Reactive power is the virtual oscillating power in the power system that enables active power to do the work in the system.So it is necessary for the power syst...
Sunday, 4 May 2014
Define the following: Average demand, Maximum demand, Demand factor, Load factor.
• Average Demand: the average power requirement during some specified period of time of considerable duration is called the average demand of installation.• Maximum Demand: The maximum demand of an installation is defined as the greatest of all the demand, which have occurred during a given period. It is measured accordingly to specifications, over a prescribed time interval during a certain period.• Demand Factor: It is defined as the ratio of actual maximum demand made by the load to the rating of the connected load.• Load Factor: It is defined...
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 gr...
State the methods of improving power factor?
Methods of improving power factor:• By connecting static capacitors in parallel with the load operating at lagging power factor. • A synchronous motor takes a leading current when over excited and therefore behaves like a capacitor.• By using phase advancers to improve the power factor of induction motors. It provides exciting ampere turns to the rotor circuit of the motor. By providing more ampere-turns than required, the induction motor can be made to operate on leading power factor like an overexcited synchronous mot...
Mention the disadvantages of low power factor? How can it be improved?
Disadvantages of low power factor:• Line losses are 1.57 times unity power factor.• Larger generators and transformers are required.• Low lagging power factor causes a large voltage drop, hence extra regulation equipment is required to keep voltage drop within prescribed limits.• Greater conductor size: To transmit or distribute a fixed amount of power at fixed voltage, the conductors will have to carry more current at low power factor. This requires a large conductor si...
Define the following terms:- • Reliability, • Maximum demand, • Reserve-generating capacity, • Availability (operational).
Reliability: It is the capacity of the power system to serve all power demands without failure over long periods.Maximum Demand: It is maximum load demand required in a power station during a given period.Reserve generating capacity: Extra generation capacity installed to meet the need of scheduled downtimes for preventive maintenance is called reserve-generating capacity.Availability: As the percentage of the time a unit is available to produce power whether needed by the system or n...
Comment on the working principle of operation of a single-phase transformer
Working principle of operation of a single-phase transformer can be explained asAn AC supply passes through the primary winding, a current will start flowing in the primary winding. As a result, the flux is set. This flux is linked with primary and secondary windings. Hence, voltage is induced in both the windings. Now, when the load is connected to the secondary side, the current will start flowing in the load in the secondary winding, resulting in the flow of additional current in the secondary winding. Hence, according to Faraday’s laws of electromagnetic...
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. As the coil moves into the influence of the next pole- pair, the current in it must reverse. This reversal of current in a coil is called commutation. Several coils undergo commutation simultaneously. The reversal of current is opposed by the static coil emf and therefore must be aided in some fashion for smooth current reversal, which otherwise would result in sparking at the brushes. The aiding emf is...
Explain different types of D.C motors? Give their applications
Different type of DC motors and their applications are as follows:-• Shunt motors: It has a constant speed though its starting torque is not very high. 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.• Service motors: It has high starting torque & its speed is inversely proportional to the loading conditions i.e. when lightly loaded, the speed is high and when heavily loaded, it is low. Therefore, motor is used in lifts,...
Explain different losses in a transformer.
There are two types of losses occurring in transformer:• Constant losses or Iron losses: The losses that occur in the core are known as core losses or iron losses. 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.• Variable losses or copper losses: when the transformer...
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 behi...
State Norton’s Theorem
The Norton’s theorem explains the fact that there are two terminals and they are as follows:• One is terminal active network containing voltage sources • Another is the resistance that is viewed from the output terminals. The output terminals are equivalent to the constant source of current and it allows giving the parallel resistance. The Norton’s theorem also explains about the constant current that is equal to the current of the short circuit placed across the terminals. The parallel resistance of the network can be viewed from the...
State Thevenin’s Theorem:
According to thevenin’s theorem, the current flowing through a load resistance Connected across any two terminals of a linear active bilateral network is the ratio open circuit voltage (i.e. the voltage across the two terminals when RL is removed) and sum of load resistance and internal resistance of the network. It is given by Voc / (Ri + R...
Name the 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 moto...
What are the different methods for the starting of a synchronous motor.
Starting methods: Synchronous motor can be started by the following two methods:• By means of an auxiliary motor: The rotor of a synchronous motor is rotated by auxiliary motor. Then rotor poles are excited due to which the rotor field is locked with the stator-revolving field and continuous rotation is obtained.• By providing damper winding: Here, bar conductors are embedded in the outer periphery of the rotor poles and are short-circuited with the short-circuiting rings at both sides. The machine is started as a squirrel cage induction motor...
Explain the application of storage batteries.
Storage batteries are used for various purposes, some of the applications are mentioned below:
• For the operation of protective devices and for emergency lighting at generating stations and substations.• For starting, ignition and lighting of automobiles, aircrafts etc.• For lighting on steam and diesel railways trains.• As a supply power source in telephone exchange, laboratories and broad casting stations.• For emergency lighting at hospitals, banks, rural areas where electricity supplies are not possibl...
What is slip in an induction motor?
lip 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. It is usually expressed as a percentage of synchronous speed (Ns) and represented by the symbol ‘...
Why back emf used for a dc motor? highlight its significance.
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 ze...
What are the various kind of cables used for transmission?
Cables, which are used for transmitting power, can be categorized in three forms:• Low-tension cables, which can transmit voltage upto 1000 volts.• High-tension cables can transmit voltage upto 23000 volts.• Super tension cables can transmit voltage 66 kV to 132 ...
How can you relate power engineering with electrical engineering?
Power engineering is a sub division of electrical engineering. 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.e. called on grid systems and they might work on off grid systems that are not connected to the syst...
What is reverse power relay?
Reverse power relay are used for the protection of the generating stations. The generating stations feed power to the grid and in case when the generating stations are off, there is no generation in the plants and it pulls off power from the grid. To stop this power flow from the grid to the generating stations, reverse power relays are us...
Why is negative feedback more preferable in control system?
The role of feedback system in control system is to check deviation from the desired result, that is, it takes the sample output back to the input and compares the output signal with the input signal for any kinds of error. Negative feedback results in a better stability for the system. It rejects any kind of disturbance signal and is less sensitive to any kind of parameter variatio...
Why AC systems are more used or preferred over DC systems?
The reasons why AC systems are more widely used and more preferable than DC systems are as follows:
It is comparably easy to maintain and modify the voltage of AC electricity for transmission and distribution than it is for DC electricity.
The entire plant cost, ie, circuit breakers, isolators, transformers, etc is much lower in comparison to that of DC transmission.
All the power stations produce AC electricity, and hence it is advantageous and also easy to use AC rather than converting it and using DC.
AC is practically a sine wave current,...
What are the differences between an alternator and a generator?
Alternator and generator, both devices have the same basic functionality. They both convert mechanical energy into electrical energy and they both have the same principal of electromagnetic induction. The main and only difference between these two devices lies in the construction. Alternator has a stationary armature and a rotating magnetic field for high voltages, while generator uses stationary magnetic field and rotating conductor which rolls on the armature with slip rin...
Why is star delta starter preferred with induction motor?
Star delta starter is preferred with induction motor because it reduces the starting current about 3-4 times of the direct current which causes the voltage to drop and hence it causes less damages and losses. Also, it increases the starting torque and hence prevents from the damaging of the motor windi...
Why a Auto Transformer is used in a grid?
auto-transformers, where they can be used (depends on the transformation ratio), are cheaper than "regular" transformers. Other than that, they operate exactly like a conventional transformer. Transformers do not care about the direction of the power flow, they can always operate with MW flow from HV to LV or vice-versa. Tap changers are used to help control voltage, particularly if you have on-load tap changers. These devices are usually installed at the HV side of the transformer (so they operate at lower current levels). The direction...
Advantages of Power factor improvement and Correction
Advantages of Power factor improvement and Correction:
Following are the merits and benefits of improved Power factor;
Increase in efficiency of system and devices
Low Voltage Drop
Reduction in size of a conductor and cable which reduces cost of the Cooper
An Increase in available power
Line Losses (Copper Losses) I2R is reduced
Appropriate Size of Electrical Machines (Transformer, Generators etc)
Eliminate the penalty of low power factor from the Electric Supply Company
Low kWh (Kilo Watt per hour)
Saving in the...
Why We Need to Install a Starter with a Motor?
Motor below 1 Hp is directly connect without starter because their armature resistance is very high and they have ability to afford the high current due to high resistance. So the Armature winding safe from the high starting current.
But a large size of motors has a very low armature resistance. if connect this type of motor direct to Supply (3-phase Supply) then the large current will destroy the armature wading due to low resistance because motor is not running in this time. Why motor is not running in this time when we connect motor...
Why alternator rated in kVA. Not in kW?
The power √3 VL IL Cos φ delivered by the alternator for the same value of current, depends upon p.f. (Power Factor=Cos φ) of the load. But the alternator conductors are calculated for a definite current and the insulation at magnetic system are designed for a definite voltage independent of p.f. (Cos φ) of the load. For this reason apparent power measured in kVA is regarded as the rated power of the alternat...
Under what condition is D.C supply applied safely to the primary of a transformer?
As i mentioned ,This is a simple electrical interview question. So if some one ask this question in interview or somewhere..then the right answer will be this....I think u got the main Idea AlsoAs we know that Transformer works only on A.C, in case of D.C Supply, the primary of transformer may start to smoke and burn....But this is the only one condition where we can operate a Transformer on D.C (although the Circuit is Useless)...For...
How To Calculate Your Electricity Bill. Easy and simple Explanation.
A consumer consumes 1000 watts load per hour daily for one month.Calculate The Total Energy bill of the consumer if per unit rate is 9 (In INR ,Rs, $, DHR, or Riyal etc) [Take 1 month = 30 Days]
Solution
1unit = 1kWh.
So Total kWh = 1000 x 24 x 30 = 720000 watts/hour
we Want to convert it into Units, Where is 1unit = 1kWh.So total Consumed units. 720000/1000...... (k=kilo=1000)Total Units = 720.Cost of per unit is 9.So total Cost or bill= 720 x 9 = 6480. (In INR,Rs, $, DHR, or Riyal etc)
[Also note that…why we multiply with 24 although...
What is the purpose of ground wires in over-Head Transmission lines?
Ground wires are bare conductors supported at the top of transmission towers. They serve to shield the line and intercept lighting stroke before it hits the current carrying conductors below. Ground wires normally do not carry current. Therefore, they are often made of steel. The ground wires are solidly connected to ground at each tow...
Why the reactance of a system under fault condition is low and faults currents may raise dangerously high value. ? (With simple example)
Because the total Power is same, and under fault condition (Short circuit)...There is no load (Impedance (Z), or Reactance ( XL)= Resistance, and in case of no load, there will be no reactance or resistance, so current will be high) in this condition...So current will be too high, and when power is same, and current increases, voltage will be decrease.
Example,
Suppose, (In normal condition)
P= 10 watt, V = 5 Volts, and Current = 2 Amp.
But in Short circuit Condition, (When current is too high)
Then,
P = 10 Watts, ...
Will a D.C Shunt Motor operate on an A.C Supply?
The Shunt winding has a large number of turns so that it has appreciable inductance. When A.C is applied to a shunt motor, the large inductive reactance of shunt winding will reduce the field current too much. Consequently, Shunt motor will not usually run on A.C Supp...
Why A.C needs more insulation than D.C at same voltage level?
For the same working voltage, the potential stress on the insulation is less than in case of DC system than that AC system. therefore , a DC line require less insulation.in other words A DC System has a less potential stress over AC system for same Voltage level, this is why AC needs more insulation over DC sys...
What is The Difference between a VOLTAMETER and a VOLTMETER?
A VOLTA-METER is a device used to carry out electrolytes and a VOLTMETER is a high resistance device used for measuring potential difference or voltage between two points in an electrical Circuit...
What is the difference between a battery and a capacitor?
There are many differences..but the major one is that Electrical Energy is stored in battery or cell in the form of chemical energy, and transformed again in the form of electrical energy, while in a capacitor, electrical charge or energy stored in the form of electrostatic field.&nb...
What is the normal or average life expectancy of a Transformer ?
When a Transformer is operated under ANSI / IEEE basic loading conditions (ANSI C57.96), its normal lifeexpectancy is about 20 years. The ANSI / IEEE basic loading conditions for Transformer are: i. The Transformer is continuously loaded at rated kVA (kilo Volt Ampere) and rated Voltages (Transformer must be operated at the rated Voltage and kVA) ii. The average temperature of the ambient air during any 24-hour period is equal to 30°C (86 °F) and at no time exceeds 40°C (104 °F). iii. The height where the transformer is installed,does...
Why the Circuit Power factor (Cos θ) Decreases, when Inductance (L) or inductive reactance (XL) increases, In inductive circuit?
Explanation:
Suppose,
when Inductance (L) = 0.02H
V=220, R= 10 Ω, L=0.02 H, f=50Hz.
XL = 2πfL = 2 x 3.1415 x 50 x 0.02 = 6.28 Ω
Z = √ (R2+XL2) = √ (102 + 6.282) = 11.8 Ω
Cos θ = R/Z = 10/11.05 = 0.85
Now we increases Inductance (L) form 0.02 H to 0.04 H,
V=220, R= 10 Ω, L=0.04 H, f=50Hz.
XL = 2πfL= 2 x 3.1415 x 50 x 0.04 = 12.56 Ω
Z = √ (R2+XL2) = √ (102 + 12.562) = 16.05 Ω
Cos θ = R/Z = 10/16.05 = 0.75
Conclusion:
We can see that, When inductance (L) was 0.02,...
Why the circuit Current (I) decrease, when Inductance (L) or inductive reactance (XL) increases in inductive circuit?
Explain the statement that " In Inductive circuit, when Inductance (L) or inductive reactance (XL) increases, the circuit Current (I) decrease" ORWhy the circuit Current (I) decrease, when Inductance (L) or inductive reactance (XL) increases in inductive circuit?Explanation:We know that, I = V / R, but in inductive circuit, I = V/XLSo Current in inversely proportional to the Current ( in inductive circuit.Let's check with an example.. Suppose, when Inductance (L) = 0.02H
V=220, R= 10 Ω, L=0.02...
A (50/60 Hz) Transformer. Which one will give more Output? (When operates on 50 or 60 Hz frequency)
A Transformer is designed to be operated on both 50 & 60 Hz frequency.
For the Same rating, which one will give more out put; when,
Operates on 50 Hz
Operates on 60 Hz
Obviously! It will give more out put when we operate a transformer (of same rating) on 50 Hz instead of 60 Hz.
Because in previous posts, we proved that, in inductive circuit, when frequency increases, the circuit power factor decreases. Consequently, the transformer out put decreases.
Let’s consider the following example.
Suppose,
When...
Why Power in pure Capacitive Circuit is Zero (0)?
+power+in+pure+capacitive+circuit.jpg)
We know that in Pure capacitive circuit, current is leading by 90 degree from voltage ( in other words, Voltage is lagging 90 Degree from current) i.e the phase difference between current and voltage is 90 degree.So If Current and Voltage are 90 Degree Out of Phase, Then The Power (P) will be zero. The reason is that,We know that Power in AC CircuitP= V I Cos φif angle between current and Voltage are 90 ( φ = 90) Degree. thenPower P = V I Cos...
Why Power in pure Inductive Circuit is Zero (0).
.+power+in+pure+inductive+circuit.jpg)
We know that in Pure inductive circuit, current is lagging by 90 degree from voltage ( in other words, Voltage is leading 90 Degree from current) i.e the pahse difference between current and voltage is 90 degree. So If Current and Voltage are 90 Degree Out of Phase, Then The Power (P) will be zero. The reason is that,We know that Power in AC CircuitP= V I Cos φif angle between current and Voltage are 90 ( φ = 90) Degree. thenPower P =...
Why Power in a circuit is Zero (0), in which Current and Voltage are 90 Degree out of phase?
If Current and Voltage are 90 Degree Out of Phase, Then The Power (P) will be zero. The reason is that, We know that Power in AC Circuit P= V I Cos φif angle between current and Voltage are 90 ( φ = 90) Degree. thenPower P = V I Cos ( 90) = 0[ Note that Cos (90) = 0] So if you put Cos 90 = 0→Then Power will be Zero (In pure Inductive circu...
A Voltmeter, an ammeter (Ampere meter) and a battery cell are connected in series. It is observed that ammeter practically shows No Deflection. Why?
Due to the large resistance of the voltmeter, the circuit resistance becomes very high. As a result, very small current will flow in the circuit. this small current f on passing through the coil of voltmeter will produce some deflection. However, in case of ammeter (Ampere meter), most of this small current will flow through the shunt. Consequently the deflection of the ammeter will be practically ni...
The Main difference between Active and passive Commonest (Very Easy Explanation with Examples)
Active Components:Those devices or components which required external source to their operation is called Active Components. For Example: Diode, Transistors, SCR etc...Explanation and Example: As we know that Diode is an Active Components. So it is required an External Source to its operation. Because, If we connect a Diode in a Circuit and then connect this circuit to the Supply voltage., then Diode will not conduct the current Until the supply voltage reach to 0.3(In case of Germanium) or 0.7V(In case of Silicon). I...
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