How a single phase motor is similar to 3 phase motor?
A single phase induction motor is very similar to a 3-phase squirrel cage induction motor. It has
(i) a squirrel-cage rotor identical to a 3-phase motor
(ii) a single-phase winding on the stator 

What is major difference between a 3-phase induction motor and a single-phase induction motor?

Unlike a 3-phase induction motor, a single-phase induction motor is not self- starting but requires some starting means. 

Why single phase induction motor does not self starts?

The single-phase stator winding produces a magnetic field that pulsates in strength in a sinusoidal manner. The field polarity reverses after each half cycle but the field does not rotate. Consequently, the alternating flux cannot produce rotation in a stationary squirrel-cage rotor. 

What is basic method to start single phase induction motor and why it is not preferred?

If the rotor of a single-phase motor is rotated in one direction by some mechanical means, it will continue to run in the direction of rotation. As a matter of fact, the rotor quickly accelerates until it reaches a speed slightly below the synchronous speed. Once the motor is running at this speed, it will continue to rotate even though single-phase current is flowing through the stator winding. This method of starting is generally not convenient for large motors. Nor can it be employed fur a motor located at some inaccessible spot.

Why Power Plant Capacity Rated in MW and not in MVA?

In a Generating station, the prime mover (Turbine) generates only and only Active Power. That’s why we rated a power plant capacity in MW instead of MVA. Its mean no matter how large your generator is, but it depends on the capacity of the engine (Prime mover/Turbine) I.e. a 50MW turbine connected to a 90MVA alternator in a power plant will generate only 50MW at full load. In short, a power plant rating is specified in terms of prime mover /Turbine (Turbine rating may be seen by nameplate rating which is in MW or Horsepower (HP) not in MVA) and not by the alternator set coupled to it.

Another thing is that, electric power company charges their consumer for kVA while they generate kW (or MW) at the power station (Power plant).They penalize their consumer for low Power factor because they are not responsible for low power factor and kVA but you. Moreover, in power plant, power factor is 1 therefore MW is equal to MVA …… (MW = MVA x P.f).

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 system.

Comparison between Star (Y) and Delta (Δ) Connections

Star (Y) Connection	Delta (Δ) Connection
In STAR connection, the starting or finishing ends (Similar ends) of three coils are connected together to form the neutral point. A common wire is taken out from the neutral point which is called Neutral.	In DELTA connection, the opposite ends of three coils are connected together. In other words, the end of each coil is connected with the start of another coil, and three wires are taken out from the coil joints
There is a Neutral or Star Point	No Neutral Point in Delta Connection
Three phase four wire system is derived from Star Connections (3-Phase, 4 Wires System) We may Also derived 3 Phase 3 Wire System from Star Connection	Three phase three wire system is derived from Delta Connections (3-Phase, 3 Wires System)
Line Current is Equal to Phase Current. i.e. Line Current = Phase Current IL = IPH	Line Voltage is Equal to Phase Voltage. i.e. Line Voltage = Phase Voltage VL = VPH
Line Voltage is √3 times of Phase Voltage. i.e. VL = √3 VPH	Line Current is √3 times of Phase Current. i.e. IL = √3 IPH
The Total Power of three phases could be found by P = √3 x VL x IL x CosФ …. Or P = 3 x VPH x IPH x CosФ	The Total Power of three phases could be found by P = √3 x VL x IL x CosФ … or P = 3 x VPH x IPH x CosФ
The speeds of Star connected motors are slow as they receive 1/√3 voltage.	The speeds of Delta connected motors are high because each phase gets the total of line voltage
In Star Connection, the phase voltage is low as 1/√3 of the line voltage, so, it needs low number of turns, hence, saving in copper.	In Delta connection, The phase voltage is equal to the line voltage, hence, it needs more number of turns.
Low insulation required as phase voltage is low	Heavy insulation required as Phase voltage = Line Voltage.
In Power Transmission, Star Connection system is general and typical to be used.	In Power Distribution and industries, Delta Connection is general and typical to be used.

(1) Real Power: (P)
Alternative words used for Real Power (Actual Power, True Power, Watt-full Power, Useful Power, Real Power, and Active Power)
In a DC Circuit, power supply to the DC load is simply the product of Voltage across the load and Current flowing through it i.e., P = V I. because in DC Circuits, there is no concept of phase angle between current and voltage. In other words, there is no Power factor in DC Circuits.
But the situation is Sinusoidal or AC Circuits is more complex because of phase difference between Current and Voltage. Therefore average value of power (Real Power) is P = VI Cosθ is in fact supplied to the load.
In AC circuits, When circuit is pure resistive, then the same formula used for power as used in DC as P = V I.
You may also read about Power Formulas in DC, AC Single Phase and and AC Three Phase Circuits.
Real Power formulas:
P = V I (In DC circuits)
P = VI Cosθ (in Single phase AC Circuits)
P = √3 VL IL Cosθ or (in Three Phase AC Circuits)
P = 3 VPh IPh Cosθ
P = √ (S2 – Q2)or
P =√ (VA2 – VAR2) or
Real or True power = √ (Apparent Power2– Reactive Power2) or
kW = √ (kVA2 – kVAR2)

(2) Reactive Power: (Q)
Also known as (Use-less Power, Watt less Power)
The powers that continuously bounce back and forth between source and load is known as reactive Power (Q)
Power merely absorbed and returned in load due to its reactive properties is referred to as reactive power
The unit of Active or Real power is Watt where 1W = 1V x 1 A.
Reactive power represent that the energy is first stored and then released in the form of magnetic field or electrostatic field in case of inductor and capacitor respectively.
Reactive power is given by Q = V I Sinθ which can be positive (+ve) for inductive, negative (-Ve) for capacitive load.
The unit of reactive power is Volt-Ampere reactive. I.e. VAR where 1 VAR = 1V x 1A.
In more simple words, in Inductor or Capacitor, how much magnetic or electric field made by 1A x 1V is called the unit of reactive power.
Reactive power formulas:
Q = V I Sinθ
Reactive Power=√ (Apparent Power2- True power2)
VAR =√ (VA2 – P2)
kVAR = √ (kVA2 – kW2)
(3) Apparent Power: (S)
The product of voltage and current if and only if the phase angle differences between current and voltage are ignored.
Total power in an AC circuit, both dissipated and absorbed/returned is referred to asapparent power
The combination of reactive power and true power is called apparent power
In an AC circuit, the product of the r.m.s voltage and the r.m.s current is calledapparent power.
It is the product of Voltage and Current without phase angle
The unit of Apparent power (S) VA i.e. 1VA = 1V x 1A.
When the circuit is pure resistive, then apparent power is equal to real or true power, but in inductive or capacitive circuit, (when Reactances exist) then apparent power is greater than real or true power.
Apparent power formulas:
S = V I
Apparent Power = √ (True power2 + Reactive Power2)
kVA = √kW2 + kVAR2
AlsoNote that;
Resistor absorbs the real power and dissipates in the form of heat and light.
Inductor absorbs the reactive power and dissipates in the form of magnetic field
Capacitor absorbs the reactive power and dissipates in the form of electric or electrostatic filed

Q:Which motor has high Starting Torque and Staring current DC motor, Induction motor or Synchronous motor?

A:DC Series motor has high starting torque. We can not start the Induction motor and Synchronous motors on load, but can not start the DC series motor without load.

Is Flux in Primary Winding always greater than the Secondary winding in Transformer?
Ans:
Flux in Primary and Secondary Winding is always equal.
In ideal case, the flux produced in Primary winding will pass through the secondary winding, so generated flux in primary winding will be same as in secondary winding.
Here is a very simple example to explain my answer.
Suppose,
              A 50Hz Single phase Transformer has 525 Primary turns and 70 Secondary Turns. If the primary is connected to 3300 Volts supply, Find the secondary Voltage. If neglecting losses, What is the primary current when the secondary current is 250 Amperes? Also Prove that the Flux produced in Both Primary and Secondary Winding is Same.?
Solution:
Given Data;
Primary Number of Turns N1 = 524,
Secondary Number of Turns N2 = 70
Primary Input Voltage V1= 3300 Volts.
Secondary Current I2= 250 A.
Find/Calculate?
Secondary Voltage V2 =?
Primary Current I1=?
Φm 1 = Φm2
We Know that,
N2/N1 = V2/V1====> V2 = (N2 x V1)/N1
Putting the Values
V2= (70 x 3300)/525 = 440 Volts Ans.
Now if Neglecting Losses,
V1I1= V2I2 ====> I1/I2 = V2/ V1 …..Or…..I1 = (V2 x I2) / V1
Putting the Values,
I1= 440 x 250/3300 = 33.3 Amp Ans.
Now turn around the Transformer equation.
E1= 4.44 f N1 φm1
φm1= E1 / 4.44 f N1
Putting the Values
Φm 1 = 3300 / (4.44 x 50 x 525) = 0.0283 Weber’s
Φm 1 = 28.3mWeber’s = Flux in Primary Windings
Same is on the other side,
E2= 4.44 f N2 φm2
Φm2= E2 / 4.44 f N2
Putting the values,
Φm2 = 440 / (4.44 x 50 x 70) = 0.0283 Weber’s
Φm2 = 28.3mWeber’s = Flux in secondary Windings
So You can see the flux (Φm) produced in Both Primary and Secondary Winding is same.

1. These batteries have connected in ___________

.


1.Series
2.Parallel

Answer: 

 2. Parallel
Explanation: As we can see that Positive connected to positive terminal and Negative connected to Negative Terminal. So the batteries configuration in Parallel.How can we connect a load in this configuration, it is shown in the figure below

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).

Passive Components:

Those devices or components which do not required external source to their operation is called Passive Components.

For Example: Resistor, Capacitor, Inductor etc…
Explanation and Example: Passive Components do not require external source to their operation.
Like a Diode, Resistor does not require 0.3 0r 0.7 V. I.e., when we connect a resistor to the supply voltage, it starts work automatically without using a specific voltage

1. 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 φ

if angle between current and Voltage are 90 ( φ = 90) Degree. then

Power 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 circuit)

2. Why Power in pure Inductive Circuit is Zero (0).

                 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 Circuit

P= V I Cos φ

if angle between current and Voltage are 90 ( φ = 90) Degree. then

Power 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 circuit)

3. Why Power in pure Capacitive Circuit is Zero (0)?

            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 Circuit

P= V I Cos φ

if angle between current and Voltage are 90 ( φ = 90) Degree. then

Power 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 capacitive circuit)

                                          

Why Battery rating in Ah (Ampere hour) and not in VA.
                  Battery stores charge in the form of chemical energy and then converts it into electrical energy to utilize for a specific time. The amount of available charge is the capacity of a cell or battery which may be expressed in Ah (Ampere-hour). Moreover, in a charged battery, the numbers of molecules are limited to create a flow of electron in electric circuits, so, there must be a limited number of electrons in a cell/battery which they motivate through a circuit tofully discharge. Now we have the option to rate the battery capacity in Number of flowing electrons for a specific time, but, it would be a headache, because there are a vast number of electrons in it. So we have another option (1C (Coulomb) = 6.25 x 1018electrons, or 6,250,000,000,000,000,000 electrons.
In addition, 1A (Ampere) = 1 coulomb of electrons per second and,
1h = 3600 Seconds
Therefore;
1Ah = (1A) x (3600s) = (C/s) x (3600s) = 3600 C.
∴ A (1 Ampere) = 1 Coulomb per second = C/s

Q.Why Motor rated in kW/Horsepower instead of kVA?
                      We know that Transformer rating may be expressed in kVA as well as Generator and Alternator rated in kVA Designer doesn’t know the actual consumer power factor while manufacturing transformers and generators i.e. the P.F (Power factor) of Transformer and Generator/Alternator depends on the nature of connected load such as resistive load, capacitive load, and inductive load as Motors, etc. But Motorhas fixed Power factor, i.e. motor has defined power factor and the rating has been mentioned in KW on Motor nameplate data table. That’s why we are rated Motor in kW or HP (kilowatts/ Horsepower) instead of kVA.In addition, Motor is a device which converts Electrical power into Mechanical power. In this case, the load is not electrical, but mechanical (Motor’s Output) and we take into the account only active power which has to be converted into mechanical load. Moreover, the motor power factor does not depend on the load and it works on any P.F because of its design.

Q.Why we can’t store AC in Batteries instead of DC.or Can we store AC in batteries instead of DC?
A.We cannot store AC in batteries because AC changes their polarity upto 50 (When frequency = 50 Hz) or 60 (When frequency = 60 Hz) times in a second. Therefore the battery terminals keep changing Positive (+ve) becomes Negative (-Ve) and vice versa, but the battery cannot change their terminals with the same speed so that’s why we can’t store AC| in Batteries.
Also when we connect a battery with AC Supply, then It will charge during positive half cycle and discharge during negative half cycle because the Positive (+ve) half cycle cancel the negative (-Ve) half cycle, so the average voltage or current in a complete cycle is Zero. So there is no chance to store AC in the Batteries.
Also note that Average Voltage x Average Current ≠ Average Power.

                                                                

Q.Explain the process of commutation in a dc machine. Explain what are inter-poles and why they are required in a dc machine.

A.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 dynamically induced into the coils undergoing commutation by means of compoles or interpoles, which are series excited by the armature current. These are located in the interpolar region of the main poles and therefore influence the armature coils only when these undergo commutation.

Q:what happen if we give 220 volts dc supply to d bulb r tube light?

A:Bulbs [devices] for AC are designed to operate such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so high that it may damage the bulb element.

Q.What is the principle of operation of tube light?

               A fluorescent lamp is a glass tube containing some mercury vapour, with a phosphorescent coating on the interior surface of the tube. The mercury vapour is made to glow by using a high voltage across its electrodes that sets off an electric arc discharge in the tube. The discharge can also be described as conducting plasma.

               The resulting flow of current through the mercury vapour is stable and well-controlled by external ballast, or loading device, which consists of a high-inductance choke-coil that is connected in series with the tube. The continuous discharge glow is invisible to the human eye but it causes the phosphorescent coating on the interior surface of the tube to emit visible light.

Q.Which is more dangerous-shock due to AC or DC?
A.Shock due to DC is more dangerous than AC

Question:What is an exciter and how does it work?

Answer:There are two types of exciters, static exciter and rotory exciter.purpose of excitor is to supply the excitation dc voltage to the fixed poles of generator.Rotory excitor is an additional small generator mounted on the shaft of main generator. if it is dc generator, it will supply dc to the rotory poles through slip ring and brushes( conventional alternator). if it is an ac excitor, out put of ac excitor is rectified by rotating diodes and supply dc to main fixed poles.ac excitor is the ac generator whose field winding are stationary and armature rotates. initialvoltage is built up by residual magnetism.It gives the starting torque to the generator.

Q:Why ELCB can't work if N input of ELCB do not connect to ground?

A:ELCB is used to detect earth leakage fault. Once the phase and neutral are connected in an ELCB, the current will flow through phase and that much current will have to return neutral so resultant current is zero. Once there is a ground fault in the load side, current from phase will directly pass through earth and it will not return through neutral through ELCB. That means once side current is going and not returning and hence because of this difference in current ELCB wil trip and it will safe guard the other circuits from faulty loads. If the neutral is not grounded, fault current will definitely high and that full fault current will come back through ELCB, and there will be no difference in current

What is the difference between earth and neutral?

Neutral is a return current path for equipments and earthing is protection for humans. Earth is a support for neutral. If neutral is not present, the machine we are using will get in full phase and we will get a shock. So earthing is done to avoid such shocks. 

What is the difference between an Insulator and a Dielectric?

Dielectrics are used to store the electric charges, while insulators are used to block the flow of electric charges. All dielectrics are insulators (they don't allow the flow of electric charges through them) but all insulators aren't dielectric because they can't store charges unlike dielectrics.

Is Cu loss affected by power factor? Why?

Yes, Cu loss varies inversely with power factor

Cu loss depends on current in the primary and secondary windings. It is well-known that current required is higher when power factor is lower.

Q.Why AC systems are preferred over DC systems?

A.Due to following reasons, AC systems are preferred over DC systems:

a. It is easy to maintain and change the voltage of AC electricity for transmission and distribution.

b. Plant cost for AC transmission (circuit breakers, transformers etc) is much lower than the equivalent DC transmission

c. From power stations, AC is produced so it is better to use AC then DC instead of converting it.

d. When a large fault occurs in a network, it is easier to interrupt in an AC system, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.

Q.Why star delta starter is preferred with induction motor?

A.Star delta starter is preferred with induction motor due to following reasons:

• Starting current is reduced 3-4 times of the direct current due to which voltage drops and hence it causes less losses.

• Star delta starter circuit comes in circuit first during starting of motor, which reduces voltage 3 times, that is why current also reduces up to 3 times and hence less motor burning is caused.

• In addition, starting torque is increased and it prevents the damage of motor winding.

Q.What is Corkscrew Rule?
A.As current flows along a wire, the magnetic field rotates in the direction of a corkscrew

Q:What is the main use of rotary phase     converter ? 

A:Rotary phase converter will be converting single phase power into true balanced 3 phase power,so it is often called as single phase to three phase converter .Often the advantages of 3 phase motors, and other 3 phase equipment, make it worthwhile to convert single phase to 3 phase so that small and large consumers need not want to pay for the extra cost of a 3 phase service but may still wish to use 3 phase equipment.

Q:What is the difference between synchronous generator & asynchronous generator?

A: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.

Q:Why syn. generators are used for the production of electricity?

A:synchronous machines have capability to work on different power factor (or say different imaginary power varying the field emf. Hence syn. generators are used for the production of electricity.

Q:How do you select a cable size (Cu & Al) for a particular load?

A:At first calculate the electrical current of the load, after that derate the electrical current considering derating factor(depending on site condition and laying of cable) after choose the cable size from cablecatalog considering derating electrical current.After that measure the length of cable required from supply point of load to load poin. Calculate the voltage drop which will max 3% (resistance and reactance of cable found from cable catalog of selecting cable) if voltage drop>3% then choose next higher size of cable.

Q.State the difference between generator and alternator.

A.Generator and alternator are two devices, which converts mechanical energy into electrical energy. Both have the same principle of electromagnetic induction, the only difference is that their construction. Generator persists stationary magnetic field and rotating conductor which rolls on the armature with slip rings and brushes riding against each other, hence it converts the induced emf into dc current for external load whereas an alternator has a stationary armature and rotating magnetic field for high voltages but for low voltage output rotating armature and stationary magnetic field is used.

Q.What is Scott- connected transformer?

A.A Scott-T transformer (also called a Scott connection) is a type of circuit used to derive two-phase electric power (2-φ, 90-degree phase rotation) from a three-phase (3-φ, 120-degree phase rotation) source, or vice-versa. The Scott connection evenly distributes a balanced load between the phases of the source.

Q.What is Phase Swinging?

A.When a synchronous motor is used for driving a varying load, then a condition known as phase swinging or hunting may occur. Hunting may also be caused if supply frequency is pulsating (as in the case of generators driven by reciprocating internal combustion engines).

Q.Is corona more in AC or DC?

A.Corona Loss p = .00241[(f+25)/$]*[root of (r/d)]*square of[Vp-Vo]

f= supply frequency 

$= air density correction factor

Vp = operating voltage

Vo= critical disprutive voltage

r= radius of conductor

d= distance of separation for fine weather

Now for DC f=0, there is a certain power loss by corona but less than AC corona loss.

What are the advantages and disadvantages of Compact Fluorescent Lamps?

The advantages are

· CFLs are up to four times more efficient than incandescent bulbs

· While initially they may cost more, CFLs are less expensive in the long run as they last much longer than incandescent bulbs

· CFLs are highly versatile and can be used in any setting that you would normally use incandescent bulbs

The disadvantages are

· CFLs when used outdoors need to be covered and protected from the elements. They are also sensitive to temperature

· While CFLs are supposed to last about 10,000 hours, turning them on and off too frequently can reduce that lifetime substantially

· CFLs are not suitable for focused or spot lights or where narrow beams of light are required. They are meant only for ambient light.

What is a meant by a Phase-Splitter?

It is a device consisting of a number of capacitors so connected in the motor circuit that it produces, from a single input wave, three output waves which differ in phase from each other.

Q.What is a synchronous capacitor?
A.An over excited synchronous motor is called synchronous capacitor, because, like a capacitor, it takes a leading current

Q:How tube light circuit is connected and how it works?

A:A choke is a connected in one end of the tube light and a starter is in series with the circuit. When supply is provided the starter will intrupt the supply intermittant cycle of AC. Due to the sudden change of supply the chock will generate around 1000volts . This volt will capable of to break the electrons inside the tube to make elctrone flow. once the current passess through the tube the starter circuit will be out of part. now there is no change of supply causes choke voltage normalised and act as minimise the current.

Q:why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?

A:Link is provided at a Neutral common point in the circuit from which various connection are taken for the individual control circuit and so it is given in a link form to withstand high Amps. But in the case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is calculated for the particular circuit (i.e load) only.So if any malfunction happen the fuse connected in the particular control circuit alone will blow off.

Q.What is a variable frequency transformer?

A.A variable frequency transformer is used to transmit electricity between two asynchronous alternating current domains. A variablefrequency transformer is a doubly-fed electric machine resembling a vertical shaft hydroelectric generator with a three-phase wound rotor, connected by slip rings to one external ac power circuit. A direct-current torque motor is mounted on the same shaft. Changing the direction of torque applied to the shaft changes the direction of power flow; with no applied torque, the shaft rotates due to the difference in frequency between the networks connected to the rotor and stator.The variablefrequency transformer behaves as a continuously adjustable phase-shifting transformer. It allows control of the power flow between two networks .

Q:Why Delta Star Transformers are used for Lighting Loads?

A:For lighting loads, neutral conductor is must and hence the secondary must be star winding. and this lighting load is always unbalanced in all three phases. To minimize the current unbalance in the primary we use delta winding in the primary. So delta / star transformer is used for lighting loads.

Q: What is actually a zero watt bulb?

Answer: A zero-watt bulb is a lightbulb that uses little power. Contrary to the name, these bulbs are not in fact zero watts. The reason they are colloquially called "zero watt" bulbs is because when they were first made, they only had a power consumption rate of 15W but testing equipment at the time was unable to detect such low wattage causing people to think they didn't use any power. Todays "zero watt" bulbs re as little as 10W.

Q.What is the difference between a capacitor and a condensor?
A.There is no difference with the exception or semantics and argument. Tesla use to call a capacitor a condenser we now call a condenser a capacitor. Schematics symbol for both are the same!
Main diff. b/w Condensers and capacitors are i. Condensers are rotating while capacitors are static ii.Smooth flow in KVAR can be obtained through condensers while it cannot be obtained through switching capacitor banks.

Q.What is reactive power? Is it necessary for power system operation?

A.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 system.

Reactive power is one which is useful for internal activation of system. if reactive power is zero the system o/p is zero.

Question:When voltage increases then current also increases then what is the need of over voltage relay and over current relay? Can we measure over voltage and over current by measuring current only? 

Answer:No.We can't sense the over voltage by just measuring the current only because the current increases not only for over voltages but also for under voltage(As most of the loads are non-linear in nature). So,the over voltage protection & over current protection are completely different. Over voltage relay meant for sensing over voltages & protect the system from insulation break down and firing. Over current relay meant for sensing any internal short circuit, over load condition ,earth fault thereby reducing the system failure & risk of fire. So, for a better protection of the system.It should have both over voltage & over current relay. 

Q.why do we need three phase power for heavy load?????

A.With a single phase, the voltage drops to 0 every half-cycle. Therefore, the amount of power you can get out of the line is not constant over time, it has a very large 120-hz ripple. With a large motor, this can cause vibration, and also means that larger wires are needed for the same power load. The same is true for so-called "2- phase" power, 2 voltages 180 degrees apart. They both cross zero at the same time, which is why this is normally also referred to as single- phase. It isn't any better than single phase. With 3-phase power, however, there is never a time when all the voltages go to 0. In fact, if you are extracting power from all 3 phases, (like in a motor) the power stays constant throughout the whole cycle. This gives a vibration-free drive (and also allows for simpler motor design because 3- phase power has an inherent "direction" which 2-phase or 1-phase power does not).

Q:Two bulbs of 100W and 40W respectively connected in series across a 230V supply which bulb will glow bright and why?

A: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. 

Q:What are HRC fuses and where it is used?

A: HRC stand for "high rupturing capacity" fuse and it is used in distribution system for electrical transformers.

Q.If one lamp connects between two phases it will glow or not?

A.If the voltage between the two phase is equal to the lamp voltage then the lamp will glow. When the voltage difference is big it will damage the lamp and when the difference is smaller the lamp will glow depending on the type of lamp

Q.What is universal motor?

A.The universal motor is so named because it is a type of electric motor that can operate on both AC and DC power. It is a commutated series-wound motor where the stator's field coils are connected in series with the rotor windings through a commutator. It is often referred to as an AC series motor. The universal motor is very similar to a DC series motor in construction, but is modified slightly to allow the motor to operate properly on AC power. This type of electric motor can operate well on AC because the current in both the field coils and the armature (and the resultant magnetic fields) will alternate (reverse polarity) synchronously with the supply. Hence the resulting mechanical force will occur in a consistent direction of rotation, independent of the direction of applied voltage, but determined by the commutator and polarity of the field coils.Universal motors have high starting torque, can run at high speed, and are lightweight and compact. They are commonly used in portable power tools and equipment, as well as many house hold appliances.

Q.What is traction motor?

Traction motor refers to an electric motor providing the primary rotational torque to a machine, usually for conversion into linear motion (traction).

Q:what happen if we give 220 volts dc supply to d bulb r tube light?

A:Bulbs [devices] for AC are designed to operate such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so high that it may damage the bulb element.

Q. What is Vernier motor?

A.Vernier motor is a PM motor and has a high torque at low speed. It operates on the principles of "magnetic gearing effect".The vernier motor is mainly used where require low speed and high torque.

Q:Why in a three pin plug the earth pin is thicker and longer than the other pins?

A:It depends upon R=rho l/a where area(a) is inversely proportional to resistance (R), so if (a) increases, R decreases & if R is less the leakage current will take low resistance path so the earth pin should be thicker. It is longer because the The First to make the connection and Last to disconnnect should be earth Pin. This assures Safety for the person who uses the electrical instrument.

Q: why, when birds sit on transmission lines or current wires doesn't get shock?

A:Its true that if birds touch the single one line (phase or neutral) they don't get electrical shock... if birds touch 2 lines than the circuit is closed and they get electrical shock.. so if a human touch single one line(phase) then he doesn't get shock if he is in the air (not touching - standing on the ground if he is standing on the ground then touching the line (phase) he will get a shock because the ground on what we standing is like line (ground bed - like neutral)। and in the most of electric lines the neutral is grounded... so that means that human who touch the line closes the circuit between phase and neutral.

Q:what is stepper motor.what is its uses?

A: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.

Why Negative Feedback is Prefered 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.

Qwhat is 2 phase motor?
A: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.

Q: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? 

A:The motor has max load current compare to that of transformer bcoz 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.

Q: what is the diff. btwn. electronic regulator and ordinary rheostat regulator for fans?

           A:The difference between the electronic and ordinary regulator is that in electronic reg. power losses are less i.e.for as we decrease the speed the electronic reg. give the power needed for that perticular speed but in case of ordinary rh type reg. the power wastage is same for every speed and no power is saved.In electronic regulator triac is employed for speed cntrl. by varying the firing angle speed is controlled but in rheostatic ctrl resistance is decreased by steps to achieve speed control.

Q:What is the difference between Isolator annd Circuit Breaker?

                 A:Isolator is a off load device which is used for isolating the downstream circuits from upstream circuits for the reason of any maintanance on downstream circuits. it is manually operated and does not contain any solenoid unlike circuit breaker. it should not be operated while it is having load. first the load on it must be made zero and then it can safely operated. its specification only rated current is given.But circuit breaker is onload automatic device used for breaking the circuit incase of abnormal conditions like shortcircuit, overload etc., it is having three specification 1 is rated current and 2 is short circuit breaking capacity and 3 is instantaneous tripping current.

What is the difference between MCB & MCCB, Where it can be used?

                 MCB is miniature circuit breaker which is thermal oprated 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.

Why star delta starter is preferred with induction motor?

Star delta starter is preferred with induction motor due to following reasons:
• Starting current is reduced 3-4 times of the direct current
due to which voltage drops and hence it causes less losses.
• Star delta starter circuit comes in circuit first during starting of motor, which reduces voltage 3 times, that is why current also reduces up to 3 times and hence less motor burning is caused.
• In addition, starting torque is increased and it prevents the damage of motor winding.

Why Battery rated in Ah (Ampere hour) and not in VA?

Battery stores charge in the form of chemical energy and then converts it into electrical energy to utilize for a specific time. The amount of available charge is the capacity of a cell or battery which may be expressed in Ah (Ampere-hour). Moreover, in a charged battery, the numbers of molecules are limited to create a flow of electron in electric circuits, so, there must be a limited number of electrons in a cell/battery which they motivate through a circuittofully discharge. Now we have the option to rate the battery capacity in Number of flowing electrons for a specific time, but, it would be a headache, because there are a vast number of electrons in it.  So we have another option (1C (Coulomb) = 6.25 x 10¹⁸electrons, or 6,250,000,000,000,000,000 electrons.

In addition, 1A (Ampere) = 1 coulomb of electrons per second and,

1h = 3600 Seconds

Therefore;

1Ah = (1A) x (3600s) = (C/s) x (3600s) = 3600 C.

∴ A (1 Ampere) = 1 Coulomb per second = C/s

But,

Why make up a new unit for battery capacity rating when an old one unit is doing just fine? L

Of course! To make your lives as technicians and students more difficult.   

As they do for electricity units… i.e. 1 Unit of Electricity = 1kWh = 1 board of Trade Unit…