Star-delta starter

Starting a large induction motor directly on-line draws 6 to 8 times its rated current, which dips the supply voltage and stresses the machine. A star-delta starter starts the motor with reduced voltage on each winding, then switches to full voltage once it is up to speed.

Star and delta connections

A 3-phase motor has three windings, with six terminals: U1-U2, V1-V2, W1-W2.

• Star (Y): U2, V2 and W2 are joined together (the star point), and L1, L2, L3 go to U1, V1, W1. Each winding gets the phase voltage: 400 V ÷ √3 ≈ 230 V.
• Delta (Δ): the windings are connected in a ring: U1-W2, V1-U2, W1-V2. Each winding gets the full line voltage: 400 V.

In star, each winding has √3 times less voltage, so the line current and the starting torque are both about one third of a direct delta start.

The three contactors

The sequence

1. START: KM3 (star) and KM1 (main) close. The motor accelerates in star.
2. After the star time (typically 5–15 s, when the motor is near full speed) KM3 opens.
3. After a short pause (30–80 ms) KM2 (delta) closes. The motor runs in delta at full power.
4. STOP: KM1 and KM2 open.

The pause ensures the star contactor's arc is out before delta closes. KM2 and KM3 must never be closed together: that would short-circuit the supply. They are interlocked twice: mechanically and with each other's NC auxiliary contacts in the coil circuits, and also in the PLC program.

PLC program (simplified)

Run      := (Start OR Run) AND Stop_OK AND OL_OK
KM1      := Run
KT_star  := TON(IN := Run, PT := 8 s)
KM3      := Run AND NOT KT_star.Q AND NOT KM2_fb
KT_pause := TON(IN := KT_star.Q, PT := 60 ms)
KM2      := Run AND KT_pause.Q AND NOT KM3_fb

See PLC timers and ladder logic.

Overload setting

When the overload relay sits after KM1 in the line to U1/V1/W1, it carries the winding (phase) current, which in delta is the line current ÷ √3. So set it to 0.58 × the motor's rated current. If it is placed in the incoming supply, set it to the full rated current.

Which motors can be started star-delta?

The motor must be designed to run in delta at your supply voltage. On a 400 V supply you need a motor marked 400 V Δ / 690 V Y. A 230 V Δ / 400 V Y motor runs in star on 400 V and cannot use star-delta. Also, the load must allow a low starting torque: fans and pumps are fine; a fully loaded conveyor may not accelerate in star.

Drawbacks and alternatives

• The open changeover causes a current and torque peak.
• Six motor cables instead of three.
• Soft starters and VFDs give smoother starts and are now often the better choice.

The PLC Sim Lab Playground has a 6-terminal star-delta motor, contactors and overload relays to wire it yourself.