it is all about automatic dc stater

1. SUBJECT:- Elements of Electrical design
TOPIC:- Automatic D.C. Starters
PREPARED BY :
Vijay Helaiya (150110109014)
Ravi Jethva (150110109016)
Yash kothadia (150110109019)

2. At starting ,the speed of motor is zero so that the back e.m.f.
In the armature is zero.
Armature resistance is so low, if it is connected to power
supply directly ; huge current will pass through armature.
The huge current may damage the machine or armature coil.
Ia = V/Ra
NECESSITY OF STARTER

3. FUCTION OF STARTER
 Start and stop the motor.
 Limit inrush current where necessary.
 Permit automatic control when required.
 Protect motor and other connected equipments
from over voltage, no voltage, under voltage etc.

4. TYPES OF AUTOMATIC STARTER
(For DC motor)
1. Back EMF Starter
2. Time Delay Starter

5. BACK EMF STARTER

6. 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.

7.  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 –PB2-PB1- CM –
Negative terminal(-)

8.  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.

9.  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 across 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.

10.  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 continuous
operation.

11. TIME DELAY STARTER

12. 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:
Positive 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.

13.  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.

14.  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.