Three-phase motors are used in every building. A Three-phase induction motor has 3 separate coils that are used to produce a rotating magnetic field. When we pass an AC current through a coil, the coils will each produce a magnetic field which changes the intensity and polarity.
If we connect each coil to a different phase, the electrons in each phase will change direction, so the magnetic field will change in intensity and polarity at a different time compared to the other phases. On the top or at the side of the motor we have a terminal box. We have Phase 1 coil connected to the two U terminals, the Phase 2 coil connected to the two V terminals, and the phase 3 coil connected to the W terminals. The coil terminals are in a different arrangement.
For the motor to run we need the circuit to complete. There are two ways to do this.
The first is the delta connection. We always connect the terminals from U1 to W2, V1 to U2, and W1 to V2. This will give us the delta configuration.
When we pass a current through different phases, electricity flows from one phase to another. Therefore we have the terminals at different arrangements because we can connect across and allow electricity to flow between the phases.
The other way we can connect the terminals is by using the star configuration. We always connect between W2, U2, and V2 on only one side of the motor terminals. When we pass current through the coils the electrons are shared between the phases.
Operation & Working of Automatic Star Delta Starter
The phase current flows to thermal overload contact, then OFF push-button, One push button, and then C3. This way, the circuit is completed.
Contactor coil C3 and timer coil (I1) are energized at once and the motor winding then connected in Star.
When C3 is energized, its auxiliary open links will be closed.
Since winding is connected in Star, hence each phase will get √3 times less than the line voltage i.e. 230V. Hence Motor starts safely.
The close contact of C3 in the Delta line opens because of which there would be no chance of activation of contactor 2 (C2).
After leaving the push button, Timer coil and coil 3 will receive a supply through Timer contact by Holding contact 3 and the close contact 2 of C2.
When Contactor 1 (C1) is energized, then the two open contacts in the line of C1 and C2 will be closed.
For the specific time in which the motor will be connected in star, after that the Timer contact will be open.
Contactor 3 will be off, because of which the open link of C3 will be close thus C2 will also energize.
C2 will be closed when c3 is off. Therefore, the motor winding will be connected to the Delta connection.
In addition, Contact 2 will open, by which, there would not be any chance of activation of coil 3 (C3)
Since the motor is connected in Delta connection now, each phase of the motor will receive full line voltage (400V) and the motor will start to run in full motion.
Advantages & Disadvantages of Star Delta Starter
Simple Design and Operation
Comparatively cheaper than other voltage controlling methods
Torque and Current performance of the Star delta starter is well.
It draws two times the starting current of the FLA of the connected motor.
It reduced the starting current to one-third as compared to DOL.
Starting Torque is reduced to one-third because starter reduces the starting current to one-third of the rated current.
It required Six leads or terminals Motor.
We might not use star delta starter if the required torque is more than 50% of the three-phase induction motors rated torque
Characteristics & Features of Star-Delta Starter
The starting current is 33% of the load current.
The peak starting current is 1.3 to 2.6 of load current.
Star-Delta Starter can be used only for low to high power induction motors.
It has reduced starting current and torque.
The 6 connection cables are needed for the terminal box.
Applications of Star-Delta Starter
Star Delta starter can only be used in cases for low to medium voltage. Star Delta Starter can be only used for light load during starting the 3-phase induction motor. Otherwise, the heavy load motor won’t start due to low torque that needs to accelerate the motor to rated speed while converting to the Delta connection.
Types of Star-Delta Starters
Based on the switching action between the Star Connection and Delta Connection, the Star Delta Starters are basically classified into Manual Starters and Automatic Starters. We will see some of the common types of Star-Delta Starters here.