Star Delta starter wiring diagram with Full Explanation

Star delta starter wiring diagram:

Here will see the practical star delta starter wiring diagram. What I have given in this wiring diagram the same will be used in many industrial. All the panel manufacture use this drawing only. This drawing has two circuir. One is power circuit and another one is control circuit. Power circuit consists of contactor MPCB, Thermal Over load relay, Timer and connector etc. Control drawing consists of mcb, control cable, terminal block etc.

Also see: Why star delta is preferred for higher HP motor.

How the circuit works:

Whenever the operator presses the start button, the star contactor and main contactor M holds and the motor starts rotating with reduced starting current. At the same time the star delta timer starts counting the star holding time. After reaches the presetting time, the timer cut the supply to the star contactor means the timer TNC becomes NO and the TNO become NC. By this changes in TNO to TNC the delta contactor holds. Then motor come to normal operational delta mode. Here Delta NC and Star NC works as reverse polarity protection means whenever the star contactor inline the delta contactor should not hold. Vice versa.


Also See: What is NO and NC

Materials Required:

MCB: 1 For controlling the input control supply

OLR: thermal over current protection as per your motor rating

ON and OFF push button: 1 each

Star delta timer:1 any make

NO and NC: as required

Control cable and power cable: as required

Contactor : 3 as per your motor rating (2è Identical high rating, 1è Low rating).

Power connectors: 6 to connect the starter terminal and output power cable

MPCB or MCCB: as per rating.

Power circuit:

Control circuit:

Star Delta starter Control Circuit


Factors Considering before building star delta starter:

  1. Contactor, MCCB or MPCB and OLR should not load more than 70 % of its full load current. Means always select one step higher version. The same starter can be useable for both exiting motor and other onstep higher HP motor also

Example: If I design a starter for 90 KW motor the same starter has to usable for 110 KW motor also. Contactor will be normal size but OLR and MCCB should protect both motor. If 90 KW motor with Full load current is equal to 156 amps but in 110 KW motor full load current is 180 amps. Since you have to select the relay range between 145 Amps to 200 Amps. And so on..

  1. In case, If you use CT operated thermal over load relay you must calculate the current carefully. Also see: How to calculate the Current setting.
  2. Putting the thermal over load relay in the output of the main contactor gives more effective and reduce in size and cost.

How cost reduces:

After delta change over the load current reduces to 1/root 3 times so that we no need to go for full motor rating relays. Please refer the power circuit.

  1. Phase sequence must be properly checked. Negligence in this 100 % leads to motor winding failure.
  2. Check all the bolt and nut tightness. Use 8.8 T bolts which gives best results.

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