Self Excited or DC Shunt Generator with circuit Diagrams

     

    In this type of dc machine, the field winding is connected parallel to the alternator. The simple parallel connection plays a major role in voltage built up in the shunt generator. we will see about what are the interesting things happen in magnetization and load characteristics of self excitation machine.

 Applying kirchoff voltage law to the circuit,

 Vt = Ea - IaRa - Vb

Vt- Terminal voltage

Ea- generated or induced voltage

Ia- Armature current

Ra- Armature resistance

Applying kirchoff current law,

Ia = IL + Ish

    Practically, there will be voltage drop due to brush but it may be around one or two volt per brush. So, we neglect the Vb.

Vt = Ea - IaRa

Observing the shunt generator, we can classify into open circuit or magnetization characteristics and terminal characteristics

Open circuit or magnetization characteristics of dc shunt generator

    In this case, the load of the dc machine is removed. At this point the load current flowing through the circuit is zero.(IL= 0)

    We can say,

Ia=Ish

Ea = Vt - IshRa

    The terminal voltage is equal to the field winding of the machine. because it is connected across the armature. When the rotor is rotated, the residual magnetic flux present in the poles induces emf in that armature.

    Because of the induction of emf, there will be minimum amount of current flowing through the field winding and this current aid the magnetic flux in the air gap. So, the voltage develops like a loop up to the saturation of point.

    The minimum value of resistance to excite the shunt field is known as critical resistance. If the resistance of field winding is beyond the critical resistance there will be no further increase in induced emf.

Rsh < Rcritical

The shunt resistance should less than the critical resistance.

Terminal or load characteristics of dc shunt generator 

    In practical if we increase the load, the demand of load current is high results in decrease in the terminal voltage Vt. because of armature voltage drop IaRa across the generator. Under no load conditions the terminal voltage is nearly equal to the induced emf because of IaRa drop on no load is small.

    The drop due to demagnetization effect of armature reaction. Because of increase in load current causes the flux weakening there will be reducing magnetic flux.

    To neglect the armature reaction the compensating winding is provided in between the field winding of self excited machine.

    The load current increases with decrease in terminal voltage due to decrease in field current Ish. In separate excited dc generator the field current is kept as constant in case of self excitation field current is variable.

Problem

    For example,

    A 10kw 250v dc shunt machine has armature field resistance of 0.1 ohm and 125 ohm.

    If we analyse the total armature power,

Ia = IL + Ish

Ish = 250/125 = 2A

IL = 40A

Ia = 42A

Ea = Vt + IaRa

             =250 + (42*0.1)

Ea = 254.20 volt

Conclusion

Notice that only field current is 2A and load current is 40A

The total Armature power developed Pa = 10.67 KW

comments

What is the advantage of shunt machine while working as motor?

Separately Excited DC generator with Diagrams

  

    The name separately excited tells you, The field winding is excited by an external independent DC power supply.i.e, There is no electrical connection between armature winding and field winding.

    The DC generator has no load and load characteristics. The load characteristics is again classified by internal and external characteristics.

Open Circuit or magnetization or No load characteristics of separately excited dc generator

    Consider the load is removed from the dc generator. In this situation the load current (IL=0).

By Kirchoff voltage law, We can write the equation,

Vt = Ea - IaRa

Vt- Terminal voltage

Ea- generated voltage

Ia- Armature current

Ra- Armature resistance

    Practically, there will be voltage drop across the brush in the range of one or two volt per brush. So, it is neglected to get a round off  voltage. 

    Even though the field current is zero, There will be some emf present in armature. Because of residual magnetism in the poles.

From the emf equation of DC generator,

 we can say generator voltage is directly proportional to the speed of the Electrical Machine.

Ea α N

    At No load condition, generation of voltage across the dc generator varies with field current at fixed RPM N1,N2,N3.The graph shows that for the generation of voltage is increased with increase in fixed rpm for different values.

Load characteristics:

External characteristic (I vs Vt)

The plot between output parameters terminal voltage vs load current gives terminal characteristic. 

When the terminal voltage Vt decrease with increase in demand of current from the more load. Because of drop across the armature ohmic drop. There is No load at terminal the Load current is Zero (Ia=0).

Vt = Ea - IaRa

Vt = Ea

Internal characteristics (Ia vs Ea)

    We all know that compensating winding is provided in between the poles for neglecting Armature Reaction. 

    Consider dc generator is not present with compensating winding.If the load current increases IL= Ia for separately excited dc generator. The armature reaction causes flux weakening, there will be reducing flux so the terminal voltage will be decreases.

Conclusion

    The Major advantage of the separately excited machine is separate DC power supply to field winding.This is more stable than other machine when we operate.  

   For example, The separately excited dc generator has a terminal voltage of 240v and induced emf of 250v. If Ra = 0.1 ohm, What will be full load current and output power?

EMF equation of DC Generator

The generator of emf in dc machine is contributed by the faraday's law of electromagnetic induction.

    Whenever the change in magnetic flux in the coil, the emf is induced in the coil.

Practically the rate at which the conductor is cutting the magnetic field induces emf in that coil.

The total magnetic flux cut by armature conductor = flux per pole × no of  poles

                                                                                 = P × Փ

p =  Number of poles

Փ = Magnetic flux per pole

N is the speed of the armature conductor in revolution per minute (RPM)

We can take n revolution in one minute or 60 seconds

                                N number of Revolution = 1 minute = 60 seconds

The time required to take one revolution     

1 revolution = 60/N seconds

The emf  induced per coil,

Z = Total no of conductors

A =  Number of parallel paths

Consider A number of parallel paths in armature and Z number of conductors. So, Z/A gives the number of conductors in each path.

In Lap winding,

            Number of parallel paths = Number of poles in dc generator

                                                A=P

Replace A=P

 In Wave winding,

    Number of parallel paths is always two 

For example, 6 pole dc machine having 480 conductors driven at a speed of 1200 rpm and  flux per pole is 0.012 wb

case 1

    Consider a lap winding

     A=P

case 2

    Consider Wave winding

    

    Number of parallel paths is always two

    A=2

Conclusion

    In dc generator emf is generator by converting mechanical energy into electrical energy is the other hand the have to give supply to dc motor in this case emf generator in the armature opposes the supply voltage called as back emf Eb

Comments 

    Is Back emf Eb equal to generated emf in dc generator?

Principle of Operation of DC Generator in Easy Ways

 

    The first electromagnetic generator was invented in 1831 by British scientist Michael Faraday. The reverse operation of  electric DC generator is DC motor.

Laws contributing in dc generator

    We have to understand the concept of laws contributing in dc generator before understand the operation of dc generator.

    Wherever the change in magnetic flux near the coil, the emf is induced in that coil. This is called as faradays law of electromagnetic induction.

    Just take a piece of bar magnet and the LED circuit with coil. If we move the bar magnet front and back near the coil, the LED will be glowing.

 

Fleming right hand rule

    In general, the Flemings right hand rule in defines the operation of dc generator. The thumb represents the direction of force in conductor.

    The fore finger represents the direction of magnetic field and the middle finger represents the direction of current in the coil.

 

Operation or dc generator

    In dc generator, we should rotate the shaft of armature through Non-conventional sources. The Armature shaft having conductors converts mechanical energy in to electrical energy. 

The armature conductors are going to rotate and magnetic field and at stationary so is known as dynamically induced emf.

    The emf induced in the conductor is minimum when the conductor is parallel to the magnetic field.

    

 e=0

    

The conductor in inclination to the magnetic field.

 

e=-nblvsinƟ

 

    The negative sign is derived from Lenz law. The induced emf produces current in the circuit always opposes the rate of change of flux. Ɵ is an angel between flux and conductor and v is the velocity of the conductor.

    The maximum emf is induced in the conductor when the conductor is perpendicular to the magnetic field.

e=-nblv

 

Consider a simple dc generator with permanent magnet. The coil is placed in between the permanent magnet. The magnetic field lines are from north to south pole. The coil is having two conductor AB and CD. The conductor AB is always connected with S2 and B2 likely the conductor CD is always connected with load.

    consider the generator armature is always rotated in the clockwise direction.

 

Positive cycle

    When the conductor is parallel to the magnetic field there is no magnetic field cut by conductor so, emf is zero in the conductors.

    The conductors are inclined to the magnetic field partially the magnetic field is cut by conductors so, there is induced emf in that conductors according to Fleming right hand rule. The current is flowing from the conductor ABCD to load through B1 B2.

    When the conductor is perpendicular to the magnetic field the maximum emf is induced in the conductors. The current is flowing from ABCD the conductor to load through B1 B2.

 

Negative cycle

    Remember the conductors are always connected to the slip rings S1 and S2. In this case the emf is zero because of no magnetic fields are cut by conductors.

    When the conductors are in inclined angle partially the emf is induced in the conductors but, the current is flowing from the conductors DCBA to load through B2 B1. 

    The emf is maximum when conductor is perpendicular to magnetic field likely the current is flowing from the conductor DCBA to load through B2 B1.

    The number of cycles per second called hertz.

 

Conclusion 

    There is no major difference between the operation of dc generator and dc motor. The mechanical energy is converted in to electrical energy vice versa. 

 

Comments

     Which material is used in brush? Why?

    

 

 

 

    

 

    

 

Basic Construction of DC Generator and it parts

    

    Do you know? There is no major difference between construction of DC generator and DC motor. In general, DC generator is a machine which converts Mechanical energy into Electrical energy. In motor the operation is vise versa.

Lets talk about the Constructional Features of DC generator. The main parts of generator are discussed one by one.

image credit: Gillett&#039, distributed under a CC BY 2.0 licenses.

Yoke

image credit: Toshinori baba, distributed under a CC BY-SA 3.0 licenses.

    The outer body or frame of the dc generator is known as yoke. The inner side of the yoke is fixed with field poles for creating the magnetic flux in the air gap.

    In general, the yoke serves the two main function

    It should give mechanical support for the field poles and protect the dc generator from surroundings. On the other hand, it provides the magnetic flux produced by the electromagnetic poles. The material used for making yoke is cast iron or cast steel

 

Pole shoe and pole core

    The arrangement of magnetic field winding is provided by inner part of yoke is known as pole shoe. By the help of former the field winding is wound in exact number of turns and then fixed with pole shoe. They spread the flux in the air gap and reduce the reluctance. 

    The pole core and pole shoe are made with lamination of steel to reduce the losses. The thickness is varied from 1 to 0.25 mm. 

 

Field coil

    To create the magnetic field between the poles the electromagnetic field coils are implemented. These coils are usually made up of copper to carry the current.

     First of all, the winding is made over the former for correct dimension and number of turns and the former wound is put over the pole cores.

 

Armature core

    The main part of dc generator is armature. the armature consists of number of conductors placed in the slots to produce the electricity when the magnetic fields are cut by armature conductors.

     It is drum shaped with laminated core. The Armature is perforated with air duct which allows the flow of air through the armature for reducing temperature.

     We can see the keyway in the armature for fix the shaft. We can reduce the losses in armature by increasing multiple number of laminations. Because of increasing resistance by decreasing the thickness of laminations.

 

Armature winding

    These are current carrying conductors when the conductors cut magnetic field produces the direction of current by Fleming right hand rule. It could also former wound and then wound on armature by automated coil puller machine. 

    The windings are made with two methods lap and wave windings. In lap winding number of paths is equal to number of conductors in wave winding the parallel path is always two.

     conductors are wound form the front end of commutator through armature. The conductors are placed in the armature slots which are wound with insulating material.

 

 Commutator

image credit: Rs1421, distributed under a CC BY-SA 3.0 licenses.

    The one of the important parts in the dc generator in commutator. The function of commutator is to facilitate collection of current from all the conductors wound on the armature. So, we can realize the number of coils is equal to number of segments.

     it converts the alternating current induced in the armature conductors in to unidirectional current in the external current having load. It is cylindrical shaped and built up with wedge shape of high conductivity.

     The segments are insulated from each other by the layers of higher resistance like mica.

 

Brushes and bearings

    The Brushes are used to collect current from the commutator. They are made with carbon or graphite and it is rectangular structure.

    The roller bearings are frequently employed for higher reliability. The ball and rollers are packed in hard oil for reduced bearing wear.

 

Conclusion

    Construction of dc motor is basic prerequisite for further with operations and winding methodology.

 

Comments

    Why the armature core is laminated in all DC generator?