Star and delta in an electric motor: connection principle and differences

Theory

As already mentioned, star and delta connection schemes are characteristic not only of an electric motor, but also for transformer windings, heating elements (for example, electric boiler heating elements) and other load.

To understand why these circuits for connecting elements of a three-phase circuit are called so, they need to be slightly modified.

In a "star", the load of each of the phases is interconnected by one of the terminals, this is called the neutral point. In the "triangle", each of the load terminals is connected to opposite phases.

Everything said in the article below is true for three-phase asynchronous and synchronous machines.

Let's consider this issue using the example of connecting the windings of a three-phase transformer or a three-phase motor (in this context, it does not matter).

In this figure, the differences are more noticeable, in the "star" the beginning of the windings are connected to the phase conductors, and the ends are connected together, in most cases, the neutral wire from the supply generator is connected to the same load point or transformer.

The dot marks the beginning of the windings.

That is, in the "triangle" the end of the previous winding and the beginning of the next are connected, and the supply phase is connected to this point. If you confuse the end and the beginning, the connected machine will not work.

What is the difference

If we talk about connecting single-phase consumers, we will briefly analyze it using the example of three electric heaters, then in the "star", if one of them burns out, the other two will continue to work. If two of the three burns out, none of them will work at all, since they are connected in pairs to line voltage.

In the triangle circuit, even if 2 heating elements are burned out, the third will continue to work. There is no neutral wire in it, there is simply nowhere to connect it. And in the "star" it is connected to a neutral point, and it is needed to equalize the phase currents and their symmetry in in the case of different loads in phases (for example, in one of the branches 1 heating element is connected, and in the rest 2 parallel).

But if, with such a connection (with different loads in phases), zero burns out, then the voltages will not be the same (where the load will sag more, and where it will be less, it will increase). We wrote more about this in the article about phase imbalance.

It should be borne in mind that it is impossible to connect ordinary single-phase devices (220V) between phases, at 380V. Either the devices must be designed for such a power supply, or the network must be with U-linear 220V (as in power networks with isolated neutral some specific objects, for example, ships).

But, with connecting a three-phase motor, zero is often not connected to the midpoint of the star, since this is a symmetrical load.

Power, current and voltage formulas

To begin with, there are two different voltages in the star circuit - linear (between linear or phase wires) and phase (between phase and zero). Ulinear is 1.73 (root of 3) times greater than Uphase. In this case, the linear and phase currents are equal.

Ul = 1.73 * Uph

Il = Iph

That is line and phase voltage correlate so that with a linear voltage of 380V, the phase is equal to 220V.

In the "triangle" Ulinear and Uphase are equal, and the currents differ by 1.73 times.

Uл = Uф

Il = 1.73 * If

Power in both cases, they are considered according to the same formulas:

• full S = 3 * Sph = 3 * (Ul / √3) * I = √3 * Ul * I;
• active P = √3 * Ul * I * cos φ;
• reactive Q = √3 * Ul * I * sin φ.

When connecting the same load to the same Uphase and Ulinear, the power of the connected devices will differ 3 times.

Let's say there is a motor that operates on a 380 / 220V three-phase network, and its windings are designed to be connected via a "star" to an electrical network with U-linear at 660V. Then, when connected to the "triangle", the supply U-linear should be 1.73 times less, that is, 380V, which is suitable for connecting to our network.

Here are the calculations to show what differences for the motor will be when switching the windings from one circuit to another.

Suppose that the stator current when connected to a triangle in a 380V network was 5A, then its total power is equal to:

S = 1.73 * 380 * 5 = 3287 VA

Switch the electric motor to a "star" and the power will decrease 3 times, since the voltage on each the winding decreased by 1.73 times (it was 380 per winding, and now it is 220), and the current is also 1.73 times: 1,73*1,73=3. This means that, taking into account the reduced values, we will calculate the total power.

S = 1.73 * 380 * (5/3) = 1.73 * 380 * 1.67 = 1070 VA

As you can see, the power has dropped 3 times!

But what will happen if there is another electric motor and it worked in a "star" in a 380V network and a stator current of the same 5A, respectively, and the windings designed for connection to a "triangle" for 220V (3 phases), but for some reason they were connected exactly in a "triangle" and connected to 380V?

In this case, the power will increase 3 times, since the voltage on the winding, on the contrary, has now increased by 1.73 times and the current by the same amount.

S = 1.73 * 380 * 5 * (3) = 9861 VA

The engine power has become more than the nominal in these same 3 times. It means it will simply burn out!

Therefore, it is necessary to connect the electric motor according to the winding connection scheme that corresponds to their rated voltage.

Practice - how to choose a scheme for a specific case

Most often, electricians work with a 380 / 220V network, so let's consider how to connect, with a star or a triangle, an electric motor to such a three-phase power network.

In most electric motors, the connection diagram of the windings can be changed, for this there are six terminals in Brno, they are arranged in such a way that with the help of a minimum set of jumpers it was possible to assemble the circuit you need. In simple words: the terminal of the beginning of the first winding is located above the end of the third, the beginning of the second, above the end of the first, the beginning of the third above the end of the second.

You can see how to distinguish between the two options for connecting an electric motor in the figure below.

Let's talk about which scheme to choose. The connection diagram of the electric motor coils does not have a particular effect on the operating mode of the engine, provided that the nominal parameters of the motor of the supply network correspond to the nominal parameters of the motor. To do this, we look at the nameplate and determine what voltages your electrical machine is specifically designed for.

Typically, the marking is:

Δ / Y 220/380

It stands for this:

If the line-to-line voltage is 220, collect the windings in a triangle, and if 380 - in a star.

To simply answer the question "How to connect the windings at the motor?" we made a table for choosing a connection scheme for you:

Star-delta switching for soft start

When starting the motor, high starting currents are observed. Therefore, to reduce the starting currents of asynchronous motors, a starting circuit with switching the windings from star to delta is used. In this case, as mentioned above, the electric motor must be designed for connection to the "triangle" and work under the U-linear of your network.

Thus, in our three-phase power grids (380 / 220V), for such cases, motors with nominal "380/660" Volts are used, for "Δ / Y", respectively.

At start-up, the windings are switched on as a "star" at a low voltage of 380V (relative to the nominal 660V), the engine starts to pick up speed and at a certain moment time (usually by a timer, in complicated versions - by a signal from the current and rpm sensors) the windings switch into a "triangle" and work already at their nominal 380 volts.

The illustration above describes this method of starting the motors, but a toggle switch is shown as an example, in practice they use two additional contactors (KM2 and KM3), although it is more complicated than the usual circuit for connecting an electric motor, but this is not it disadvantage. But she has a number of advantages:

• Less load on the grid from inrush currents.
• Accordingly, there are less voltage drops and the likelihood of stopping related equipment decreases.
• Soft start of the engine.

There are two main drawbacks to this solution:

1. It is necessary to lay two 3-core cables from the contactor location directly to the motor terminals.
2. The starting torque drops.

Conclusion

As such, there are no differences in performance when connecting the same electric motor in a star or delta scheme (it will simply burn out if you make the wrong choice). Also, there are no advantages and disadvantages of any of the schemes. Some authors cite as an argument that the current in the "star" is less. But with the same power of two different motors, one of which is designed for connection in the "star", and the second in the "triangle" to the network, for example, 380V - the current will be the same. And one and the same engine cannot be switched "at random" and "it is not clear for what", as it will simply burn out. The main thing is to choose the option that corresponds to the voltage of the supply network.

We hope that now you have become more clear about what constitutes a star and triangle diagram in electric motor, what is the difference in connecting each of the methods and how to choose a circuit for a specific case. We hope the information provided was useful and interesting for you!

Related materials:

• What is the difference between alternating current and direct current
• What are phase, zero and grounding for?
• Connection diagrams for a three-phase electric meter