Starters are used for starting of large three-phase induction motor:

Explanation:

Starters are essential components used in the operation of large three-phase induction motors. Their primary function is to manage the current drawn by the motor during its start-up phase, which is typically much higher than the current required for normal operation.

When a large three-phase induction motor is initially started, it draws a significantly high inrush current, often 6 to 8 times its full-load current. This high starting current can cause several issues, including voltage drops in the power supply network, potential damage to the motor windings, and excessive mechanical stress on the motor components. To mitigate these issues, starters are employed.

Correct Option:

The correct option is:

Option 3: to decrease supply current at start

This is the primary reason for using starters in large three-phase induction motors. By reducing the initial inrush current, starters help in minimizing the adverse effects associated with the high starting current, thereby ensuring a smoother and safer start-up process.

Additional Information

There are several types of starters used for this purpose, including:

• Direct-On-Line (DOL) Starter: This type of starter directly connects the motor to the power supply. It is simple and inexpensive but results in a high inrush current. DOL starters are typically used for smaller motors where the high starting current is manageable.
• Star-Delta Starter: This starter initially connects the motor windings in a star (Y) configuration, reducing the voltage across each winding to 1/√3 (approximately 58%) of the line voltage. This results in a lower starting current. After the motor reaches a certain speed, the windings are reconnected in a delta (Δ) configuration for normal operation.
• Autotransformer Starter: This starter uses an autotransformer to reduce the voltage supplied to the motor during start-up. The reduced voltage results in a lower starting current. After the motor accelerates, the autotransformer is bypassed, and the full line voltage is applied to the motor.
• Soft Starter: A soft starter gradually increases the voltage supplied to the motor, thereby smoothly ramping up the motor speed and reducing the inrush current. Soft starters provide better control over the starting process and are often used in applications requiring a gentle start.

Analysis of Other Options:

To further understand the analysis, let’s evaluate the other options:

Option 1: to increase its efficiency

This option is incorrect. The primary function of a starter is not to increase the efficiency of the motor. While proper starting techniques can contribute to the longevity and reliability of the motor, they do not directly impact its efficiency.

Option 2: to decrease losses in the machine

This option is also incorrect. While reducing the starting current can help in minimizing losses during the start-up phase, the primary purpose of a starter is to manage the inrush current rather than to decrease the overall losses in the machine.

Option 4: to maintain constant speed

This option is incorrect. Starters are used to control the starting current and ensure a smooth start-up process. They do not play a role in maintaining the constant speed of the motor during its normal operation. Speed control is typically managed by other means, such as variable frequency drives (VFDs) or mechanical governors.

Conclusion:

In conclusion, starters are crucial for the safe and efficient operation of large three-phase induction motors. Their primary function is to decrease the supply current at start, thereby mitigating the potential issues associated with high inrush currents. By understanding the role of starters and the different types available, one can ensure the optimal performance and longevity of the motor.