What is the modified starter called when the holding coil circuit is made independent of the field circuit? 

Explanation:

4-Point Starter

Definition: A 4-point starter is a type of electrical starter used for starting and protecting DC motors. It is called a 4-point starter because it has four terminals: L (Line), F (Field), A (Armature), and N (No-volt release coil). Unlike the 3-point starter, the 4-point starter's holding coil circuit is made independent of the field circuit, which eliminates the drawback of the 3-point starter where the no-volt release (NVR) coil could potentially disengage during operation due to variations in the field current.

Working Principle: The 4-point starter operates on the principle of electromagnetic force. When the motor is started, the handle of the starter is moved from the OFF position towards the ON position. As the handle moves, the resistance connected in series with the armature is gradually reduced, allowing a controlled increase in armature current. The no-volt release coil ensures that the motor stops in case of a power failure or if the handle is not in the ON position. Since the NVR coil is connected directly across the supply voltage, its operation is independent of the field circuit, which ensures better stability and reliability.

Advantages:

• The holding coil circuit is independent of the field circuit, ensuring stable operation even if the field current fluctuates.
• Provides better protection to the motor against power supply interruptions or faults.
• Prevents the motor from restarting automatically after a power failure, thus ensuring safety.

Disadvantages:

• More complex design compared to a 3-point starter, leading to higher manufacturing costs.
• Requires additional components, such as a separate connection for the no-volt release coil.

Applications: 4-point starters are widely used in DC motors, especially in applications requiring precise control and protection, such as in industrial machinery, elevators, and cranes.

Correct Option Analysis:

The correct option is:

Option 3: 4-point starter

The 4-point starter is specifically designed to address the limitation of the 3-point starter by making the holding coil circuit independent of the field circuit. This design ensures that the no-volt release coil remains energized irrespective of variations in the field current, thereby providing better operational stability and protection for the motor.

Additional Information

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

Option 1: 2-point starter

The 2-point starter is used for smaller DC motors and is rarely employed in practical applications. It does not have a no-volt release coil, which makes it less reliable for protecting the motor during power supply interruptions or faults. This type of starter does not solve the issue of making the holding coil circuit independent of the field circuit, so it is not the correct answer.

Option 2: 3-point starter

The 3-point starter is commonly used for starting DC motors. However, in this design, the no-volt release coil is connected in series with the field winding. This dependency can lead to operational issues if the field current fluctuates, as it may cause the no-volt release coil to disengage and stop the motor. This limitation is addressed in the 4-point starter, making the 3-point starter an incorrect option for the given question.

Option 4: 5-point starter

The concept of a 5-point starter is not commonly used or defined in standard electrical engineering practices. This option is invalid and does not pertain to the question.

Option 5: (No option provided)

This option is left blank, so it is not a valid choice.

Conclusion:

The 4-point starter effectively eliminates the drawback of the 3-point starter by making the holding coil circuit independent of the field circuit. This design ensures stable and reliable operation of the motor, even in the presence of field current fluctuations. The 4-point starter is therefore the correct answer for the given question and is widely used in industrial applications requiring precise motor control and protection.