When a three phase induction motor of 5 HP or more power is started from three phase supply, then the starting current of that motor is very high. This current is 5 to 6 times more than the full load current of that motor. The motor body may overheat due to excessive current. Or the motor winding may also burn out. Therefore the motor of 5 to 20 HP is started or stopped with the help of star delta starter.
When the motor is started with a star delta starter, power is supplied to the star connection of the motor’s winding. having star connection
Phase voltage = line voltage × 3 This relation is stable. Means when the three phase induction motor is started in star connection, then the voltage received by the winding of that three phase induction motor is 3 times less. If the voltage gets less then the current flowing in the winding of the motor also flows less in that proof. Using this low current, the motor is started safely by initially supplying power to the star connection of the three phase induction motor winding.
When three phase induction motor is started in star connection. Then in a short time it attains 75 to 80 percent of its original speed. With the help of star delta starter after the motor attains speed, the star connection of the motor is converted into delta connection at the same moving state of the motor.
In a delta connection, the line voltage and phase voltage are the same. After connecting to the delta connection of the motor, the motor winding starts getting more voltage than the voltage received earlier. And the motor attains its highest speed as soon as it comes to the delta connection.
Star Delta Starter Types
There are three main types of star delta starter.
- Manually Operated Star Delta Starter
- automatic star delta starter
- Semi Automatic Star Delta Starter
Manually Operated Star Delta Starter
This type of star delta starter has contact strips mounted on a fiber rod. An insulated handle is also attached to one end of the same fiber rod. On the side of the contact strips, there are three terminals L1, L2 and L3. In the series of these three terminals, O.L.C. (Over Load Coil) are added.
These three terminals are for connecting to three phase supply. It would have six more terminals which are U1, V2, V1, W2, W1, U2 as follows. These terminals are connected to the 6 terminals of the three phase induction motor. NVC (No Volt Coil) and Stop Button These are connected in series with each other in parallel to any 2 phase out of the three phases.
How does a manually operated star delta starter work?
The starter is connected to a three-phase supply with the handle of the starter pointing down, then the three terminals W2,U2,V2 are simultaneously shorted.
L1-U1, L2-V1, L3- W1 In this way the three phase supply motor winding gets connected to the motor star connection. In star connection, the starting current of the motor decreases and the motor starts rotating at a slow speed. After a while, when the motor reaches 75 to 80 percent of its original speed, the starter handle is turned upwards. Because of which the motor moves from star connection to delta connection. The contact straps point upwards as the handle is raised.
And L1 U1 V2, L2 V1 W 2, L3 W1 U2 In this way the motor gets connected to the delta connection by shorting the terminals.
Thus in the running position of the motor, the motor starts rotating at its maximum speed by adding full supply voltage to the motor winding.
Possible faults caused by a manually operated star delta starter
There is always the possibility of some mistakes while operating the manually operated star delta starter handle by hand.
The motor must be connected to the first star connection with the first starter handle facing down. After the motor reaches 75 to 80% of its original speed, the motor has to be connected to the delta connection by turning the handle upwards. But when the handle is pressed down, many times the contacts are not done correctly. At the same time the motor is likely to be connected to a direct delta connection when the handle is turned upwards.
The purpose of using a manually operated star delta starter is not fulfilled because the motor is connected to a direct delta connection. To remove this type of error, automatic star delta starter is used more so that the motor can go from star connections to delta connection automatically.
Automatic star delta starter
This type of starter has three contactors
- Main Contactor
- Star Contactor
- Delta Contactor
And a timer is installed, the timer has normally open and normally close contacts. According to the time setting, the normally open contact is closed at a certain time and at the same time the normally close contact is opened.
How Automatic Star Delta Starter Works?
The internal configuration of Automatic Star Delta Starter is shown in the figure.
In this starter one has contactor second star contactor and third delta contactor. There is a timer in which an N.O. (Normally Open) and one N.C. (Normally Close) There are two such contacts. N.O. at a certain time according to the time setting of the timer. Contact N.C. and at the same time N.C. Contact N.O. it happens.
How Automatic Star Delta Starter Works?
As shown in figure. L1,L2,L3 are connected to 3 phase supply. And U1,V2,W1,U2 and V1,W2 these terminals are connected to 6 terminals of 3 phase induction motor. When the start button of this starter is pressed, L3 passes through a phase thermal switch, start button and timer to a terminal on the NVC of the star contactor. The NVC which needs 2 phase to operate. The second phase of the L2 star contactor is given direct to the second terminal of the NVC, as shown in the figure.
When the NVC of the star contactor gets 2 phases by pressing the ON push button, the three terminals U1, V1 and W1 are shorted when the star contactor becomes ON. If you look carefully in Akruti, then you will understand well.
At the same time, many NVCs also get 2 phase contactors in the Star Delta starter at the same time. And the main contactor also turns ON. And the U2,V2 and W2 contacts of 3 phase induction motor get L1,L2 and L3 3 phase supply. And the 3 phase induction motor is connected to star connection, the starting current is reduced, the motor starts smoothly.
The amount of time it takes for a 3 phase induction motor to achieve 70 to 80% of its speed is set in the timer. Therefore, the timer operates after the set time, which means after 3 to 12 seconds. As the timer operates, its NO contact changes to NC, and NC contact changes to NO. At the same time the NVC of the delta contactor gets 2 phase supply. And the delta contactor becomes ON. At the same time the L3 supply to the NVC of the star contactor is cut. And the star contactor turns OFF. Now at this time only delta contactor and main contactor are ON as shown in akruti
In this way the terminals of the three phase induction motor are shorted and reflected in the delta connection.
U1- W2 to L1
V1- V2, L2
L3 to W1-U2
In this way the 3 phase supply joins the motor delta connection. In this way the motor winding gets full phase voltage and the motor starts rotating at its maximum speed.