 |
| 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.
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
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
