Sunday, 18 February 2018

Widely used Application In The Work For Starting Of Motor!!??

In Real Life Widely  Useful Application Is Three Phase Starter,  How Its Work And How Its Really  Operating In Real Life




Function Of Starter,  Working Principle Of Starter And Full Detail About The Three Phase Starter.



Function of starter 


The device which is used to start the d.c. motor is called a Starter .The function of the starter is to limit the starting high current to a safe value.






NECESITY OF A STARTER


EMF equation for Armature Circuit
E =V - IR


The current drawn by a Motor Armature is
I=(V - E) /R


In case of d.c. motor the back emf develop is directly proportional to the speed because
E =(§ZNP) /(60A)






When the motor is at reast the speed of the motor is zero hence back emf Eb will be zero so if a d.c.

motor is connected directly to the supply main it will drawn a very Large Current.


The current drawn by the aature becomes
I=V/R




  • The resistance of the Armature Winding r, is very small (hardly 1 to 2 ohm), as such the current flowing in the armature winding under starting condition becomes quite large. 
  • The Starting Current may be 5 to 7 times that of full load current. 
  • The Starting Current drawn by the de motor can be limited to safe value either by using a starter or by inserting a variable rheostat in series with the armature circuit as shown in Figer.

  •  The Starter is used only for the short duration (5 to 10 sec) which limits the starting high current to a safe value.


Very small d c. motors may be started directly from the supply lines because of the follawing reasons


(1) Relatively higher armature resistance R compared to large motors and
(2) Law moment of intertia.
The dc shunt ar compound motor is started by the following two types af starters:
(1)Three pointt starter and
(2)Four paint starter


in figer shows a wiring diagram of a d c. three point starter.

It consists graded starting resistances, rs which is connected in series with armature, two protective devices namely no volt and overload release, starting arm and the brass arc segment s.




Generally three terminals of the Starter L F and A are brought out, which are connected to the positive line terminal, shunt field and armature terminal of the motor respectively.

  • For starting the motor, the dc supply is switched on by closing the switch and then the starting arm is slowly moved to the right (clockwise). 
  • As soon as the suarting arm makes contact with stud and brass are segment s, whole of the starting resistance r.
  •  inserted in series with the armature and at the same time the circuit is directly connected across the supply through the segment s and the holding coil (NVC) is also energized.

Therefore the current drawn by the armature at this instant will be
I=V/(Ra+Rs)


As the Starting Arm is further moved, the starting resistance r, gradually cut out and when the arm reaches the RUN position, the entire starting resistance is cut out. No volt and overload release coils are the protective devices provided in the starter.


Questions  Related To Three Phase Starter :

1) give thr Function of starter ???
Ans:The device which is used to start the d.c. motor is called a starter.
The function of the starter is to limit the starting high current to a safe value.


2)When the motor is at reast the speed of the motor is zero hence back emf Eb will be ________????
Ans: zero


3)The starting current may be _______ that of full load current. ??
Ans:5 to 7 times


4)The dc shunt and compound motor is started by the how many types af starters and which????? 
Ans : Two
(1)Three pointt starter and
(2)Four paint starter


5) The starter is used only for the short duration ______ time which limits the starting high current to a safe value.???
Ans:the short duration.



May You Like To Read This Related Articles :


0 comments: