Download Function Of Starter

Design of starters for AC
motors
Group no - 3
Name
Chaudhari Darshan M.
Chaudhari Hardik D
Chavda Mayur N.
Joshi Ankur S.
Pathak Dhaval K
Enrolment no
130090109007
130090109008
130090109010
130090109024
130090109048
Guided By:
Prof. Chetan K. Lad
Prof. Hetal Patel
C.K. PITHAWALA COLLAGE OF
ENGINEERING & TECHNOLOGY
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Function Of Starter
The device which is used to start the 3-phase induction motor is called a
starter.
Necessity Of Starter
Function of starter is to limit the starting high current to a safe value.
The magnitude of E2 depends upon the flux linking with the rotor
conductors and its relative speed. The strength of the rotor flux depends
upon the applied voltage. At the instant of applying rated voltage to the
stator winding, rotor is stationery and as such the ship is unity. So if full
rated voltage is given to the stator winding, then the magnitude of the emf
induced in the rotor conductors will be high, because the relative speed
between the rotor conductors and stator revolving flux is very high i.e.
equal to the synchronous speed of the stator flux. Further the rotor
conductors are short circuited and thus have low impedance.
Hence, the current drawn by the stator winding or motor is very large,
approximately 5 to 7 times the full load current.
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Direct- on- line starter (D.O.L. starter)
Manual primary resistance starter
Primary resistance automatic starter
Automatic auto transformer starter
Manual star – delta starter
Automatic star – delta starter
Manual rotor resistance starter
Automatic rotor resistance starter
L1, L2, L3 Lines
M Main contacts
MA Auxiliary or maintaining contact
S1 START PUSH button
S2 OFF PUSH button
OLC Overload relay coil
OL Overload relay contact
C Magnetic coil or operating coil
Fig (1) : DOL starter
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A starter which connects a motor directly across the line is called a DirectO3-Line starter , i.e. in this method, the motor is connected by means of a
starter across the full supply voltage.
Fig. (1) shows a wiring diagram of a push button type Direct-On-Line
starter, which is commonly used.
It is very simple, inexpensive, easy to install and maintain.
It consists of a ‘START’ and ‘OFF’ push buttons, Electromagnetic
contactor and overload relay.
Switching by this starter is directly from line without any provision to
control the starting current i.e. there is no device to reduce the starting
current in this starter.
Fig (2) Control Circuit
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Operation
When the START push button ‘S1’ is pressed, the circuit is completed
from L1 through S2, S1, C (operating coil) and normally closed contacts
(O.L.) of the overload relay to L2.
i.e. L1 – S2 – S1 – C - O.L. – L2
Thus, the coil C is energized, and it closes contacts M and connects the
motor across the line.
An Auxiliary or Maintaining contact MA is used to keep the holding or
operating coil energized after the finger’s removed from the ‘’START’
push button S1.
When the ‘OFF’ push button ‘S2’ is pressed, the supply through the
contactor coil C is disconnected.Since the coil C is de-energised, the
main contacts M are opened.
The supply to the motor is disconnected and the motor stops.
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In this starter, there is no current limiting device to limit the starting high
current, only protection against under voltage and overload is provided.
Under Voltage Protection
When the supply voltage is below a certain value or in the event of failure
of
power supply, the coil C is de-energized, therefore motor will be
disconnected'
from the main supply.
Overload Protection
The motor is protected against overload by a thermal overload relay which
open circuits. The control circuit when over load occurs.
In case of an overload on the motor, overload relay coils are energised.
The normally closed contacts DOL Is opened and the contactor coil C is
de-energized the motor from the supply.
In this method of starting. The rate of temperature rise is high and motor
may get damaged if the starting period is large, which may be due to
excessive load or excessive voltage drop in the supply lines.
So small size squirrel cage inductor. Motors up to 5 kW may be this
method.
Fig(1) : Manual Primary resistance starter
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In this method of starting of 3- phase induction motor, primary resistances
are connected in all the three phase of the stator winding, as a result the
applied voltage across the stator winding at the instant of starting is
reduced to a fraction K of the rated voltage of the motor.
Therefore the initial high starting current will also reduce by the same
fraction.
The purpose of primary or starting resistors is to drop some voltage and
hence reduce the voltage applied across the motor terminals.
The torque developed by the motor is directly proportional to the square of
applied voltage, so if the voltage applied across the motor terminals is
reduced by fraction K, but the starting torque is reduced by a fraction K@
of that obtainable with direct switching.
The drawbacks of this method of starting is reduced starting torque and
large power consumption and heating of resistors.
Hence this method of starting is used for small motors only.
L1, L2, L3 Lines
S1,S2 Start contents
SA Auxiliary or maintaining contact
Cs Magnetic coil or operating coil
for start contacts
Cr Magnetic coil for running contacts
TRc Time delay relay coil
TR Time delay contact
R1, R2, R3 Running contacts
R1, r2, r3 primary resistors
OLC Overload relay coil
OL Overload relay contact
PB1 START push button
PB2 STOP push button
Fig (1) : Primary diagram of a primary resistance automatic starter
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Operation
When the ‘START’ push button PB1 is pressed, the circuit is completed
from L1 through PB2, PB1, coil Cs and normally closed contacts (OL) of
the overload relays to line L2.
i.e. L1 - PB2 – PB1 - Cs – OL – L2
Thus, the coil Cs ( operating coil for start contacts) is energized and it
closes all starting contacts S1, S2, S3 and SA.
The auxiliary or maintaining contact SA is used to keep the operating coil
Cs energized after the finger id removed from this START push button
PB1.
The primary resistors r1, r2, and r3 in series with line will reduce the
starting current drawn from the line.
Also, at the same time, the coil TRc of a time delay relay connected across
a and b is energized and set the time mechanism.
After a definite time delay, contact TR close and circuit is completed
through operating coil CR.
Pressing the ‘STOP’ push button de-energizes all contactors and thus the
supply to the motor Is disconnected and the motor stops.
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This coil CR becomes energized, close the run contacts .
The primary resistors now bypassed, thus connected the motor across the
line.
Pressing the ‘STOP’ push button de – energizes all contactors and thus
the supply to the motor is disconnected and the motor stops.
Fig (1) : Manual auto transformer starter
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In this method, 3-phase auto transformer with fixed tappings is used to
obtain reduced voltage for starting the 3-phase induction motors.
Normally 50% to 60% tappings can be used to obtain a safe value of
starting current.
Thus, 50% to 60% of the rated voltage is applied at starting and the auto
transformer is cut out of the motor circuit, when the motor has picked up
the speed about 70% to 80% of the normal speed.
Hence, during normal running condition, the voltage across the stator
winding is of rated value. Fig (1) shows the schematic diagram of. Manual
auto transformer starter.
One problem that occurs with a primary resistor starter is that all the
voltage that is dropped through the resistors is turned into heat. The
amount of heat may become very large and cause problem, Auto
transformer starter is able to provide
The same type of voltage reduction without building up large amount of
heat. Further in this method of starting, the voltage is reduced by
transformation and not by dropping voltage in the resistors and hence the
current and power drawn from the supply are also reduced in comparison
to primary resistance starter.
This starter also provides the large amount of torque per ampere of line
current. This method are use in both star and delta connected motors.
L1, L2, L3 Lines
S1,S2 Start contents
SA Auxiliary or maintaining contact
Cs Operating coil for Start contacts
R1, R2, R3 Running contacts
Cr Magnetic coil or operating coil
for start contacts
ST start contacts
Cst Operating coil for start contacts
TR Pneumatic timer
T.O. Time opening Contact
T.C. Time closing contact
OLC Overload relay coil
OL Overload relay contact
PB1 START push button
PB2 STOP push button
Fig (1) : Automatic auto transformer starter
Control Circuit
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Operation
When the ‘START’ push button PB1 is pressed, the circuit is completed
from L1 through OFF push button PB2, PB1, operating coil Cs and
normally closed contacts (OL) of the overload relays to line L2.
i.e. L1 – PB2 – PB1 – Cs - OL – L2
Thus, the coil Cs ( operating coil for start contacts) is energized and it
closes all the starting contacts S1, S2, S3 and SA.
Further, on staring pressing the ‘START’ push button PB1,at the same
time, one alternative path is completed as follows:
L1 - PB2 – PB1 – T.O. - CsT – OL – L2
Hence, start coil CST et energized, which closes start contacts ‘ST’.
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The star contacts connected the three coil ends of the auto transformer
together to from the start point and auto transformer connected in series
with the motor.
Thus the voltage is reduced.
After a per – determined time, the pneumatic timing relay opens the star
contactors and simultaneously closes time closing switch T.C.
At the same time operating coil CR get energized by following path:
L1 – PB2 - Auxi. Contact SA – T.C. – ST – CR – OL - L2
All the run contacts (r1, r2 and r3) are than closed placing the motor
directly across the line and motor runs with full supply voltage.
Pressing the OFF push button PB2 or in case of an overload on the motor
de –energized all contactors and therefore disconnect the motor from the
supply line.
Fig (1) : Manual Star – Delta Starter
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L1, L2, L3 Lines
S Start contents
Cs Operating coil for Start contacts
D Delta contacts
Cd Operating coil for Delta contacts
M Line contacts
CM operating coil for contacts M
TR Pneumatic timer
TO Time opening Contact
TC Time closing contact
OLC Overload relay coil
OL Overload relay contact
PB1 START push button
PB2 STOP push button
Fig (1) : Wiring diagram and control circuit
of automatic star-delta starter
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The operating coil CM get energized by following path:
‘ L1 – PB2 – PB1 – CM – OL – OL – OL – L2’
At the same time, the operating coil Cs get energized by following path :
‘ L1 – PB2 – PB1 – TO –D –CS – OL – OL – OL – L2’
The operating coil CD is energized by following path
'L1 - PB2 –MA – TC – S – CD – OL – OL – OL - L2'
Pressing the OFF PUSH button, de-energizes all contactors and
disconnects the motor from the line.
3 – Phase
Ac
Supply
Fig (1) : Manual rotor resistance starter
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This method of starting Is only used for slip ring induction motors because
in case of squirrel cage induction motors, rotor has copper bars, which are
short circuited at both ends.
Hence it is not possible to add any external resistance in the rotor circuit.
In case of slip ring induction motors, rotor has 3-phase star connected
winding whose terminals are connected to three respective slip rings.
In rotor resistance starter the three terminals of the rotor winding are
connected to a variable external resistances through slip rings. Full supply
voltage is applied across the stator. Resistances are fully in the circuit at
starting, so that the starting current is reduced.
The External variable resistance connected in each phase of the rotor
circuit not only reduces the current at starting but increases the starting
torque also due to improvement in power factor.
The rotor circuit resistance is gradually cut out, as the motor speeds up
and during normal running condition, the rotor circuit resistance is
completely cut out and the sliprings are short circuited.
Fig (1) : Manual rotor resistance starter
Fig. (2) Control circuit
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L1, L2, L3 Lines
S Starting contacts
Cs Operating coil or Magnetic
coil for starting contacts
R Running contacts
CR Operating coil for running
contacts
CTR Time delay relay coil
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TR Time delay relay contact
SA Auxiliary starting contact
OLC Overload relay coil
OL Overload relay contact
PB1 START push button
PB2 OFF push button
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Operation
Fig (1) shows the wiring diagram of an automatic rotor resistance starter
which is commonly used for starting slip ring induction motors and Fig (2)
shows its control circuit. This starter consists of start and run contactors,
time delay relay, overload relays, START and OFF push buttons.
When the START push button PB, is pressed, the circuit is completed
from line L1 through OFF push button PB2, PB1 operating coil Cs and
normally closed contacts of overload relays of L2.
Therefore, the operating coil Cs is energized, closing the starting contacts
S and auxiliary contact SA and the motor starts with external starting
resistances in rotor circuit which will limit the starting current and
improve the starting torque.
At the same time, the coil CTR of a time delay relay is also energized and
set the timing mechanism.
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After a predetermined time, contact TR will close and the operating coil
for running contacts CR will energized through following path :
‘L1 – pb2 –SA –TR –CR – OL – OL – OL – L2’
Thus, coil CR is energized and causes running contacts R to close and
short circuit the starting resistances. Thus external starting resistances get
short circuited during running condition and motor runs with full rated
speed.