Tuesday, April 15, 2014

Star Delta Starter and applications

Motors up-to 7.5 KW are started using direct online starters but above 7.5 KW induction motors are started using Star-Delta Starters as if motors above 7.5 KW are started directly then there will be voltage disturbances in line due to large starting current surges. So let’s discuss how this Star-Delta Starter works.

Working Principal of Star-Delta Starter:

First of all you must know what Star and Delta connections are:-

Star Connection or Wye Connections:-

In Star connections, 
Phase current= Line current and
Phase voltage= Line voltage/ √3.

This means that during starting voltage get reduced by factor 1/√3.

So Torque also gets reduced by factor 1/3 as Torque is directly proportional to square of voltage.
Current also get reduced by 1/√3 in induction motor if motor is Started in Star connections.
Delta Connections:-

In Delta connections

Phase Voltage = Line Voltage and 

Phase current= Line Current/ √3.

After Starting motor in star connections then transition is done from Star connections to Delta connections after certain delay.

Star-delta Starter Consists following components:
1)     Main, star and delta contactorsà 3 No’s
2)     Timer Star-Deltaà 1 No.
3)     Overload relayà 1No.
4)     Main MCCB or Fuse Unit

Power Circuit of Star Delta Starter:
Main Power circuit of Star-Delta Starter is as Shown below:-




Star Delta Power circuit; Star Delta Starter
Add caption

In this Circuit there are three power contactors  C1 for Main C2 for Delta and C3 for Star.


Control Circuit of Star-Delta Starter (Open Transition):
Control circuit for Star-Delta Starter is as shown below:-



Star Delta Starter Control Circuit; Why Star Delta Starter Control Circuit

In that circuit when Start PB is pressed Star and Main contactor gets holds and motor get started in Start connections and after certain delay which was set in timer star contactor get disengage and delta contactor comes in line.
Star and Delta contactor NC’S are take in line both in Delta and contactor line so that only 1 contactor either star or Delta may comes in line.

In Star Delta Starter since Six leads goes at Motor end so in each lead phase current will flow so Overload relay used for Star-Delta circuit is usually for phase current not line current.

There will be always question arises that why Induction motor kept running in Star then visit link below:-

http://electrialstandards.blogspot.com/2014/04/induction-motors-not-kept-running-in.html

 Application of Star-Delta Starter:

In Most of industries for motors above 7.5 KW Star-Delta Starters are used. Star Delta Starters are used as during starting start current get reduced by 1/3 times and also torque get reduced by 1/3 times.

They have low maintenance cost compared to VFD or Soft-Starters which will be discussed later.