Electrical Engineering

What’s a Volt? A unit of electric force, measuring the potential strength of current.

Electricity travels at 6,696,000 miles per hour.

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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 less because of core loss.

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.

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.

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.

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

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.

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.  

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.