Which of the following testing methods is a direct testing of D C machine?  

Explanation:

Direct Testing of DC Machines

Definition: Direct testing of a DC machine involves operating the machine under actual working conditions and measuring its performance directly. This method provides accurate results as the machine is tested in real-time under actual load conditions. Among the various testing methods available for DC machines, the Brake Test is one of the most common direct testing methods.

Correct Option Analysis:

The correct option is:

Option 4: Brake Test

The Brake Test is a direct testing method where the DC machine is tested under load conditions, and its performance is evaluated. In this method, the machine is loaded mechanically using a brake drum, and the torque produced by the machine is measured. The output power is calculated based on the torque and speed. The input power is measured using electrical instruments to evaluate the efficiency of the machine.

Working Principle of Brake Test:

• A belt or rope is wound around the brake drum, which is mounted on the shaft of the DC machine.
• The ends of the belt or rope are connected to a spring balance and a fixed weight. The tension in the rope creates a braking torque on the drum.
• The torque produced by the machine is measured by the difference in readings of the spring balance on either side of the drum.
• The rotational speed of the shaft is measured using a tachometer.
• Electrical instruments are used to measure the input power supplied to the machine.
• The output power is calculated using the formula:

Output Power, P_out = 2 × π × N × T

• Where:
• N = Rotational speed of the shaft in revolutions per second (rps)
• T = Torque produced by the machine (measured using the spring balance)
• The efficiency of the DC machine can be calculated using the formula:

Efficiency, η = (P_out / P_in) × 100%

Advantages of Brake Test:

• Provides accurate results as the machine is tested under actual working conditions.
• Simple and straightforward method for performance evaluation.

Disadvantages of Brake Test:

• Cannot be used for large machines due to the difficulty in dissipating the heat generated during testing.
• Requires a mechanical setup, which may involve additional costs and effort.

Applications:

• Commonly used for small and medium-sized DC machines to evaluate their performance.
• Widely used in laboratories for educational and research purposes.

Additional Information

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

Option 1: Swinburne's Test

Swinburne's Test is an indirect testing method used for DC machines. It involves measuring the no-load losses of the machine and estimating its performance under load conditions. This method is not a direct test as the machine is not tested under actual working conditions. It is suitable for machines where loading is difficult or impractical, such as large DC machines.

Option 2: Field Test

The Field Test is a specific method used for testing two identical DC machines connected back-to-back, where one machine acts as a motor and the other as a generator. This method is not a direct testing method, as it involves testing two machines simultaneously under controlled conditions.

Option 3: Hopkinson’s Test

Hopkinson’s Test, also known as the Regenerative Test, is another indirect testing method. It involves connecting two identical DC machines back-to-back, with one operating as a motor and the other as a generator. The power circulating between the machines reduces the external power requirement. This method is suitable for large machines but is not considered a direct testing method.

Conclusion:

The Brake Test is the correct answer as it is a direct testing method where the DC machine is tested under actual load conditions. While other methods like Swinburne's Test, Field Test, and Hopkinson’s Test are valuable for specific applications, they are not direct testing methods. The Brake Test provides accurate results and is widely used for evaluating the performance of small and medium-sized DC machines.