Ward - Leonard controlled DC drives are generally used for: 

Explanation:

Ward-Leonard Controlled DC Drives

Definition: The Ward-Leonard system is an electrical drive system used to control the speed of DC motors. It utilizes a motor-generator set to provide a variable DC voltage to the armature of the DC motor, allowing for precise and wide-range speed control. This system is particularly useful in applications requiring smooth and efficient speed variation under heavy load conditions.

Working Principle:

The Ward-Leonard system consists of three main components:

• AC Motor: This motor operates on a fixed AC supply and drives the DC generator.
• DC Generator: This generator is mechanically coupled to the AC motor and provides a variable DC voltage output. The voltage can be adjusted by controlling the field excitation of the generator.
• DC Motor: The variable DC voltage from the generator is supplied to the armature of the DC motor, which drives the load. The speed of the DC motor can be precisely controlled by varying the generator's output voltage.

By adjusting the generator’s excitation, the output voltage can be controlled, thereby allowing precise speed regulation of the DC motor. The Ward-Leonard system offers a wide range of speed control, both above and below the base speed, and is known for its smooth operation and ability to handle heavy loads efficiently.

Advantages:

• Provides a wide range of speed control, both above and below the base speed.
• Ensures smooth and stepless speed variation, making it suitable for heavy-duty applications.
• Offers excellent torque characteristics, which is critical in applications involving heavy loads.

Disadvantages:

• High initial cost due to the need for a motor-generator set.
• Lower efficiency compared to modern electronic drives due to mechanical losses in the system.
• Requires significant maintenance because of the mechanical components involved.

Applications: Ward-Leonard controlled DC drives are commonly used in applications where precise speed control and high torque are necessary, such as in heavy-duty excavators, rolling mills, elevators, and cranes.

Correct Option Analysis:

The correct option is:

Option 3: Heavy duty excavators

Ward-Leonard controlled DC drives are particularly suitable for heavy-duty applications like excavators due to their ability to provide precise speed control and high torque under varying load conditions. Heavy-duty excavators require robust and reliable speed control systems to handle challenging operational environments, and the Ward-Leonard system excels in delivering the required performance. Its ability to operate efficiently under heavy mechanical loads makes it an ideal choice for such applications.

Additional Information

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

Option 1: Light duty excavators

Ward-Leonard controlled DC drives are not commonly used for light-duty excavators. Light-duty applications typically require less complex and less expensive speed control systems, such as variable frequency drives (VFDs) or simpler DC drives. The high cost and complexity of the Ward-Leonard system make it unsuitable for light-duty applications where such precision and robustness are not necessary.

Option 2: Medium duty excavators

While the Ward-Leonard system can be used for medium-duty applications, it is not the most cost-effective solution. Medium-duty excavators may use more modern electronic drives that offer good performance at a lower cost. The Ward-Leonard system is better suited for heavy-duty operations where its advantages can be fully utilized.

Option 4: Low duty excavators

This option is incorrect because low-duty excavators, similar to light-duty ones, do not require the precision, robustness, or high torque capabilities of the Ward-Leonard system. Simpler and less expensive drive systems are typically used for such applications.

Conclusion:

The Ward-Leonard system is a reliable and robust method for controlling the speed of DC motors, making it highly suitable for heavy-duty applications like excavators. Its ability to provide precise speed control and excellent torque characteristics under heavy loads makes it the preferred choice for such demanding environments. However, due to its high cost and complexity, it is not commonly used in light-duty, medium-duty, or low-duty applications, where simpler drive systems are more appropriate.