Wednesday, November 20, 2013

electricals interviews questions

electrical review basis

electricals basic review

Review a list of frequently asked Electrical interview questions for electrical graduates. Frequently asked interview questions with answers under the subjects like electrical machines,Transmission and distribution,Power electronics and some general basic questions.

1. What is a System?
When a number of elements or components are connected in a sequence to perform a specific function, the group of elements that all constitute a System
2. What is Control System?
In a System the output and inputs are interrelated in such a manner that the output quantity or variable is controlled by input quantity, then such a system is called Control System.
The output quantity is called controlled variable or response and the input quantity is called command signal or excitation.

3. What are different types of Control Systems?
Two major types of Control Systems are 1) Open loop Control System 2) Closed Loop Control Systems
Open loop Control Systems:The Open loop Control System is one in which the Output Quantity has no effect on the Input Quantity. No feedback is present from the output quantity to the input quantity for correction.
Closed Loop Control System:The Closed loop Control System is one in which the feedback is provided from the Output quantity to the input quantity for the correction so as to maintain the desired output of the system.

4. What is a feedback in Control System?
The Feedback in Control System in one in which the output is sampled and proportional signal is fed back to the input for automatic correction of the error ( any change in desired output) for futher processing to get back the desired output.
5. Why Negative Feedback is preffered in the Control System?
The role of Feedback in control system is to take the sampled output back to the input and compare output signal with input signal for error ( deviation from the desired result).
Negative Feedback results in the better stability of the system and rejects any disturbance signals and is less sensitive to the parameter variations. Hence in control systems negative feedback is considered.

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6. What is the effect of positive feedback on stability of the system?
Positive feedback is not used generally in the control system because it increases the error signal and drives the system to instability. But positive feedbacks are used in minor loop control systems to amplify certain internal signals and parameters
7. What is Latching current?
Gate signal is to be applied to the thyristor to trigger the thyristor ON in safe mode. When the thyristor starts conducting the forward current above the minimum value, called Latching current, the gate signal which is applied to trigger the device in no longer require to keep the scr in ON position.
8. What is Holding current ?
When scr is conducting current in forward conduction state, scr will return to forward blocking state when the anode current or forward current falls below a low level called Holding current
Note: Latching current and Holding current are not same. Latching current is associated with the turn on process of the scr whereas holding current is associated with the turn off process. In general holding current will be slightly lesser than the latching current.

9. Why thyristor is considered as Charge controlled device?
During the triggering process of the thyristor from forward blocking state to forward conduction state through the gate signal, by applying the gate signal (voltage between gate and cathode) increases the minority carrier density in the p-layer and thereby facilitate the reverse break over of the junction J2 and thyristor starts conducting. Higher the magnitude of the gate current pulse, lesser is the time required to inject the charge and turning on the scr. By controlling the amount of charge we can control the turning on time of the scr.
10. What are the different losses that occur in thyristor while operating?
Different losses that occur are
a)Forward conduction losses during conduction of the thyristor
b)Loss due to leakage current during forward and reverse blocking.
c)Power loss at gate or Gate triggering loss.
d)Switching losses at turn-on and turn-off.

11. What is meant by knee point voltage?
Knee point voltage is calculated for electrical Current transformers and is very important factor to choose a CT. It is the voltage at which a CT gets saturated.(CT-current transformer).
12. What is reverse power relay?
Reverse Power flow relay are used in generating stations's protection. A generating stations is supposed to fed power to the grid and in case generating units are off,there is no generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we use reverse power relay.
13. What will happen if DC supply is given on the primary of a transformer?
Mainly transformer has high inductance and low resistance.In case of DC supply there is no inductance ,only resistance will act in the electrical circuit. So high electrical current will flow through primary side of the transformer.So for this reason coil and insulation will burn out.
14. What is the difference between isolators and electrical circuit breakers? What is bus-bar?
Isolators are mainly for switching purpose under normal conditions but they cannot operate in fault conditions .Actually they used for isolating the CBs for maintenance. Whereas CB gets activated under fault conditions according to the fault detected.Bus bar is nothing but a junction where the power is getting distributed for independent loads.
15. What are the advantage of free wheeling diode in a Full Wave rectifier?
It reduces the harmonics and it also reduces sparking and arching across the mechanical switch so that it reduces the voltage spike seen in a inductive load.
16. Mention the methods for starting an induction motor?
The different methods of starting an induction motor:
a)DOL:direct online starter
b)Star delta starter
c)Auto transformer starter
d)Resistance starter
e)Series reactor starter 

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17. What is the power factor of an alternator at no load?
At no load Synchronous Impedance of the alternator is responsible for creating angle difference. So it should be zero lagging like inductor.
18. 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.
19. 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, in automation parts it is used.
20. 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 its not consuming.. hence the load current in the transformer is because of core loss so it is minimum.
21. What is SF6 Circuit Breaker?
SF6 is Sulpher hexa Flouride gas.. if this gas is used as arc quenching medium in a Circuitbreaker means SF6 CB.
22. What is ferrantic effect?
Output voltage is greater than the input voltage or receiving end voltage is greater than the sending end voltage.
23. What is meant by insulation voltage in cables? explain it?
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.
24. What is the difference between MCB & MCCB, Where it can be used?
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.
25. Where should the lighting arrestor be placed in distribution lines?
Near distribution transformers and out going feeders of 11kv and incomming feeder of 33kv and near power transformers in sub-stations.
26. Define IDMT relay?
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.
27. What are the transformer losses?
TRANSFORMER LOSSES - Transformer losses have two sources-copper loss and magnetic loss. Copper losses are caused by the resistance of the wire (I2R). Magnetic losses are caused by eddy currents and hysteresis in the core. Copper loss is a constant after the coil has been wound and therefore a measurable loss. Hysteresis loss is constant for a particular voltage and current. Eddy-current loss, however, is different for each frequency passed through the transformer.

28. what is the full form of KVAR?
We know there are three types of power in Electricals as Active, apparent & reactive. So KVAR is stand for ``Kilo Volt Amps with Reactive component.
29. Two bulbs of 100w and 40w respectively connected in series across a 230v supply which bulb will glow bright and why?
Since two bulbs are in series they will get equal amount of electrical current but as the supply voltage is constant across the bulb(P=V^2/R).So the resistance of 40W bulb is greater and voltage across 40W is more (V=IR) so 40W bulb will glow brighter.
30. Why temperature rise is conducted in bus bars and isolators? 
Bus bars and isolators are rated for continuous power flow, that means they carry heavy currents which rises their temperature. so it is necessary to test this devices for temperature rise.
31. What is the difference between synchronous generator & asynchronous generator?
In simple, synchronous generator supply's both active and reactive power but asynchronous generator(induction generator) supply's only active power and observe reactive power for magnetizing.This type of generators are used in windmills.
32. What is Automatic Voltage regulator(AVR)?
AVR is an abbreviation for Automatic Voltage Regulator.It is important part in Synchronous Generators, it controls theoutput voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator.
33. Difference between a four point starter and three point starter?
The shunt connection in four point stater is provided separately form the line where as in three point stater it is connected with line which is the drawback in three point stater
34. 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.

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

36. Operation carried out in Thermal power stations?
The water is obtained in the boiler and the coal is burnt so that steam is obtained this steam is allowed to hit the turbine , the turbine which is coupled with the generator generates the electricity.
37. What is 2 phase motor?
A two phase motor is a motor with the the starting winding and the running winding have a phase split. e.g;ac servo motor.where the auxiliary winding and the control winding have a phase split of 90 degree.

38. What is the principle of motor?
Whenever a current carrying conductor is placed in an magnetic field it produce turning or twisting movement is called as torque.

39. What is meant by armature reaction?
The effect of armature flu to main flux is called armature reaction. The armature flux may support main flux or opposes main flux.
40. What is the difference between synchronous generator & asynchronous generator?
In simple, synchronous generator supply's both active and reactive power but asynchronous generator(induction generator) supply's only active power and observe reactive power for magnetizing.This type of generators are used in windmills.
41. Whats is MARX CIRCUIT?
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.
42. What are the advantages of speed control using thyristor?
Advantages :
1. Fast Switching Characterstics than Mosfet, BJT, IGBT
2. Low cost
3. Higher Accurate.

43. What is ACSR cable and where we use it?
ACSR means Aluminium conductor steel reinforced, this conductor is used in transmission & distribution.
44. Whats the one main difference between UPS & inverter ? And electrical engineering & electronics engineering ?
Uninterrupt power supply is mainly use for short time . means according to ups VA it gives backup. ups is also two types : on line and offline . online ups having high volt and amp for long time backup with with high dc voltage.but ups start with 12v dc with 7 amp. but inverter is startwith 12v,24,dc to 36v dc and 120amp to 180amp battery with long time backup.
45. What will happen when power factor is leading in distribution of power?
If their is high power factor, i.e if the power factor is close to one:
a)Losses in form of heat will be reduced,

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