A star-delta motor starter is a type of motor starter that reduces the starting current of an induction motor by connecting the motor windings in a star configuration during startup. Once the motor has accelerated to a certain speed, the starter switches the motor windings to a delta configuration, which provides full voltage to the motor.
Diagram of a Star-Delta Motor Starter
Working Principle of Star-Delta Motor Starter:
The star-delta motor starter consists of the following components:
- Main line contactor: This contactor controls the power supply to the motor.
- Star contactor: This contactor connects the motor windings in a star configuration.
- Delta contactor: This contactor connects the motor windings in a delta configuration.
- Timer: This timer delays the switching from star to delta connection.
How it works
When the motor is started, the main line contactor and the star contactor are closed. The delta contactor is open. This connects the motor windings in a star configuration and reduces the voltage applied to the motor.
After a period of time, the timer switches the delta contactor on and the star contactor off. This connects the motor windings in a delta configuration and applies the full voltage to the motor.
The working of a star-delta starter can be explained through the following equations:
Star Connection
In a star connection, the motor windings are connected together at a common point, known as the star point. The voltage applied to each winding is reduced by a factor of 1/√3, and the current flowing through each winding is also reduced by a factor of 1/√3.
The following equation shows the relationship between the line voltage (VL) and the phase voltage (VP) in a star connection:
VP = VL / √3
The following equation shows the relationship between the line current (IL) and the phase current (IP) in a star connection:
IP = IL / √3
Delta Connection
In a delta connection, the motor windings are connected in a loop. The full voltage is applied to each winding, and the current flowing through each winding is also equal to the line current.
The following equation shows the relationship between the line voltage (VL) and the phase voltage (VP) in a delta connection:
VP = VL
The following equation shows the relationship between the line current (IL) and the phase current (IP) in a delta connection:
IP = IL
Switching from Star to Delta
When the motor is started, it is connected in a star configuration to reduce the starting current. Once the motor has accelerated to a certain speed, it is switched to a delta configuration to provide the motor with its full starting torque.
The following equation shows the ratio of the starting current in a star connection to the starting current in a delta connection:
Starting current ratio = 1 / √3
This equation shows that the starting current in a star connection is one-third of the starting current in a delta connection.
Advantages of Star-Delta Starters
- Reduced starting current: Star-delta starters can reduce the starting current of a motor by up to 33%. This is because the motor is initially connected in a star configuration, which reduces the voltage applied to the motor windings.
- Reduced starting torque: Star-delta starters can also reduce the starting torque of a motor by up to 33%. This can be beneficial in applications where a high starting torque is not required.
- Smooth start: Star-delta starters can provide a smooth start for a motor, which can help to reduce wear and tear on the motor and the driven load.
Disadvantages of Star-Delta Starters
- Increased complexity: Star-delta starters are more complex than other types of motor starters, such as direct online starters. This is because they require additional contactors and timers.
- Increased cost: Star-delta starters are more expensive than other types of motor starters.
Applications of Star-Delta Starters
Star-delta motor starters are commonly used in industrial applications where high starting currents and starting torques are not required. Some examples of applications where star-delta motor starters are used include:
- Conveyors
- Fans
- Pumps
- Compressors
- Lifts