Ladder Diagram for Star Delta Starter: Complete PLC Program with Timer, Interlocks, and I/O Table
This is a free printable ladder diagram for star delta starter: download the diagram as SVG or open it and print to paper or PDF.
A star-delta starter (also written Y-Δ starter) reduces the inrush current of a 3-phase induction motor at start-up by initially connecting the stator windings in star configuration, then switching to delta for full-speed running. The PLC ladder diagram for a star-delta starter uses three contactors (main KM1, star KM3, and delta KM2), a TON timer to control the Y-to-Δ transition, and critical electrical interlocks to prevent KM2 and KM3 from closing simultaneously — an event that would short-circuit two motor phases.
Why star-delta starters exist: a direct-on-line (DOL) 3-phase induction motor draws 5 to 7 times its full-load current (FLA) during start-up. For a 37 kW motor with a FLA of 70 A that means a starting surge of 350–490 A. This stresses the supply transformer, causes voltage dips that affect other equipment on the same busbar, and subjects the motor windings to high electromagnetic forces. The star connection reduces the voltage applied to each winding from line voltage (e.g. 400 V) to phase voltage (400/√3 ≈ 231 V), cutting starting current to approximately one-third of the DOL value. Torque is also reduced to one-third, which is acceptable for low-inertia loads (fans, centrifugal pumps, compressors) but unsuitable for high-starting-torque applications (conveyors under full load, crushers).
Understanding the three operating states:
State 1 — Star (Y): KM1 (main) closes first, then KM3 (star) closes to join the motor's U2/V2/W2 terminal end-connections together at a neutral star point. The motor accelerates under reduced voltage. This state holds for the timer preset period, typically 5–10 seconds depending on the motor's inertia and the required speed ramp.
State 2 — Open transition: At the end of the timer preset, KM3 (star) opens first. A brief dead-time (2–5 complete scan cycles, or a dedicated short timer of 50–100 ms) is observed before KM2 (delta) closes. This dead-time is the open-circuit transition interval — the motor is momentarily disconnected from the supply entirely. The disconnection causes a transient current spike when delta reconnects as the motor's residual magnetic field may be out of phase with the supply. A closed-circuit transition (where the motor is never disconnected) eliminates this spike but requires a more complex circuit with a fourth impedance contactor.
State 3 — Delta (Δ): KM2 (delta) closes, connecting U1-W2, V1-U2, W1-V2 (standard terminal delta connection). The motor now receives full line voltage across each winding and runs at rated speed.
Power circuit connections: on the line side, KM1 feeds all three phases (L1/L2/L3) to the motor's top terminals (U1/V1/W1). KM3 (star) shorts together the motor's bottom terminals (U2/V2/W2) to form a neutral point — the star centre. KM2 (delta) connects U1 to W2, V1 to U2, and W1 to V2, forming the delta configuration. The overload relay (OL or F1) is always installed in the KM1 circuit, measuring the actual motor current in all three phases.
I/O allocation table: - I0.0: Start pushbutton (momentary NO) - I0.1: Stop pushbutton (NC, normally closed for fail-safe) - I0.2: Overload relay contact (NC, fail-safe) - Q0.0: Main contactor KM1 - Q0.1: Delta contactor KM2 - Q0.2: Star contactor KM3 - M0.0: Run memory bit (seal-in latch)
The complete ladder program — rung by rung:
Rung 1 — Start/Stop seal-in with overload: XIC Start_PB (I0.0) in parallel with XIC Run_M (M0.0), all in series with XIO Stop_PB (I0.1) and XIO OL_Contact (I0.2). Output: OTE Run_M (M0.0). This is the standard motor start/stop seal-in circuit. When Start is pressed, M0.0 energises and seals itself in. Stop or overload breaks the rung.
Rung 2 — Main contactor KM1: XIC Run_M (M0.0). Output: OTE KM1 (Q0.0). The main contactor closes whenever the Run bit is set.
Rung 3 — Star contactor KM3 (with delta electrical interlock): XIC Run_M (M0.0) AND XIO KM2 (Q0.1) AND XIO T_Star.DN. Output: OTE KM3 (Q0.2). The star contactor closes when running, provided the delta contactor is NOT closed (XIO Q0.1 is the electrical interlock) and the star timer has NOT completed. This is the critical safety rung — if the delta output ever becomes TRUE while this rung is executing, XIO Q0.1 immediately opens and KM3 de-energises.
Rung 4 — Star-to-delta TON timer (T_Star, PRE = 7 s): XIC Run_M (M0.0) AND XIC KM3 (Q0.2). Enables TON T_Star with a preset of 7 000 ms. The timer only runs while both the Run bit and the star contactor output are TRUE — if the star contactor fails to close for any reason, the timer does not run and the transition never occurs. When ACC = PRE, T_Star.DN goes TRUE.
Rung 5 — Delta contactor KM2 (with star electrical interlock): XIC Run_M (M0.0) AND XIC T_Star.DN AND XIO KM3 (Q0.2). Output: OTE KM2 (Q0.1). The delta contactor closes only after the star timer completes and the star contactor output is de-energised (XIO Q0.2 is the electrical interlock in the delta direction). This mutual interlock — KM2 has XIO Q0.2, and KM3 has XIO Q0.1 — is implemented in both the ladder and as hardwired contacts in the contactor auxiliary circuit.
Why electrical interlock alone is insufficient and mechanical interlock is also required: the ladder electrical interlock operates at software scan speed, typically 1–20 ms. If a PLC CPU fault causes the output image to become corrupted, both Q0.1 and Q0.2 could momentarily be TRUE. Mechanical interlocks on the contactor bodies operate independently of the PLC program — a physical lever linkage between the two contactor arms prevents both from closing simultaneously regardless of coil signal state. IEC 60947-4-1 categorises this as a mechanically interlocked assembly. Both layers of protection are required in a compliant installation.
Open-circuit vs closed-circuit transition: in open-circuit transition (the standard approach described above), the motor is briefly disconnected between star and delta. The residual rotor flux creates a back-EMF that may be phase-shifted from the mains when KM2 closes, generating a reconnection transient. For most applications this is acceptable. In closed-circuit transition (also called soft transition), a fourth impedance contactor inserts a set of resistors in the circuit during the transition to limit the reconnection spike. This is more expensive but required for very large motors or sensitive supply networks.
Star-delta vs soft starter vs VFD: a soft starter uses power electronics (SCRs) to gradually ramp the applied voltage, eliminating the hard transition spike entirely. A VFD (variable frequency drive) controls both voltage and frequency, providing the smoothest start and full speed control. Star-delta is the lowest-cost option and remains appropriate for applications where a 1/3-torque start is sufficient and the motor runs continuously at one speed.
Timer setting guidance: the transition timer preset (T_Star.PRE) should be set long enough for the motor to reach close to synchronous speed in star before switching to delta. For most centrifugal pump and fan applications, 5–8 seconds is typical. Check the motor's speed-torque curve — if the motor is still accelerating rapidly at the end of the timer, extend the preset. If the motor is fully up to speed well before the timer expires, shorten it to minimise wear on the star contactor.
Draw your own star-delta PLC ladder diagram in circuitdiagrammaker — add your specific I/O addresses, set the timer preset, label the interlocks, and export a print-ready PDF for your panel documentation.
How to wire ladder diagram for star delta starter
- Rung 1 — Start/Stop seal-in with overload protection XIC Start_PB (I0.0) in parallel with XIC Run_M (M0.0), all in series with XIO Stop_PB (I0.1) and XIO OL_Contact (I0.2). Output: OTE Run_M (M0.0). Pressing Start sets M0.0 and seals it in through the parallel XIC. Stop (NC) or overload (NC) breaking breaks the rung and de-energises M0.0, stopping all motor contactors.
- Rung 2 — Main contactor KM1 XIC Run_M (M0.0). Output: OTE KM1 (Q0.0). The main contactor connects three-phase supply to the motor's U1/V1/W1 terminals. It remains closed throughout both the star and delta phases — it only opens when the motor is stopped.
- Rung 3 — Star contactor KM3 with delta electrical interlock XIC Run_M (M0.0) AND XIO KM2_Delta (Q0.1) AND XIO T_Star.DN. Output: OTE KM3 (Q0.2). The XIO Q0.1 electrical interlock is the safety-critical element: if the delta output is ever TRUE for any reason, this contact immediately opens and de-energises the star contactor. The XIO T_Star.DN gate causes the star contactor to drop out when the transition timer completes.
- Rung 4 — Star-to-delta transition TON timer (T_Star, PRE = 7 s) XIC Run_M (M0.0) AND XIC KM3 (Q0.2). Enables TON T_Star with preset 7 000 ms. The timer requires both the run bit AND the star contactor output to be TRUE before timing starts. This prevents timing without confirmation that the star contactor actually closed. When done, T_Star.DN = TRUE.
- Rung 5 — Dead-time delay (optional but recommended for open transition) Insert a TON T_Deadtime (PRE = 100 ms) with XIC T_Star.DN as the enable. This short dead-time ensures KM3 has physically opened before KM2 closes, preventing even a sub-millisecond overlap between star and delta contacts. Output T_Deadtime.DN enables the delta rung.
- Rung 6 — Delta contactor KM2 with star electrical interlock XIC Run_M (M0.0) AND XIC T_Star.DN (or T_Deadtime.DN if using dead-time rung) AND XIO KM3_Star (Q0.2). Output: OTE KM2 (Q0.1). The XIO Q0.2 interlock prevents KM2 from closing if KM3 is still energised. The delta contactor latches in through the Run bit — it stays closed until the stop or overload condition breaks Rung 1.
- Verify mechanical interlock is fitted In the panel, confirm that KM2 and KM3 are mechanically interlocked (a lever linkage between the contactor bodies prevents both from closing simultaneously regardless of PLC output state). This hardware interlock is mandatory in addition to the electrical interlock in Rungs 3 and 6.
- Test the sequence in simulation Using circuitdiagrammaker or your PLC simulator, force I0.0 (Start) TRUE. Verify: KM1 (Q0.0) energises immediately; KM3 (Q0.2) energises; T_Star begins counting. After 7 s, verify T_Star.DN goes TRUE, KM3 de-energises, then KM2 (Q0.1) energises. Confirm KM2 and KM3 are NEVER simultaneously TRUE.
Specifications
| Motor type | 3-phase induction motor with 6 winding terminals (dual-voltage or star-delta rated) |
|---|---|
| Supply voltage (delta) | Equal to line voltage (e.g. 400 V AC 50 Hz or 460 V AC 60 Hz) |
| Voltage in star phase | Line voltage ÷ √3 ≈ 58% of rated delta voltage |
| Starting current reduction | ~1/3 of DOL inrush current |
| Starting torque reduction | ~1/3 of DOL starting torque |
| Contactors required | KM1 (main), KM2 (delta), KM3 (star) — minimum; 4th contactor for closed-transition |
| Timer instruction | TON — typical preset 5–10 s (application dependent) |
| Electrical interlock | XIO Q0.1 on star rung; XIO Q0.2 on delta rung (both required) |
| Mechanical interlock | Mandatory hardware interlock between KM2 and KM3 per IEC 60947-4-1 |
| Overload relay position | Always in the main (KM1) circuit, measuring full motor current |
| Transition type | Open-circuit (standard); closed-circuit optional with 4th impedance contactor |
| PLC I/O count | 3 inputs (Start, Stop, OL) + 3 outputs (KM1, KM2, KM3) |
Safety warnings
- KM2 and KM3 must be both mechanically and electrically interlocked. Energising both simultaneously causes a phase-to-phase short circuit that can destroy the motor, start a fire, and pose a lethal arc-flash hazard. Do not commission without confirming both interlock layers are in place.
- All work on contactor panels must comply with local electrical safety regulations (NFPA 70E in the USA, BS 7671 in the UK, or equivalent national standards). Isolate and lock out/tag out (LOTO) the supply before working on any live panel.
- The overload relay must be correctly set to the motor FLA. An incorrectly set or bypassed overload relay leaves the motor unprotected against phase failure and sustained overload — the most common cause of motor winding burnout.
- Open-transition switching produces a reconnection voltage transient. For motors connected to sensitive instrumentation or shared supply busbars, specify a closed-transition starter to avoid voltage disturbances affecting other equipment.
Tools needed
- PLC programming software (TIA Portal, Studio 5000, OpenPLC, CX-Programmer, or circuitdiagrammaker.com for diagram drafting)
- Three contactors rated for the motor FLA: KM1 (main), KM2 (delta), KM3 (star) — mechanically interlocked pair for KM2/KM3
- Overload relay rated to motor FLA (electronic or thermal-magnetic, Class 10 or 20)
- TON timer instruction with adjustable preset (5–10 s typical, verified from motor data sheet)
- Motor data sheet showing full-load current (FLA), starting current multiplier, and inertia constant J
- I/O allocation table completed before writing the first rung
Common mistakes
- Fitting the overload relay in the star contactor (KM3) circuit instead of the main contactor (KM1) circuit — in the star phase motor current is measured at star-connected values; during delta the star-side OL is bypassed entirely, providing no protection in the run state
- Omitting the mechanical interlock and relying solely on the ladder electrical interlock — a PLC CPU fault or stuck output can override software interlocks; the mechanical interlock is a hardware last line of defence
- Setting the star-phase timer too short for the connected load, causing delta reconnection while the motor is still accelerating — this produces a high reconnection transient equal to or worse than DOL starting current
- Connecting only three motor terminals (DOL style) rather than all six — a star-delta starter requires access to both ends of each winding (U1/V1/W1 and U2/V2/W2)
- Using the same output address for two separate contactors due to copy-paste errors — duplicate OTE coils mean the second rung's state overwrites the first each scan, creating unpredictable contactor switching
Troubleshooting
- Motor runs in star but never switches to delta
- Cause: T_Star timer never completes because either the timer enable rung de-energises before the preset (M0.0 drops out due to stop or OL) or T_Star.PRE is set to an extremely large value by error Fix: Monitor T_Star.ACC in online view while the motor runs. If ACC is counting but never reaching PRE, verify the preset value. If ACC resets to zero unexpectedly, trace why the enable rung (M0.0 AND KM3) drops out prematurely — check for nuisance OL trips.
- Delta contactor KM2 energises and immediately de-energises (chatters)
- Cause: The KM3 output (Q0.2) is not fully de-energised before the delta rung evaluates — the XIO Q0.2 interlock on the delta rung is TRUE for only part of the scan, causing the delta output to flicker Fix: Add a short dead-time TON (PRE = 100 ms) between T_Star.DN and the delta rung enable. This guarantees KM3 has mechanically opened before KM2 is commanded to close.
- Overload relay trips immediately when transitioning to delta
- Cause: Open-transition reconnection transient exceeds the OL relay's Class 10 trip curve; or the motor was not fully accelerated in star, so the delta reconnection adds to a still-high starting current Fix: Increase T_Star.PRE to allow full acceleration. Switch to a Class 20 or Class 30 overload relay which tolerates higher transient currents for longer. For persistent problems, specify a closed-transition starter.
- Motor hums and does not accelerate in star phase
- Cause: One or more motor terminals (U2/V2/W2) are not connected to the star contactor KM3 output, so the star neutral point is not formed and the motor sees imbalanced phase voltages Fix: Verify power circuit wiring with a multimeter: with KM1 and KM3 both closed (motor stopped, test with simulation), check continuity between U2-V2-W2 at the motor terminal box. All three should show continuity to each other.
- KM1 (main contactor) chatter on start-up
- Cause: The Start pushbutton (I0.0) is wired as NC instead of NO, or the seal-in contact (M0.0) address is wrong, so the rung de-energises as soon as Start is released Fix: Confirm I0.0 is a normally open momentary contact wired so its PLC input reads TRUE when pressed. Confirm the seal-in XIC contact uses the M0.0 bit address — not a different memory bit — so it holds the rung energised after Start is released.
Frequently asked questions
Why is star connection used first in a star-delta starter?
In star connection each motor winding receives line voltage divided by the square root of 3 (approximately 58% of line voltage). Because motor torque is proportional to the square of voltage, this reduces starting torque to about one-third of its DOL value and reduces starting current to about one-third, easing stress on the supply network and motor windings during acceleration.
How much does a star-delta starter reduce starting current?
A star-delta starter reduces starting current to approximately one-third (33%) of the direct-on-line starting current. For a motor with a 6× FLA DOL starting current, a star-delta starter limits inrush to approximately 2× FLA during the star phase.
What is the typical timer setting for a star-delta starter?
The transition timer (T_Star) is typically set to 5–10 seconds. The correct value depends on the motor's moment of inertia and load torque curve — the motor should have reached close to full speed in star before the delta transition fires. Setting too short causes the delta contactor to close while the motor is still accelerating, creating a high reconnection current. Setting too long subjects the star contactor to prolonged operation at higher-than-rated current.
Why must the star and delta contactors be interlocked?
If KM2 (delta) and KM3 (star) both close simultaneously, two motor phases are directly short-circuited through the delta crossover connections. This creates an immediate phase-to-phase fault, destroying the contactors, blowing fuses, and potentially damaging the motor. Both mechanical interlock (hardware) and electrical interlock (ladder XIO contacts) are required under IEC 60947-4-1.
What is the difference between open-transition and closed-transition star-delta?
Open-transition briefly disconnects the motor from supply between star and delta phases. The residual rotor field may be out of phase with the mains when delta reconnects, creating a current transient that is often as large as the original DOL inrush. Closed-transition inserts impedance (resistors or reactors) via a fourth contactor so the motor is never fully disconnected, eliminating the reconnection spike. Closed-transition is more expensive but required for very large motors or networks sensitive to voltage dips.
Can a star-delta starter be used on any 3-phase motor?
Only on motors designed for delta connection at the supply voltage and which have all six winding terminals (U1/V1/W1 and U2/V2/W2) brought out to the terminal box. A motor with only three terminals (internally delta-connected) cannot be used with a star-delta starter. Also unsuitable for high-starting-torque applications (loaded conveyors, compressors) because the one-third torque in star may be insufficient to accelerate the load.
What is the difference between a star-delta starter and a soft starter?
A star-delta starter switches mechanically between two fixed voltage levels (58% in star, 100% in delta), producing a step change and a reconnection transient at the transition. A soft starter uses SCRs to continuously ramp the applied voltage from zero to full over a programmable ramp time, eliminating the step and the transient. Soft starters also allow adjustable ramp time, current limiting, and built-in overload protection, but cost significantly more than a star-delta contactor set.
Why does my motor trip on overload when transitioning from star to delta?
The reconnection transient in open-transition star-delta produces a brief current spike that can exceed the overload relay trip threshold, especially for fast-tripping electronic overloads. Solutions: increase the overload trip class (e.g. from Class 10 to Class 20) to tolerate the transient; use a closed-transition circuit; or increase T_Star.PRE so the motor is fully accelerated in star and the delta reconnection speed mismatch is minimised.