In a three-phase squirrel-cage induction motor, the increased starting torque can be obtained by:

A) Using deep-bar rotors

B) Increasing the frequency of operation

C) Increasing the number of poles

D) Using a double-cage rotor

Explanation:

Three-Phase Squirrel-Cage Induction Motor

Definition: A three-phase squirrel-cage induction motor is a type of induction motor which is widely used in industrial and commercial applications. It is named "squirrel-cage" because the rotor structure resembles a squirrel cage. The rotor consists of short-circuited bars, typically made of aluminum or copper, which are connected by end rings. This type of motor is known for its rugged construction, reliability, and low maintenance requirements.

Increasing Starting Torque: In a three-phase squirrel-cage induction motor, various methods can be employed to increase the starting torque. Starting torque is crucial for applications where the motor needs to start under load. Here are some ways to achieve higher starting torque:

• Using Deep-Bar Rotors: Deep-bar rotors have rotor bars that are deeper and have a non-uniform cross-section. This design creates higher resistance during startup, which increases the starting torque. As the motor speeds up, the skin effect reduces the resistance, allowing the motor to operate efficiently at full speed. This method is effective because it provides high starting torque without compromising the motor's performance at normal operating speeds.
• Increasing the Number of Poles: By increasing the number of poles in the motor, the synchronous speed decreases. This lower synchronous speed results in higher slip at startup, which increases the starting torque. However, this method also reduces the motor's rated speed, which may not be desirable for all applications. It is a trade-off between starting torque and operating speed.
• Using a Double-Cage Rotor: Double-cage rotors have two sets of rotor bars with different electrical properties. The outer cage has higher resistance and lower reactance, providing high starting torque. The inner cage has lower resistance and higher reactance, which improves the motor's performance at normal operating speeds. This design allows for a good balance between high starting torque and efficient operation.

Correct Option Analysis:

The correct option is:

Option 1: A) Using deep-bar rotors, C) Increasing the number of poles, and D) Using a double-cage rotor.

This option correctly identifies the methods that can be used to increase the starting torque in a three-phase squirrel-cage induction motor. These methods are effective in improving the motor's starting performance while maintaining efficient operation at normal speeds.

Additional Information

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

Option 2: B) Increasing the frequency of operation, C) Increasing the number of poles, and D) Using a double-cage rotor.

Increasing the frequency of operation does not directly increase the starting torque. Instead, it affects the motor's speed and performance characteristics. Higher frequency operation typically increases the synchronous speed, which may not be desirable for applications requiring high starting torque. Therefore, this option is not suitable for increasing the starting torque.

Option 3: A) Using deep-bar rotors, B) Increasing the frequency of operation, and C) Increasing the number of poles.

As mentioned earlier, increasing the frequency of operation does not contribute to higher starting torque. While using deep-bar rotors and increasing the number of poles are effective methods, the inclusion of frequency increase makes this option incorrect for the specific goal of increasing starting torque.

Option 4: A) Using deep-bar rotors, B) Increasing the frequency of operation, and D) Using a double-cage rotor.

Similar to option 3, this option includes increasing the frequency of operation, which does not directly enhance the starting torque. While using deep-bar rotors and a double-cage rotor are valid methods, the inclusion of frequency increase renders this option incorrect for increasing starting torque.

Conclusion:

Understanding the methods to increase the starting torque in a three-phase squirrel-cage induction motor is crucial for selecting the appropriate approach based on the application requirements. Using deep-bar rotors, increasing the number of poles, and using a double-cage rotor are effective methods to achieve higher starting torque without compromising the motor's performance at normal operating speeds. These methods provide a balance between starting performance and operational efficiency, making them suitable for various industrial and commercial applications.