Control the speed of the given three phase squirrel cage induction motor using the applicable methods: i) autotransformer, ii) VVVF.

 Title: Speed Control of a Three-Phase Squirrel Cage Induction Motor Using Autotransformer and VVVF Methods 

Abstract

The purpose of this experiment is to control the speed of a three-phase squirrel cage induction motor using two different methods: Autotransformer and VVVF (Variable Voltage Variable Frequency) control. The Autotransformer method provides a simple and cost-effective way to reduce the voltage applied to the motor, thereby controlling its speed. The VVVF method, on the other hand, offers a more precise and efficient speed control mechanism by varying both the voltage and frequency supplied to the motor. This report covers the setup, testing, and performance analysis of both methods.

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Introduction

Induction motors are commonly used in various industrial and commercial applications due to their robustness, reliability, and simplicity. Speed control of three-phase squirrel cage induction motors is often required in processes where the motor's speed must be adjusted for optimal performance. The following methods are commonly used to control the speed of these motors:

1. Autotransformer Method: This method is often used for reducing the starting current and controlling the speed. It involves using an autotransformer to step down the voltage applied to the motor.
2. VVVF Control Method: This method is more advanced and involves controlling both the voltage and frequency supplied to the motor, providing a wide range of speed control.

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Materials and Methods

Materials:

• Three-phase squirrel cage induction motor
• Autotransformer
• Variable Frequency Drive (VFD) or VVVF controller
• Tachometer (for measuring speed)
• Digital ammeter, voltmeter, and wattmeter
• Load (dynamometer or mechanical load)
• Power supply (three-phase AC)
• Switchgear and control devices (such as contactors, fuses, etc.)

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Method 1: Speed Control Using Autotransformer (Autotransformer Method)

Procedure:

1. Autotransformer Setup:

   • Connect the induction motor to a three-phase power supply.
   • Use an autotransformer to reduce the voltage applied to the motor. Typically, the autotransformer reduces the voltage to about 50%-80% of the rated voltage, which reduces the starting current and torque.
   • Connect the autotransformer’s primary side to the power supply and its secondary side to the motor.

2. Test Execution:

   • Start the motor using the autotransformer.
   • Measure the current drawn by the motor during startup.
   • Measure the motor speed (using a tachometer) and compare it with the rated speed.
   • Gradually increase the load on the motor and observe the motor’s behavior under different load conditions.
   • If possible, vary the tapping on the autotransformer to control the voltage applied to the motor, and observe the changes in speed.

3. Results Analysis:

   • With an autotransformer, the voltage and thus the speed of the motor are reduced at the start.
   • Speed control is not as flexible with this method; you are limited to step-wise changes in voltage.
   • Typically, the motor speed is about 60-80% of the rated speed, depending on the autotransformer settings.

Key Formula for Speed Control Using Autotransformer:
The speed of an induction motor is directly proportional to the supply frequency, so reducing the voltage reduces the torque and the motor's effective speed.

$$N = \frac{120 \times f}{P}$$

Where:

• $N$ = Speed (in rpm)
• $f$ = Supply frequency (Hz)
• $P$ = Number of poles in the motor

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Method 2: Speed Control Using VVVF (Variable Voltage Variable Frequency) Method

Procedure:

1. VVVF Setup:

   • Connect the induction motor to the Variable Frequency Drive (VFD).
   • Ensure the VFD is set up to control both the voltage and frequency supplied to the motor. The VFD can adjust the voltage and frequency independently, allowing for continuous speed control.
   • Set the VFD to vary the frequency from its rated frequency (e.g., 50 Hz) to lower values, such as 40 Hz, 30 Hz, etc.

2. Test Execution:

   • Start the motor using the VFD and observe the speed as the frequency is varied.
   • Measure the motor speed at different frequencies (e.g., 50 Hz, 40 Hz, 30 Hz, etc.) using a tachometer.
   • Record the current drawn by the motor at each speed setting.
   • Measure the input power (voltage, current, and power factor) and observe how the motor's efficiency changes as the speed is varied.

3. Results Analysis:

   • As the frequency is reduced, the motor speed decreases in direct proportion to the supply frequency.
   • The voltage will also be reduced proportionally by the VFD, maintaining the torque characteristics of the motor.
   • The VVVF method allows for smooth and continuous speed variation without steps, providing precise control over the motor's speed.

Key Formula for Speed Control Using VVVF:
Speed and frequency are directly proportional in an induction motor:

$$N = \frac{120 \times f}{P}$$

Where:

• $N$ = Speed (in rpm)
• $f$ = Supply frequency (Hz)
• $P$ = Number of poles in the motor

In this case, the VFD allows you to vary $f$, and thus $N$, smoothly across a wide range.

Results and Data Analysis

• Autotransformer Method:

  • The motor will run at reduced speed, typically 60-80% of the rated speed.
  • The torque delivered by the motor will also be reduced due to the reduced voltage, making this method suitable for applications that don’t require full torque during startup or under load.

• VVVF Method:

  • The motor will run at a wide range of speeds, from zero to its rated speed, depending on the frequency set by the VFD.
  • The voltage and frequency are smoothly adjustable, which allows for fine-tuned control over speed, making this method suitable for applications requiring precise speed control.

Discussion/Analysis

1. Autotransformer Method:

   • This method is relatively simple and inexpensive, but it has limitations in terms of the range of speed control. The motor operates at a lower voltage and thus lower speed, which is not ideal for applications requiring a wide range of speeds.
   • The key limitation is that the speed control is step-based and not continuous.

2. VVVF Method:

   • The VVVF control method provides superior speed control over a wide range of speeds.
   • By varying both voltage and frequency, the motor’s performance can be optimized for specific load conditions. This method is more energy-efficient and offers smoother operation compared to the autotransformer method.
   • It also maintains the motor's torque characteristics across the speed range, which is an advantage in applications requiring precise speed control.

Conclusion

Both the Autotransformer method and the VVVF method are effective ways to control the speed of a three-phase squirrel cage induction motor. However, the VVVF method provides more flexibility, allowing for smooth, continuous speed control over a wide range, whereas the Autotransformer method is more suited for applications where only reduced-speed operation is required, typically at the startup phase. The choice between these methods depends on the specific needs of the application, such as cost, precision, and energy efficiency.