It deals with generation, transmission and distribution of energy in electrical form. Design of all power equipments also comes under power engineering. Power engineers may work on the design and maintenance of the power grid i. What are the various kind of cables used for transmission? Why back emf used for a dc motor? The induced emf developed when the rotating conductors of the armature between the poles of magnet, in a DC motor, cut the magnetic flux, opposes the current flowing through the conductor, when the armature rotates, is called back emf.
Its value depends upon the speed of rotation of the armature conductors. In starting, the value of back emf is zero. What is slip in an induction motor? Slip can be defined as the difference between the flux speed Ns and the rotor speed N.
Speed of the rotor of an induction motor is always less than its synchronous speed. Explain the application of storage batteries. What are the different methods for the starting of a synchronous motor.
Then rotor poles are excited due to which the rotor field is locked with the stator-revolving field and continuous rotation is obtained. The machine is started as a squirrel cage induction motor first.
When it picks up speed, excitation is given to the rotor and the rotor starts rotating continuously as the rotor field is locked with stator revolving field. Name the types of motors used in vacuum cleaners, phonographic appliances, vending machines, refrigerators, rolling mills, lathes, power factor improvement and cranes. The output terminals are equivalent to the constant source of current and it allows giving the parallel resistance. The parallel resistance of the network can be viewed from the open circuit terminals when all the voltage and current sources are removed and replaced by the internal resistance.
State Maximum power transfer theorem The Maximum power transfer theorem explains about the load that a resistance will extract from the network. This includes the maximum power from the network and in this case the load resistance is being is equal to the resistance of the network and it also allows the resistance to be equal to the resistance of the network. This resistance can be viewed by the output terminals and the energy sources can be removed by leaving the internal resistance behind.
Explain different losses in a transformer. Two types of iron losses are: o eddy current loss o 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. These losses depend upon the loading conditions of the transformers.
Therefore, these losses are also called as variable losses. Explain different types of D. C motors? Therefore, it is suitable for constant speed drive, where high starting torque is not required such as pumps, blowers, fan, lathe machines, tools, belt or chain conveyor etc.
Therefore, motor is used in lifts, cranes, traction work, coal loader and coal cutter in coalmines etc. Its advantage is, it can run at NIL loads without any danger. This motor will therefore find its application in loads having high inertia load or requiring high intermittent torque such as elevators, conveyor, rolling mill, planes, presses, shears and punches, coal cutter and winding machines etc.
Explain the process of commutation in a dc machine. Explain what are inter-poles and why they are required in a dc machine. Commutation: It is phenomenon when an armature coil moves under the influence of one pole- pair; it carries constant current in one direction.
Answer : It is the converter whose output frequency is less than the input frequency. What Is Step Down Cyclo-converter? Answer : It is the converter whose output frequency is more than the input frequency. What Is Ac Voltage Controller? Answer : It is the one which converts fixed alternating voltage to a variable voltage without change in frequency. What Is Inverter? Answer : A device which converts dc power into ac power at desired output voltage and frequency is called as Inverter.
What Is Duty Cycle? Answer : It is the ratio of the on time of the chopper to total time period of the chopper. Generally, fuses have a DC voltage rating that is half of the maximum AC voltage rating. Answer : SCR or thyristor will have three regions of operations based on the mode in which the device is connected in the circuit. Reverse blocking region: When the cathode of the thyristor is made positive with respect to the anode and no gate signal is applied.
In this region scr exhibits the reverse blocking characteristics similar to diode. Forward blocking region: In this region the anode of the thyristor is made positive with respect to the cathode and no gate signal is applied to the thyristor.
A small leakage current flow in this mode of operation of the thyristor. Forward conduction region: when the forward voltage applied between the anode and cathode increases at particular break over voltage avalanche breakdown takes place and thyristor starts conducting current in forward direction.
By this type of triggering the device damages the scr. Hence a gate signal is applied before the forward break over voltage to trigger the scr. What Is Latching Current? Answer : 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 Holding Current? Answer : 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.
Answer : 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. Answer : Thyristor can be triggered by increasing the forward voltage between anode and cathode, at forward break over voltage thyristor starts conducting. However this process may damage the thyristor, so thyristor is advices to trigger on through the gate pulse. When a gate signal is applied thyristor turns on before reaching the break over voltage.
Forward voltage at which the thyristor triggers on depends on the magnitude of the gate current. Higher is the gate current lower is the forward break over voltage. Answer : Connecting a capacitor across a generator always improves power factor, but it will help depends up on the engine capacity of the alternator, other wise 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. Answer : Generally capacitor gives infinite resistance to dc components i. Answer : Circuit Breaker is one which makes or breaks the circuit. Answer : Isolator is a off load device which is used for isolating the downstream circuits from upstream circuits for the reason of any maintenance on downstream circuits. But circuit breaker is onload automatic device used for breaking the circuit in case of abnormal conditions like short circuit, overload etc.
Answer : Only one of the terminals is evident in the earth resistance. In order to find the second terminal we should recourse to its definition: Earth Resistance is the resistance existing between the electrically accessible part of a buried electrode and another point of the earth, which is far away. Answer : A lock-out relay is generally placed in line before or after the e-stop switch so the power can be shut off at one central location.
This relay is powered by the same electrical source as the control power and is operated by a key lock switch. The relay itself may have up to 24 contact points within the unit itself. This allows the control power for multiple machines to be locked out by the turn of a single key switch.
Answer : At no load Synchronous Impedance of the alternator is responsible for creating angle difference. So it should be zero lagging like inductor. 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 Z. 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.
Answer : mA is a standard range used to indicate measured values for any process. The reason that 4ma is chosen instead of 0 mA is for fail safe operation. For example- a pressure instrument gives output 4mA to indicate 0 psi, up to 20 mA to indicate psi, or full scale. Due to any problem in instrument i. So if range is mA then we can differentiate whether it is due to broken wire or due to 0 psi.
Answer : 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 Is Reverse Power Relay? Answer : Reverse Power flow relay are used in generating station'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. Answer : 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.
Definition Of Power Electronics? Answer : Power electronics refers to control and conversion of electrical power by power semiconductor devices wherein these devices operate as switches. Answer : The main task of power electronics is to control and convert electrical power from one form to another.
Rectifier applications: Variable speed dc drives, Battery chargers, DC power supplies and Power supply for a specific application like electroplating.
Inverter applications: Emergency lighting systems, AC variable speed drives, Un-interrupted power supplies and Frequency converters. Cycloconverter applications: It is rarely used. Can be used for controlling the speed of an AC traction motor. In this region SCR exhibits the reverse blocking characteristics similar to diode. Answer : Forward conduction losses Loss due to leakage current during forward and reverse blocking Switching losses at turn on turn off Gate triggering loss.
Load current waveform is also improved. Explain The Function Of Cyclo-converter? What Is An Inverter? Define Circuit Turn Off Time. Answer : It is defined as the time during which a reverse voltage is applied across the thyristor during its commutation process.
Answer : If the circuit turn off time is less than the thyristor turn off time the device may turn on at an undesired instant resulting in commutation failure.
Answer : A dc Chopper is equivalent to the transformer in ac circuit. It is a static switch used to get the variable dc voltage from a constant dc voltage. Answer : Natural commutation Forced commutation. What Is Natural Commutation? Answer : The process of the current flowing through the thyristor goes through a natural zero and enable the thyristor to turn off is called as natural commutation.
What Is Forced Commutation? Answer : The process of the current flowing through the thyristor is forced to become zero by external circuitry is called as forced commutation. What Is Step Down Chopper? Answer : In step down chopper, the average output voltage is less than the input supply voltage. It is also known as Buck converter. What Is Step Up Chopper? Answer : In step up chopper, the average output voltage is more than the input supply voltage.
It is also known as Boost converter. What Is Voltage Commutation? Answer : The process of a charged capacitor momentarily reverse biases the conducting SCR and turns it off is called as voltage commutation. What Is Current Commutation? Answer : The process of a current pulse is made to flow in the reverse direction through the conducting SCR and thus made the net SCR current becomes zero, consequently turn off the SCR is called as current commutation.
Answer : The advantages of current commutated chopper is;. What Is Load Commutation? Answer : In load commutation, the load current flowing through the thyristor either becomes zero or is transferred to another device from the conducting SCR. Answer : To turn on these kind of devices single pulse of short duration is sufficient. Continuous gate voltage of entire on time is not required. It will avoid the hard triggering. Answer : Lower hate requirements Lower switching losses Smaller snubber circuit requirements.
Answer : Because the controlling parameter is gate-emitter voltage. Answer : Because the output drain current can be controlled by gate-source voltage. Answer : Because the output collector current can be controlled by base current. Answer :. Answer : A thyristor can be turned off by making the current flowing through it to zero. Define Latching Current?
Answer : The latching current is defined as the minimum value of anode current which it must attain during turn on process to maintain conduction when gate signal is removed.
Define Holding Current? Answer : The holding current is defined as the minimum value of anode current below which it must fall to for turning off the thyristor. What Is A Snubber Circuit?
Answer : It consists of a series combination of a resistor and a capacitor in parallel with the thyristors. Answer : Forward conduction losses Loss due to leakage current during forward and reverse blocking.
Switching losses at turn-on and turn-off. Gate triggering loss. Define Hard-driving Or Over-driving? Answer : When gate current is several times higher than the minimum gate current required, a thyristor is said to be hard-fired or over-driven. I need them to build up my educational knowledge level up, for a Telecom interview. Sir I wish to improve my basic knowledge level in electronics and communication field.
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