Question

Explain working of autotransformer starter for a 3 phase induction motor with neat diagram.

Answer 1

### Autotransformer Starter for a 3-Phase Induction Motor

An **autotransformer starter** is a device used to start 3-phase induction motors by reducing the initial voltage supplied to the motor during startup. This helps in minimizing the high inrush current that can occur when the motor is started directly on-line. Let’s explore how an autotransformer starter works, along with a neat diagram for better understanding.

---

### Working Principle

The autotransformer starter operates on the principle of using a variable transformer (the autotransformer) to step down the voltage applied to the induction motor during startup. Here’s how it works step-by-step:

1. **Reduced Voltage During Startup**:
   - When starting a 3-phase induction motor, the autotransformer reduces the voltage supplied to the motor. This helps in limiting the starting current, which can be several times higher than the rated current. The reduction in voltage results in a proportionate reduction in current.

2. **Autotransformer Configuration**:
   - The autotransformer has multiple taps that allow for different voltage levels to be obtained. Typically, the starting voltage is set at about 50% or 80% of the supply voltage. The specific tap used depends on the design requirements and the motor's characteristics.

3. **Connection to the Motor**:
   - The autotransformer is connected to the supply and the motor. Initially, only the reduced voltage from the autotransformer is applied to the motor.

4. **Transition to Full Voltage**:
   - After a predetermined time (or when the motor reaches a certain speed), a switch or relay is used to disconnect the autotransformer from the circuit. This action allows the motor to be supplied with the full line voltage, enabling it to operate at its rated speed and torque.

5. **Protection and Control**:
   - The autotransformer starter includes control elements like overload relays to protect the motor from overheating and damage during the starting and running phases.

---

### Diagram

Below is a simple diagram illustrating the connections and operation of an autotransformer starter for a 3-phase induction motor:

```
                     +----------+
                     |          |
                     |   Load   |
                     |  (Motor) |
                     +-----+----+
                           |
                           |
                           |
                    +------+------+
                    |  Autotrans- |
                    | Transformer  |
                    +------+-------+
                           |
                     +-----+-----+
                     |   Taps    |
                     +-----------+
                           |
                           |
                     +-----+-----+
                     |   Switch   |
                     +-----+-----+
                           |
                           |
                     +-----+-----+
                     |   Supply   |
                     |   (3-Phase)|
                     +-----------+
```

### Components of the Autotransformer Starter

1. **Autotransformer**: A transformer with one winding that serves as both the primary and secondary winding, which allows voltage adjustment.

2. **Switch**: A mechanism to switch the motor from reduced voltage to full voltage.

3. **Overload Relay**: Protects the motor from overload conditions by disconnecting it if the current exceeds safe levels.

4. **Control Circuit**: Used to manage the timing of the switch operation, ensuring the motor starts smoothly and transitions to full voltage at the correct time.

### Advantages of Using an Autotransformer Starter

1. **Reduced Inrush Current**: Helps to limit the inrush current to safe levels during startup, which can prolong the life of the motor and associated equipment.

2. **Smooth Starting**: The gradual increase in voltage allows for smoother acceleration, reducing mechanical stress on the motor and connected load.

3. **Simple Design**: The autotransformer starter has a relatively simple design compared to other types of starters, making it easier to implement and maintain.

4. **Flexibility**: Different taps can be used to adjust the starting voltage according to the motor and application requirements.

### Disadvantages

1. **Cost**: Autotransformers can be more expensive than other starter types, such as direct-on-line starters.

2. **Size**: Autotransformers can be bulky, which may be a consideration in space-constrained environments.

3. **Efficiency**: There may be slight energy losses in the transformer, affecting overall efficiency.

---

### Conclusion

The autotransformer starter is an effective solution for starting 3-phase induction motors by providing reduced voltage at startup, thus minimizing inrush current and mechanical stress. Its design and operational principles allow for smooth motor acceleration and enhanced reliability, making it a widely used method in industrial applications. Understanding its workings, benefits, and limitations is crucial for engineers and technicians involved in motor control systems.

Answer 2

An autotransformer starter is a type of starting device used to start three-phase induction motors. It is designed to reduce the starting current and limit the starting torque of the motor, thereby reducing mechanical stress on the motor and electrical stress on the power supply. Here's a detailed explanation of its working along with a neat diagram.

### Working Principle

1. **Autotransformer Construction:**
   - The autotransformer consists of a single winding that acts as both the primary and secondary winding. It has three terminals: one common terminal and two other terminals (high-voltage and low-voltage). This design allows for a variable voltage to be applied to the motor.

2. **Starting Sequence:**
   - **Initial Stage:**
     - When the motor is started, the autotransformer is connected to the motor circuit. The transformer is designed to provide a reduced voltage to the motor during the starting phase. This reduction in voltage limits the starting current and torque.
     - The reduced voltage is applied to the motor’s stator windings, which lowers the starting current and torque compared to direct-on-line starting.
   - **Running Stage:**
     - Once the motor reaches a certain speed (usually around 70-80% of its rated speed), a centrifugal switch or an equivalent device disconnects the autotransformer from the motor circuit. This is done to apply full line voltage to the motor, allowing it to operate at its rated performance.

### Diagram

Here is a simplified diagram of an autotransformer starter for a three-phase induction motor:

```
              L1     L2     L3
              |      |      |
              |      |      |
          +---+---+---+---+---+
          |                  |
         [A]                [M]
          |                  |
          +------------------+
              |       |
              |       |
         +----+-------+----+
         |                 |
         |                 |
       [Auto-           [M]
       Transformer]    (Motor)
         |                 |
         +-------+---------+
                 |
                 |
              N1 | N2
               |   |
               |   |
           ----+---+----
                |
                |
               GND
```

### Components in the Diagram:

1. **L1, L2, L3:** Three-phase supply lines.
2. **[A]:** Autotransformer.
3. **[M]:** Motor (Induction Motor).
4. **N1, N2:** Terminals of the autotransformer.
5. **GND:** Ground.

### Operation:

1. **Starting Phase:**
   - During start-up, the autotransformer provides a reduced voltage to the motor. This is achieved by using the appropriate tapping on the autotransformer which provides a lower voltage ratio to the motor.
   - The motor starts with lower current and torque, which reduces the strain on both the motor and the electrical system.

2. **Running Phase:**
   - Once the motor reaches a certain speed, the autotransformer is bypassed or disconnected from the motor circuit. The motor then receives full line voltage, allowing it to operate at its rated conditions.

3. **Transition Mechanism:**
   - A switch or a relay is used to change the connection from the reduced voltage circuit to the full voltage circuit. This transition is controlled either manually or automatically based on the motor's speed.

### Advantages:

- **Reduced Starting Current:** By applying reduced voltage during start-up, the autotransformer limits the inrush current, reducing the load on the power supply.
- **Smooth Starting:** It provides a gradual increase in voltage, which results in smoother acceleration of the motor.
- **Lower Mechanical Stress:** Reduced starting torque lowers mechanical stress on the motor and connected machinery.

### Disadvantages:

- **Size and Cost:** Autotransformers can be bulky and costly compared to other starting methods.
- **Complexity:** The starting system can be more complex due to the need for switching mechanisms and controls.

In summary, an autotransformer starter provides a controlled method of starting a three-phase induction motor by initially applying a reduced voltage and then switching to full voltage once the motor is up to speed. This helps in minimizing the starting current and mechanical stress.