Wye-Delta Motor Starting: Diagram and Limits

Short answer

Wye-delta starting starts the motor with the windings in wye (star), which cuts line current and torque to a third, and then switches them to delta. It needs a six-terminal motor whose delta rated voltage equals the supply voltage, three contactors and a timer. It suits loads that start almost unloaded, not loads that need torque at the start.

What wye-delta starting is

Wye-delta (Y-Δ, or star-delta) starting is a reduced-voltage method for squirrel-cage motors. It takes advantage of the fact that a winding can be connected in two ways: in wye (star), where each winding receives the line voltage divided by √3, or in delta, where it receives the full line voltage.

The motor starts in wye, with reduced voltage on each winding. Once it has accelerated, a timer switches it to delta, which is its normal running connection. Starting current falls to a third of what it would draw in direct-on-line starting; so does torque.

Conditions for using it

The motor must have six terminals accessible (U1, V1, W1, U2, V2, W2) in the terminal box.
The supply voltage must equal the rated delta voltage shown on the nameplate. A 400/690 V motor on a 400 V supply meets this; the same motor on 690 V runs in wye and cannot start this way.
The load must start with low torque: wye torque is a third of the direct-on-line starting torque. Centrifugal pumps and fans with a closed valve or low flow usually qualify; loaded belts, loaded compressors or mills do not.
Starting must be short and infrequent, because the transition stresses the motor and the supply.

Why current and torque drop to a third

In wye, each winding sees 1/√3 of the line voltage (on a 400 V supply, about 231 V). Winding current drops by that factor. In delta, line current is √3 times winding current; in wye, line current equals winding current. Combining both effects:

Line current in wye = 1/3 of the line current in delta.
Torque in wye = 1/3 of the torque in delta, because torque is proportional to the square of the winding voltage.

That one-third factor is what justifies the method and also what limits it.

Power circuit

Three contactors are used:

Contactor	Usual name	What it connects
KM1	Line (main)	L1, L2, L3 to U1, V1, W1 of the motor
KM2	Wye (star)	Ties U2, V2, W2 together to form the wye neutral point
KM3	Delta	Connects U2 to V1, V2 to W1 and W2 to U1

The overload relay goes in series with the line or with the windings, depending on the arrangement. If installed in series with KM1, it measures line current and is set to the nameplate delta current. If installed inside the delta, it measures winding current and is set to that current (line current divided by √3). The relay nameplate and the manufacturer diagram show which case applies, and confusing them leaves the motor poorly protected. Other methods are autotransformer starting and starting with stator or rotor resistors, explained in autotransformer and resistor motor starting.

The power sequence is:

KM1 and KM2 close: the motor is in wye and starts.
KM2 opens: the motor is supplied without the wye connection.
KM3 closes: the motor is in delta and runs at rated conditions.

With KM2 and KM3 closed at the same time, the three phases are bolted together in a short circuit. That is why the control design guarantees they never overlap.

Control circuit

The sequence is governed by an on-delay timer and the auxiliary contacts:

The start command energizes KM1 and KM2 and starts the timer.
When the time is up, the timer opens the KM2 circuit.
When KM2 drops out, its closed (NC) auxiliary contact allows KM3 to be energized.
KM3 seals in and de-energizes the timer.
Each connection contactor (KM2 and KM3) has an NC contact of the other in series with its coil: electrical interlocking.

Some starters add a mechanical interlock between KM2 and KM3, and a dead time of a few tens of milliseconds between opening the wye and closing the delta, which lets the KM2 arc extinguish before KM3 closes. Dedicated wye-delta timers that include this pause also exist.

Worked example (illustrative)

A 15 kW, 400 V (delta) motor, rated current 28 A, assumed direct-on-line starting current of 7 × In and assumed direct-on-line starting torque of 1.8 times rated (82 N·m, in the example of the induction motor guide). Assumed data, not from a commercial model.

Quantity	Direct-on-line	Wye
Line current	196 A	65 A
Starting torque	148 N·m	49 N·m
Voltage per winding	400 V	231 V

If the load needs 60 N·m to begin moving, the motor in wye (49 N·m) cannot start it, and wye-delta starting is unsuitable. If the load needs 20 N·m at the start, there is a reasonable margin.

The transition: the critical point

During the change from wye to delta the motor is left without supply for an instant (open transition). The motor keeps turning, but the residual magnetic flux generates a voltage that, when the delta closes, may not be in phase with the supply. The result is a current peak that can approach direct-on-line values, and a torque jolt. If the motor is still slow, the peak is almost equivalent to a direct-on-line start.

To reduce it:

The time in wye must be long enough for the motor to reach a speed close to rated. Too short a value switches to delta with the motor slow; too long makes the motor run needlessly in a low-torque regime and overheat.
In systems with continuity requirements there is closed transition, with resistors that keep the motor supplied during the change; it adds complexity and a fourth contactor.

The changeover time is set with the real start and a current measurement, not by a fixed rule. A useful indicator is that the current falls to a value close to rated before the change.

Comparison with other methods

Method	Starting current	Starting torque	Remark
Direct-on-line	Maximum	Maximum	Simplest; needs a strong supply
Wye-delta	1/3 of direct	1/3 of direct	Six-terminal motor; transition peak
Autotransformer	Per the tap	Per the tap	More costly and bulky
Soft starter	Adjustable	Adjustable	No abrupt transition; see soft starter

Common mistakes

Applying it to a load with high starting torque, so the motor does not accelerate and the timer switches to delta with a current peak.
Setting the timer too short, so the change happens with the motor slow.
Using a motor whose delta does not match the supply, that is, a 400/690 V motor on a 690 V supply.
Wiring errors on KM3, which connect the terminals wrongly and cause a phase-to-phase short circuit when the delta closes.
Overload relay set wrongly by confusing line current with winding current.
No interlocking between KM2 and KM3.
Forgetting the direction of rotation: swapping two supply phases between KM1 and the motor affects both modes, but swapping the terminals of one winding wrongly produces a field that does not correspond.

Checklist

Six-terminal motor and supply voltage equal to the nameplate delta voltage.
Load torque during the start lower than the wye torque, with margin.
Contactors rated for the current of each function (KM1 and KM3 in service; KM2 only at start).
Electrical interlocking between KM2 and KM3 verified.
Overload relay set according to its location in the circuit.
Timer setting verified by measuring current during the real start.
Direction of rotation checked.
Overload protection tested after the modification.

Frequently asked technical questions

Does it work for medium-voltage motors? Rarely. Medium-voltage motors usually have their windings connected in wye internally and are started with other methods, as covered in medium-voltage motor starters.
Is it equivalent to a soft starter? No: the soft starter adjusts voltage continuously and avoids the jump at the changeover; see soft starter vs variable frequency drive.
Can it be replaced by a soft starter in an existing panel? In many cases yes, but it changes the motor wiring and the protection; the whole assembly is reviewed.

How we can help

Before modifying a starter, calculate the torque the load asks for and the voltage drop each method produces. At Tiza Group we carry out that study and coordinate the protection remotely, worldwide; we then test and commission the panel in Colombia, or elsewhere on request. See engineering design and studies, or write to us through contact.

References

IEC 60947-4-1, electromechanical contactors and motor-starters.
WEG, Electric motor specification guide, starting methods section.
Revista Ingeniería Eléctrica, article on wye-delta starting.

Frequently asked questions

Which motors allow wye-delta starting?

Those with six terminals accessible in the terminal box, whose nameplate gives the supply voltage as the delta voltage. A 400/690 V motor on a 400 V supply allows it; on a 690 V supply it does not, because it must run in wye.

Why does the starting torque drop to a third?

Torque depends on the square of the voltage applied to each winding. In wye the winding receives 1/√3 of the line voltage, and the square of that factor is 1/3.

What happens if the motor has not accelerated when it switches to delta?

The motor sees a current close to direct-on-line starting, with an additional peak at the transition. That cancels the advantage of the method and can trip the protection. It is the symptom of a load that asks for more torque than wye gives.

Why can't the wye and delta contactors close at the same time?

With both closed, the motor phases are short-circuited through the two contactors. That is why they are interlocked electrically with each other and, in many cases, mechanically as well.