Question

With neat sketch state the working principle of Star-Delta starter.

Answer 1

The Star-Delta starter is a method used to reduce the starting current of a three-phase induction motor. Here’s a concise explanation of its working principle along with a neat sketch description.

### Working Principle

1. **Starting in Star Configuration**:
   - When the motor starts, it is initially connected in a **star (Y)** configuration. In this configuration, the line voltage is divided by √3, which reduces the voltage across each winding.
   - This results in a lower starting current (approximately 1/3 of what it would be in delta configuration) and reduces the mechanical stress on the motor.

2. **Switching to Delta Configuration**:
   - After the motor reaches about 70-80% of its full speed (usually within a few seconds), the starter switches the motor connection from star to **delta (Δ)**.
   - In the delta configuration, the motor operates at full line voltage, allowing it to deliver its rated power.

### Sketch Description

1. **Star Configuration**:
   - Three windings of the motor are connected to form a star point.
   - Each phase winding is connected to the line through a contactor.

2. **Delta Configuration**:
   - The windings are interconnected in a triangular formation.
   - Each winding connects to two other windings, allowing full line voltage across each winding.

3. **Control Circuit**:
   - A timer or relay controls the switching of the contactors from star to delta after a specified time.

### Sketch Elements

- **Motor**: Represented as a circle labeled "Induction Motor."
- **Contactors**: Three contactors for star (Y) and delta (Δ) configurations, with a switch mechanism.
- **Power Supply**: Three-phase power supply connections (R, Y, B) leading to the contactors.
- **Winding Connections**: Lines representing how the windings connect in star and delta formations.

### Summary

The Star-Delta starter effectively reduces the inrush current during motor startup, minimizing electrical and mechanical stress. This method is particularly useful for large induction motors, enhancing their performance and longevity.

Answer 2

The star-delta starter is a type of electrical circuit used to start induction motors. Its primary purpose is to reduce the starting current and torque by initially connecting the motor windings in a star (Y) configuration and then switching to a delta (Δ) configuration once the motor reaches a certain speed. Here’s a detailed explanation of how it works:

### Working Principle of Star-Delta Starter

1. **Initial Star Connection:**
   - **Purpose:** When an induction motor starts, it initially requires high starting torque and current. To limit these values, the motor windings are connected in a star configuration.
   - **Star Configuration (Y):** In this configuration, each of the three windings of the motor is connected to a common point (the neutral point). This reduces the voltage applied to each winding to 1/√3 (approximately 57.7%) of the line voltage.
   - **Result:** The reduced voltage results in a lower starting current and torque. The current flowing through the motor is approximately 1/√3 of the current that would flow if the motor were connected directly to the line voltage.

2. **Transition to Delta Connection:**
   - **Timing:** After the motor reaches about 70-80% of its full speed (this depends on the motor and load), the starter switches the motor windings from the star to the delta configuration.
   - **Delta Configuration (Δ):** In this configuration, the windings are connected in a triangle shape, with each end of the winding connected to the line voltage directly. This allows the motor to operate at its full rated voltage and achieve its full torque and speed.
   - **Result:** The motor now operates with its full capacity, and the starting torque and current are no longer a concern.

### Circuit Diagram

The star-delta starter circuit typically includes the following components:

1. **Contactor for Star Connection (K1):** This contactor connects the motor windings in the star configuration during the starting phase.

2. **Contactor for Delta Connection (K2):** This contactor switches the motor windings to the delta configuration once the motor has accelerated to the required speed.

3. **Timer:** A time delay relay is used to switch from the star to the delta configuration after the motor has started. This delay ensures that the motor has sufficient time to reach the necessary speed.

4. **Overload Relay:** Protects the motor from overheating by disconnecting it in case of excessive current.

### Operation Sequence

1. **Starting Phase:**
   - When the motor is started, the K1 contactor engages, connecting the motor windings in the star configuration.
   - The motor accelerates under reduced voltage, leading to lower starting current and torque.

2. **Transition Phase:**
   - After a preset time (determined by the timer), the K1 contactor disengages, and the K2 contactor engages.
   - This switches the motor windings to the delta configuration, allowing the motor to run at its full rated voltage.

3. **Running Phase:**
   - The motor operates in the delta configuration, with full voltage applied, and runs at its rated speed and torque.

### Advantages

- **Reduced Starting Current:** By reducing the voltage during the starting phase, the starting current is significantly lower, which helps to avoid excessive stress on the electrical supply.
- **Lower Starting Torque:** Reduces mechanical stress on the motor and connected equipment during startup.
- **Cost-effective:** Star-delta starters are relatively simple and cost-effective compared to other motor starting methods.

### Limitations

- **Not Suitable for All Motors:** The star-delta starter is typically used for motors with a high starting torque requirement and is not suitable for all types of loads or motors.
- **Speed of Motor:** The motor must be capable of accelerating to its rated speed within the time allowed by the timer. If the motor cannot accelerate quickly enough, it may stall or not start properly.

### Neat Sketch

The following sketch illustrates the basic arrangement of the star-delta starter:

```
          +------------------+
          |                  |
          |                  |
  L1 -----|---- K1 (Star) ----|---- Motor
          |                  |
  L2 -----|---- K2 (Delta) ---|---- Motor
          |                  |
  L3 -----|-------------------|---- Motor
          +------------------+
```

In this diagram:
- **K1** is the contactor for the star connection.
- **K2** is the contactor for the delta connection.
- **L1, L2, L3** are the power supply lines.
- **Motor** represents the three-phase induction motor.

By using this system, the motor can be started with lower inrush current and then switched to full voltage for normal operation, improving the efficiency and longevity of both the motor and the electrical system.