Working principle ,construction , types and preventive of DC generator

 

DC generator

the DC generator is a  rotating machine . which is convert mechanical energy into electrical energy at form of dc emf  is called dc generator .

operating principle of dc generator

first of all the DC excitation  that is the supply of dc current to  is given to the field winding of the machine  and magnetic flux is produced in the machine . the armature of the machine is rotate by prime movers. now the rotating armature conductor are cut by the magnetic flux in the machine and according to faraday law of electro magnetic induction  emf or voltage  is induced . the induced emf or voltage at form of ac which is finally converted in to dc by commutator of machine and this appeared across the carbon brash.

Construction of DC generator

construction of dc generator

the dc machine is a rotating electrical machine which may be either generator or motor. if input  is mechanical and output is electrical  then the dc machine is simply called generator . if input is electrical and output is mechanical then the dc machine is called motor . the dc machine has following part .

Construction of DC generator

yoke 

the  yoke is the outermost frame of machine which acts the protecting  cover for the whole machine that means its gives mechanical  protection for whole machine .

field pole

that part of dc machine in which field winding is done and dc excitation is given.

armature 

the armature is the rotating part of the machine which has armature conductor  that is armature winding , armature shaft etc .

commutator  

the part of dc machine which convert the AC voltage generated in armature  into DC voltage which appears across the carbon brush . so it is also called mechanical rectifier.

carbon brash 

the part of dc machine which connected  the external load to the commutator

Type of dc generator

The following are the type of dc generator

Separately excited dc generator

2)     Self-excited  dc generator

1)Separately excited dc generator

The dc generator machine there is no electrical connection between armature winding and filed winding is called separately excited dc generator dc generator shown in figure

V_dc= dc voltage

I_f = field current

R_f  = field resistance

I_a = armature current

R_{a =}  armature resistance

E_{g =}  ZQNP/60A

   = generated voltage

I_L    = Current through the load

R_l = load resistance

V= terminal voltage

Applying KVL to the armature circuit

E_g - I_aR_a- v=0

E_g=I_aR_A+V

In the field circuit

I_f =v_dc/R_f

2) self excited  dc generator

The dc generator machine there is  electrical connection between armature winding and filed winding is called separately excited dc generator dc generator . the self excited dc generator is the following three type

a) shunt generator

b) series generator

c) compound generator

shunt generator

                                  the dc generator in which the field winding is connected in parallel with the armature winding is called shunt generator .the circuit diagram for shunt generator is shown as shown in the figure

I_f= field current

R_f = field resistance

Ia = armature current

Ra =  armature resistance

 E_g  = generated voltage

I_L    = Current through the load

R_L = load resistance

V= terminal voltage

Applying KVL to the load circuit

Eg - IaRa- v=0

Eg=IaRa+V

This is the relation between emf and terminal voltage also

I_f= V/R_f

Ia = I_f+ I_L

Series generator

                                  the dc generator in which the field winding is connected in series with the armature winding is called series generator .the circuit diagram for series generator is shown as shown in the figure

I_f= field current

R_F = field resistance

Ia = armature current

Ra =  armature resistance

 Eg  = generated voltage

I_L    = Current through the load

R_L = load resistance

V= terminal voltage

Applying KVL to the load circuit

Eg – IaRa-I_FR_F- V=0

Eg=IaRa+V+ I_FR_F

This is the relation between emf and terminal voltage also

Ia = If= IL

Compound generator

The dc generator in which there is two step of field winding such as series field winding and shunt field winding is called compound generator the following are the two type of compound generator .

1)     Long shunt compound generator

2)     Short shunt compound generator

 

Long shunt compound generator

The compound generator in which the shunt field winding is connected across the load while the series field winding is connected in series with the armature is called long shunt compound generator . the circuit diagram of long shunt compound generator is shown in figure .

R_SE= resistance of series field winding

R_SH= resistance of shunt field winding

I_F   = field current

I_L    = load current

R_a = armature resistance

I_a = armature current

E_g= generated voltage

V = terminal voltage

Applying KVL to the load circuit

E_g-I_aR_a-I_aR_se-V=0

E_g= IaRa+IaRse+V

This is the required relation between generated voltage and terminal voltage for long shunt compound generator .

I_f=V/R_sh

I_a=I_f+ I_L

Short  shunt compound generator

The compound generator in which the shunt field winding is connected across the armature while the series field winding is connected in series with the load is called short shunt compound generator . the circuit diagram of short shunt compound generator is shown in figure .

R_se= resistance of series field winding

R_sh= resistance of shunt field winding

I_f   = field current

I_L   = load current

Ra = armature resistance

Ia = armature current

Eg= generated voltage

V = terminal voltage

Applying KVL to the load circuit

Eg-IaRa-IseR_se-V=0

Eg= IaRa+IseR_se+V

Applying KVL to the field circuit

Eg-IaRa-IshR_sh=0

I_a = I_L+I_F

( note  I_L=Ise AND I_{F  =} Ish )

 a) shunt generator ( load characteristics )

the characteristics curve which shows the variation of voltage across the load with the load current is called load characteristics of generator . if there no load on the generator then armature current equal  to the field  current which is very small so , the drop in armature is also very small which can be neglected  so the terminal voltage is equal to the generated emf but when load is connected then the I_a=I_L +I_{F    so the drop in the armature is also increase  load hence the terminal voltage decrease , thus the shunt generator has dropping voltage characteristics the nature of curve shown in figure.}

characteristics of dc series generator

the characteristics curve which shows the variation of terminal voltage with the load current is called loaded characteristics of series generator is shown .

if the load current increases then the value of field current also increase due to this magnetic flux Q increasing and hence the generated voltage is increase and finally terminal voltage increase so the series generator has rising voltage characteristics curve .

                    characteristics of dc series generator

PREVENTIVE MAINTENANCE

Preventive maintenance  prevent the breakdown by timely inspection lubrication ,adjustment, and repair . preventive maintenance is maintenance carried out before the breakdown  occur.

its tries to minimize problems of breakdown maintenance ,which means problems like excessive delay in production .time loss and productivity loss etc.

the maintenance engineer locates the weak spot like bearing ,part under vibration , part under high temperature etc. then inspect , repair there by reducing the danger of breakdown .

OBEJECTIVE OF PREVENTIVE MAINTENACE 

1)to locating any condition which may lead to a breakdown.

2) to make the machinery and equipment always ready to use and maintain then optimum production efficiency.

3)to found maximum productivity by the minimum repair cost ensure the safely  of works.

ADVANTAGE

1) reduced breakdown and repair time.

2) low maintenance cost

3) better production quality and lower unit cost of manufacture. 

4)no delay in delivering the product to the customer .

DISADVANTAGE

1)preventive maintenance required more workers compare to breakdown maintenance.

2)cost of preventive maintenance is high due to regular inspection of equipment.

preventive maintenance of generator

why we do to preventive maintenance 

1)reduction in  generator failure and breakdown.

2) increasing the efficiency of generator .

3)reduction   generator  operating  and maintenance cost.

4)safely of the generator set.

type of preventive maintenance of generator

A- check

B-check

C-check

D-check

A-CHECK -A-check is done on daily basis such as check fuel level, check  tube oil level, check coolant level, check battery voltage, check V- belt condition, check leakage , check all filter condition

B-CHECK

B- check is done after (300 t0 350) hours of cycle or six months whichever is earlier. process of B-check flow below point

 perform the A- check 

change the tube oil

change the fuel filter

change the tube oil filter

change the air filter

change the water separator filter

radiator cleaning 

change the coolant level if its low than add the coolant
 

C-CHECK

c-check is done after (800-1500)  hrs of cycle or 3 year which is earlier . perform A-check and B-check and check  all terminal ,thimbles ,insulation of alternator winding and pollution level

D-check 

d- check is done after (3000-5000) hours of cycle in this check  replace the nozzles add piston bearing , piston cylinder also need to be change, serving of fuel pump, cylinder head and injector pin.