Control Circuit of Star Delta Starter

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The control circuit of a star-delta starter uses relay logic, a timer, and electrical interlocks to sequence the main, star, and delta contactors safely — ensuring correct transition from reduced-voltage starting to full-voltage running.

While the power circuit of a star-delta starter deals with the motor terminals and the high-voltage, high-current switching between star and delta configurations, the control circuit is the brain that governs the sequence. It operates at a reduced control voltage — typically 110 V AC or 24 V AC/DC from a control transformer — and governs which contactor energises at which moment and in which order.

The control circuit sequence works as follows:

1. The operator presses the START pushbutton (SB1 — normally open, momentary). This energises the main contactor coil (KM) and the star contactor coil (KY) simultaneously, through the normally closed (NC) stop button (SB2) and the normally closed overload relay contact (OL-NC).

2. An auxiliary normally open (NO) contact of KM seals in the start circuit — this is the holding or latching contact. Once KM is energised and its auxiliary NO contact closes across the start pushbutton, the start button can be released and the circuit remains latched.

3. An auxiliary NO contact of KM simultaneously supplies power to the star-delta timer (T1) coil, starting the timing period.

4. During the star period, KM and KY are both energised. The NC auxiliary contact of KY is in series with the KD coil circuit — so KD cannot energise while KY is closed. This is the electrical interlock preventing simultaneous star and delta connection.

5. When the timer reaches its set time, its normally open timed contact closes. This energises the delta contactor (KD) coil — but before KD can energise, the KY coil must de-energise. The timer circuit also opens the supply to KY, or the KD coil circuit energising opens the KY coil supply via KD's NC auxiliary contact.

6. With KY de-energised and KD energised, the motor runs in delta.

7. The STOP pushbutton (SB2 — NC) opens the control circuit entirely, de-energising KM, KD, and T1, returning all contactors to their rest state.

The overload relay contact (OL-NC) is wired in series with the entire control circuit such that any overload trip instantly de-energises all contactors and stops the motor.

How to wire control circuit of star delta starter

Establish the control supply Connect a control transformer (if used) between two supply phases or from line and neutral. The control transformer's secondary output — typically 110 V AC or 24 V AC — supplies the control circuit. Fuse the secondary with an appropriately rated fuse (typically 1–4 A depending on the number and type of contactor coils in the circuit).
Wire the stop pushbutton in series as the first element Connect one side of the control supply (L) to one terminal of the stop pushbutton (SB2, which is a normally closed momentary contact). All control current must pass through the stop button — this ensures that pressing STOP immediately breaks the control circuit and de-energises all contactors and the timer. Never place the stop button in a position where pressing it only affects one branch of the circuit.
Wire the overload relay NC contact in series after the stop button Connect the output of the stop button to the normally closed contact of the thermal overload relay (OL-NC). The overload relay's NC contact opens when a motor overload is detected, de-energising the entire control circuit in the same way as the stop button. After OL-NC, connect to the common rail of the parallel start/holding contact branch.
Wire the start pushbutton in parallel with the KM holding contact Connect the start pushbutton (SB1, normally open momentary) between the common rail and the KM coil. In parallel with SB1, wire the KM auxiliary NO (self-latching) contact. When the start button is pressed, current flows through SB1 to KM. When KM energises, its auxiliary NO contact closes, providing a parallel path that maintains KM energised after SB1 is released.
Wire the KY coil in parallel with KM (simultaneous energisation at start) The star contactor coil (KY) must energise simultaneously with KM on start. Wire KY coil in parallel with KM coil, supplied from the same control circuit rail (after the overload contact and stop button). Place the KD auxiliary NC contact in series with the KY coil — this is the interlock that prevents KY from energising if KD is already closed.
Wire the timer relay and the KD transition circuit Wire the timer relay (T1) coil in series with a KM auxiliary NO contact, so the timer starts counting from the moment KM energises. The timer's timed-to-energise NO contact wires to the KD coil circuit. The KD coil also has KY auxiliary NC contact in series (interlock). Some control circuits also include a timer contact that opens the KY circuit before closing KD — ensure the specific timer and contactor sequence is designed so KY fully de-energises before KD energises, even if only by the contactor's mechanical travel time.
Test the control circuit under no-load conditions Before connecting the motor, test the control circuit with the power circuit disconnected. Apply the control supply and press START. Observe: KM and KY should energise simultaneously. After the set timer period, KY should de-energise and KD should energise. Press STOP — all contactors should de-energise immediately. Simulate an overload trip by manually opening the OL-NC contact and confirm all contactors drop out.

Specifications

Control supply voltage (typical)	110 V AC (from control transformer) or 24 V AC/DC
Control circuit fuse (typical)	1–4 A, matched to transformer secondary VA rating and contactor coil count
Stop button type	Normally closed (NC), momentary, red, clearly labelled STOP or HALT
Start button type	Normally open (NO), momentary, green, clearly labelled START or RUN
Interlock contact type	Normally closed (NC) auxiliary contact of KY in series with KD coil; NC contact of KD in series with KY coil
Timer type	On-delay (energise delay) — contact closes a set time after coil is energised
Overload relay contact in control circuit	Normally closed (NC) — opens on overload trip, de-energising all control circuit elements
Applicable standards	IEC 60947-4-1 (motor starters); IEC 60947-5-1 (control circuit devices); IEC 60364 (installation)

Safety warnings

The control circuit of a star-delta starter is connected to and controls high-voltage, high-current power circuits. Even if working only on the low-voltage control circuit, the power circuit must be isolated, locked out, and verified dead before working inside the starter panel. Use lock-out/tag-out procedures at the main isolator. Three-phase motor circuits at 380–415 V are immediately lethal.
Electrical interlocking between KY and KD is a mandatory safety requirement, not an optional enhancement. Hardware interlocking through auxiliary contacts must be present in the control circuit — software or timer-based interlock alone is not sufficient. Verify interlock wiring by attempting to manually energise both contactors simultaneously and confirming the circuit prevents this.
Do not bypass or defeat the overload relay under any circumstances. The overload relay is the primary protection for the motor against sustained overcurrent — bypassing it can result in motor burn-out, fire, and personnel injury.
Follow IEC 60947-4-1 for contactor and motor starter sizing, and IEC 60364 or the applicable national wiring standard for the installation. Star-delta starters must be installed by a licensed electrician.
Arc flash hazards exist inside energised motor control panels. Wear arc flash rated PPE (arc-rated face shield, gloves, and clothing) when working on or near energised starter panels, and comply with the site's arc flash risk assessment.

Tools needed

Digital multimeter (AC and DC voltage, continuity, resistance)
Insulated screwdrivers (for terminal wiring)
Torque screwdriver (for terminal connections — undertorqued connections cause arcing and overheating)
Clamp meter (for measuring control circuit current during commissioning)
Wiring diagram and control sequence diagram for the specific starter
Lock-out/tag-out equipment
Personal protective equipment: arc flash rated face shield, arc-rated gloves and clothing

Common mistakes

Wiring the KM holding contact after the overload relay NC contact rather than in parallel with the start button — if wired incorrectly, the holding contact may bypass the overload trip, allowing the motor to run regardless of an overload condition.
Omitting or incorrectly wiring the electrical interlock between KY and KD, relying only on the timer to prevent simultaneous energisation — timer contact bounce or malfunction can allow simultaneous contact closure without a hardware interlock.
Connecting the timer coil without a KM auxiliary contact in series — the timer starts when control power is applied rather than when the motor starts, allowing the timer to elapse before the start button is pressed.
Using a timer set too short, switching to delta before the motor has accelerated sufficiently, causing excessive transition current and potential overload tripping.
Not including a reset circuit after an overload trip — some control circuits require the overload relay to be manually reset at the relay before the circuit can be re-energised; omitting a clear reset procedure in the panel layout causes operator confusion.
Undertorquing control wiring terminals — in the vibrating environment of an industrial panel, loose terminals cause high-resistance joints that heat up, arc, and eventually open the control circuit at a critical moment.

Troubleshooting

Motor starts in star but never transitions to delta: Cause: Timer relay faulty or its timed NO contact not closing; wiring error in the delta contactor circuit Fix: With the motor running in star (motor disconnected from the mechanical load for safety), measure voltage at the timer relay's timed contact output after the set time elapses. If voltage does not appear, the timer is faulty. If voltage appears but KD does not energise, check the KY NC interlock contact and the KD coil wiring.
Delta contactor energises immediately at start without any star period: Cause: Timer relay wired incorrectly — its contact is wired to KD without the timer delay, or timer set to minimum (near zero) delay Fix: Verify the timer relay wiring: the timer coil input should receive a delayed output. Check the timer set-point — many timers have their adjustment at the minimum end of their range as default. Increase the set-point to the required value and confirm the delay occurs correctly.
Contactor chattering or buzzing during start: Cause: Insufficient control voltage at contactor coil due to voltage drop in control wiring or control transformer overloaded by too many contactor coils Fix: Measure AC voltage at the contactor coil terminals during start. If significantly below the rated coil voltage, the control transformer may be undersized for the combined inrush current of all contactor coils at the moment of starting. A control transformer rated at the sum of all contactor coil currents is required.
Overload relay trips during normal operation: Cause: Overload set below motor FLC, extended star period causing higher-than-expected temperature build-up, or the motor is overloaded by the mechanical system Fix: Verify the overload relay setting matches the motor nameplate FLC. Measure line current in delta running condition with a clamp meter and compare to FLC. If current is above FLC in delta, the motor is mechanically overloaded. If current is normal and the relay still trips, the relay trip class may be too sensitive for the starting profile.
Motor stops immediately after START button is released: Cause: Self-latching (holding) auxiliary contact of KM not wired correctly, or the KM auxiliary contact is faulty Fix: Verify the wiring of the KM NO auxiliary contact in parallel with the start pushbutton SB1. With the motor running (if briefly possible), test whether the KM auxiliary contact has closed by measuring continuity across it. If the wiring is correct but the contact is not closing, the auxiliary contact block on KM is faulty and must be replaced.

Frequently asked questions

Why must the star and delta contactors be electrically interlocked?

If both KY (star) and KD (delta) close simultaneously, the motor end terminals are connected in star (shorted together by KY) while simultaneously being connected to supply phases in a delta cross-connection by KD. This creates direct phase-to-phase short circuits through the motor windings, destroying the motor, the contactors, and potentially tripping the upstream supply protection. Electrical interlocking — NC auxiliary contact of each contactor wired in series with the other's coil — prevents this regardless of timer or wiring faults.

What is the role of the self-latching (holding) auxiliary contact?

The start pushbutton is momentary — it does not latch mechanically. The holding contact is an auxiliary NO contact of the main contactor (KM) wired in parallel with the start pushbutton. When KM energises, this contact closes, providing an alternative current path that keeps KM energised after the start button is released. Without this contact, the motor would stop the moment the operator released the start button.

Can the control circuit use a PLC instead of relay logic?

Yes. A PLC (programmable logic controller) can replicate the entire relay logic sequence in software, using its digital outputs to switch contactor coils via small interface relays. The sequence logic — start, star period, timer, delta transition, stop, overload trip — is programmed rather than hardwired. PLC control adds flexibility, diagnostic capability, and the ability to add interlocks with other machines, but still requires the physical electrical interlocks between KY and KD as a hardware backup.

What happens if the timer fails in the closed position (timer contact stays closed)?

If the timer energises the delta contactor (KD) prematurely or immediately, the sequence attempts to switch to delta before the motor has accelerated. The result is a high inrush current similar to direct-on-line starting, potentially tripping the overload relay or blowing fuses. If the timer contact fails open (never closes), the motor runs continuously in star, overheating the windings. Either failure mode requires timer replacement.

Why is the control circuit supplied from a reduced voltage via a control transformer?

A control transformer reduces the supply voltage — typically from 380–415 V three-phase to 110 V single-phase — for the control circuit. This makes the control circuit safer to work on (110 V is less likely to be fatal than 400 V) and allows the use of smaller, lighter, less expensive relay and contactor coils. It also isolates the control circuit from the power circuit, reducing the risk of a power circuit fault energising control wiring.

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