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