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AUTOMATIC STAR DELTA STARTER Presented by: Group no. 20 Under guidance of: Er. I. C. Choudhury
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Page 1: Automatic Star Delta Staretr Ppt

AUTOMATIC STAR DELTA STARTER

Presented by:Group no. 20

Under guidance of: Er. I. C. Choudhury

Page 2: Automatic Star Delta Staretr Ppt

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

Page 3: Automatic Star Delta Staretr Ppt

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

Page 4: Automatic Star Delta Staretr Ppt

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.

Page 5: Automatic Star Delta Staretr Ppt

Star delta starter

Page 6: Automatic Star Delta Staretr Ppt

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?

Page 7: Automatic Star Delta Staretr Ppt

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.

Page 8: Automatic Star Delta Staretr Ppt

Now let’s have a look what happens when the motor is connected in STAR, i.e. in the starting condition.

Page 9: Automatic Star Delta Staretr Ppt

Comparision between star and delta current

Page 10: Automatic Star Delta Staretr Ppt

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

Page 11: Automatic Star Delta Staretr Ppt

Practical ckt of automatic star delta

starter

Page 12: Automatic Star Delta Staretr Ppt

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.

Page 13: Automatic Star Delta Staretr Ppt

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.

Page 14: Automatic Star Delta Staretr Ppt

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.

Page 15: Automatic Star Delta Staretr Ppt

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

Page 16: Automatic Star Delta Staretr Ppt

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.

Page 17: Automatic Star Delta Staretr Ppt

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.

Page 18: Automatic Star Delta Staretr Ppt

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.

Page 19: Automatic Star Delta Staretr Ppt

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.

Page 20: Automatic Star Delta Staretr Ppt

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.

Page 21: Automatic Star Delta Staretr Ppt

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

Page 22: Automatic Star Delta Staretr Ppt

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