Transcript active learning assignment topic name :

ACTIVE LEARNING ASSIGNMENT
TOPIC NAME :- DC MOTOR STARTERS
PREPARED BY:DEEP CHOTAI (130110109007)
SHREYANSH JETHWA (130110109017)
KHILAN JOSHI (130110109018)
ELEMENTS OF ELECTRICAL DESIGN (2150904 )
Course co-ordinator:SAMIR B PATEL
AY-2015-16
THREE POINT STARTER
 It consists of graded starting
resistances, rs which is connected in
series with armature.
 It also consists of two protective
devices namely no volt and
overload release , starting arm and
the brass arc segment s.
 Generally three terminals of starter
L,F & A are brought out, which are
connected to positive line terminal ,
shunt field & armature terminal of
the motor respectively.
CONTINUE
FUNCTION OF NO VOLT RELEASE COIL (NVC)
 When the handle is in the 'RUN' position, soft iron piece connected to the handle gets
attracted by the magnetic force produced by NVC. Design of NVC is such that it
holds the handle in 'RUN' position against the force of the spring as long as supply to
the motor is proper. Thus NVC holds the handle in the 'RUN' position and hence also
called hold on coil.
 NVC consists of a thin wire of many turns & is connected in series with the shunt
field winding of motor. The coil is magnetized when the current flows through the
shunt field winding .
 NVC performs the similar action under low voltage conditions and It releases the
handle and goes to OFF position by spring tension due to the failure of supply and
disconnects the motor from supply.
FUNCTION OF OVERLOAD RELAY COIL (OLC)
 It’s function is to demagnetize the NVC in case of fault or overload of the machine
OR protect the motor from overload.
 It consists of a few turns of thick wire & is connected in series to armature . This coil
is magnetized when excessive current flows through armature due to overload or
some fault & it attracts the tripping plunger C , which is short circuited the terminal of
NVC.
 NVC will be demagnetized & release the starting arm which will come to OFF
position due to spring tension & thus motor stops.
DISADVANTAGE OF 3 POINT STARTER
 In this starter, the NVC and the field winding are in series. So while
controlling the speed of the motor above rated, field current is reduced
by adding an extra resistance in series with the field winding. Due to
this, the current through NVC also reduced. Due to this, magnetism
produced by NVC also reduces. This may release the handle from its
RUN position switching off the motor. To avoid the dependency of
NVC and the field winding, four point starter is used, in which
NVC and the field winding are connected parallel.
FOUR POINT STARTER
4 POINT STARTER
 Figure shows a wiring diagram of it
which is connected to a long shunt
compound motor.
 In this starter NVC is connected directly
to the line through protective resistance.
when starting arm touches stud no. 1
line current divides into 3 parts:
1) one part passes through starting
resistance Rs, series field & armature
2) Second part passes through shunt field
winding.
3) Current passes through the NVC &
protective resistance r.
CONTINUE
 With this arrangement , NVC is independent of shunt field circuit,
therefore it will not be affected by the change of current in shunt field
circuit , it means electromagnetic pull exerted by NVC will always
be sufficient & will prevent the spring from restoring the starting
arm to OFF position.
 The possibility of accidentally opening the field circuit is quite
remote; hence greater acceptance of 4 point starter over 3 point
starter.
BACK EMF STARTER
 Figure shows a wiring diagram and control circuit of a back emf type
automatic starter used for starting d.c. shunt and compound motors.
 It consists of start and stop push buttons PB1 and PB2, M and MA are normally
open contacts, which are actuated by the main operating coil CM, a,b, and c are
normally open contacts, which are actuated by the operating coils Ca,Cb and
Cc respectively. r1,r2 and r3 are starting resistances.
 MA is the auxiliary or maintaining contact which is used to keep the operating
coil CM energized after the finger is removed from the ‘START’ push button
PB1.
BACK EMF STARTER
 When the START push button PB1 is
pressed, the circuit is completed from
positive terminal through L, normally
closed contact OL,PB2,PB1, main
operating coil CM to the negative terminal
of the line.
 I.e. Positive terminal(+) – L – OL –PB2PB1- CM –Negative terminal(-)
BACK EMF STARER
 Thus, the operating coil CM is energized and closes its contacts M and MA.
Closing of contacts the armature of the motor to the supply through full starting
resistance in series and field circuit directly across the supply, thus the motor
accelerate.
 As the speed of the motor increases, its back emf increases, which is available
across the coils Ca,Cb and Cc. When the back emf become equal to the
operating voltage of the coil CA, it is energized and closes its contact ‘a’, thus
cutting down the starting resistance r1 of the first section. The motor further
accelerates causing the back emf to further increase.
BACK EMF STARTER
 When the back emf reaches the operating voltage of the coil Cb, it is energized and
closes its contacts b, cutting down r2. similarly contact ‘c’ get closed after sufficient
increase in back emf is achieved. Now, all the starting resistances are cut down and
the motor armature is connected directly accross the supply.
 When the STOP push button PB2 is pressed, the supply through the contactor coil
CM is disconnected. Sinnce the operating coil CM is de-energized, the contacts M
and MA are opened, the supply to the motor is disconnected and the motor stops.
 Also in case of an overload on the motor, overload relay coil OLC is energized and
opens the contact OL. Thus the coil CM is disconnected from the supply and
therefore disconnect the motor from the supply.
DISADVANTAGE OF BACK EMF STARTER
 The disadvantage of back emf type starter is that the closing of contacts a,b
and c connected across starting resistances r1, r2 and r3 depends upon the
amount of back emf developed by the motor.
 Hence, in case, if the accelerations of the motor and hence building of back
emf is delayed due to load condition, the starting resistance will continue to
be in line for a prolonged period and may get damaged due to overheating as
they are not designed for continupous operation.
TIME DELAY STARTER
 This starter overcomes the drawback of the back emf starter. In this section
resistances are cut out at definite time intervals. Figure shows the wiring
diagram and control circuit of automatic time delay type d.c. starter.
 When the START push button PB1 is pressed, the operating coil CM is
energized through the following path:
Posive terminal (+) – OL – PB2 – PB1 – CM – Negative terminal (-)
 Thus, closes its contacts M and MA. The closing of contacts M connect the
armature of the motor across the supply, thus the motor across main supply
through full starting resistances and field circuit directly across the supply,
thus the motor starts rotating.
TIME DELAY STARTER
 The closing of contacts MA energizes the coil
Ca which closes its contacts ‘a’ after a pre-set
time delay. Hence the first section resistance r1
is cut out. At the same time, coil Cb is
energized and after a pre-determined time
delay, closes its contact ‘b’, short circuiting the
starting resistance r2 and also energizing the
coil Cc.
 As the coil Cc is energized, the contact ‘c’ is
closed and cut out the starting resistance r3
after pre-set time delay. In this way the entire
starting resistance is cut out and the motor
armature is connected directly across the
supply.
TIME DELAY STARTER
 The protection against overload is provided by the overload relay OLC
which is connected in series with the motor armature. In case of an
overload on the motor, overload relay coil OLC is energized and opens
the contact OL. Thus disconnect the motor from the supply.
 Pressing the STOP push button PB2, de-energizes all contactors and
thus the supply to the motor is disconnected and the motor stops.