What is a star-delta starter?
A star-delta starter is a starting method for three-phase induction motors in which the motor windings are first connected in star (Y) and, after a few seconds, in delta (Ξ), so that starting current and starting torque fall to about one third of a direct-on-line start. This limits the current surge on the supply and the mechanical stress on the driven machine without expensive power electronics. The method is also called star-delta starting, Y-Ξ starting or, in North America, wye-delta starting.
In OT environments you will find star-delta starters in production lines, pump systems, fans and compressors, where a direct start would put too much load on the supply or the installation.
π§ How does a star-delta starter work?
A three-phase motor has three windings. In star, each winding sees the phase voltage: the line voltage divided by β3, which is about 230 V on a 400 V supply. In delta, each winding sees the full 400 V line voltage. Because torque varies with the square of the voltage, the motor delivers only one third of its torque in star, and the line current is also one third of the delta value.
The start takes three steps:
- Star - the line contactor and the star contactor close. The motor starts at roughly one third of the direct-on-line starting current. A time-delay relay or the PLC counts down the set star time, usually 6 to 10 seconds, sometimes longer for a load with high inertia.
- Changeover pause - the star contactor opens. Only after a short pause of about 50 ms does the delta contactor close, so the arc in the star contactor has been extinguished and no phase-to-phase short circuit can occur.
- Delta - the motor receives the full line voltage, accelerates to rated speed and keeps running in delta.
As a rule of thumb, the motor should have reached at least about 80% of rated speed at the moment of changeover; in practice, values from 75 to 90% are quoted. If you switch too early, you still get a current peak that can approach that of a direct start.
βοΈ What values are typical for a star-delta start?
| Quantity | Direct-on-line (DOL) | Star-delta, star stage | Note |
|---|---|---|---|
| Voltage per winding | 400 V | approx. 230 V (1/β3) | On a 400 V supply with a 400 V Ξ motor |
| Starting current | 6β8 Γ Iβ | approx. 2β2.7 Γ Iβ | One third of DOL |
| Starting torque | 100% of DOL torque | approx. 33% of DOL torque | The load must start easily |
| Star time | n/a | usually 6β10 s | Depends on load inertia |
| Changeover pause | n/a | approx. 50 ms | Prevents a short circuit via the arc |
| Speed at changeover | n/a | approx. 80% of rated or more | Too early gives a high current peak |
The torque figures immediately show the main limitation: a load that needs a lot of torque at low speed, such as a fully loaded conveyor or a reciprocating compressor under pressure, will not get up to speed in star.
π§ How is a star-delta starter built?
A classic star-delta starter, often part of a motor starter in an MCC or control cabinet, consists of three contactors to IEC 60947-4-1 in utilisation category AC-3:
| Component | Function | Typical sizing |
|---|---|---|
| Line contactor (K1) | Feeds the winding starts U1, V1, W1 | approx. 58% of Iβ |
| Star contactor (K2) | Connects U2, V2 and W2 into a star point | approx. 33% of Iβ |
| Delta contactor (K3) | Links U1βW2, V1βU2, W1βV2 | approx. 58% of Iβ |
| Overload relay | Thermal protection in the winding circuit | Setting Iβ Γ 0.58 |
| Time-delay relay or PLC | Star time and changeover pause | 6β10 s and approx. 50 ms |
Because the line and delta contactors only carry the winding current, they can be smaller than for a direct start. For the same reason, the overload relay usually sits in the winding circuit and you set it to 0.58 Γ the rated motor current: for a 30 A motor that is 17.4 A. Some designers choose three identical contactors to simplify spare parts management. Always follow the manufacturerβs selection tables.
Motor requirements
- Six terminals - U1, V1, W1 and U2, V2, W2 must all be brought out, with no links fitted on the terminal board.
- Delta voltage equal to line voltage - on a 400 V supply, as in the Netherlands and most of Europe, the nameplate should read 400 V Ξ / 690 V Y. A 230 V Ξ / 400 V Y motor already runs in star on this supply and is not suitable.
- Correct phase sequence in delta - link the delta so that the rotating field keeps the same phase sequence after changeover as in star. A wrongly linked delta makes the motor brake briefly at changeover, causing a severe current and torque surge.
π Where does a star-delta starter fit, and where not?
Suitable loads start easily: centrifugal pumps with a closed discharge valve, fans in HVAC systems, centrifugal compressors, machine tools and woodworking machines. Less suitable are loads with high breakaway torque or high inertia, applications with many starts per hour, and processes in which you want to control the speed.
| Method | Starting current | Speed control | When to choose |
|---|---|---|---|
| DOL | 6β8 Γ Iβ | No | Small motors and a strong supply |
| Star-delta | approx. 1/3 of DOL | No | Medium-sized motors with an easy start |
| Soft starter | Adjustable, typically 2β4 Γ Iβ | No | Smooth start and stop, for example against water hammer |
| Variable frequency drive | Around Iβ | Yes | Variable speed and energy savings |
| Auto-transformer start | Adjustable via taps | No | Large motors with strict supply limits |
As power electronics have become cheaper, many designers choose a soft starter or variable frequency drive for new installations. The star-delta starter remains popular where simplicity, robustness and low cost weigh more heavily, and in existing installations that have run reliably for years.
π How do you control a star-delta start safely from OT?
In modern installations a PLC increasingly takes over the role of the time-delay relay, for example in ladder logic according to IEC 61131. That brings benefits, but also demands discipline:
- Interlock in hardware - the star and delta contactors must never close at the same time. Always provide an electrical interlock with auxiliary contacts, or a mechanical interlock, and do not rely on software alone. This also prevents a faulty change to the PLC program from causing a phase-to-phase short circuit.
- Monitor feedback - read in the auxiliary contacts of the contactors and raise a fault if a contactor fails to drop out or sticks.
- Use the data - star time, number of starts and motor current are valuable inputs for condition monitoring and predictive maintenance. Smart overload relays and motor starters with IO-Link or a fieldbus connection deliver this data directly to SCADA or the HMI.
- Standards - the electrical equipment of machines falls under IEC 60204-1; low-voltage installations in the Netherlands fall under NEN 1010.
Practical example
A sewage pumping station has a 45 kW, 400 V Ξ pump with a rated current of about 80 A. A direct start would peak at around 500 A; in star that is roughly 170 A. You set the overload relay to 80 Γ 0.58 β 46 A and the star time to about 8 seconds. During commissioning you use a clamp meter to check that the current has clearly dropped before changeover and that the pump is close to full speed. If not, the star time is too short or the pump is starting against too much back pressure.
Also pay attention to new motors. Since 1 July 2021, 2- to 8-pole three-phase motors from 0.75 to 1,000 kW in the EU must meet at least efficiency class IE3, and since 1 July 2023 IE4 applies to 2-, 4- and 6-pole motors from 75 to 200 kW (Ecodesign Regulation (EU) 2019/1781, with exceptions such as brake motors and explosion-protected designs). Such motors can have a higher starting current ratio than their predecessors, so check the datasheet and the starter settings when you replace a motor.
β Frequently asked questions
Why does the starting current drop to one third with star-delta?
In star, each winding receives 1/β3 of the line voltage, so the winding current drops by a factor of β3. Because in star the winding current equals the line current, while in delta the line current is β3 times the winding current, the line current with a star-delta starter ends up at one third of a direct start.
How long should the motor run in star?
With a star-delta starter the star time is usually between 6 and 10 seconds. The right time has been reached when the motor reaches at least about 80% of rated speed and the current has fallen substantially. A star time that is too short produces a current peak at changeover that is almost as high as a direct start.
Can every three-phase motor be started star-delta?
No, a star-delta starter only works with a motor designed for the line voltage in delta and with six terminals brought out. On a 400 V supply that means a nameplate reading 400 V Ξ / 690 V Y. A 230 V Ξ / 400 V Y motor is not suitable.
Why is the overload relay set to 0.58 Γ Iβ?
In a star-delta starter the overload relay usually sits in the winding circuit behind the line contactor. In delta, only 1/β3, or 58%, of the line current flows there. If you set the relay to the full rated current, the motor is not adequately protected.
What is the difference between open and closed transition?
A standard star-delta starter uses open transition: the motor is briefly disconnected, which causes a current and torque peak when it is reconnected. A closed-transition starter uses an extra contactor with resistors that keep the motor energised during changeover, so the peak is much smaller.
Is a soft starter better than a star-delta starter?
A soft starter provides a stepless, adjustable ramp-up and a soft stop without a changeover peak. A star-delta starter is simpler and has no power electronics producing heat or harmonics. For loads that are hard to start or must start frequently, a soft starter or variable frequency drive is usually the better choice.
π In summary
A star-delta starter starts a three-phase motor first in star and then in delta, so that starting current and starting torque fall to about one third of a direct-on-line start.
The method is simple, robust and inexpensive, but it requires a 400 V Ξ motor with six terminals, a load that starts easily, and a well-tuned star time with a short changeover pause. Set the overload relay to 0.58 Γ Iβ, interlock the star and delta contactors in hardware, and use the start data in your OT systems for maintenance. Where you want a softer start or speed control, a soft starter or variable frequency drive is the logical alternative.