Engineering · PLC · Project 3 of 15 · Motor control · Level 3

Star-delta starter

Start a big motor gently in star, then switch to delta after a few seconds. Learn timers and three contactors.

Learning model only. This page teaches the program logic. A real machine needs a proper control panel with an isolator, fuses, motor protection and a hard-wired emergency stop. Mains and motor wiring is for qualified electricians. A PLC output never drives a motor directly.

You will learn

In star the motor sees about 58 % of the line voltage, so the starting current is roughly one third. After the timer the program drops star and pulls in delta. Star and delta must never be on together: each rung checks the other with an NC contact.

How it works

Press START. Main and star close. The motor spins slowly at about 58 % speed in the animation.
After 4 s the star contactor opens and the delta contactor closes. The motor reaches full speed.
Press STOP. All three drop out and the timer resets.

In real life

Used on pumps, compressors and fans above about 5.5 kW where the supply cannot take the full starting current. Soft starters and drives now do this job too.

Inputs, outputs and addresses

The same signals on seven PLC brands. Inputs and outputs are numbered in order; check your own hardware and software, because addresses depend on the model and module position.

Name	What it is	Type	Siemens S7-1200	Allen-Bradley CompactLogix	Mitsubishi iQ-F FX5U	Delta DVP / AS	Omron CP2E	Schneider M241	Inovance H5U
START	START push-button	Input	`%I0.0`	`Local:1:I.Data.0`	`X0`	`X0`	`0.00`	`%IX0.0`	`X0`
STOP	STOP push-button (N.C.)	Input	`%I0.1`	`Local:1:I.Data.1`	`X1`	`X1`	`0.01`	`%IX0.1`	`X1`
OL	Overload contact (N.C.)	Input	`%I0.2`	`Local:1:I.Data.2`	`X2`	`X2`	`0.02`	`%IX0.2`	`X2`
KM_MAIN	Main contactor	Output	`%Q0.0`	`Local:2:O.Data.0`	`Y0`	`Y0`	`100.00`	`%QX0.0`	`Y0`
KM_STAR	Star contactor	Output	`%Q0.1`	`Local:2:O.Data.1`	`Y1`	`Y1`	`100.01`	`%QX0.1`	`Y1`
KM_DELTA	Delta contactor	Output	`%Q0.2`	`Local:2:O.Data.2`	`Y2`	`Y2`	`100.02`	`%QX0.2`	`Y2`

Ladder diagram (shown for Siemens, live state)

Green wires carry power in the moment captured below. Switch the brand and the language in the simulator.

Structured text, Siemens SCL

(* Siemens S7-1200, CPU 1214C DC/DC/DC - TIA Portal - SCL *)
(* Rung 1: Main contactor with seal-in *)
"KM_MAIN" := ("START" OR "KM_MAIN") AND "STOP" AND "OL";
(* Rung 2: Star time (4 s) *)
"T1"(IN := "KM_MAIN", PT := T#4s);
(* Rung 3: Star while the timer runs *)
"KM_STAR" := "KM_MAIN" AND NOT "T1".Q AND NOT "KM_DELTA";
(* Rung 4: Delta after the timer, star must be off *)
"KM_DELTA" := "KM_MAIN" AND "T1".Q AND NOT "KM_STAR";

Structured text, Allen-Bradley

(* Allen-Bradley CompactLogix 5069 (L306ER) - Studio 5000 Logix Designer - Structured Text *)
(* Rung 1: Main contactor with seal-in *)
KM_MAIN := (START OR KM_MAIN) AND STOP AND OL;
(* Rung 2: Star time (4 s) *)
Timer1.PRE := 4000;   (* ms *)
Timer1.TimerEnable := KM_MAIN;
TONR(Timer1);
(* Rung 3: Star while the timer runs *)
KM_STAR := KM_MAIN AND NOT Timer1.DN AND NOT KM_DELTA;
(* Rung 4: Delta after the timer, star must be off *)
KM_DELTA := KM_MAIN AND Timer1.DN AND NOT KM_STAR;

Try this

Watch the timer value count up inside the TON block.
Change the preset in your own program: try 2 s and 8 s.

Parts to buy

The links open a search on each store, so you see what is in stock today. Check the model, voltage and price before you order.

#	Item	Buy / compare
1	PLC with 3 inputs and 3 outputs	Amazon.in · IndiaMART · Robu.in
2	3 contactors (main, star, delta)	Amazon.in · IndiaMART · Robu.in
3	Overload relay	Amazon.in · IndiaMART · Robu.in
4	Motor with all six terminals brought out	Amazon.in · IndiaMART · Robu.in

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This is a learning tool. It teaches the logic, not the exact keystrokes of each vendor’s software. Check the manual of your own PLC.