We will address a type of indirect starter for three-phase motors, this type of indirect starter, better known as a triangle star, is very widespread due to its robustness of operation, ease of installation and its benefit in relation to a direct starter.
The drive method using a star-delta starter increases the life of a motor, decreasing the starting current, generating less heating due to the joule effect, conserving the motor insulation, which allows the electrical insulation to be preserved for a longer time.
The departure star triangle
The starter is used in three-phase motors with six terminals or twelve, popularly known as six-point or twelve-point motors, generally, this type of starter is used in motors with powers above 40CV.
When we cannot use a direct start, often due to the high load starting torque, as to break the load inertia, and for the motor to reach a speed close to synchronous speed, it will require a high electrical current from the network causing peaks and disturbances, so direct starting is not applicable. (The starting torque is also called torque or moment, it is the measure of the effort required to rotate an axis)
The starter has an excellent beneficial cost and is a solution widely used in drives since it reduces disturbances or peaks of current by about 66%. In order to use the star-triangle indirect start, the power supply must be at the rated voltage of the motor triangle, otherwise, you can burn the motor when the key closes in a triangle, so the drive will not fulfill the purpose of reducing the current starting point since instar it will be receiving the nominal voltage in this case.
Principle of operation star-triangle departure
To exemplify the operation of the star-delta starter, let’s give an example of a 220V / 440V motor, we assume that the mains supply voltage is 220V, so in this type of motor, the triangle closure would be to supply the motor with 220V and the star closure would be for the 440V voltage.
In this case, when the three-phase electric motor is started, it starts in star, remembering that for example, this closure is for the highest voltage at 440V, but when we start the star motor, it is applied to the 220V terminals, so , we have the phase voltage at the time of star game is 220V / √3 = 73.3 V . At this time, the starting current will be 33.3% of the motor starting rating, this is due to the drop in voltage applied to the coils, and the existence of more electrical resistance, when the motor coils are closed in star.
In the starter, there is a noticeable loss of torque, also in the starting torque, but in counter-starting, we will not have a high starting belt, because in a conventional direct start it can reach about 13X the motor nominal.
Observe the diagram showing the connection of the star motor, and the voltage found in the line before the voltage found in the motor phases at the moment of the activation of the star switch:
This way, when the motor is started, it will start smoothly, until it reaches enough speed to allow a closing turn to triangle without the current peak effect. When the turn occurs, the motor will be close to the synchronous speed of the rotating magnetic field, allowing the motor not to suffer from a high starting torque and sudden current oscillations.
Observe the line and phase voltages at the moment of turning to the delta closing, the line voltages are equal to the phase voltage so the motor will be in maximum power and torque:
To better understand the issues addressed here and the step-by-step operation, we suggest reading another article that deals with triangle star closure .
Star triangle match in practice
Observe the electrical diagram of a drive using a star-delta starter as follows:
The operation of the control consists of the following way, when pressing the on button the contactors KM1 and KM3 start operating simultaneously, note that the contactor KM3 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 a 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, since in 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.
Sizing of a star-delta starter:
Dimensioning a star-delta starter is not a simple thing to do, many electricians get lost right now, undersizing the circuit or over-dimensioning, but in this step by step we will show how to calculate the main components of a star-delta circuit in a simple and easy way .
Dimensioning of KM1 and KM2 power contactors
It should be noted that KM1 (load contactor), is subject to the phase / line current when starting in STAR, that is, the current will be 1/3 of the starting rating at the moment the motor is driven in star. After closing in Triangle, KM1 will be subject to the motor phase current, following the mathematical model of the concept:
For KM2, in the starter start, the contactor is disconnected, but when the circuit is switched to Triangle, K2 becomes subject to the phase current, just like K1 so we use the same form of KM1 to calculate the current in KM2.
It is worth mentioning that when the motor is closed in a triangle the current is divided approximately equally between KM1 and KM2 so we use the mathematical model presented. Then for the calculation of the current of the KM1 and KM2 contactor we use the formula below, remembering that IN is the motor plate current:
Scaling of KM3
We should note that KM3 is subject to the phase / line current at the starter start, as we have shown previously, at the moment of the motor starting the current will be 33.3% of the starting nominal, this current circulates in KM3. When the key turns to a triangle, KM3 will be disconnected. Thus, to perform the correct dimensioning of the KM3 contactor, we must use the mathematical model below:
Design of the thermal relay – RT
It should be noted that the thermal relay is subject to the phase / line current in the starter as well as the KM1 contactor, that is, the current will be 33.3% of the starting nominal and at the moment of turning the circuit into a triangle, the thermal relay will be subject to the phase current of the motor, ie:
(We use 1.25 or 1.15 depending on the type of thermal relay)
Fuses have the function of protecting our circuit, in case of a high current surge due to a short circuit or an overload for a longer period of time, the fuses break, interrupting the passage of electrical current. These components are used to protect the circuit as a whole, cables, contactors and thermal relays.
In order to choose the best fuse, we must observe three conditions:
- Fuse dimensioning based on the starting current of the motor:
“These data can be found on the motor nameplate and plug of the fuse manufacturer”
- We need to make sure that the fuse current must have a nominal current of at least 20% more than the rated current of the electric motor, just multiply the rated current of the motor by 1.20.
- We need to make sure that the fuse current must protect the contactors and thermal relays in our circuit, so we need to observe the specifications of the component manufacturers. The maximum current of the components cannot be less than that of the fuse.
It is necessary that you also understand the activation of motors by direct start, I suggest the video below that deals with the assembly of a drive step by step, when understanding this drive you will have an easier time with the star triangle drive.v