Star connection! What is it and where is it applied?


Most lovers of the electrical world, who are interested in the area, have heard about star and triangle connections in three-phase motors, three-phase transformers, and others. With that in mind, we will cover technical concepts on the topic, we want you to really understand how to use it, and where to apply these connections and closures, in addition to showing in which cases we use starter connections and drives.

To understand this concept we need to talk about the nominal voltage of equipment, this is the voltage margin that a piece of equipment can operate without compromising its insulation.

The vast majority of equipment such as three-phase motors, which work in three-phase systems, are supplied with multiple rated voltages, made possible by the winding terminals being reclosable, and the main type of connection for motors and transformers that allow multiple voltages is the star-delta connection.

To exemplify the situation, let’s take the example of a three-phase 6-point motor. In the triangle connection, the winding of each phase has the two ends brought out of the motor, each phase will receive line voltage or mains voltage in the case of 220V.

Star connection. What and what is its application?

In case we connect the three phases instar, the motor can be connected to a voltage line in the amount of 220V x √3 = 380V (the root of three and a constant that comes from the deduction of the lag between the phases of the 120º three-phase system, is that we apply the rule of the parallelogram and using the law of cosines to arrive at this deduction), so we do not change the voltage in the winding, even applying the 380V line voltage.

This type of connection requires at least six terminals on the motor and is suitable for any double rated voltages, as long as the second voltage is the same as the first multiplied by √3.
Example: 220V / 380V, 380V / 660V and 440V / 760V.

In the 380V / 660V and 440V / 760V examples, the higher declared voltage serves to indicate that a motor can be started using a star-triangle starter, not for use in the normal regime.

Where the star connection is used.

But what are all the connections and closings for electric motors? We know that when driving a three-phase cage motor it can reach about 8 to 12 times its rated current, for example, a motor with a nominal current of 25 A can reach a starting current of 240 A when being driven by direct start, in this if this current at the start can cause a series of surges in a three-phase system, such as a high voltage drop in the network.

It is important to remember that the power utilities limit the voltage drop in the network, imposing fines when necessary, in addition, it is worth noting that in these cases it is necessary that the protection systems and cables be oversized, to meet the peaks of the inrush current.

Wirig diagram for star closure.

When a direct start is not possible, especially when there is a high torque, that is, a high load on the engine, a solution that has a reduced cost is to use the star-triangle starter, thus stopping a direct starter for a starter indirect.

We use the star-delta starter when the motor torque curve is high enough. It is advisable that the star inrush current is between 25% to 33% of the triangle inrush current.

Star-triangle starting is also recommended in cases where there is a need for susceptible starts, that is, in a short period of time. The star-triangle starter is more robust than other solutions, such as starting via frequency inverters and starters that have electronic components and have a high investment cost.

How to make a triangle star match

To assemble a star-delta starter, it is necessary to assemble a power circuit with three contactors, protection fuses for each phase, a protection circuit breaker compatible with the load and a thermal relay.

Connection diagram for delta closing.

The starting system is controlled by a command circuit, which has a timer relay, used to activate the power contactors at the start, that is, at the moment of starting the motor is closed in star and after winning the starting torque the command closes through the contactors, then the motor starts to operate in a triangle connection. At this moment, the motor works in a normal regime, avoiding large current peaks.

Let’s see the diagram below:Star triangle departure. What is it and what is it for?

Electrical diagram for star triangle departure.

When pressing the on button, contactors KM1 and KM3 start operating simultaneously, note that the KM3 contactor closes the terminals 4-5-6 of the motor (star closure), at the same time KM1 energizes the terminals 1-2-3 of the motor , so the motor starts in star with reduced current and working with a reduced phase voltage.

At this moment, the normally open contact 13/14 of the KM1 contactor energizes the star-delta relay KA1, it counts for a certain time and removes the contactor KM3 and places the contactor KM2, being responsible for closing the motor in delta.

Note that at the moment of transition from KM3 to KM2, the closing of the coils with the supply phases should be with the following combination 1-6 in phase R, 2-4 in phase S and 3 -5 in phase T.

At this moment it is very important that when activating KM2 the closing is carried out in this way, because otherwise you will cause a short between the phases. Another way to protect your circuit, which is extremely recommended, is to interlock the contactors KM3 and KM2, as there is no way they can be activated together, otherwise there will be a short between phases in KM3.

To interlock the contactors, it is enough that you pass the connection line of the coil KM2 in an open contact of KM3 and the supply line of KM3 in the open contact of KM2, in this way one contactor will not enter when the other is switched.

The benefit of using a relay suitable for star and triangle starts is that it makes it possible to simplify the command, as it will be necessary to perform the seal only in KM1. This seal serves to sustain the electrical voltage in the contactor, even after you stop pressing the on button. To make a seal, just pass the KM1 contactor supply line into an open contact.

As a safety measure, an emergency switch and two contacts of the KM1 and KM2 thermal relays were installed next to the main power supply line of the controller, as their function is to cut the power supply to the control circuit in cases of overloads and phase failure. , a motor circuit breaker was also installed to protect against short-circuit current.

So that you can better understand the relationship between the star-triangle connection and the concept for phase voltage and line voltage, which are extremely important in the area of ​​electricity. The video below explains exactly what phase voltage and line voltage are.

We finish this article here and we hope to have helped you to clear all doubts about star connection! If you still have any doubts or curiosity about this matter, leave it in the comments and we will respond.v