Question

Why star delta starter is preferred with induction motor?

Answer 1

A star-delta starter is preferred with induction motors because it helps reduce the starting current and provides smoother and more efficient operation. Here’s why:

### 1. **Reduction of Starting Current**:
   - When an induction motor starts, it draws a large inrush current (often 6-8 times the normal running current). This high current can damage the motor and the electrical supply system.
   - In a **star connection**, the motor windings are connected in a way that reduces the voltage applied to the motor by a factor of √3 (about 58% of the line voltage). This significantly reduces the starting current.
   - Once the motor reaches a certain speed (around 70-80% of its full speed), it is switched to a **delta connection**, where the full line voltage is applied, and the motor runs at its normal rated speed and torque.

### 2. **Reduced Mechanical Stress**:
   - When a motor starts with full voltage (as in direct-on-line starting), it experiences a sudden surge of mechanical stress. This can wear out the motor components more quickly.
   - The star-delta starter helps to ease this stress during startup by initially limiting the voltage and then gradually applying full voltage as the motor accelerates.

### 3. **Cost-Effective**:
   - Using a star-delta starter is cost-effective because it reduces the need for expensive components (like large capacitors or resistors) that would otherwise be necessary to limit the starting current in other types of starters.

### 4. **Energy Saving**:
   - By reducing the starting current and preventing power surges, the star-delta starter helps save energy and reduces power loss, especially in applications where motors start frequently.

### 5. **Simple and Reliable**:
   - The star-delta starter is easy to design, cost-effective, and reliable. It’s a popular choice for motors with a power rating up to around 15 kW (but it may not be suitable for larger motors because of the relatively high torque during star operation).

### 6. **Smoother Start**:
   - With a reduced voltage at the start (star connection), the motor accelerates more gradually and smoothly, avoiding jerks or sudden movements that could damage connected machinery or the motor itself.

### When to Use Star-Delta Starter:
Star-delta starters are mainly used for **medium-sized induction motors**, typically rated between 5 to 15 kW, where reducing starting current is essential, but the load doesn’t require very high starting torque.

In summary, a star-delta starter is preferred because it reduces the starting current, minimizes mechanical stress, is cost-effective, and provides a smooth start for the induction motor.