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Title: Direct Load Test on a Three-Phase Squirrel Cage Induction Motor 

Abstract

The purpose of this experiment is to conduct a direct load test on a three-phase squirrel cage induction motor to analyze its performance characteristics. The test involves measuring the input electrical power, output mechanical power, and motor speed under varying load conditions. Key parameters such as efficiency, power factor, motor current, and torque-slip characteristics are plotted to understand the motor's behavior under different operating conditions. The results show the relationship between load and these performance parameters, which are essential for evaluating the efficiency and suitability of the motor for different applications.

Introduction

A three-phase squirrel cage induction motor is one of the most widely used types of electric motor due to its robustness, reliability, and simple construction. The performance of an induction motor depends on various factors such as load, speed, efficiency, power factor, and current. The direct load test allows for the measurement of these parameters at different load conditions, providing insight into the motor's behavior and performance characteristics. This experiment aims to perform a direct load test on a three-phase squirrel cage induction motor and plot the following characteristics:

  • Efficiency vs. Output
  • Power Factor vs. Output
  • Power Factor vs. Motor Current
  • Torque-Slip/Speed Characteristics

Materials and Methods

Materials:

  • Three-phase squirrel cage induction motor
  • Digital wattmeter
  • Tachometer
  • Ammeter
  • Voltmeter
  • Variable load (e.g., dynamometer or brake drum)
  • Stopwatch
  • Thermometer (optional, for monitoring motor temperature)

Procedure:

  1. Connect the three-phase squirrel cage induction motor to the power supply.
  2. Set the motor to no-load condition and measure the no-load current, voltage, and power.
  3. Gradually apply the load using a dynamometer or brake drum, and measure the following parameters at various load levels (e.g., 25%, 50%, 75%, and 100% of rated load):
    • Input power (using a wattmeter)
    • Output mechanical power (using the dynamometer)
    • Motor current (A)
    • Voltage (V)
    • Motor speed (rpm) using a tachometer
  4. Record the data for efficiency, power factor, motor current, and torque at each load point.

Results

Once you have completed the experiment and collected the data, use it to calculate the following parameters at each load condition:

  1. Efficiency:
    Efficiency is calculated using the formula:

    η=Output Power (mechanical)Input Power (electrical)×100\eta = \frac{\text{Output Power (mechanical)}}{\text{Input Power (electrical)}} \times 100

  2. Power Factor:
    Power factor is calculated as:

    Power Factor=Real Power (W)Apparent Power (VA)\text{Power Factor} = \frac{\text{Real Power (W)}}{\text{Apparent Power (VA)}}

    Where Apparent Power (VA) is given by:

    Apparent Power=Voltage×Current\text{Apparent Power} = \text{Voltage} \times \text{Current}

  3. Motor Current: This is directly measured from the ammeter at each load.

  4. Torque: Torque can be calculated from the output power and speed:

    Torque=Output Power (W)2π×Speed (rpm)/60\text{Torque} = \frac{\text{Output Power (W)}}{2\pi \times \text{Speed (rpm)} / 60}

Once you have all the data, you can create the required plots.

Graphs to Plot

  1. Efficiency vs. Output

    • Plot the output power (in kW or HP) on the x-axis and the efficiency (%) on the y-axis.
    • This graph will show how efficiently the motor operates at different loads.

  2. Power Factor vs. Output

    • Plot the output power (in kW or HP) on the x-axis and the power factor on the y-axis.
    • This graph will show how the power factor changes with the load.

  3. Power Factor vs. Motor Current

    • Plot the motor current (in amperes) on the x-axis and the power factor on the y-axis.
    • This shows how the power factor varies with the current drawn by the motor.

  4. Torque-Slip/Speed Characteristics

    • Plot the speed (in rpm) on the x-axis and the torque (in Nm) on the y-axis for different load conditions.
    • The speed will typically decrease as the load increases, while the torque will increase.

Discussion/Analysis

  • Efficiency: The efficiency of the motor typically increases as the load increases, but after a certain point, it may begin to level off or decrease due to losses in the motor.
  • Power Factor: At light loads, the power factor is generally low, and as the load increases, the power factor improves. This is because at low loads, the motor draws more reactive power.
  • Motor Current: The current increases with the load, as more electrical power is needed to deliver more mechanical power.
  • Torque-Slip/Speed Characteristics: The speed decreases as the load increases, and torque increases. This behavior is characteristic of an induction motor.

Conclusion

The direct load test on the three-phase squirrel cage induction motor provided valuable insights into its performance under varying load conditions. The efficiency of the motor increased with the load, and the power factor improved as the load was applied. The motor current increased with the load, which is expected, and the torque-slip characteristics showed a typical decrease in speed as the load increased. These results demonstrate the motor’s typical performance profile and provide useful data for selecting the motor for specific applications.