Measure the open circuit voltage ratio of the three-phase slip ring induction motor and perform the speed control by insertion of resistance in rotor circuit for slip ring induction motor.

 

 Title: Measurement of Open Circuit Voltage Ratio and Speed Control of Three-Phase Slip Ring Induction Motor by Insertion of Resistance in Rotor Circuit 

Abstract

This experiment focuses on two key aspects of the three-phase slip ring induction motor: measuring the open circuit voltage ratio and performing speed control by inserting external resistance into the rotor circuit. The open circuit voltage ratio is used to assess the motor's efficiency and performance under no-load conditions. Speed control is achieved by inserting resistance into the rotor circuit, which effectively reduces the motor's speed by increasing the rotor resistance, thus changing the slip and controlling the motor's speed. The relationship between rotor resistance and speed is explored, along with the impact on current and efficiency.

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Introduction

Induction motors are commonly used in various applications due to their simplicity and rugged design. Specifically, the three-phase slip ring induction motor provides a means of controlling speed more effectively compared to the standard squirrel cage induction motor. The open circuit voltage ratio gives insights into the efficiency of the motor, and the insertion of external resistance into the rotor circuit allows for speed control by varying the slip and consequently adjusting the rotor speed.

1. Open Circuit Voltage Ratio: This ratio helps in understanding the performance characteristics of the motor, particularly under no-load conditions. It is measured between the stator and rotor under open circuit conditions and used to determine the efficiency and effectiveness of the motor.

2. Speed Control via Rotor Resistance: In slip ring motors, external resistance can be inserted into the rotor circuit, which increases the resistance of the rotor windings. This increases the slip of the motor, causing a decrease in the motor's speed. This method allows for controlling the speed of the motor without altering the supply frequency.

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Materials and Methods

Materials:

• Three-phase slip ring induction motor
• Rheostat or external variable resistance for rotor circuit
• Digital voltmeter and ammeter
• Tachometer (for measuring motor speed)
• Power supply (three-phase AC)
• Wattmeter
• Rotor circuit connection setup (for inserting resistance into the rotor)
• Switchgear and control devices (such as fuses and circuit breakers)

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Procedure:

1. Measuring Open Circuit Voltage Ratio

1. Initial Setup:

   • Connect the three-phase slip ring induction motor to a three-phase power supply.
   • Ensure the rotor is not connected to any load or external resistance (i.e., the rotor circuit is open).
   • Measure the stator voltage using the voltmeter, and record it as the supply voltage.
   • Use a voltmeter to measure the induced voltage in the rotor (open-circuit voltage) under no-load conditions.

2. Measurements:

   • Measure and record the stator voltage ($V_1$) and the rotor voltage ($V_2$) under open circuit conditions.
   • The open-circuit voltage ratio is given by: $$\text{Voltage Ratio} = \frac{V_2}{V_1}$$
   • This ratio helps in understanding how the voltage is induced in the rotor relative to the stator voltage.

3. Repeat the measurements for different load conditions if necessary, though typically no-load voltage ratios are the focus.

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2. Speed Control by Insertion of Resistance in Rotor Circuit

1. Initial Setup:

   • Connect the three-phase slip ring induction motor to the three-phase power supply.
   • Insert a variable resistor or rheostat into the rotor circuit to control the resistance.

2. Test Execution:

   • Start the motor at its rated voltage without any resistance in the rotor circuit (this is the base speed).
   • Measure and record the motor speed (using a tachometer), current, and power at no-load and full-load conditions.
   • Slowly insert resistance into the rotor circuit. As the resistance increases, measure the changes in the motor speed.
   • Record the following parameters at different rotor resistances:
     • Rotor resistance ($R_r$) - this will be varied by adjusting the rheostat.
     • Motor speed ($N$) - as the resistance is increased, the speed will decrease.
     • Stator current ($I_s$) and rotor current ($I_r$).

3. Results Analysis:

   • Observe how the motor speed decreases with an increase in rotor resistance. This is due to an increase in the slip, which causes the motor to slow down.
   • The increase in resistance results in higher power losses in the rotor, which further reduces the efficiency of the motor.
   • Plot the motor speed versus rotor resistance and observe the linear or nonlinear relationship.

4. Power and Efficiency:

   • Record the power input and compare the changes in efficiency as the rotor resistance is varied. Efficiency decreases as more resistance is added due to increased losses in the rotor windings.
   • Efficiency can be calculated using the formula: $$\eta = \frac{\text{Output Power}}{\text{Input Power}} \times 100$$
   • Output power is the mechanical power delivered by the motor, and input power is the electrical power supplied to the motor.

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Results and Data Analysis

Open Circuit Voltage Ratio:

• Stator Voltage ($V_1$): [Measured Value]
• Rotor Voltage ($V_2$): [Measured Value]
• Open Circuit Voltage Ratio: $$\frac{V_2}{V_1} = \text{[Calculated Value]}$$

This ratio shows the voltage induced in the rotor compared to the stator. The open-circuit voltage ratio is a critical parameter in understanding the motor’s performance and efficiency.

Speed Control by Rotor Resistance Insertion:

• Base Speed (No Resistance): [Measured Speed at No Load]
• Speed at Various Rotor Resistances:
  • Rotor Resistance ($R_r$) = [Value]: Speed = [Measured Speed]
  • Rotor Resistance ($R_r$) = [Value]: Speed = [Measured Speed]
  • Rotor Resistance ($R_r$) = [Value]: Speed = [Measured Speed]

The motor speed will decrease as the rotor resistance is increased. This is due to the increase in slip, which increases the losses and reduces the speed of the motor.

• Current Measurements:

  • Stator Current ($I_s$) at various rotor resistances.
  • Rotor Current ($I_r$) at various rotor resistances.

• Efficiency: As the rotor resistance increases, the motor’s efficiency decreases due to the increased rotor losses.

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Discussion/Analysis

1. Open Circuit Voltage Ratio:

   • The open-circuit voltage ratio is an important parameter for understanding how much voltage is induced in the rotor relative to the stator. This ratio gives insight into the magnetizing characteristics of the motor and its no-load performance.

2. Effect of Rotor Resistance on Speed:

   • Inserting external resistance in the rotor circuit increases the slip, which causes the motor speed to decrease. The more resistance inserted, the slower the motor operates. This speed control method is beneficial for applications where precise speed control is needed, although it can lead to higher losses and reduced efficiency.

3. Power Losses and Efficiency:

   • As resistance increases, rotor losses increase, leading to lower efficiency. The speed reduction is achieved at the expense of additional losses, which may limit the practical use of this method in high-efficiency applications.

4. Advantages and Disadvantages:

   • Advantages: Simple method for speed control, useful in applications where only a limited speed range is needed.
   • Disadvantages: Reduced efficiency, increased power losses in the rotor, and limited control range compared to other methods like VVVF (Variable Voltage Variable Frequency).

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Conclusion

The open circuit voltage ratio and speed control via rotor resistance insertion in a three-phase slip ring induction motor were successfully measured and analyzed. The open-circuit voltage ratio gives insight into the motor's efficiency, while the insertion of external resistance in the rotor circuit provides a practical method for controlling the motor's speed by increasing slip. However, this method comes with the trade-off of reduced efficiency due to increased rotor losses.