Star-Delta Control Circuit Diagram: Timer, Contactors, and Mandatory Interlocks
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A star-delta control circuit diagram shows how a timer relay and three contactors — KM1, KM2, and KM3 — sequence a three-phase motor from star starting to delta running, with mandatory electrical interlocks to prevent simultaneous star and delta energisation.
Star-delta starting is the most widely used reduced-voltage starting method for three-phase induction motors in the 5 kW to 150 kW range. Its purpose is to reduce the inrush current at startup, which — in direct-on-line (DOL) starting — can be 6 to 8 times the motor's full-load current. Star-delta starting reduces this initial inrush to approximately one-third of the DOL value, which eases the demand on the supply and reduces mechanical shock to the driven load.
The method works by briefly connecting the motor windings in star configuration during starting (lower voltage per winding), then switching to delta configuration (full voltage per winding) once the motor has accelerated to near-synchronous speed.
The control circuit requires three contactors, designated by convention as KM1, KM2, and KM3: - KM1 is the main contactor. It connects the motor terminals to the three-phase supply at all times when the motor is running. - KM2 is the star contactor. It shorts the far ends of the three motor windings together to form the star point during the starting phase. - KM3 is the delta contactor. It cross-connects the motor terminals to form the delta configuration during the running phase.
A timer relay (TR) controls the duration of the star phase. The timer is set to the time required for the motor to accelerate to approximately 80–90% of synchronous speed in star — typically between 5 and 15 seconds depending on the motor size and load inertia.
The most critical aspect of the control circuit is the interlocking between KM2 (star) and KM3 (delta). If both contactors energise simultaneously, they create a phase-to-phase short circuit across the supply. To prevent this, the control circuit must include electrical interlocks: a normally-closed auxiliary contact of KM3 is wired in series with the coil circuit of KM2, and a normally-closed auxiliary contact of KM2 is wired in series with the coil circuit of KM3. Relying on mechanical interlocks or software-only interlocks alone is insufficient for safety — both are required.
The transition from star to delta produces a current transient because the motor is momentarily disconnected before being reconnected in delta. This transition transient can be as high as DOL inrush in some conditions — this is a known limitation of the open transition star-delta starter.
How to wire star delta control circuit diagram
- Verify the motor is delta-connected at nameplate voltage Star-delta starting only works on a motor whose windings are rated for the full line voltage in delta configuration. The motor nameplate must show the delta connection voltage matching the supply. For example, a motor rated 400 V delta (690 V star) is correctly connected for star-delta starting on a 400 V supply.
- Size and select three contactors: KM1, KM2, KM3 KM1 (main) must be rated for the motor full-load current. KM2 (star) and KM3 (delta) are rated at approximately 58% of the motor full-load current, because in star/delta they carry only phase winding current, not full line current. Each contactor must also include auxiliary contact blocks for interlocking and control circuit use.
- Select and size the overload relay The overload relay (OL) protects the motor against sustained overcurrent. In a star-delta starter, the overload relay is typically placed in the motor winding circuit (after KM3 in the delta circuit), not in the line circuit. Set the overload relay to the motor full-load current divided by 1.732 (the ratio for winding current in delta).
- Wire the main power circuit Connect the three-phase supply to the line side of KM1. The load side of KM1 connects to the motor terminals (U1, V1, W1) and also to the line side of KM3. The load side of KM3 connects to the motor winding far ends (U2, V2, W2). The line side of KM2 connects to U2, V2, W2, and the load side of KM2 shorts all three together to form the star point.
- Wire the control circuit start/stop logic The control circuit typically operates at 24 V DC, 110 V AC, or 230 V AC via a control transformer. Wire a start pushbutton (normally open) in series with a stop pushbutton (normally closed) and the coil of KM1. Add a latching auxiliary contact from KM1 in parallel with the start button so KM1 seals in when released.
- Wire the star contactor and timer relay Wire the coil of KM2 (star contactor) in series with: the normally-closed auxiliary contact of KM3 (electrical interlock), and an auxiliary contact from KM1 (so KM2 only energises when KM1 is closed). Wire the timer relay coil in parallel with KM2's coil so it starts timing simultaneously with the star connection.
- Wire the delta contactor with interlocks The timer's normally-open timed contact drives the coil of KM3 (delta). Wire a normally-closed auxiliary contact of KM2 in series with KM3's coil (electrical interlock against simultaneous energisation). Also wire the timer's contact to drop out KM2 before KM3 closes — achievable using a normally-closed instantaneous contact from KM3 in the KM2 circuit. Verify both interlocks are present before commissioning.
Specifications
| Starting current reduction vs DOL | Reduced to 1/3 of DOL inrush current |
|---|---|
| Starting torque reduction vs DOL | Reduced to 1/3 of DOL starting torque |
| Number of contactors required | 3 (KM1 main, KM2 star, KM3 delta) |
| KM2 / KM3 current rating (relative to KM1) | Approximately 58% of KM1 (FLC / 1.732) |
| Typical star phase timer setting | 5–15 seconds (motor and load dependent) |
| Ripple frequency of pulsating winding voltage in star | N/A — this is a motor starting method, not a rectifier |
| Applicable motor type | Three-phase squirrel cage induction motor, winding rated for supply voltage in delta |
Safety warnings
- Simultaneous energisation of KM2 (star) and KM3 (delta) causes a three-phase short circuit. Electrical interlocking via normally-closed auxiliary contacts in series with each contactor's coil is not optional — it is a mandatory safety requirement. Verify interlocks before commissioning.
- All motor starter panel work must comply with IEC 60364, IEC 60204-1, NEC/NFPA 70, BS 7671, or the applicable local standard. The panel must be installed, wired, and commissioned by a qualified and licensed electrician.
- Always isolate the main supply, lock out the isolator, and verify dead with an approved voltage tester before working inside the motor starter panel. The capacitance in contactor snubbers may hold charge briefly after isolation.
- The star-delta transition produces a mechanical torque transient. Verify that the driven load and coupling can withstand this transient before commissioning. For sensitive loads, consider a soft-starter or variable-frequency drive instead.
- The overload relay must be correctly set. An incorrectly set overload relay provides false protection — set it to the motor winding current (FLC / 1.732) not to the motor line current if placed in the winding circuit.
Tools needed
- Digital multimeter (voltage, continuity, resistance)
- Insulation resistance tester (megohmmeter)
- Clamp meter (for measuring starting and running current)
- Screwdrivers (flat and cross-head, insulated)
- Wire stripper and crimping tool
- Ferrule crimping tool and ferrules (for terminal connections)
- Calibrated timer (for verifying timer relay setting)
Common mistakes
- Omitting the electrical interlock between KM2 and KM3 — relying only on a mechanical interlock bar without the auxiliary contact in the coil circuit leaves the circuit vulnerable to interlock failure.
- Setting the timer too short, causing the delta contactor to close before the motor has adequately accelerated, which produces a transition transient as severe as DOL starting — the opposite of the intended benefit.
- Placing the overload relay in the main line (after KM1) rather than in the motor winding circuit — in a star-delta starter, line current and winding current differ by a factor of 1.732 in delta, so an overload relay in the line circuit must be set differently.
- Using contactors of identical current rating for all three positions — KM2 and KM3 can legitimately be sized smaller than KM1 because they carry winding current, not full line current, reducing panel cost.
- Failing to check motor winding connection before applying star-delta — a motor whose windings are rated for the supply voltage in star (not delta) will run at reduced voltage in delta, damaging the motor.
Troubleshooting
- Motor starts but does not transition to delta — remains in star
- Cause: Timer relay is not timing out (coil not energised, timer set too long, or timer fault) or KM3 coil circuit is open (interlock contact stuck closed on KM2, or open wire in KM3 coil circuit) Fix: Measure voltage at the timer coil terminals — if absent, trace back through the KM1 auxiliary contact feeding the timer coil circuit. If the timer is energised, verify its timed contact closes after the set time by measuring across it. If the timer contact closes but KM3 does not energise, measure voltage at the KM3 coil terminals and check the KM2 normally-closed interlock contact.
- Fuse or breaker trips immediately on pressing start
- Cause: KM2 and KM3 are closing simultaneously (interlock failure or wiring error), creating a short circuit; or a motor winding is shorted to earth Fix: Do not reset and retry without investigation. Isolate supply. Check that KM2 and KM3 coil circuits have the correct interlock contacts wired in series. Test motor insulation resistance between each terminal and earth — a healthy motor reads several hundred megohms or more.
- Overload relay trips during star-to-delta transition
- Cause: Transition transient exceeds the overload relay trip threshold; overload relay is not calibrated for winding current; or overload relay has no time-delay on the transition transient Fix: Verify the overload relay is set to the correct winding current value (FLC / 1.732 for delta winding current). If the relay trips specifically during transition, check whether the relay type supports a brief overload delay. If transition transients are severe, consider a closed-transition starter.
Frequently asked questions
What do KM1, KM2, and KM3 represent in a star-delta control circuit?
KM1 is the main line contactor, which remains closed whenever the motor is running in either star or delta. KM2 is the star contactor, which closes to form the star point during starting. KM3 is the delta contactor, which closes to connect the motor in delta for normal running. KM2 and KM3 must never close simultaneously.
Why is electrical interlocking mandatory between the star and delta contactors?
If KM2 (star) and KM3 (delta) close simultaneously, they create a three-phase short circuit across the supply. The resulting fault current will destroy the contactors and motor winding instantly. Electrical interlocking — a normally-closed auxiliary contact of each contactor wired in series with the other's coil circuit — is the primary safeguard against this.
How is the star-to-delta transition timer set?
The timer is set to the time the motor requires to accelerate to approximately 80–90% of synchronous speed under the starting load, typically 5–15 seconds. Setting it too short causes the delta contactor to close before the motor is fast enough, generating a high transition transient. Setting it too long wastes energy and increases starting time without benefit.
Does star-delta starting reduce starting current to one-third of DOL inrush?
Yes. In star configuration, each motor winding receives 1 / 1.732 (approximately 57.7%) of the line voltage, which reduces the winding current to 1/3 of the DOL inrush value. However, the starting torque is also reduced to 1/3 of DOL starting torque, which limits star-delta starting to loads that do not require high torque at zero speed.
What is the transition transient in a star-delta starter?
The transition transient is a brief current spike that occurs when the motor is disconnected from star and reconnected in delta. During this open-transition period, the motor continues to rotate and generate a back-EMF that may be out of phase with the supply when reconnected. In the worst case, this transient can approach DOL inrush levels. Closed-transition starters use impedance bridging to reduce this spike.
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