Ans 1 :
Excitation means producing a electrical magnetic field.
If we rotate a wire in a constant magnetic field only then a electric current is produced. To get a constant magnetic field we have to apply a DC voltage to a coil. Only DC voltage can give a fixed magnetic field. Ac current can also make a electric magnetic fiels, but it will be a fluctuating field, which will not produce electric voltage in alternator.
Ans 2:
If we rotate a wire in a constant magnetic field only then a electric current is produced. To get a constant magnetic field we have to apply a DC voltage to a coil. Only DC voltage can give a fixed magnetic field. Ac current can also make a electric magnetic fiels, but it will be a fluctuating field, which will not produce electric voltage in alternator.
Ans 2:
Alternators generate electricity by the same principle as DC generators. When magnetic field lines cut across a conductor, a current is induced in the conductor. In general, an alternator has a stationary part (stator) and a rotating part (rotor). The stator contains windings of conductors and the rotor contains a moving magnetic field. The field cuts across the conductors, generating an electrical current, as the mechanical input causes the rotor to turn.
Ans 3 :
Ans 3 :
Rating of any electrical device corresponds to the maximum heat that it can bear and heat losses in an electrical device are I^2/R, proportional to current. now the actual current in the circuit in a transformer or alternator depends upon the actual power in the circuit not just the active power. KW is active power while KVA is Active power plus reactive power. So the ratings are given in KVA not in KW
Ans 4 :
Rather than have six leads from the three phase alternator, one of the leads from each phase may be connected to form a common junction. The stator is then called wye or star connected.
Ans 5:
n a DC machine, the main field is produced by field coils. In both the generating and motoring modes, the armature carries current and a magnetic field is established, which is called the armature flux. The effect of armature flux on the main field is called the armature reaction.
Ans 6: Because the power factor of load is not known while it is leading or lagging so transformers are rated in kVA.
Ans 7:
In general for the efficiency to be maximum for any device the losses must be minimum. Between the iron and copper losses the iron loss is the fixed loss and the copper loss is the variable loss. When these two losses are equal and also minimum the efficiency will be maximum.
Therefore the condition for maximum efficiency in a transformer is
Iron loss = Copper loss ( which ever is minimum)
Ans 8:
deally the voltage regulation voltage of a transformer should be zero. It means when you change the load from no load to short circuit (theoretically, normally you don't want to burn the transformer windings) the output voltage doesn't change and remains equal to the no load voltage.
Ans 9:
When ferranti effect occurs at light load conditions the load side voltage is higher than the supply voltage. Thus there will be a negatice V.R.
Ans 10:
Poor voltage regulation can damage the transformer therefore we use capacitive load to counter it.
Ans 11:
An electric motor converts electrical energy into a mechanical energy which is then supplied to different types of loads. A.c. motors operate on an a.c. supply, and they are classified into synchronous, single phase and 3 phase induction, and special purpose motors. Out of all types, 3 phase induction motors are most widely used for industrial applications mainly because they do not require a starting device.
A 3 phase induction motor derives its name from the fact that the rotor current is induced by the magnetic field, instead of electrical connections.
The operating principle of a 3 phase induction motor is based on the production of r.m.f.
Ans 12:
If total current supplied from source is Io, it will have one component in phase with supply voltage V1 and this component of the current Iw is core loss component. This component is taken in phase with source voltage, because it is associated with active or working losses in transformer. Other component of the source current is denoted as Iμ.
This component produces the alternating magnetic flux in the core, so it is watt-less; means it is reactive part of the transformer source current.
Ans 13:
Like any electric motor, a 3-phase induction motor has a stator and rotor. The stator carries a 3-phase winding (called stator winding) while the rotor carries a short circuited winding (called rotor winding). Only the stator winding is fed from 3-phase supply. The rotor winding derives its voltage and power from the externally energized stator winding through electromagnetic induction and hence the name acquired. The induction motor may be considered to be a transformer with a rotating secondary and it can, therefore, be described as a "transformer-type" a.c. machine in which electrical energy is converted into mechanical energy.
Ans 14:
The power factor at no load is low because the magnetizing component of input cur rent is a large part of the total input current of the motor. When the load on the motor is increased, the in-phase current supplied to the motor increases, but the magnetizing component of current remains the same. This means that the resultant line current is more nearly in phase with the voltage and the power factor is improved when the motor is loaded, compared with an unloaded motor which has its magnetizing current as a major component of the input current.
Ans 15:
starters are used in dc motor to minimize the high starting current and also protect the motor. starters are used in dc motor to minimize the high starting current and also protect the motor.
Ans 16:
Unlike transformer and generators the Motor has fixed power factor and also the ratings of motor has been mentioned in kw on the numberplate of all motors that is why motor is rated in KW or HP(kilowatts/horsepoer) instead of KVA.Motor.
Ans 17:
1. Used in both Synchronous motors and induction motors.
2. Used in electrical apparatus testing labs since the voltage can be smoothly and continuously varied.
3. They find application as boosters in AC feeders to increase the voltage levels.
Ans 18:
The stator core is made up of lamination of steel alloys or magnetic iron, to minimize the eddy current losses.
Ans 19:
Because Single phase stator produces a nonrotating, pulsating magnetic field.
Ans 20:
1. Split phase induction motor.
2. Capacitor start inductor motor.
3. Capacitor start capacitor run induction motor.
4. Shaded pole induction motor.
Ans 4 :
Rather than have six leads from the three phase alternator, one of the leads from each phase may be connected to form a common junction. The stator is then called wye or star connected.
Ans 5:
n a DC machine, the main field is produced by field coils. In both the generating and motoring modes, the armature carries current and a magnetic field is established, which is called the armature flux. The effect of armature flux on the main field is called the armature reaction.
Ans 6: Because the power factor of load is not known while it is leading or lagging so transformers are rated in kVA.
Ans 7:
In general for the efficiency to be maximum for any device the losses must be minimum. Between the iron and copper losses the iron loss is the fixed loss and the copper loss is the variable loss. When these two losses are equal and also minimum the efficiency will be maximum.
Therefore the condition for maximum efficiency in a transformer is
Iron loss = Copper loss ( which ever is minimum)
Ans 8:
deally the voltage regulation voltage of a transformer should be zero. It means when you change the load from no load to short circuit (theoretically, normally you don't want to burn the transformer windings) the output voltage doesn't change and remains equal to the no load voltage.
Ans 9:
When ferranti effect occurs at light load conditions the load side voltage is higher than the supply voltage. Thus there will be a negatice V.R.
Ans 10:
Poor voltage regulation can damage the transformer therefore we use capacitive load to counter it.
Ans 11:
An electric motor converts electrical energy into a mechanical energy which is then supplied to different types of loads. A.c. motors operate on an a.c. supply, and they are classified into synchronous, single phase and 3 phase induction, and special purpose motors. Out of all types, 3 phase induction motors are most widely used for industrial applications mainly because they do not require a starting device.
A 3 phase induction motor derives its name from the fact that the rotor current is induced by the magnetic field, instead of electrical connections.
The operating principle of a 3 phase induction motor is based on the production of r.m.f.
Ans 12:
If total current supplied from source is Io, it will have one component in phase with supply voltage V1 and this component of the current Iw is core loss component. This component is taken in phase with source voltage, because it is associated with active or working losses in transformer. Other component of the source current is denoted as Iμ.
This component produces the alternating magnetic flux in the core, so it is watt-less; means it is reactive part of the transformer source current.
Ans 13:
Like any electric motor, a 3-phase induction motor has a stator and rotor. The stator carries a 3-phase winding (called stator winding) while the rotor carries a short circuited winding (called rotor winding). Only the stator winding is fed from 3-phase supply. The rotor winding derives its voltage and power from the externally energized stator winding through electromagnetic induction and hence the name acquired. The induction motor may be considered to be a transformer with a rotating secondary and it can, therefore, be described as a "transformer-type" a.c. machine in which electrical energy is converted into mechanical energy.
Ans 14:
The power factor at no load is low because the magnetizing component of input cur rent is a large part of the total input current of the motor. When the load on the motor is increased, the in-phase current supplied to the motor increases, but the magnetizing component of current remains the same. This means that the resultant line current is more nearly in phase with the voltage and the power factor is improved when the motor is loaded, compared with an unloaded motor which has its magnetizing current as a major component of the input current.
Ans 15:
starters are used in dc motor to minimize the high starting current and also protect the motor. starters are used in dc motor to minimize the high starting current and also protect the motor.
Ans 16:
Unlike transformer and generators the Motor has fixed power factor and also the ratings of motor has been mentioned in kw on the numberplate of all motors that is why motor is rated in KW or HP(kilowatts/horsepoer) instead of KVA.Motor.
Ans 17:
1. Used in both Synchronous motors and induction motors.
2. Used in electrical apparatus testing labs since the voltage can be smoothly and continuously varied.
3. They find application as boosters in AC feeders to increase the voltage levels.
Ans 18:
The stator core is made up of lamination of steel alloys or magnetic iron, to minimize the eddy current losses.
Ans 19:
Because Single phase stator produces a nonrotating, pulsating magnetic field.
Ans 20:
1. Split phase induction motor.
2. Capacitor start inductor motor.
3. Capacitor start capacitor run induction motor.
4. Shaded pole induction motor.
Ans 21:
The most important uses and application of Transformer are:
· It can rise or lower the level of level of Voltage or Current ( when voltage increases, current decreases and vice virsa because P =V x I, and Power is same ) in an AC Circuit.
· It can increase or decrease the value of capacitor, an inductor or resistance in an AC circuit. It can thus act as an impedance transferring device.
· It can be used to prevent DC from passing from one circuit to the other.
· it can isolate two circuits electrically.
Ans 22:
In practical cases power circuits such as lighting require a constant voltage. Shunt and series generators cannot be used for such situations.Because, as load current of shunt generators increases its terminal voltage falls and as load current of series generator increases the terminal voltage also increases .
In compound generators both series and shunt windings are placed in each field pole .The series winding so connected that flux produced by it aids the flux produced by the shunt winding. (Such generators are called Cumulatively compounded generators).So as load current increases , flux produced by series winding compensates the loss of flux due to armature reaction.Then the terminal voltage of the generator remains constant.
Ans 23:
If voltage remains constant flux will decrease and more current will flow.
Ans 24:
Thee core will go in saturation region and voltage will produce.
Ans 25:
Because there are separate branches for the flow of armature current and the field current in DC shunt Motor
Ans 26:
When a DC motor is initially energized, there is no back emf because the rotor is not turning. That means that the motor will draw very high current. If it is a small battery-powered motor, there is not much problem caused by the brief surge of high current while the motor gets up to speed. If the motor is larger, the high current can damage the commutator, the windings or the power supply equipment. Therefore, a starter is needed to insert a current limiting resistor or sequence of resistors until the motor gets up to speed.
Ans 27:
It indicates
· Manufactures Name
· Rated Volts And Full Load Amps
· Rated Frequency
· RPM
· Synchronous speed
· Torque
· Locked Rotor Indicating Code Letter
· Bearing Part Numbers,etc
The most important uses and application of Transformer are:
· It can rise or lower the level of level of Voltage or Current ( when voltage increases, current decreases and vice virsa because P =V x I, and Power is same ) in an AC Circuit.
· It can increase or decrease the value of capacitor, an inductor or resistance in an AC circuit. It can thus act as an impedance transferring device.
· It can be used to prevent DC from passing from one circuit to the other.
· it can isolate two circuits electrically.
Ans 22:
In practical cases power circuits such as lighting require a constant voltage. Shunt and series generators cannot be used for such situations.Because, as load current of shunt generators increases its terminal voltage falls and as load current of series generator increases the terminal voltage also increases .
In compound generators both series and shunt windings are placed in each field pole .The series winding so connected that flux produced by it aids the flux produced by the shunt winding. (Such generators are called Cumulatively compounded generators).So as load current increases , flux produced by series winding compensates the loss of flux due to armature reaction.Then the terminal voltage of the generator remains constant.
Ans 23:
If voltage remains constant flux will decrease and more current will flow.
Ans 24:
Thee core will go in saturation region and voltage will produce.
Ans 25:
Because there are separate branches for the flow of armature current and the field current in DC shunt Motor
Ans 26:
When a DC motor is initially energized, there is no back emf because the rotor is not turning. That means that the motor will draw very high current. If it is a small battery-powered motor, there is not much problem caused by the brief surge of high current while the motor gets up to speed. If the motor is larger, the high current can damage the commutator, the windings or the power supply equipment. Therefore, a starter is needed to insert a current limiting resistor or sequence of resistors until the motor gets up to speed.
Ans 27:
It indicates
· Manufactures Name
· Rated Volts And Full Load Amps
· Rated Frequency
· RPM
· Synchronous speed
· Torque
· Locked Rotor Indicating Code Letter
· Bearing Part Numbers,etc
Ans 28:
Open circuit test or no load test on a transformer is performed to determine 'no load loss (core loss)' and 'no load current I0'.
While we do impedence test or short circuit test to find copper loss.
Ans 29:
There is slight change in curve of voltage regulation in Separately Excited dc Generator but it cost effective.
While there is a rapid change in Shunt dc generator .
Ans 30:
1. Synchronous motors are inherently not self starting. They require some external means to bring their speed close to synchronous speed to before they are synchronized.
2. The speed of operation of is in synchronism with the supply frequency and hence for constant supply frequency they behave as constant speed motor irrespective of load condition
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