3-Phase Motor Starter Diagram: Direct-On-Line and Star-Delta Starting Methods

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A 3-phase motor starter connects an induction motor to the 3-phase supply in a controlled manner, either directly via a direct-on-line (DOL) contactor or in reduced-voltage stages via a star-delta arrangement to limit inrush current.

Starting a 3-phase induction motor places severe mechanical and electrical stress on the motor, the starter, and the supply network. At the moment of energisation, an induction motor can draw six to eight times its full-load current. For small motors (typically up to 7.5 kW on a stiff supply network), this inrush is acceptable and a direct-on-line (DOL) starter is appropriate. For larger motors, a reduced-voltage starting method such as star-delta is used to limit the demand.

Direct-On-Line (DOL) Starter:

A DOL starter consists of a main contactor (K1), a thermal overload relay (OLR), and a control circuit with start/stop push-buttons. The main contactor connects all three phases to the motor simultaneously on start. The overload relay monitors current in two or three phases; if the motor is overloaded, the relay trips and de-energises the contactor coil, disconnecting the motor.

The control circuit is powered from a reduced control voltage (commonly 230 V AC single phase derived from two of the three supply phases, or from a control transformer). The STOP button is normally closed; the START button is normally open. When START is pressed, K1 coil is energised, the contactor closes, and a K1 auxiliary contact seals in the circuit (latches) so the operator can release the START button. The motor runs until STOP is pressed or the overload relay trips.

Star-Delta Starter:

A star-delta starter uses three contactors: main contactor (KM), star contactor (KS), and delta contactor (KD). The motor must have all six winding terminals (U1, V1, W1, U2, V2, W2) accessible on the terminal block.

On start, KM and KS close simultaneously, connecting the motor windings in star configuration. In star, the voltage across each winding is the phase voltage (230 V on a 400 V system), reducing the starting current to approximately one-third of the DOL value, and starting torque to approximately one-third as well.

After a set time (typically 5–15 seconds, controlled by a timer relay), KS opens and KD closes, reconnecting the windings in delta configuration. In delta, full line voltage (400 V) is applied across each winding, and the motor runs at full performance.

Mechanical and electrical interlocks between KS and KD prevent both contactors from closing simultaneously, which would short-circuit the supply.

All starter installations must be protected by appropriate upstream fuses or motor protection circuit breakers and must comply with IEC 60947 (switchgear), IEC 60034 (motors), and applicable national wiring codes.

How to wire 3 phase starter diagram

Select the starter type for the motor and load Determine the motor kW rating, full-load current (FLC), starting torque requirement, and supply network constraints. For motors up to approximately 7.5 kW with a stiff supply and light starting loads, DOL is typically acceptable. For larger motors or supply networks with inrush restrictions, use star-delta or a soft starter. Consult the motor nameplate and the network operator's requirements.
Isolate and verify dead at the incomer Open the upstream isolator and circuit breaker. Apply lock-out/tag-out to all isolation points. Verify absence of voltage on all three phases at the incomer terminals with a calibrated tester. All three phases must be confirmed dead before any work begins.
Wire the power circuit (DOL) Connect three-phase supply (L1, L2, L3) to the main contactor top terminals. Connect the bottom terminals of the main contactor to the top terminals of the thermal overload relay (T1, T2, T3). Connect the bottom terminals of the overload relay to the motor terminals (U, V, W). Use cables rated for the motor FLC with appropriate derating for ambient temperature and installation method.
Wire the power circuit (star-delta) Connect three-phase supply to main contactor KM top terminals. KM bottom terminals connect to motor terminals U1, V1, W1 (first-end winding terminals) and also to delta contactor KD top terminals. Delta contactor KD bottom terminals connect to motor terminals U2, V2, W2 (second-end winding terminals). Star contactor KS connects U2, V2, W2 together (forming the star point) when closed. Wire the overload relay in the main contactor (KM) circuit.
Wire the control circuit For DOL: wire control supply across two phases or via control transformer. Wire normally-closed (NC) STOP button, then NC overload relay contact, in series with the K1 coil. Wire normally-open (NO) START button in parallel with a K1 auxiliary contact (self-hold). For star-delta: add a timer relay coil in parallel with KS coil, and wire the timer NC contact to break KS and NO contact to make KD after the set time period. Wire electrical interlocks between KS and KD auxiliary contacts.
Set the overload relay Set the thermal overload relay current adjustment to the motor's full-load current (FLC) as shown on the motor nameplate. Do not set it above FLC — this defeats the overload protection. If the motor has a service factor, the overload may be set at FLC × service factor (e.g. FLC × 1.15), but this must be done per the applicable code and motor design.
Restore power and perform no-load test Restore power and test the start and stop functions with the motor running unloaded. Verify the motor runs in the correct direction (swap any two phase leads at the motor terminals if reversal is needed). For star-delta, verify the transition from star to delta occurs smoothly after the set timer period. Measure running current on all three phases and confirm they are balanced and below FLC.

Specifications

Typical supply voltage (Europe/AU/UK)	400 V AC, 3-phase, 50 Hz
Typical supply voltage (North America)	208 V, 460 V, or 480 V AC, 3-phase, 60 Hz
DOL inrush current (typical)	6–8 × motor full-load current (FLC) at starting
Star-delta starting current	Approximately 1/3 of DOL inrush (2–2.7 × FLC)
Star-delta starting torque	Approximately 1/3 of DOL starting torque
Star-to-delta transition time	3–15 seconds (adjust to allow motor to reach near-synchronous speed in star)
Overload relay setting	Motor FLC (as shown on motor nameplate)
Contactor duty rating for motor loads	AC3 (IEC 60947-4-1)

Safety warnings

Three-phase electrical installations involve voltages (typically 400 V line-to-line in Europe/AU, 208 V or 480 V in North America) that are immediately lethal. All work on 3-phase motor starters must be carried out by a licensed electrician in accordance with IEC 60947, IEC 60364, NEC/NFPA 70, BS 7671, or the applicable national wiring code. Always isolate all three phases, verify absence of voltage on all phases, and apply lock-out/tag-out before opening any starter panel.
Verify that phase rotation is correct before the first motor run. Wrong phase rotation causes the motor to run in reverse, which can damage driven machinery. Check rotation direction with the motor uncoupled from the load on the first start. Swap any two phase leads at the motor terminals to reverse direction.
The overload relay protects against sustained overcurrent but does not protect against short circuits. Always provide upstream short-circuit protection (fuses or MPCB) rated for the available fault current at the installation point.
In a star-delta starter, ensure that the star (KS) and delta (KD) contactors are mechanically and electrically interlocked. If both contactors close simultaneously, a dead short occurs across the supply, causing immediate fuse blow or circuit breaker trip, and potential destruction of the contactors and associated wiring.
After any fault trip (overload or motor protection circuit breaker), investigate and resolve the fault before resetting and restarting. Repeated forced resets without fault diagnosis can cause motor winding failure from thermal overload and overheating.

Tools needed

Three-phase digital multimeter (voltage, current, and resistance measurement)
Clamp meter (for motor current measurement under load)
Lock-out/tag-out kit
Screwdrivers (flathead and Pozidrive for terminal screws)
Wire stripper and ferrule crimping tool
Torque screwdriver (for terminal torque specification compliance)
Non-contact voltage tester
Insulation resistance tester (megger, for motor winding testing)

Common mistakes

Setting the overload relay above the motor's FLC, negating overload protection and allowing the motor to operate in thermal overload without tripping.
In star-delta starters, failing to interlock KS and KD both mechanically and electrically. A wiring error that allows both contactors to close simultaneously creates a phase-to-phase short circuit.
Connecting motor terminals in the wrong sequence for star-delta, causing the winding configuration to be incorrect in the delta phase. The motor may run but with incorrect performance, vibration, or draw unbalanced phase currents.
Using a star-delta starter on a motor with insufficient starting torque in star configuration (one-third of DOL torque). If the load requires high starting torque, the motor stalls in star and the timer runs out before adequate speed is achieved, causing a damaging current transient on delta transition.
Forgetting to size the control circuit wiring and contactors for the correct AC duty rating. Contactors have different AC1, AC2, AC3, AC4 ratings for different load types. Motor loads are AC3 (squirrel cage induction) — using an under-rated contactor causes premature contact erosion.

Troubleshooting

Overload relay trips shortly after motor starts: Cause: Overload relay set too low; motor stalled or mechanically jammed; high-inertia load requiring longer acceleration time; star-delta timer set too short causing delta transition before motor has accelerated Fix: Check overload relay setting against motor FLC nameplate. Verify the driven machine is not mechanically jammed. For star-delta, extend the timer period. If the overload trips repeatedly at correct settings, investigate the mechanical load for overload or bearing failure.
Motor hums but does not rotate (single-phasing): Cause: One of the three supply phases is absent — blown fuse, open contactor contact, or broken supply conductor Fix: Immediately stop the motor — a single-phasing motor draws excessive current in the two remaining phases and will overheat rapidly. Measure voltage on all three phases at the starter input and at the motor terminals to identify the missing phase. Replace blown fuse or repair the open circuit.
Motor runs in wrong direction: Cause: Phase sequence at motor terminals is reversed relative to the desired direction Fix: Stop and isolate the motor. Swap any two of the three phase wires at the motor terminal block (e.g. swap U and V connections). Restore power and verify rotation direction before coupling to load.
Star-delta: contactor chatters or motor jerks during star-to-delta transition: Cause: Transition timer period too short — motor has not reached sufficient speed in star; severe current transient on delta energisation Fix: Extend the star-to-delta transition timer. Verify the load can accelerate to near-synchronous speed within the star period. For high-inertia loads, consider a closed-transition (Korndörfer) star-delta starter or a soft starter.
Main contactor does not latch (motor runs only while START button held): Cause: Self-hold auxiliary contact of the contactor not wired correctly, or auxiliary contact is faulty Fix: Verify wiring of the normally-open auxiliary contact of K1 (or KM) in parallel with the START button. Test the auxiliary contact with a multimeter — it should show continuity when the contactor is manually held in. Replace contactor if auxiliary contact is faulty.

Frequently asked questions

What is the difference between a DOL and star-delta starter?

A DOL starter connects the motor directly to full supply voltage in one step — simple and cheap but produces full inrush current (6–8× FLA). A star-delta starter connects the motor in star first (reducing winding voltage by 1/√3), then switches to delta after the motor accelerates. This limits starting current to approximately one-third of the DOL value, reducing stress on the supply network.

Why does a star-delta starter need six motor terminals?

In DOL starting, the motor windings can be pre-connected internally in star or delta, exposing only three terminals. Star-delta starting requires the starter to externally reconfigure the winding connections. This requires access to both ends of all three windings — six terminals in total (U1, V1, W1 for one end; U2, V2, W2 for the other end).

What does the overload relay protect against?

The thermal overload relay protects the motor windings from sustained overcurrent caused by mechanical overload, phase failure, or blocked rotor. It does not protect against short circuits — that is the role of the upstream fuses or motor circuit breaker. The overload relay is calibrated to the motor's full-load current (FLC) and trips on sustained overcurrent by heating bimetallic strips.

What causes a current transient (current spike) during the star-to-delta transition?

When the star contactor opens and the delta contactor closes, there is a brief moment where the partially rotating motor is reconnected to the supply out of phase with the residual rotor flux. This can cause a current transient comparable to or exceeding the original DOL inrush. A transition timing delay and sometimes a closed-transition starter (which inserts a resistor during changeover) are used to minimise this spike.

Can I use a star-delta starter on any 3-phase motor?

Only on motors with six accessible winding terminals designed to run in delta on the supply voltage (e.g. a motor rated 400 V delta/690 V star, used on a 400 V supply). A motor designed for star connection on the supply voltage cannot be star-delta started. Also, star-delta start provides only one-third of full-load torque during the star phase, which is insufficient for high-inertia or high-starting-torque loads.

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