits and these are read as Pico-Farads. 

Question No. 04: What is the different between digital phase converter and ordinary phase converter? 

Answer: Digital phase converter is a recent development in phase converter technology that utilizes proprietary software in a powerful microprocessor to control solid state power switching components. This microprocessor, called a digital signal processor (DSP), monitors the phase conversion process, continually adjusting the input and output modules of the converter to maintain perfectly balanced three-phase power under all load conditions.

Question No. 05: What is meant by regenerative braking?
Answer: When the supply is cut off for a running motor, it still continue running due to inertia. In order to stop it quickly we place a load (resistor) across the armature winding and the motor should have maintained continuous field supply, so that back e.m.f voltage is made to apply across the resistor and due to load the motor stops quickly. This type of breaking is called as "Regenerative Breaking".

Question No. 06: What is the difference between MCB & MCCB, Where it can be used?
Answer: MCB is miniature circuit breaker which is thermal operated and use for short circuit protection in small current rating circuit. MCCB moulded case circuit breaker and is thermal operated for over load current and magnetic operation for instant trip in short circuit condition. Under voltage and under frequency may be inbuilt. Normally it is used where normal current is more than 100A.

Question No. 07: Why ac solenoid valves attract the plunger even though we interchange the terminal? Will the poles changes?
Answer: Yes because the pole changes for every half-cycle of ac voltage so the polarity of AC voltage is continuously changing for every half cycle. So, interchanging of terminals in ac system does not show any difference. That's why the ac solenoid attracts the plunger even though its terminals are interchanged. 

Question No. 08: What is the main use of rotary phase converter? 
Answer: Rotary phase converter will be converting single phase power into true balanced 3 phase power, so it is often called as single phase to three phase converter .Often the advantages of 3 phase motors, and other 3 phase equipment, make it worthwhile to convert single phase to 3 phase so that small and large consumers need not want to pay for the extra cost of a 3 phase service but may still wish to use 3 phase equipment.

Question No. 09: Why electricity in India is in the multiples of 11 like 11kv, 22kv, 33kv?
Answer: Transformer Induced voltage equation contains 4.44 factors. 
E = 4.44 × f × T × π 
E - Induced emf per phase 
T - Number of turns 
f - Frequency 
π - Maximum flux per pole 
From the equation we see that E is proportional to 4.4 and it is intern multiple of 11. So always transmission voltage is multiple of 11 

Question No. 10: What are the types of power in electrical power?
Answer: There are normally three types of power are counted in electrical power. They are,
  • Apparent power
  • Active power
  • Reactive power

Question No. 11: Name the types of motors used in vacuum cleaners, phonographic appliances, vending machines, refrigerators, rolling mills, lathes, power factor improvement and cranes.
Answer: Following motors are used: -
  • Vacuum cleaners- Universal motor.
  • Phonographic appliances – Hysteresis motor.
  • Vending machines – Shaded pole motor.
  • Refrigerators – Capacitor split phase motors.
  • Rolling mills – Cumulative motors.
  • Lathes – DC shunt motors.
  • Power factor improvement – Synchronous motors.

Question No. 12: Explain different losses in a transformer.
Answer: There are two types of losses occurring in transformer:

Constant losses or Iron losses: The losses that occur in the core are known as core losses or iron losses. Two types of iron losses are:
  • eddy current loss
  • Hysteresis loss. 
These losses depend upon the supply voltage, frequency, core material and its construction. As long as supply voltage and frequency is constant, these losses remain the same whether the transformer is loaded or not. These are also known as constant losses.

Variable losses or copper losses: when the transformer is loaded, current flows in primary and secondary windings, there is loss of electrical energy due to the resistance of the primary winding, and secondary winding and they are called variable losses. These losses depend upon the loading conditions of the transformers. Therefore, these losses are also called as variable losses.

Question No. 13: Explain forward resistance, static resistance and dynamic resistance of a PN junction diode.
  • Forward Resistance: Resistance offered in a diode circuit, when it is forward biased, is called forward-resistance.
  • DC or Static Resistance: DC resistance can be explained as the ratio of the dc-voltage across the diode to the direct current flowing through it.
  • AC or Dynamic Resistance: It can be defined as the reciprocal of the slope of the forward characteristic of the diode. It is the resistance offered by a diode to the changing forward current.

Question No. 14: Why is the starting current high in a DC motor?
Answer: In DC motors, Voltage equation is V = Eb - IaRa
Where, (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 like 0.01ohm.i.e, Ia will become enormously increased.

Question No. 15: Define IDMT relay?
Answer: It is an inverse definite minimum time relay. In IDMT relay its operating is inversely proportional and also a characteristic of minimum time after which this relay operates. It is inverse in the sense; the tripping time will decrease as the magnitude of fault current increase.

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