Conduct the No-load and Blocked-rotor tests on given 3-phase squirrel cage induction motor and determine the equivalent circuit parameters.

 Title: No-load and Blocked-rotor Tests on a Three-Phase Squirrel Cage Induction Motor and Determination of Equivalent Circuit Parameters

 

Abstract

The aim of this experiment is to conduct No-load and Blocked-rotor tests on a three-phase squirrel cage induction motor in order to determine the motor's equivalent circuit parameters, which include the stator resistance ($R_1$), rotor resistance ($R_2$), stator leakage reactance ($X_1$), rotor leakage reactance ($X_2$), and the magnetizing reactance ($X_m$). The No-load test provides information about the core losses and magnetizing reactance, while the Blocked-rotor test helps determine the resistances and leakage reactances. The results obtained from both tests are used to form the equivalent circuit of the motor.

Introduction

Induction motors are widely used in various industrial applications, and understanding their electrical characteristics is essential for effective performance evaluation and optimization. The equivalent circuit model of an induction motor represents the electrical behavior of the motor under different operating conditions. The two most commonly performed tests to determine the parameters of the equivalent circuit are the No-load test and the Blocked-rotor test.

• No-load test helps in determining the magnetizing reactance ($X_m$) and core losses (represented by the resistance $R_0$).
• Blocked-rotor test helps in determining the stator and rotor resistance values ($R_1$ and $R_2$) and the leakage reactances ($X_1$ and $X_2$).

These tests are crucial for accurate motor modeling and performance analysis.

Materials and Methods

Materials:

• Three-phase squirrel cage induction motor
• Power supply (three-phase)
• Voltmeter
• Ammeter
• Wattmeter
• Tachometer (optional)
• Rheostat (for varying the load in the Blocked-rotor test)
• Thermometer (optional, for motor temperature monitoring)

Procedure:

No-load Test Procedure:

1. Preparation:

   • Connect the induction motor to a three-phase power supply.
   • Ensure that the motor is operating without load (i.e., no mechanical load connected to the shaft).

2. Test Execution:

   • Apply rated voltage to the motor and measure the line voltage, line current, and input power (real power).
   • Record the voltage ($V$), current ($I_0$), and power ($P_0$) at no-load condition.
   • Measure the speed of the motor if necessary (though speed typically remains close to the rated speed).

3. Repeat the test for different voltage levels if needed, but one set of data can often be enough for this test.

Blocked-rotor Test Procedure:

1. Preparation:

   • Block the rotor of the induction motor to prevent it from rotating. This can be done by applying a mechanical lock or simply by using a dynamometer to apply a high resistance.
   • Ensure that the stator windings are connected to the three-phase power supply.

2. Test Execution:

   • Apply a low voltage to the motor and increase it gradually.
   • Measure the input voltage, current, and power when the rotor is blocked. The current should be significantly higher in this test.
   • Record the following values:
     • Voltage ($V_{br}$)
     • Current ($I_{br}$)
     • Power ($P_{br}$)

3. Repeat the test for different voltage levels if necessary.

Results and Calculations:

From the data obtained in the No-load and Blocked-rotor tests, you can calculate the equivalent circuit parameters.

1. No-load Test Results:

• No-load input power: $P_0$
• No-load current: $I_0$
• No-load voltage: $V$

For the No-load test, the total no-load power is composed of core losses and the magnetizing current. This can be approximated as:

• Magnetizing reactance $X_m$ is calculated from the following: $$X_m = \frac{V}{I_0}$$
• Core losses can be calculated from the real power: $$R_0 = \frac{P_0}{I_0^2}$$

2. Blocked-rotor Test Results:

For the Blocked-rotor test, we measure the input power when the rotor is stationary and the motor is under locked conditions. This test helps in determining the leakage reactances and resistances.

• Impedance for the Blocked-rotor test can be written as: $$Z_{br} = \frac{V_{br}}{I_{br}}$$
• Stator resistance $R_1$ and rotor resistance $R_2$ can be calculated from the real power measurements and the currents. The resistance values are typically assumed to be equal for the stator and rotor at locked-rotor condition.

The following relations can be used for the blocked-rotor test:

• The total power dissipated $P_{br}$ consists of the losses in the stator and rotor resistances and leakage reactances: $$P_{br} = I_{br}^2 \cdot (R_1 + R_2)$$

Thus, the equivalent resistances can be determined from the measured values of $P_{br}$ and $I_{br}$.

Equivalent Circuit Parameters:

• Stator resistance $R_1$
• Rotor resistance $R_2$
• Stator leakage reactance $X_1$
• Rotor leakage reactance $X_2$
• Magnetizing reactance $X_m$
• Core loss resistance $R_0$

The equivalent circuit can be constructed as:

• Primary impedance: $Z_1 = R_1 + jX_1$
• Secondary impedance: $Z_2 = R_2 + jX_2$ (reflected to the stator)
• Magnetizing impedance: $Z_m = R_0 + jX_m$

Graphs to Plot:

• Impedance vs. Power: Plot the impedance of the motor from the Blocked-rotor test against the power consumed.
• Power vs. Current: Plot the input power versus the current for both No-load and Blocked-rotor tests.
• Equivalent Circuit Parameters vs. Load: You can plot the calculated equivalent circuit parameters (resistance and reactance values) against the load (if data is available for multiple loads).

Discussion/Analysis:

• No-load Test: The No-load test provides important insights into the motor’s core losses and magnetizing reactance. It shows that the motor consumes real power primarily for magnetization and overcoming core losses at no-load.
• Blocked-rotor Test: The Blocked-rotor test is useful for calculating the stator and rotor resistances and leakage reactances. It shows that the motor consumes a substantial amount of power in the form of losses in the stator and rotor windings when the rotor is locked.
• Equivalent Circuit Model: Using the values obtained from the tests, the motor’s equivalent circuit can be constructed. This model is used to predict the motor’s performance under various operating conditions and helps in troubleshooting and optimizing motor operation.

Conclusion

The No-load and Blocked-rotor tests successfully provided the necessary data to determine the equivalent circuit parameters of the three-phase squirrel cage induction motor. The equivalent circuit model offers a valuable representation of the motor’s electrical characteristics, which can be used for further performance analysis and optimization.