2. 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
3. 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
4. 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.
5.
6. 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
7. First we will examine the normal running condition, i.e.
when the motor is connected in Delta.
8. Now let’s have a look what happens when the
motor is connected in STAR, i.e. in the starting
condition.
9.
10. 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
11.
12. 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.
13. CONSTRUCTION
A contactor has three components.
1) The contact parts
2) Electromagnet
3) enclosure
The contact parts are the current carrying parts of the
contactor.
This includes power contact, auxiliary contact and contact
spring.
14. 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.
15. 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
16. 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.
17. 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.
18. 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.
19. 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.
20. 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.
21. This 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
22. 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
