Star-delta starter simulator

A large motor started direct on line draws 6 to 8 times its rated current and gives the machine and the supply a hard jolt. A star-delta starter (wye-delta in North America) softens that: it starts with the windings in star and, once the motor has picked up speed, changes to delta. Here you can start an 11 kW motor, watch the contactors move on the power and control circuits, and compare its current with a direct-on-line start. You can also get the timer, the load or the interlock wrong, which is how you really learn.

How to use it

• Start and Stop: a tap is a short press; if you hold the button, it lasts as long as you hold it.
• Load: a fan needs little torque to start; a loaded conveyor needs 60 % of rated torque from the first moment.
• Timer KT: how long the motor stays in star before changing to delta.
• Interlock: if you remove it, KM3 can close before KM2 has fully dropped out.
• Diagrams: on the power circuit, green shows where current flows; on the control circuit, where there is voltage. Contacts are drawn as they are at each moment, not at rest.
• Chart: the blue line is the motor line current and the dashed one is what the same motor would draw direct on line. The green line is the speed.

Why start in star

In delta, each motor winding gets the line-to-line voltage: 400 V. In star it gets √3 times less, about 230 V. With less voltage, less current flows through each winding and, on top of that, in star the line current equals the winding current, while in delta it is √3 times bigger. Together, the line current in star is one third of the direct-on-line current. The torque also drops to a third, because it depends on the square of the voltage.

That third is both the advantage and the limit. A fan or a centrifugal pump needs very little torque to start, and the motor accelerates easily in star. A loaded conveyor, a compressor under pressure or a mill needs a lot of torque from the start: in star the motor cannot cope, it just hums and, when it changes to delta, the peak is almost that of a direct-on-line start. For those machines you use a soft starter or a variable frequency drive.

The motor must be rated 400/690 V

To start star-delta on a 400 V supply, the motor must run in delta at 400 V. Its nameplate will say 400/690 V Δ/Y. A 230/400 V motor runs in star on a 400 V supply and cannot take delta: each winding would get 400 V instead of 230 V and would burn out. It is the first thing to check before fitting the starter. No links go in the terminal box: all six terminals are wired to the starter, which makes the star and the delta with its contactors. The same logic applies on other supplies: the motor’s delta voltage must match the line voltage.

The sequence, step by step

1. When you press S1, KM1 (main) and KM2 (star) close together, and timer KT starts timing. KM1’s 13-14 contact keeps the circuit energised when S1 is released: that is the seal-in.
2. The motor accelerates in star, with a third of the current and torque.
3. When KT times out, its normally closed contact opens the KM2 branch and its normally open contact closes the KM3 (delta) branch.
4. KM3 cannot pull in until KM2 has dropped out, thanks to the interlock: the 61-62 normally closed contact of each contactor is in series with the other one’s coil. Real starters add a mechanical interlock between the two contactors, and dedicated star-delta timers leave a pause of about 50 ms at the changeover.
5. The motor stays in delta, at full voltage, until S0 is pressed or a protection trips.

Why so much care at the changeover? Because when a contactor opens under load its contacts part, but the current keeps flowing for a few milliseconds through the arc. If KM3 closes at that moment, the three phases are joined through KM3 and KM2: a short circuit. Remove the interlock in the simulator and look at the millisecond chart.

How long in star?

Just long enough for the motor to stop accelerating. If the changeover comes too early, the motor is still slow and the peak at the change to delta looks like a direct-on-line start, which is exactly what you wanted to avoid. If it comes too late, nothing is gained and, under load, the motor heats up, because in star it gives little torque. Typical settings are 3 to 10 seconds, depending on the inertia of the machine. In practice you set it by watching the current with a clamp meter: when it stops falling in star, that is the moment.

The overload relay: where it goes and how to set it

The most common place for overload relay F1 is right after KM1, on the cables going to U1, V1 and W1. There, with the motor in delta, it does not carry the line current but the winding current, which is √3 times smaller. That is why it is set to 0.58 times the rated current (In/√3): for this 21 A motor, about 12.2 A. Set to 21 A, it does not protect. If the relay is placed before the line splits between KM1 and KM3, then it is set to the nameplate rated current.

Typical mistakes

• Fitting a 230/400 V motor to a star-delta starter on a 400 V supply.
• Removing or not wiring the interlock between KM2 and KM3.
• Setting the timer by guesswork, too short or too long.
• Using it on machines that start under load, which cannot even begin to turn in star.
• Setting the overload relay to In when it sits in the winding branch.
• Ignoring the delta connection order the manufacturer gives for each direction of rotation: wired the other way, the peak at the changeover is higher.

Teaching a class? There is a printable worksheet for this simulator, with exercises and answers. More material on the teachers page.

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