Dc motor starters

unit iii,mechanical branch,electrical drives and control

1. Unit III
Starting methods

2. DC MOTOR STARTERS
Voltage equation of a DC motor is
V = E
b+ I
aR
a
I
a = (V -E
b) /R
a
A large current flows through the armature
during starting.
This current is large enough to damage the
armature circuit.

3. DC Motor Starters
3 point starter,
4 point starter,
2 point starter
No-load release coil starter,
Thyristor controller starter .

5. 3 Point starter
'L' Line terminal. (Connected to positive of
supply.)
'A' Armature terminal. (Connected to the
armature winding.)
'F' Field terminal. (Connected to the field
winding.)

6. Draw backs of 3 Point starter
To increase the speed of the motor field
resistance can be increased.
Therefore current through shunt field is
reduced.
Field current becomes very low which results in
holding electromagnet too weak to overcome the
force exerted by the spring.
.

7. The holding magnet may release the arm of the
starter during the normal operation of the
motor and thus disconnect the motor from the
line
This is not desirable. A four point starter is thus
used.

9. 4 Point starter
L' Line terminal. (Connected to positive of
supply.)
'A' Armature terminal. (Connected to the
armature winding.)
'F' Field terminal. (Connected to the field
winding.)
Like in the case of the 3 point starter, and in
addition to it there is, a 4th point N. (Connected
to the No Voltage Coil NVC)

10. 4 Point starter
Theremarkabledifferenceincaseofa4point
starteristhattheNoVoltageCoilisconnected
independentlyacrossthesupplythroughthe
fourthterminalcalled'N‘.

11. 2 Point starter

12. 2 Point starter
The start arm is simply moved towards right to
start the motor.
Thus, maximum resistance is connected in series
with the armature during starting
Then gradually decreased as the start arm
moves towards right.
This starter is sometimes also called as a 2 point
starter.
The no load release coil holds the start arm to
the run position and leaves it when the voltage is
lost.

13. Starters for three phase induction
motor
Directly switched ON, motor take 5 to 7 times of
full load current
Damage the motor
High voltage drop in the line
So, starters are necessary

14. Types of induction motor starters
1.DOL starter
2.Primary resister starter
3.Autotransformer starter
4.Star delta starter
5.Rotor resistance starter

15. DOL STARTER

16. PRIMARY RESISTANCE STARTER

17. Torque vs peed Curves as Function of
Voltage

18. STAR DELTA STARTER

19. Star connection of motor

20. Delta connection of motor

21. Started with star connection
Run with delta connection
Reduced voltage applied at the time of starting due
to star connection
Full voltage applied to the motor ,when rotor speed
reaches 80% of rated speed with delta connection

22. AUTO TRANSFORMER STARTER

25. Operation of autotransformer
Handle at start position
The motor is connected through the auto
transformer
Reduced voltage is applied
Starting current reduced
When the motor gets of 80% of the normal
speed,handle is moved to RUN condition
Then the motor receives full line voltage

26. advantages
Reduced line current
Smooth starting
High acceleration

27. Disadvantages
Higher cost
It is used large rated motor only

28. Over load protection(OLR)
Whenever motor overloaded
The OLR(over load relay coil) is energised
Hence the supply switched OFF

29. Low voltage protection
When supply voltage goes low or falls
Under voltage relay coil deenergizes and release
the handle to OFF position

30. ROTOR RESISTANCE STARTER

31. This starter only used for slip ring motor
External resistance connected in the rotor terminals
Motor always started with full supply voltage
The resitances is gradually cut out of the rotor
circuit as the motor gathers speed

32. iii Assignment questions
1.Draw and explain the following DC motor starters
a.2 point starter
b.3 point starter
c.4 point starter
2.Draw and explain the following induction motor
starters
a.DOL starter
b.Star delta starter
c.Auto transformer starter
d.Rotor resistance starter