Star Delta Starter Ladder Diagram (PLC, Run It Free Online)
Star-delta starting is the most common reduced-voltage starting method for squirrel-cage induction motors: the motor starts in star (Y) connection at roughly one-third the direct-on-line current, then transitions to delta (Δ) for full-speed running. Below you will find the complete I/O table, the ladder diagram rung by rung, and the timing diagram — and because this is a live runnable scenario, you can write the ladder logic for star delta starter right here in the browser and watch all three contactors sequence correctly without installing anything.
motorstar-deltasoft-starttimingsequencing
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Star-delta (Y-Δ) starting reduces inrush current during motor start-up. On START_PB, the main contactor and star contactor energise (star connection). After a 3-second timer expires the star contactor drops; a 200 ms transition delay prevents arcing; then the delta contactor energises (normal running). STAR and DELTA contactors are hard-interlocked — never both on simultaneously.
Objectives
START_PB energises MAIN_CONTACTOR and STAR_CONTACTOR together
After 3 s in star, STAR_CONTACTOR drops
After 200 ms transition delay, DELTA_CONTACTOR energises
STAR and DELTA are interlocked — never simultaneously on
STOP_PB or ESTOP de-energises all contactors immediately
RUN_LAMP illuminates when DELTA_CONTACTOR is energised (fully running)
Hints
Use a STAR_DONE latch bit: SET when T_STAR.Q fires; RESET when /RUN_BIT
T_STAR(IN := RUN_BIT AND NOT STAR_DONE, PT := 3000)
T_TRANSITION(IN := RUN_BIT AND STAR_DONE AND NOT DELTA_LATCH, PT := 200)
Use a DELTA_LATCH bit: SET when T_TRANSITION.Q; RESET when /RUN_BIT
I/O Table
Inputs
START_PB
Start push-button (momentary)
BOOL · %I0.0
STOP_PB
Stop push-button (momentary)
BOOL · %I0.1
ESTOP
E-stop (NC — true=healthy)
BOOL · %I0.2
Outputs
MAIN_CONTACTOR
Main line contactor coil
BOOL · %Q0.0
STAR_CONTACTOR
Star (Y) contactor coil
BOOL · %Q0.1
DELTA_CONTACTOR
Delta (Δ) contactor coil
BOOL · %Q0.2
RUN_LAMP
Running indicator lamp (delta mode)
BOOL · %Q0.3
Your program will be tested against:
All test cases run automatically when you submit. Assertions are hidden until you pass.
#1START_PB energises MAIN and STAR contactors
Pressing START_PB initiates star start — MAIN and STAR on, DELTA off
#2Star drops after 3 s, delta energises after 200 ms transition
Full start sequence: star phase → transition → delta running
#3STAR drops before DELTA energises — both off during transition
Immediately after star drops and before delta picks up, both contactors are off
#4STOP_PB de-energises all contactors during delta run
While running in delta, STOP_PB drops all contactors
I/O table for a star-delta starter PLC program
A star-delta starter needs three input signals and four outputs. The inputs are a momentary Start push-button (START_PB, %I0.0), a momentary Stop push-button (STOP_PB, %I0.1), and the E-stop contact wired normally-closed so a healthy circuit reads TRUE (ESTOP, %I0.2).
The outputs are the three contactors — Main (MAIN_CONTACTOR, KM1, %Q0.0), Star (STAR_CONTACTOR, KM2, %Q0.1) and Delta (DELTA_CONTACTOR, KM3, %Q0.2) — plus a Running indicator lamp (RUN_LAMP, %Q0.3) that illuminates only once the motor has fully transitioned to delta running. KM2 and KM3 are electrically interlocked in the panel; the PLC adds a software interlock on top.
The star-delta starter I/O: Start, Stop, E-stop inputs; Main (KM1), Star (KM2), Delta (KM3) contactor outputs and a Run lamp.
Ladder logic for star delta starter — main and star rung
The first ladder rungs establish the run latch and energise the Main and Star contactors together on START_PB. A RUN_BIT internal coil seals in via its own auxiliary contact, so the sequence continues after the momentary Start push-button is released. STOP_PB and the E-stop are wired normally-closed in series, so any break in that chain resets RUN_BIT immediately and drops all contactors.
With RUN_BIT high and STAR_DONE not yet latched, STAR_CONTACTOR (KM2) is energised. MAIN_CONTACTOR (KM1) is controlled solely by RUN_BIT — it stays on for the entire run, whether the motor is in star or delta. This is the standard ladder diagram for star delta starter: three separate coil rungs for KM1, KM2 and KM3, each with its own enabling conditions.
The main and star rung: START_PB seals in RUN_BIT; MAIN_CONTACTOR energises for the full run; STAR_CONTACTOR energises while STAR_DONE is not yet set.
Star-delta timing diagram — star phase, transition gap, delta running
The timing diagram reveals the three phases clearly. At time zero, START_PB is pressed: MAIN_CONTACTOR and STAR_CONTACTOR both pick up immediately while DELTA_CONTACTOR stays off — the motor is running in star (reduced voltage). The T_STAR on-delay timer (TON, preset 3 s) starts counting.
After 3 seconds T_STAR.Q fires, latching STAR_DONE and dropping STAR_CONTACTOR. There is now a 200 ms transition gap — T_TRANSITION (TON, preset 200 ms) — during which both STAR_CONTACTOR and DELTA_CONTACTOR are off, allowing the magnetic flux in KM2 to collapse before KM3 picks up. This prevents the momentary back-EMF from arcing the contacts. Once T_TRANSITION.Q fires, DELTA_LATCH sets, DELTA_CONTACTOR energises and RUN_LAMP illuminates. STAR and DELTA are never simultaneously on.
Timing diagram: Main on throughout; Star on for 3 s then off; 200 ms gap; Delta on for full-speed running. Star and Delta never overlap.
Transition rung — TON timer and star-delta interlock
The transition rung is the most critical part of the star delta PLC program. T_STAR runs while RUN_BIT is true and STAR_DONE has not yet latched. When T_STAR.Q fires, STAR_DONE is set (latched). STAR_CONTACTOR is driven by `RUN_BIT AND NOT STAR_DONE`, so it drops the instant STAR_DONE sets.
T_TRANSITION then counts 200 ms with the condition `RUN_BIT AND STAR_DONE AND NOT DELTA_LATCH`. When T_TRANSITION.Q fires, DELTA_LATCH is set and DELTA_CONTACTOR — driven by `RUN_BIT AND DELTA_LATCH` — picks up. The star-delta interlock in ladder logic is therefore structural: STAR_CONTACTOR and DELTA_CONTACTOR use mutually exclusive Boolean conditions that can never both be TRUE at the same time.
The transition rung: T_STAR (3 s) latches STAR_DONE; T_TRANSITION (200 ms) latches DELTA, then DELTA_CONTACTOR energises with an NC STAR interlock.
How to build a star-delta starter in the PLC simulator
Working through this star delta PLC program in the browser gives you immediate feedback on every rung. The simulator runs your IEC 61131-3 structured text or ladder logic against the same four test cases the real commissioning engineer would run: star phase confirmed, full star-to-delta transition, transition gap verified (both contactors off between the two phases), and stop drops all contactors instantly.
The build flow below walks you from the I/O assignment through to a passing all-green test suite. Each step maps directly to a rung or latch in the program, so you can follow the flow chart and then verify your implementation in the live scenario above.
Step-by-step build flow for the star-delta starter: from I/O assignment to a verified passing test suite.
Star delta starter PLC programming — the timer-driven transition
What makes star delta starter PLC programming different from a plain motor starter is the timed, interlocked transition from the star (Y) connection to the delta (Δ) connection. The whole star delta PLC program comes down to three coil rungs plus two timers: MAIN_CONTACTOR (KM1) follows RUN_BIT and stays on the entire run; STAR_CONTACTOR (KM2) is driven by RUN_BIT AND NOT STAR_DONE; and DELTA_CONTACTOR (KM3) is driven by RUN_BIT AND DELTA_LATCH. T_STAR (a TON, typically 3–7 s) latches STAR_DONE to end the star phase, then T_TRANSITION (a 200 ms TON) latches DELTA_LATCH after a dead-time gap so the star contactor has fully dropped before the delta contactor picks up. Because STAR and DELTA are driven by mutually exclusive Boolean conditions, the interlock is structural — they can never be true together.
This is the exact star delta starter PLC ladder diagram you can write and watch execute in the live scenario on this page — all three contactors sequence correctly with the timer-driven transition, graded against four automated test cases. To keep a copy of this star delta starter PLC ladder diagram (I/O table plus every rung) beside you while you build, press Ctrl/Cmd+P on this page and choose Save as PDF.
Frequently asked questions
What is the ladder diagram for a star-delta starter?
The ladder diagram for a star-delta starter has a Start/Stop seal-in rung that latches a RUN_BIT, a MAIN_CONTACTOR rung driven by RUN_BIT alone, a STAR_CONTACTOR rung driven by RUN_BIT AND NOT STAR_DONE, a TON timer rung (T_STAR, typically 3–7 s), and a DELTA_CONTACTOR rung driven by RUN_BIT AND DELTA_LATCH. A short transition timer (200 ms) ensures Star drops completely before Delta picks up.
Why are star and delta contactors interlocked in the PLC program?
Energising both Star (KM2) and Delta (KM3) simultaneously creates a dead short across the motor windings. The hardware interlock (mechanically linked contacts) is the last line of defence; the PLC software interlock — normally-closed STAR_CONTACTOR contacts in the DELTA rung and vice versa — prevents the command ever being issued. Belt and braces.
How long should the star-delta timer be set to?
Typical star timer presets are 3–10 seconds depending on the motor size and load. Smaller motors on light loads may complete the acceleration in 3 s; larger motors with high-inertia loads (fans, pumps) may need 6–10 s. The transition is correct when the motor has accelerated close to synchronous speed before switching to delta. In this simulator scenario the preset is 3 s.
What is the transition delay in a star-delta starter and why is it needed?
The transition delay (typically 50–300 ms) is a dead-time gap between the Star contactor dropping and the Delta contactor picking up. Without it the residual flux in KM2 and the back-EMF from the spinning motor can cause severe arcing and contact welding. The PLC implements this with a second TON timer (T_TRANSITION, 200 ms in this scenario) that only starts after STAR_DONE is latched.
Can I run a star-delta starter PLC program without a physical PLC?
Yes. This page is a live browser scenario — write the ladder logic or IEC structured text, press Run, and the simulator executes the full start sequence including the timer-driven star-to-delta transition and the interlock verification. No PLC, no panel wiring and no licence required.
What inputs and outputs does a star-delta starter PLC program need?
Inputs: Start push-button (momentary NO), Stop push-button (momentary NC or software-NC), and an E-stop (wired NC, healthy = TRUE). Outputs: Main contactor (KM1), Star contactor (KM2), Delta contactor (KM3), and optionally a Run lamp that illuminates once the motor is in delta. Two internal variables are also needed: a star-done latch bit and a delta latch bit.
What is the star delta starter PLC ladder diagram?
The star delta starter PLC ladder diagram has a Start/Stop seal-in rung that latches RUN_BIT, a MAIN_CONTACTOR (KM1) rung on RUN_BIT alone, a STAR_CONTACTOR (KM2) rung on RUN_BIT AND NOT STAR_DONE, a T_STAR TON timer rung (3–7 s) that latches STAR_DONE, a short T_TRANSITION TON (≈200 ms) that latches DELTA after a dead-time gap, and a DELTA_CONTACTOR (KM3) rung on RUN_BIT AND DELTA_LATCH. To keep a copy of this exact ladder diagram, press Ctrl/Cmd+P on this page and choose Save as PDF.
How do you write a star delta PLC program?
Latch RUN_BIT from the Start push-button (reset by Stop and E-stop in series), energise MAIN and STAR together at start, then run a TON for the star time. When that timer is done, latch STAR_DONE to drop STAR, run a second short TON for the transition dead-time, then latch DELTA to energise the DELTA contactor. Drive STAR by RUN_BIT AND NOT STAR_DONE and DELTA by RUN_BIT AND DELTA_LATCH so they are mutually exclusive. You can write and run exactly this star delta PLC program in the browser scenario on this page.
Is star delta starter PLC programming hard to learn?
No — it is one of the most approachable timed-sequence projects because it is just three contactor coil rungs plus two TON timers and two latch bits. The only subtlety is the transition dead-time: the star contactor must fully drop before the delta contactor picks up, which a 200 ms transition timer handles. The live scenario on this page lets you write the rungs and immediately see the timer-driven star-to-delta transition graded by four automated test cases.
