List of Frequently asked interview questions with answers under the subjects like electrical machines, Transmission and distribution, Power electronics and some general basic questions.
Part 1 : Question 1 to 25.
1. What is a Sytem?
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.
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
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 cycle.so, 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?
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.