For small capacity motors (up to 3 Hp), what voltage can be applied at startup? 

Voltage Application at Startup for Small Capacity Motors (Up to 3 Hp):

Correct Option: Full normal voltage can be applied at startup for small capacity motors (up to 3 Hp).

Explanation:

Small capacity motors, such as those up to 3 Hp, are designed to handle the application of full normal voltage at startup without adverse effects. These motors typically have robust designs and lower power ratings, which allow them to start directly with full voltage. This method of starting is known as "direct-on-line" (DOL) starting.

When full normal voltage is applied directly to the motor at startup, the motor achieves its rated speed almost instantly, resulting in quicker operation. This technique is suitable for smaller motors because they require relatively low starting current and do not pose a significant risk to the electrical system or the motor itself.

Advantages of Applying Full Normal Voltage:

• Simplicity: Direct application of full voltage eliminates the need for complex starting mechanisms, making it a simple and cost-effective method.
• Quick Startup: The motor reaches its rated speed rapidly, reducing the time needed for startup and enhancing operational efficiency.
• Compact Design: Small capacity motors are designed to withstand the initial surge of current during startup, ensuring reliable operation.
• Cost-Effective: No additional equipment, such as soft starters or variable frequency drives, is required, reducing overall costs.

Disadvantages:

• Although this method is suitable for small motors, applying full normal voltage to larger motors can cause excessive current draw, leading to potential damage to the motor and the electrical system.
• Direct-on-line starting may result in a mechanical shock to the connected equipment, which could lead to wear and tear over time.

Important Information

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

Option 1: Zero voltage.

This option is incorrect because applying zero voltage at startup would prevent the motor from starting entirely. Motors require a certain level of electrical energy to overcome inertia and initiate rotation. Without voltage, the motor remains stationary and cannot perform any operation.

Option 2: Double the normal voltage.

Applying double the normal voltage to a motor at startup is highly unsafe and impractical. Excessive voltage can lead to overheating, insulation breakdown, and permanent damage to the motor. Additionally, the electrical system may experience significant stress, causing potential hazards such as short circuits or equipment failure.

Option 3: Half the normal voltage.

This option involves applying reduced voltage at startup, which is not recommended for small capacity motors. While reduced voltage methods, such as soft starters or star-delta starters, are used for larger motors to limit inrush current, small motors are designed to handle full normal voltage without requiring such techniques. Applying half voltage may result in slower startup, lower torque, and inefficient operation.

Option 4: Full normal voltage.

This is the correct option. As explained above, small capacity motors (up to 3 Hp) are designed to start directly with full normal voltage, ensuring quick and efficient operation without compromising the motor's performance or safety.

Conclusion:

For small capacity motors, the application of full normal voltage at startup is the most suitable approach. It ensures simplicity, cost-effectiveness, and efficient operation. Other options, such as zero voltage, double voltage, or half voltage, are either impractical or unsuitable for these motors. Understanding the characteristics of small motors and their ability to handle direct-on-line starting is crucial for selecting the appropriate starting method.