Electrical Interview Questions and Answers

Q1. What is electricity ?
Electricity is a general term used for all phenomena caused by electric charge whether static or in motion.

Q2. What happens if I connect a capacitor to a generator load?
Connecting a capacitor across a generator always improves power factor, but it will help depends up on the engine capacity of the alternator, otherwise the alternator will be over loaded due to the extra watts consumed due to the improvement on pf. Secondly, don’t connect a capacitor across an alternator while it is picking up or without any other load.

Q3. Define Pole Pitch?
The distance between the centres of two adjacent poles is called pole pitch. One pole pitch is equals to 180 electrical degrees. It is also defined as the number of slots per pole.

Q4. What are the types of electricity ?
There are two types
1. Static Electricity and
2. Current Electricity.

Q5. What Is The General System Requirements Of Alternator?
For the generation of emf, there should be two basic systems.
magnetic field system to produce the magnetic field
Armature system which houses the conductors on which the EMF is to be induced

Q6. Why the capacitors works on ac only?
Generally capacitor gives infinite resistance to dc components (i.e., block the DC components). It allows the ac components to pass through.

Q7. Explain the working principal of the circuit breaker?
Circuit Breaker is one which makes or breaks the circuit. It has two contacts namely fixed contact & moving contact. Under normal condition the moving contact comes in contact with fixed contact thereby forming the closed contact for the flow of current. During abnormal & faulty conditions (when current exceeds the rated value) an arc is produced between the fixed & moving contacts & thereby it forms the open circuit, Arc is extinguished by the Arc Quenching media like air, oil, vacuum etc.

Q8. What is static electricity ?
Static electricity means electricity at rest in contradistinction to dynamic or current electricity the effects of which are purely due to the electrostatic field produced by the charge. As it is obtained by rubbing two substance such as glass and silk it is also called frictional electricity.

Q9. How many types of colling system it transformers?
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)

Q10. What is the function of anti-pumping in circuit breaker?
when breaker is close at one time by close push button, the anti-pumping contactor prevent re close the breaker by close push button after if it already close.

Q11. Do you know the advantages of speed controlusing thyristor?
Advantages:
1. Fast Switching Characterstics than Mosfet, BJT, IGBT
2. Low cost
3. Higher Accurate.

Q12. What Are The Advantages Of Stationary Armature And Rotating Field System?
The stationary armature coils can be insulated easily.
Higher peripheral speed can be achieved in the rotor.
Cooling of the winding is more efficient.
Only two slip rings are required to give DC supply to the field system
Output current can be easily supplied to the load circuit. Slip-rings and brushes are not necessary.

Q13. What is current electricity ?
Current electricity means the electricity in motion the effects of which are due to the flow of electrons in a conductor.

Q14. What is stepper motor? What is its uses?
Stepper motor is the electrical machine which act upon input pulse applied to it.it is one type of synchronous motor which runs in steps in either direction instead of running in complete cycle.so, in automation parts it is used.

Q15. Name the different types of motors used in vacuum cleaners, phonographic appliances, vending machines, refrigerators, rolling mills, lathes, power factor improvement and cranes.
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.

Q16. Will The Alternators Have Rotating Armature System Or Stationary Armature System?
Generally in alternators, the armature is stationary and the field rotates. Small low-voltage alternators often have a rotating armature and a stationary field winding.But in large alternators rotating armature field type is used.

Q17. What are the types of current electricity ?
There are two types :-
(1) Direct current and
(2) Alternating current.

Q18. Tell me in detail about c.t. and p.t.?
The term C.T means current transformer, and the term P.T means potential transformer. In circuit where measurements of high voltage and high current is involved, they are used there. Electrical Interview Questions & Answers visit www.eeekenya.com Page | 2 Particularly when a measuring device like voltmeter or ammeter is not able to measure such high value of quantity because of large value of torque due to such high value it can damage the measuring device.so, CT and PT are introduced in the circuits. They work on the same principle of transformer, which is based on linkage of electromagnetic flux produced by primary with secondary. They work on the ratio to they are designed. E.g. if CT is of ratio 5000\5A and it has to measure secondary current of 8000A.then ANS=8000*5\5000=8Aand this result will be given to ammeter .and after measuring 8A we can calculate the primary current. Same is the operation of PT but measuring voltage.

Q19. In the motors starting purpose why the armature rheostats at maximum position while the field rheostat is kept in minimum position?
In the time of starting a motor the field rheostat is kept in minimum state for the high starting torque.In the time of starting a motor the armature rheostats is used to control or reduce the high starting current. This high starting current can damage the motors starting systems.

Q20. What Is Meant By Stator? What Is Meant By Rotor?
In any electrical machine ( AC/DC motor or generator) the stationary member is called as stator. Similarly in all machines the rotating member is known as rotor.

Q21.What are the different methods of producing electricity ? Give examples.
The methods are –
1. By means of frictions- Static electricity is produced.
2. By means of chemical action in cells and batteries.
3. By means of mechanical driving- Generator produces electricity in two dissimilar methods.
4. By means of heat – Thermal electricity is produced.
5. By means of lighting effect – Electricity is produced in photo electric cell.

Q22. There are a Transformer and an induction machine. Those two have the same supply. For which device the load current will be maximum? And why?
The motor has max load current compare to that of transformer because the motor consumes real power and the transformer is only producing the working flux and it’s not consuming. Hence the load current in the transformer is because of core loss so it is minimum.

Q23. What’s the other name for short circuit?
fault.

Q24. What Are The Advantages Of Three Phase Motor Over Single Phase Motor?
Three phase motors are having
Higher starting torques
Improved speed regulation
Less vibration
Quieter operation
compared to the single phase motors.

Q25.What are the sources of electricity ?
1. Battery
2. Generator and
3. Thermocouple.

 Q26. What is power factor? Whether it should be high or low? Why?
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)

Q27. Name power losess in rotating electrical machines.
Power losess in rotating electrical machines are Copper losses, core losses, mechanical losses and stray losses.

Q28. What Is Basic Principle Of Operation Of Alternators/dc Generators?
They are working on the fundamental principle based on Faraday’s Laws of Electromagnetic Induction.
This law states that, When a current carrying conductor moves in magnetic field, it induces an EMF.

Q29. What are the applications of electricity ?
Heating, Lighting, Welding, Running of Motors, Battery charging, Electroplating, Relays, telephones, Electronic Equipment, etc.

Q30. What is the difference between Isolator and Circuit Breaker?
Isolator is an off load device which is used for isolating the downstream circuits from upstream circuits for the reason of any maintenance on downstream circuits. It is manually operated and does not contain any solenoid unlike circuit breaker. It should not be operated while it is having load. First the load on it must be made zero and then it can safely operate. Its specification only rated current is given. But circuit breaker is on load automatic device used for breaking the circuit in case of abnormal conditions like short circuit, overload etc., it is having three specification 1 is rated current and 2 is short circuit breaking capacity and 3 is instantaneous tripping current

Q31. What is the significance of vector grouping in Power Transformers?
Every power transformer carries a vector group listed by its manufacturer. Fundamentally it tells you the information about how the windings are connected (delta or wye) and the phace difference betweent the current and voltage. EG. DYN11 means Delta primary, Wye Secondry and the current is at 11 o clock reffered to the voltage.

Q32. How To Minimize The Eddy Current Losses?
When the core is laminated and insulated from each other with paper or varnish the eddy current loss is minimized.

Q33. What are the effects of electricity ?
(1) Physiological effect (Electric shock), (2)Heating effect (Lamp, Heater, Fuse), (3)Magnetic effect (Fan, Motor, Electric bell), (4) Chemical effect (Battery charging and Electroplating) and (5) X – Ray effect ( X- Rays).

Q34. State the factors, for the choice of electrical system for an aero turbine.
The choice of electrical system for an aero turbine is guided by three factors:
• Type of electrical output: dc, variable- frequency ac, and constant- frequency ac.
• Aero turbine rotational speed: constant speed with variable blade pitch, nearly constant speed with simpler pitch- changing mechanism or variable speed with fixed pitch blades.
• Utilization of electrical energy output: in conjunction with battery or other form of storage, or interconnection with power grid.

Q35. What Is Meant By Turbo Alternators?
Turbo alternators are high speed alternators. Because of high speed of rotation, the rotor diameter is reduced and the axial length is increased. Two or four poles are generally used and steam turbines are used as prime movers.

Q36. What are A.C. and D.C. ?
A.C is the abbreviation of an alternating current which flows to and from in a circuit in alternate direction periodically with the variation of its magnitude in regular manner that after reaching a maximum in one direction decreases to zero, finally reversing and reaching a maximum in the opposite direction decreases to zero again the cycle of pulsation being repeated continuously. D.C. is the abbreviation of a direct current which flows in one direction only and which does not have any appreciable pulsation in its magnitude.

Q37. What Are The Various Types Of Rotors Used In The Alternators?
Salient pole rotor
Non- salient pole rotor and Cylindrical rotor