Automatic Star Delta Motor starter

Star delta starter is one of the common motor starting technique widely used in industry where there is a requirement of reducing the starting current. Let's see the principle, advantages & disadvantages of star delta starter...

Although Star delta starter is not very popular starter but still today it is used in industries to reduce motor starting current and efficient starting

Basic principle of Star Delta Starter:

Before going to the power and control circuit of star delta starter let us see the basic principle. The basic principle of the star delta starter is that during starting the motor shall have star connected winding and after sometime  when the motor have gained adequate speed the winding shall be converted to Delta winding. Advantage of this is that when the motor winding is star connected then the voltage in each phase is=(the line voltage /1.732)

Suppose a 415 volt motor is to be started by star delta starter. In that case when the motor is starting that is the winding is in Star connection voltage across each phase will be 415/1.732. Due to this reduced applied voltage the starting current will also be reduced to 33.33%(1/3) of direct starting in delta connection.

After the motor has attained certain amount speed (around 90% of the rated speed) and some voltage has been induced to its winding then the winding will be switched to Delta. And the motor will run in delta connection.

The motors that are to be started by star delta starter should have six number of terminals available, in the other form of starters the Motor has three numbers of terminals but for Star Delta starter 6 number of terminals are required because the winding has to be connected in Star mode once and Delta mode for the rest. Both the connections i.e. star and delta, are available inside the star delta starter, only the changeover is done during its operation. Thats why 6 nos. Of terminals are required.

Let us see the power circuit diagram of a star delta starter.

The power circuit of a Star Delta Starter is drawn below.

Fig 1: Automatic Star Delta Starter-Power Circuit Diagram/
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B stands for circuit breaker, which are MCCB, SFU, or MPCB.

MCCB means Moulded Case Circuit Breaker,

SFU stands for Switch Fuse Unit and MPCB for Motor Protection Circuit Breaker.

MPCB is used motor rated upto 30 kW.

CT is shown which shall be used for current measurement or protection. CT shown In this drawing is for one phase only however 3 number of line CT with ammeter selector switch can also be used for current measurement.

The next component is the Contactor. There are three contactors shown in this drawing.
1. MC or main contractor.
2. DC or Delta contactor.
3. SC or Star Contactor

Now question may come why we need this contactor when we have the circuit breaker. Basically in this configuration the circuit breakers are MCCB, SFU or MPCB. They can trip the circuit automatically during any short circuit. Also MCCB, MPCB can be tripped from remote end (away from switchboard) by using a shunt trip coil to the breakers but they can not be switched on by Electrical signal. These can be switched on by moving the grip only. So for remote operation, interlock with the process system automatic switching on off is required, which is achieved by contactor. However this explanation is applicable for the main contactor only. Star contactor and delta contactor are required to make the star and delta winding.

Bimetallic thermal over load relay is given for overload protection.

Rating of these switchgear component viz. MCCB/SFU/MPCB, contactor and thermal over load relays for a particular motor is selected as per Type 2 co ordination chart, which can be easily found in the internet.

In general motor upto 90 kW are fed by this arrangement however motor rated 110 kW and above has air circuit breaker with motor protection relay. Motor protection relay covers all the protection required, and air circuit breaker can be controlled by Electrical signal, so there contactor or bimetallic relays are not used. But for forming the star and delta winding contactors will be required.

We have seen the power circuit diagram, now we shall focus on the control circuit. The control circuit diagram of star delta starter is shown below.

Fig 2: Star Delta Starter-Control wiring diagram/
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Before starting the description please note that NO & NC stand for normally open and normally closed contact respectively. These states are applicable when the corresponding coil is at de-energized state, suppose we say NO contact of a relay coil A, means when A is de-energized then the contact is open, but when the coil A will be energized it shall be closed, similarly an NC contact will open when the coil is energized.

Now let us see the components.

MC/DC/SC- Main/Delta/Star contactor coil. Energizing which will convert NC contacts to open and NO contacts to close and vice versa when de-energized. Energizing it the main contact shall be closed to run the motor and de energizing it shall cause motor to stop.

Emergency Stop P.B- Emergency stop push button switch, it shall be installed near to the motor. It is a stay put type button that means when pressed it shall be locked at the new position. The push button is not shown in this drawing, however it shall be connected in such a manner that pressing of this switch wil stop the motor irrespective of the motor operating  from local or remote.

Start P.B- Push button switch for motor starting. It is a spring return type switch, means after pressing it shall return to its previous position. It is NO type switch.

Stop P.B- Push button switch for motor stop. It is also spring return type. It is NC type switch.

Both start and stop push buttons are located near to the motor in a panel called "Local Push Button Station".

So for starting the motor first have to close the circuit breaker or MCCB etc.

O/L relay contact shall be normally closed. That means it the relay has not picked up it will be closed, when overload occurs the relay contact will open and control supply will be disconnected.

T- T is the timer. After the set time has over it switches over the motor connection from star to Delta.

Now let us see step by step principle to understand how the motor starts with start delta starter.

Step-1

For starting the motor first the start push button to be pressed. Pressing of this start push button will cause current flows through the timer coil and since the T1-T2 and DC7-DC8 are NC contact so the SC will be also energised.

Hence the timer will start counting time, and all the NO contact of SC will be coverted to closed contact, and NC will be converted to opened contact.

Hence SC 1-2, 3-4, 5-6 and 9-10 will be closed. And SC 7-8 will be opened. If you see the diagrams, you will see that this will cause
1. The start contact of the motor winding to close. So now the motor is in star winding.
2. Since SC 7, 8 will be opened so delta contactor coil, DC will not be energised. So under no circumstances there shall be any provision of short circuit.
3. Since SC 9, 10 will be closed so, now the main contactor coil MC will be energised.

Step-2

Energization of main contactor will result in closure of contacts MC 1-2, 3-4, 5-6. Also MC 9-10 & MC 7-8.
So now even if the Start P.B is back to its NO state ( Since it is a Spring return type P.B) the main contactor coil will remain energised through MC 9-10, MC 7-8. Also the SC coil is energised. So the motor will start at the winding connected in Star configuration.

Step-3

Now when the timer T will complete its set time its contact will change their state, i.e. T1-T2 will become Opened and T3-T4 will become closed. Although T3-T4 is closed it will not result in the Delta coil charging unless the SC is de-energised. Let's see how.

1. T1-T2 has opened. So it will immediately de-energise SC. Thus will result opening of SC 1-2, 3-4, 5-6, 9-10; and closure of SC 7-8. So main contactor coil will remain energised through its own contact i.e. MC 7-8

2. Along with opening of T1-T2, T3-T4 will be closed at the same time. Now after de-energization of SC, and closure of T3-T4, DC coil will be energised, and its subsequent NO contacts will be closed, i.e. DC 1-2, 3-4, 5-6, and DC 7-8 will be opened, so there shall not be any provision of energisation of SC.

So now the motor will run at Delta configuration.

In case the motor get overloaded the over load relay will be energised and will make the O/L 1-2 contact open. So it will isolate the control supply, and the motor will stop.
Protection against short circuit and earth fault is given by MCCB, SFU OR MPCB.

Advantages:

1. Economic starter.
2. Simple circuit.
3. Draws 1/3 times current than that required for starting the motor in direct online delta.
4. Not large in size, could be easily accommodate in switchboard panel.

Disadvantages:

1. Starting torque is also reduced to 1/3 of starting torque developed in direct delta starting.

2. Very steep change in torque speed characteristics.

3. Needs 6 nos. of terminals, hence the motor needs to be specific.

4. Mainly used in low voltage system.

5. Two (2) runs of cables are required.

6. Inclusion of so many numbers of contactors make it unreliable.


Due to these disadvantages star delta starter is not oftenly used, and it is not a very recommend practice. Modern day invention of soft starter has replaced this almost.

In this Article we have shown the basic circuit and principle. however in actual Emergency push button stop button is kept in series, to isolate the entire control supply in case of any emergency.

Local remote selection is placed after the Emergency stop P.B. Remote selection is kept for operation of the motor from DCS i.e. auomated control system.

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