Star Delta motor Starter control diagram working principle

Star delta starter control diagram working principle:

How star delta starter works with control diagram:

A star delta starter usually consists of 

three contactors, an overload relay and 

a timer. This starting method can only be 

used with a motor that is delta connected during continuous run.

The basic idea behind a star-delta starter 

is that during the first part of the accel-

eration, the motor windings are connected in star, giving a reduced current. After a preset time, the connection will change to delta which will give the full current and also the full torque.

When used in delta, the voltage across 

each motor winding is same as the net-

work voltage. The motor current will split 

between two parallel windings with the 

factor 1/√3 compared to the line current. If the impedance in each motor winding is Z, then the sum of impedance for the parallel windings is Z/√3.

When the motor is star connected (Y- connected) the motor windings become serial connected. The resulting impedance will then be √3*Z, resulting in an impedance which is ((√3*Z)/( Z/√3) = 3), 3 times the impedance when delta connected. As the voltage level is the same, the resulting current when Y-connected will be 1/3 of the current when delta connected. So 

when starting using Star-Delta start, the 

star connection results in a current of 33% compared to a delta connected motor.

As the main voltage is the same the mo-

tor feels a star connection as a voltage 

reduction, as because the voltage across 

each motor windings will be 1/√3 of the 

main voltage. This lower voltage will also 

result in a torque reduction. The torque 

will be reduced with the square of the voltage, [(1/√3)* (1/√3) ≈ 0,33] ending up being 33% of the toque available when delta connected. However, this is only a theoretical value. A more true value is 25% as there are additional losses as well as other efficiency data valid when used star connected. This works well in an unloaded or very light loaded start, but it will not be possible to start heavier applications.

5) There are three type of star delta starter 
      i)   Manual star delta starter
      ii)  Semi automatic start delta starter 
     iii)  Fully  Automatic star delta starter 

Star delta starter control diagram:

 

Automatic star delta starter control diagram

S- star contactor, D-delta contactor, p- power or main contactor

TDR- Timer 

WORKING:

1)here are two contactors that are close during run,( D, P ) often referred to as the main contactor ( P ) and the delta contactor ( d ) . These are rated at 58% of the current rating of the motor. The third contactor is the star contactor ( S ) 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  rated at one third of the motor rating.

2) In operation, the Main Contactor( P )and the Star Contactor( S ) are closed initially, 

3) and then after a period of time,after operation of timer  the star contactor is opened, and then the delta contactor  is closed. 

4) The control of the contactors is by the timer  built into the starter. The Star and Delta are electrically interlocked and preferably mechanically interlocked as well. In effect, there are four states:

1) OFF State. All Contactors are open

2) 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.

3) 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.

4) 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.

This type of operation is called open transition switching because there is an open state between the star state and the delta state.

Power diagram of star delta starter :

power circuit star delta

K1 = Line contactor
K2 = Star contactor
K3 = Delta contactor
F2 = Overload relay
F1 = Backup fuse
F3 = Control circuit fuse

ADVANTAGES OF STAR DELTA STARTER:

 
1 ) Star-Delta starters are widely used due to their relatively low price.
2 ) There are no limits to the number of times they can be operated.
3) The starting current is reduced to approximately one-third. 

4) it can reduced current 1/3 time of rated current

DISADVANTAGES OF STAR DELTA STARTER:

1) The starter can only be applied to motors where the six leads or terminals can be accessed.

2)A big problem with star-delta starters appears when starting for instance pumps. 

The motor will accelerate to about 80-

85% of the rated speed before the load 

torque is equal to the motor torque and 

the acceleration ceases. To reach the rat-

ed speed, a switch over to delta position 

is necessary, and this switch over will v

tags: circuit, control, motor, power, star-delta, starter, starting,ADVANTAGES