Star Delta Starter Line Diagram and Its Working Principle

Fig: Three-phase motor connection diagram with a star-delta starter.

What is a Star-Delta Starter for Motor Starting?

A star-delta starter is an electrical motor starting device, generally used in large motors to overcome some technical limitations. Star and Delta mean here 2 separate states of motor running: first Star connection and then Delta connection.

The starting current of any heavy electric motor can be more than 4 times the normal load current it draws when it has gained speed and has reached its normal running condition. 

To overcome this initial high-current starting problem, such an arrangement needs a star connection at starting time, and if the star connection has sufficient torque to run up to 75% to %80 of full load speed, then the motor can be connected in Delta mode.

When the motor is connected to the Delta configuration, the phase voltage increases by 173% and phase currents increase by the same ratio. The line current increases three times its value in star connection.

What Problem Starting a Motor without Star-Delta?

We already know that if the motor starts simply when connected in Delta, the starting current would be huge just to be able to start the motor, not for running conditions. To do this would require -

v Large capacity circuit breakers to allow the start-up surge current to pass without immediately shutting the motor off.

v Oversized 3-phase power service cables require just for starting time, but the normal running time is not necessary.

v Large size of coils and contacts on the relays or contractors need to control the motor, but normal running time needs smaller sizes of them.

Sometimes, the utility company also does not permit the big motor to start without a Star-Delta starter because of system instability due to switching transients. 

How to Wire a Star-Delta Starter for a Motor?

Following the figure below, the power connection between magnetic contactors for Main, Star, Delta, and the motor is shown in the schematic line diagram and wiring diagram for easy wiring.

Star-Delta Starter Connection with Control Wiring

What is Important During the Star-Delta Transition Period?

It is important that the break between the Star contactor switch OFF and the Delta contactor switch is ON because the Star contactor must be reliably quenched before the Delta contactor is activated. During the transition period of switch-over, the motor must be free-running with little deceleration. It is also important that the switch-over pause is not too long; it may generate a voltage of its own, and this may add to or subtract from the applied line voltage.

That’s all for the Star-Delta starter for today; here in this article, we discussed only motor starting by 3 magnetic contactors. Star-Delta and solid-state motor starters from different companies are also available in the market now.

STAR-DELTA STARTER CALCULATOR

Starting Current, Torque & Contactor Sizing — IEC 60947-4-1

WAZIPOINT SYSTEM

MOTOR & SUPPLY PARAMETERS

Motor Rating

kW HP

Supply Voltage — Line to Line (V)

Efficiency (%) Power Factor

Starting Current Ratio (I_st / I_n)

— quick select — < 7.5 kW (~5.5) 7.5–37 kW (~6.5) > 37 kW (~7.5)

Poles 2 4 6 8 Frequency (Hz)

RESULTS

Enter parameters and click CALCULATE.

REFERENCE NOTES

Typical Starting Current Ratio

Motor Size	Ist/In
< 7.5 kW	5 – 6
7.5 – 37 kW	6 – 7
> 37 kW	6 – 8

Per IEC 60034-12 design classes — confirm against the motor nameplate/datasheet where available.

Star Time Guideline

Motor Size	Star Duration
≤ 7.5 kW	3 – 5 s
7.5 – 22 kW	5 – 8 s
22 – 55 kW	8 – 12 s
> 55 kW	12 – 20 s

Switch to delta once the motor reaches ~80–85% of synchronous speed. Switching too early causes a high transient current spike.

Overload relay placement: Most panels mount the thermal overload in the delta-connected branch (between KM3 and the motor windings), where it sees only the phase current (~58% FLC) — this is the setting shown above. If your design instead places the relay in the incoming line (in series with KM1), set it to 100% FLC. Confirm against the actual control drawing before commissioning.

Cable sizing: Six conductors run between the starter and the motor (U1, V1, W1, U2, V2, W2), each carrying the phase current shown above (~58% FLC), not the full FLC. Size these per IEC 60364-5-52 / IS 732 using that reduced design current.

Applicability: Star-delta starting reduces both starting current and starting torque to roughly ⅓ of direct-on-line (DOL) values. It requires access to all six motor winding leads, and the motor's delta-connection nameplate voltage must equal the supply voltage. Not suitable where more than ~33% of DOL starting torque is needed under load (loaded conveyors, positive-displacement pumps) — consider a soft starter or VFD instead.

STAR-DELTA CONNECTION SCHEMATIC

3-PHASE SUPPLY L1 L2 L3 KM1 — MAIN closed throughout run MOTOR TERMINALS U1 V1 W1 KM3 — DELTA closed only in RUN W2 U2 V2 KM2 — STAR closed only in START STAR POINT M 3~ 3-PHASE INDUCTION MOTOR Sequence: KM1 + KM2 close together (STAR START) → after timer delay, KM2 opens → KM3 closes (DELTA RUN). KM1 stays closed throughout.STANDARDS: IEC 60947-4-1 · IEC 60034-12 · IS 13947 · IS 8544 · IEC 60364-5-52

WAZIPOINT SYSTEM — ENGINEERING CALCULATOR