How Star-Delta starter works with induction motor?
Most induction motors start with the DOL(Direct On-Line) starter, but when a very large capacity induction motor is started with a DOL starter. Then it may cause a disturbance on the supply lines due to large starting current surges.
To limit the starting current, large induction motors are started at a reduced voltage method. Star delta starter is the popular method to start a large induction motor at a reduced voltage.
Star delta starter means two separate stat of starting. First, the motor starts with a star connection and when it reaches up to 75% to 85% of its full load speed, the motor runs in delta connection.
When the motor starts without a Star-Delta starter
⇒ Starting current would be the 4-6 times of the full load current.
⇒ An over-sized power capacity cable is required to handle the initial high starting current.
Working Principle of Star Delta Starter
Star-Delta starter is most common and extensively used to start the induction motor. This is the most common reduced voltage starter. To understand the working of first we have to understand the 3-phase connection in star and delta configuration.
In star delta starter, initially, the motor is started in STAR configuration. The voltage across each phase is reduced by √3 factors. After a period of time motor reaches 75 to 80% of full load speed then the motor runs in DELTA configuration.
When a motor is connected in a star configuration, the stator winding is connected as shown in the figure below. The voltage across each phase is reduced by 1/3. In star configuration Voltage across the line is equal to the √3 times voltage across the phase. Here line current is equal to the phase current.
When a motor is connected in a delta configuration, the stator winding is shown below. The voltage across the line is equal to the voltage across the phase. The current across the line is equal to the √3 times current across the phase.
Star Delta Starter Wiring
The star-delta starter is manufactured from three contactors, a timer, and a thermal overload relay. The below image represents the power and control wiring of the star-delta starter.
There are mainly four stats in star-delta starter:
1. OFF Stat ⇒ This is the off stat of the starter, all the contactors are in the OFF position.
2. STAR Stat ⇒ In this stat, Main and Star contactors are closed and Delta contactor is open. The motor is connected to STAR.
3. OPEN Stat ⇒ This stat is the transition stat from STAR to DELTA. The Main contactor is closed only, both the Star and Delta contactors are open. There is a voltage across only one end of the motor windings. The rotor is still rotating.
3. DELTA Stat ⇒ The Main and Delta contactors are closed and the Star contactor is open. The motor is running with full line voltage and full load speed.
The star and delta are electrically interlocked as shown in the below diagram.
Pros and Cons of Star Delta starter
Advantages of star-delta starter
⇒ Low cost.
⇒ Components require very little space to build the starter.
⇒ Limit the starting current by 1/3 of the DOL starter.
Disadvantages of star-delta starter
⇒ A specific motor is required which has six leads.
⇒ Starting torque is also reduced by 1/3.
⇒ Supply voltage must be the same as the rated voltage in the delta connection.
PLC Ladder Logic for Star Delta Starter
Here you can find the plc program ladder logic for the star-delta starter.
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