automatic star delta staretr ppt
TRANSCRIPT
AUTOMATIC STAR DELTA STARTER
Presented by:Group no. 20
Under guidance of: Er. I. C. Choudhury
Contents Motor operating principle What is star-delta starting Star-delta starter Why star-delta starting is used Comparison between star and delta current Advantages and disadvantages Practical circuit diagram Components Contactor Over load relay Timer Conclusion References
Motor operating principle
Faraday’s law:- a. Whenever a conductor cuts a
magnetic field an e.m.f. is induced in it.
b. Whenever a current carrying conductor is placed in magnetic field it experiences a force.
PREAMBLE
What is Star-Delta starting?
The motor is connected in STAR during the starting and when the motor is accelerated close to the 75% of the rated speed, the motor is connected in DELTA.
Star delta starter
With the windings in a star connection rather than the delta, the full voltage applied line to line is the same, but the voltage across the individual motor windings is reduced. The voltage is reduced by the inverse of the square root of 3 or 57.7% of full voltage.
Starting Voltage Reduction =Vline /√3
Hence starting current is reduced
Why Star-Delta Starting is used?
Let’s consider an example : motor: 120kW, 4 Pole, 380 Volt, Delta
connected, 3 Phase, 50 Hz.
First we will examine the normal running condition, i.e. when the motor is connected in Delta.
Now let’s have a look what happens when the motor is connected in STAR, i.e. in the starting condition.
Comparision between star and delta current
Advantages
As calculated above, the most significant advantage is the reduction in starting current.
The starting current will determine to a large extent:
the size of the cables used, the size of the fuses, as well as the transformers. Requiring 67% less starting current can have a
tremendous cost saving implication!Disadvantages As calculated above, the disadvantage of using star
delta starting is the reduction in starting torque.
Increased run-up time
Practical ckt of automatic star delta
starter
CONTACTORS
A contactor is an electrically controlled
switch
used for switching a power circuit.
It can be controlled by a circuit which has a
much lower power level than the switched
circuit
The physical size of contactors ranges from a
device small enough to pick up with one hand,
to large devices approximately
a meter (yard) on a side.
CONSTRUCTIONA contactor has three components.
1) The contact parts2) Electromagnet3) enclosure
The contact parts are the current carrying parts of
the contactor.
This includes power contact, auxiliary contact and
contact spring.
The electromagnet provides driving force to close the contacts. The coil input is either DC or AC depending on the contactor design.
Enclosure is a frame housing the contact and the electromagnet.Enclosure are made of insulating materials like Bakelite,Nylon 6 and Thermosetting plastic to protect and insulate the contacts.
THERMAL OVERLOAD RELAY
To protect the motor when motor
current is excessive, overload relay is
connected in series with the supply to
the motor.
Overload relays :
• allow harmless temporary overloads
(such as motor starting) without
disrupting the circuit
• will trip and open a circuit, if current
is high enough to cause motor
damage over a period of time
• can be reset once the overload is
removed
Overload relays are
rated by a trip class
which defines the
length of time it will
take for the relay to trip
in an overload
condition.
The most common
trip classes are Class
10, Class 20, and Class
30.
Overload protection can be accomplished with the use of
a bimetal overload relay. This component consists of a
small heater element wired in series with the motor and a
bimetal strip that can be used as a trip lever.
The bimetal strip is made of two dissimilar metals
bonded together. The two metals have different thermal
expansion characteristics, so the bimetal strip bends at a
given rate when heated.
Under normal operating conditions, the heat generated by the heater element will be insufficient to cause the bimetal strip to bend enough to trip the overload relay.
As current rises, heat also
rises. The hotter the bimetal
strip becomes, the more it
bends. In an overload
condition, the heat generated
from the heater will cause the
bimetal strip to bend until the
mechanism is tripped,
stopping the motor.
ELECTRONIC STAR-DELTA TIMER
For an automatic star-delta
starter timer is required to
change the connection from star
to delta after a specified time
interval.
In case of an electronics timer
we can specify the time for which
the motor operate in star
connection as well as the transit
time from star to delta.
Functions :When the supply voltage U is applied, the star-contact switches to on position and the set star-time t1 begins . After the interval t1 has expired the star-contact switches to off-position and the set transit-time t2 begins. After the interval t2 has expired the contact for the delta-contactor switches to on position.To restart the function the supply voltage must be interrupted and re-applied.
conclusionThis type of starter is used upto 50 HP motors.
This method of starting of motor is simple, cheap, effective since very less power is lost in auxiliary components
This starter is used where high starting torque is not essential requirement
References
Practical book on electrical engineering- by P.S. Dhogal www.cselectrric.co.in
Theory and performance of electrical machines- by J.B.Gupta
Basic Electrical Engineering-by V.N.Mittle