© 2014 IJIRT | Volume 1 Issue 6 | ISSN: 2349-6002
STAR DELTA STARTER OF INDUCTION MOTOR
Akshay Rohilla, Harish Chand Joshi, Anurag Negi
Department of Electrical Engineering,
Dronacharya College of Engineering,Maharshi University
Abstract - Most induction motors are started directly on line,
but when very large motors are started that way, they cause a
disturbance of voltage on the supply lines due to large starting
current surges. To limit the starting current surge, large
induction motors are started at reduced voltage and then have
full supply voltage reconnected when they run up to near
rotated speed. Two methods are used for reduction of starting
voltage is star delta starting.
INTRODUCTION
Star/Delta starters are probably the most common reduced voltage
starters in the 50Hz world. (Known as Wye/Delta starters in the
60Hz world). They are used in an attempt to reduce the start
current applied to the motor during start as a means of reducing
the disturbances and interference on the electrical supply.
Component: The Star/Delta starter is manufactured from three
contactors, a timer and a thermal overload. The contactors are
smaller than the single contactor used in a Direct on Line starter
as they are controlling winding currents only. The currents
through the winding are 1√3 = 0.58 (58%) of the current in the
line. this connection amounts to approximately 30% of the delta
values. The starting current is reduced to one third of the direct
starting current.
WORKING PRINCIPLE OF STAR DELTA STARTER
There are three states of Star-Delta starters, a). Star Connected
State, b). Open State, and c). Delta Connected State. During
starting time Main and Star Contactors remain closed and
complete Circuit. In Star Connected State, voltage applied is
reduced to 1/√3 of the Line Voltage across each winding. As and
when motor attains good rotational speed, say about 90% of full
r.p.m. after few seconds, timer connected in starter disconnects
Star Contactor first and then connects Delta Contactor. Between
these two, Star connected and Delta connected states, circuit
becomes open and motor neither remains in Star nor in Delta
State. This is called open transition switching. In Delta connected
state voltage applied to windings is equal to Line Voltage. Items
Required to Make Star Delta Starter Three Contactors ( One Main
Contactor, One Star Contactor and One Delta Contactor ), Over
Load Relay ( or OLR ) Timer, Fuse Switch Unit ( or FSU ), 2 Pole
MCB, Fuse, Start Push Button ( NO ) Stop Push Button
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WORKING
There are two contactors that are close during run, often referred
to as the main contactor and the delta contactor. These are AC3
rated at 58% of the current rating of the motor. The third contactor
is the star contactor and that only carries star current while the
motor is connected in star. The current in star is one third of the
current in delta, so this contactor can be AC3 rated at one third of
the motor rating. In operation, the Main Contactor (KM3) and the
Star Contactor (KM1) are closed initially, and then after a period
of time, the star contactor is opened, and then the delta contactor
(KM2) is closed. The control of the contactors is by the timer
(K1T) built into the starter. The Star and Delta are electrically
interlocked and preferably mechanically interlocked as well. In
effect, there are four states: OFF State. All Contactors are open
Star State. The Main and the Star contactors are closed and the
delta contactor is open. The motor is connected in star and will
produce one third of DOL torque at one third of DOL current.
Open State. The Main contactor is closed and the Delta and Star
contactors are open. There is voltage on one end of the motor
windings, but the other end is open so no current can flow. The
motor has a spinning rotor and behaves like a generator. Delta
State. The Main and the Delta contactors are closed. The Star
contactor is open. The motor is connected to full line voltage and
full power and torque are available. This type of operation is
called open transition switching because there is an open state
between the star state and the delta state.
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© 2014 IJIRT | Volume 1 Issue 6 | ISSN: 2349-6002
in starting. In starting as slip s=1, the induced EMF in rotor circuit
would be sE2=E2, hence heavy current flows in short circuit rotor
circuit, reflect to draw heavy primary current from stator circuit,
or from the mains, and hence may blow out fuses. In star/delta
starter, in starting, star connection, voltage applied to winding is
phase voltage( Vphase=Vline/sqrt3 ), and will draw less current,
once the speed picks up slip will reduced running slip(.04) from
one. E2 will reduced (.04E2) and motor winding will put to delta
connection (Vline will apply to windig coil). In wound rotor
starting resistance is connected to rotor circuit, to reduce starting
inrush current and increasing the starting torque.
CONCLUSION
For star delta starter,the basic function is to enable the motor to
start and the motor windings are configured in a star formation to
the supply voltage. The voltage applied for star delta starter to the
individual motor winding is therefore reduced by a factor of 1√3
= 0.58 this connection amounts to approximately 30% of the delta
values. The starting current is reduced to one third of the direct
starting current.
RESULT
Star delta starter is preferred with induction motor due to
following reasons:
• Starting current is reduced 3-4 times of the direct current due to
which voltage drops and hence it causes less losses.
• Star delta starter circuit comes in circuit first during starting of
motor, which reduces voltage 3 times, that is why current also
reduces up to 3 times and hence less motor burning is caused.
Due to the reduced starting torque, the star-delta-connection is
suitable for drives with a high inertia mass but a resistance torque
which is low or only increases with increased speed. It is
preferably used for applications where the drive is only put under
a load after run-up. After motor run-up, in most cases an
automatic timing relay controls the switch-over from star to delta.
The run-up using star connection should last until the motor has
reached the approximate operational speed. so that after switching
to delta, as little post acceleration as possible is required. Postacceleration in delta connection will instigate high currents as
seen with direct on-line starting.
REFERENCE
Internet and Basic Electrical Engineering ,jp tewari
• In addition, starting torque is increased and it prevents the
damage of motor winding
Starters are used to reduce inrush current by applying less voltage
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